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  • 1.
    Acosta Navarro, Juan C.
    et al.
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Ekman, Annica M. L.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Pausata, Francesco S. R.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Lewinschal, Anna
    Stockholm University, Faculty of Science, Department of Meteorology .
    Varma, Vidya
    Seland, Øyvind
    Gauss, Michael
    Iversen, Trond
    Kirkevåg, Alf
    Riipinen, Ilona
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Hansson, Hans Christen
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Future response of temperature and precipitation to reduced aerosol emissions as compared with increased greenhouse gas concentrations2017In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 30, no 3, p. 939-954Article in journal (Refereed)
    Abstract [en]

    Experiments with a climate model (NorESM1) were performed to isolate the effects of aerosol particles and greenhouse gases on surface temperature and precipitation in simulations of future climate. The simulations show that by 2025-2049, a reduction of aerosol emissions from fossil fuels following a maximum technically feasible reduction (MFR) scenario could lead to a global and Arctic warming of 0.26 K and 0.84 K, respectively; as compared with a simulation with fixed aerosol emissions at the level of 2005. If fossil fuel emissions of aerosols follow a current legislation emissions (CLE) scenario, the NorESM1 model simulations yield a non-significant change in global and Arctic average surface temperature as compared with aerosol emissions fixed at year 2005. The corresponding greenhouse gas effect following the RCP4.5 emission scenario leads to a global and Arctic warming of 0.35 K and 0.94 K, respectively.

    The model yields a marked annual average northward shift in the inter-tropical convergence zone with decreasing aerosol emissions and subsequent warming of the northern hemisphere. The shift is most pronounced in the MFR scenario but also visible in the CLE scenario. The modeled temperature response to a change in greenhouse gas concentrations is relatively symmetric between the hemispheres and there is no marked shift in the annual average position of the inter-tropical convergence zone. The strong reduction in aerosol emissions in MFR also leads to a net southward cross-hemispheric energy transport anomaly both in the atmosphere and ocean, and enhanced monsoon circulation in Southeast and East Asia causing an increase in precipitation over a large part of this region.

  • 2. Agarwal, Sahil
    et al.
    Wettlaufer, John S.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Yale University, United States; University of Oxford, United Kingdom.
    The Statistical Properties of Sea Ice Velocity Fields2017In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 30, no 13, p. 4873-4881Article in journal (Refereed)
    Abstract [en]

    By arguing that the surface pressure field over the Arctic Ocean can be treated as an isotropic, stationary, homogeneous, Gaussian random field, Thorndike estimated a number of covariance functions from two years of data (1979 and 1980). Given the active interest in changes of general circulation quantities and indices in the polar regions during the recent few decades, the spatial correlations in sea ice velocity fields are of particular interest. It is thus natural to ask, "How persistent are these correlations?'' To this end, a multifractal stochastic treatment is developed to analyze observed Arctic sea ice velocity fields from satellites and buoys for the period 1978-2015. Since it was previously found that the Arctic equivalent ice extent (EIE) has a white noise structure on annual to biannual time scales, the connection between EIE and ice motion is assessed. The long-term stationarity of the spatial correlation structure of the velocity fields and the robustness of their white noise structure on multiple time scales is demonstrated; these factors (i) combine to explain the white noise characteristics of the EIE on annual to biannual time scales and (ii) explain why the fluctuations in the ice velocity are proportional to fluctuations in the geostrophic winds on time scales of days to months. Moreover, it is shown that the statistical structure of these two quantities is commensurate from days to years, which may be related to the increasing prevalence of free drift in the ice pack.

  • 3. Agarwal, Sahil
    et al.
    Wettlaufer, John S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Yale University, USA; University of Oxford, United Kingdom.
    The Statistical Properties of Sea Ice Velocity Fields2017In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 30, no 13, p. 4873-4881Article in journal (Refereed)
    Abstract [en]

    By arguing that the surface pressure field over the Arctic Ocean can be treated as an isotropic, stationary, homogeneous, Gaussian random field, Thorndike estimated a number of covariance functions from two years of data (1979 and 1980). Given the active interest in changes of general circulation quantities and indices in the polar regions during the recent few decades, the spatial correlations in sea ice velocity fields are of particular interest. It is thus natural to ask, How persistent are these correlations?'' To this end, a multifractal stochastic treatment is developed to analyze observed Arctic sea ice velocity fields from satellites and buoys for the period 1978-2015. Since it was previously found that the Arctic equivalent ice extent (EIE) has a white noise structure on annual to biannual time scales, the connection between EIE and ice motion is assessed. The long-term stationarity of the spatial correlation structure of the velocity fields and the robustness of their white noise structure on multiple time scales is demonstrated; these factors (i) combine to explain the white noise characteristics of the EIE on annual to biannual time scales and (ii) explain why the fluctuations in the ice velocity are proportional to fluctuations in the geostrophic winds on time scales of days to months. Moreover, it is shown that the statistical structure of these two quantities is commensurate from days to years, which may be related to the increasing prevalence of free drift in the ice pack.

  • 4. Ansell, T. J.
    et al.
    Jones, P. D.
    Allan, R. J.
    Lister, D.
    Parker, D. E.
    Brunet, M.
    Moberg, A.
    Jacobeit, J.
    Brohan, P.
    Rayner, N. A.
    Aguilar, E.
    Alexandersson, Hans
    SMHI.
    Barriendos, M.
    Brandsma, T.
    Cox, N. J.
    Della-Marta, P. M.
    Drebs, A.
    Founda, D.
    Gerstengarbe, F.
    Hickey, K.
    Jonsson, T.
    Luterbacher, J.
    Nordli, O.
    Oesterle, H.
    Petrakis, M.
    Philipp, A.
    Rodwell, M. J.
    Saladie, O.
    Sigro, J.
    Slonosky, V.
    Srnec, L.
    Swail, V.
    Garcia-Suarez, A. M.
    Tuomenvirta, H.
    Wang, X.
    Wanner, H.
    Werner, P.
    Wheeler, D.
    Xoplaki, E.
    Daily mean sea level pressure reconstructions for the European-North Atlantic region for the period 1850-20032006In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 19, no 12, p. 2717-2742Article in journal (Refereed)
    Abstract [en]

    The development of a daily historical European-North Atlantic mean sea level pressure dataset (EMSLP) for 1850-2003 on a 5 latitude by longitude grid is described. This product was produced using 86 continental and island stations distributed over the region 25 degrees-70 degrees N, 70 degrees W-50 degrees E blended with marine data from the International Comprehensive Ocean-Atmosphere Data Set (ICOADS). The EMSLP fields for 1850-80 are based purely on the land station data and ship observations. From 1881, the blended land and marine fields are combined with already available daily Northern Hemisphere fields. Complete coverage is obtained by employing reduced space optimal interpolation. Squared correlations (r(2)) indicate that EMSLP generally captures 80%-90% of daily variability represented in an existing historical mean sea level pressure product and over 90% in modern 40-yr European Centre for Medium-Range Weather Forecasts Re-Analyses (ERA-40) over most of the region. A lack of sufficient observations over Greenland and the Middle East, however, has resulted in poorer reconstructions there. Error estimates, produced as part of the reconstruction technique, flag these as regions of low confidence. It is shown that the EMSLP daily fields and associated error estimates provide a unique opportunity to examine the circulation patterns associated with extreme events across the European-North Atlantic region, such as the 2003 heat wave, in the context of historical events.

  • 5.
    Beer, Christian
    et al.
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM). Max Planck Society, Germany.
    Weber, Ulrich
    Tomelleri, Enrico
    Carvalhais, Nuno
    Mahecha, Miguel
    Reichstein, Markus
    Harmonized European Long-Term Climate Data for Assessing the Effect of Changing Temporal Variability on Land-Atmosphere CO2 Fluxes2014In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 27, no 13, p. 4815-4834Article in journal (Refereed)
    Abstract [en]

    Temporal variability of meteorological variables and extreme weather events is projected to increase in many regions of the world during the next century. Artificial experiments using process-oriented terrestrial ecosystem models make it possible to isolate effects of temporal variability from effects of gradual climate change on terrestrial ecosystem functions and the system state. Such factorial experiments require two long-term climate datasets: 1) a control dataset that represents observed and projected climate and 2) a dataset with the same long-term mean as the control dataset but with altered short-term variability. Using a bias correction method, various climate datasets spanning different periods are harmonized and then combined with the control dataset with consistent time series for Europe during 1901-2100. Then, parameters of a distribution transformation function are estimated for individual meteorological variables to derive the second climate dataset, which has similar long-term means but reduced temporal variability. The transformation conserves the number of rainy days within a month and the shape of the daily meteorological data distributions, which is important to ensure that, for example, drought duration does not modify the suitability of localized vegetation type to precipitation regimes. The median absolute difference between daily data of both datasets is 5% to 20%. On average, decadal extreme values are reduced by 2% to 35%. Driving a terrestrial ecosystem model with both climate datasets shows a general higher gross primary production under reduced temporal climate variability. This effect of climate variability on productivity demonstrates the potential of the climate datasets for studying various effects of temporal variability on ecosystem state and functions over large domains.

  • 6.
    Bender, Frida A. -M.
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Engström, Anders
    Stockholm University, Faculty of Science, Department of Meteorology .
    Wood, Robert
    Charlson, Robert J.
    Evaluation of Hemispheric Asymmetries in Marine Cloud Radiative Properties2017In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 30, no 11, p. 4131-4147Article in journal (Refereed)
    Abstract [en]

    The hemispheric symmetry of albedo and its contributing factors in satellite observations and global climate models is evaluated. The analysis is performed on the annual mean time scale, on which a bimodality in the joint distribution of albedo and cloud fraction is evident, resulting from tropical and subtropical clouds and midlatitude clouds, respectively. Hemispheric albedo symmetry is not found in individual ocean-only latitude bands; comparing the Northern and Southern Hemisphere (NH and SH), regional mean albedo is higher in the NH tropics and lower in the NH subtropics and midlatitudes than in the SH counterparts. This follows the hemispheric asymmetry of cloud fraction. In midlatitudes and tropics the hemispheric asymmetry in cloud albedo also contributes to the asymmetry in total albedo, whereas in the subtropics the cloud albedo is more hemispherically symmetric. According to the observations, cloud contributions to compensation for higher clear-sky albedo in the NH come primarily from cloud albedo in midlatitudes and cloud amount in the subtropics. Current-generation climate models diverge in their representation of these relationships, but common features of the model-data comparison include weaker-than-observed asymmetry in cloud fraction and cloud albedo in the tropics, weaker or reversed cloud fraction asymmetry in the subtropics, and agreement with observed cloud albedo asymmetry in the midlatitudes. Models on average reproduce the NH-SH asymmetry in total albedo over the 60 degrees S-60 degrees N ocean but show higher occurrence of brighter clouds in the SH compared to observations. The albedo bias in both hemispheres is reinforced by overestimated clear-sky albedo in the models.

  • 7.
    Bender, Frida
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Engström, Anders
    Stockholm University, Faculty of Science, Department of Meteorology .
    Karlsson, Johannes
    Stockholm University, Faculty of Science, Department of Meteorology .
    Factors controlling cloud albedo in marine subtropical stratocumulus regions in climate models and satellite observations2016In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 29, no 10, p. 3559-3587Article in journal (Refereed)
    Abstract [en]

    This study focuses on the radiative properties of five subtropical marine stratocumulus cloud regions, on monthly mean scale. Through examination of the relation between total albedo and cloud fraction, and its variability and relation to other parameters, some of the factors controlling the reflectivity, or albedo, of the clouds in these regions are investigated. It is found that the main part of the variability in albedo at a given cloud fraction can be related to temporal, rather than spatial variability, indicating spatial homogeneity in cloud radiative properties in the studied regions. This is seen most clearly in satellite observations, but also in an ensemble of climate models. Further comparison between satellite data and output from climate models shows that there is good agreement with respect to the role of liquid water path, the parameter that can be assumed to be the primary source of variability in cloud reflectivity for a given cloud fraction. On the other hand, the influence of aerosol loading on cloud albedo differs between models and observations. The cloud-albedo effect, or cloud brightening caused by aerosol through its coupling to cloud droplet number concentration and droplet size, is found not to dominate in the satellite observations on monthly mean scale, as it appears to do on this scale in the climate models. The disagreement between models and observations is particularly strong in regions with frequent occurrence of absorbing aerosols above clouds, where satellite data contrary to the climate models indicate a scene darkening with increasing aerosol loading.

  • 8. Bjork, Goran
    et al.
    Stranne, Christian
    Borenas, Karin
    The Sensitivity of the Arctic Ocean Sea Ice Thickness and Its Dependence on the Surface Albedo Parameterization2013In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 26, no 4, p. 1355-1370Article in journal (Refereed)
    Abstract [en]

    In this study, the response of sea ice thickness to changes in the external forcing is investigated and particularly how this response depends on the surface albedo formulation by means of a one-dimensional coupled ocean-ice-atmosphere model. The main focus is on the thickness response to the atmospheric heat advection F-wall, solar radiation F-SW, and amount of snow precipitation S-prec. Different albedo parameterization schemes [ECHAM5, CSIRO, and Community Climate System Model, version 3 (CCSM3)] representing albedos commonly used in global climate models are compared together with more simplified schemes. Using different albedo schemes with the same external forcing produces large differences in ice thickness. The ice thickness response is similar for all realistic albedo schemes with a nearly linear decrease with increasing F-wall in the perennial ice regime and with a steplike transition into seasonal ice when F-wall exceeds a certain threshold. This transition occurs at an annual-mean ice thickness of 1.7-2.0 m. Latitudinal differences in solar insolation generally leads to increasing ice thickness toward the North Pole. The snow response varies significantly depending on which albedo scheme is used. The ECHAM5 scheme yields thinner ice with S-prec, the CSIRO scheme gives ice thickness nearly independent of S-prec, and with the CCSM3 scheme the ice thickness decreases with S-prec. A general result is that the modeled ice cover is rather sensitive to positive perturbations of the external heat supply when it is close to the transition such that just a small increase of, for example, F-wall can force the ice cover into the seasonal regime.

  • 9. Bjork, Goran
    et al.
    Stranne, Christian
    Borenäs, Karin
    SMHI, Core Services.
    The Sensitivity of the Arctic Ocean Sea Ice Thickness and Its Dependence on the Surface Albedo Parameterization2013In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 26, no 4, p. 1355-1370Article in journal (Refereed)
    Abstract [en]

    In this study, the response of sea ice thickness to changes in the external forcing is investigated and particularly how this response depends on the surface albedo formulation by means of a one-dimensional coupled ocean-ice-atmosphere model. The main focus is on the thickness response to the atmospheric heat advection F-wall, solar radiation F-SW, and amount of snow precipitation S-prec. Different albedo parameterization schemes [ECHAM5, CSIRO, and Community Climate System Model, version 3 (CCSM3)] representing albedos commonly used in global climate models are compared together with more simplified schemes. Using different albedo schemes with the same external forcing produces large differences in ice thickness. The ice thickness response is similar for all realistic albedo schemes with a nearly linear decrease with increasing F-wall in the perennial ice regime and with a steplike transition into seasonal ice when F-wall exceeds a certain threshold. This transition occurs at an annual-mean ice thickness of 1.7-2.0 m. Latitudinal differences in solar insolation generally leads to increasing ice thickness toward the North Pole. The snow response varies significantly depending on which albedo scheme is used. The ECHAM5 scheme yields thinner ice with S-prec, the CSIRO scheme gives ice thickness nearly independent of S-prec, and with the CCSM3 scheme the ice thickness decreases with S-prec. A general result is that the modeled ice cover is rather sensitive to positive perturbations of the external heat supply when it is close to the transition such that just a small increase of, for example, F-wall can force the ice cover into the seasonal regime.

  • 10. Brandefelt, Jenny
    Atmospheric modes of variability in a changing climate2006In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 19, no 22, p. 5934-5943Article in journal (Refereed)
    Abstract [en]

    The response of the atmospheric circulation to an enhanced radiative greenhouse gas forcing in a transient integration with a coupled global climate model is investigated. The spatial patterns of the leading modes of Northern Hemisphere atmospheric variability are shown to change in response to the enhanced forcing. An earlier study showed that the spatial patterns of the leading modes in the Southern Hemisphere changed in response to the enhanced forcing. These changes were associated with changes in the propagation conditions for barotropic Rossby waves. This is, however, not the case for the Northern Hemisphere, where the propagation conditions are unchanged. Other possible mechanisms for the changes in the spatial patterns of the leading modes are discussed.

  • 11. Brandefelt, Jenny
    et al.
    Kallen, E.
    The response of the Southern Hemisphere atmospheric circulation to an enhanced greenhouse gas forcing2004In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 17, no 22, p. 4425-4442Article in journal (Refereed)
    Abstract [en]

    The response of the atmospheric circulation to an enhanced radiative greenhouse gas forcing is investigated. It has been proposed that the response of the climate system to an enhanced forcing projects directly onto the preexisting natural modes of variability. An evaluation of this possibility and in particular of the implications of unchanged flow regimes is performed with a focus on the Southern Hemisphere extratropical atmospheric circulation. Low-pass-filtered mean sea level pressure and geopotential height at 500 and 200 hPa from a transient integration with a coupled global climate model is used. The response to an enhanced forcing projects strongly onto the leading modes of present-day variability, in agreement with other studies. However, the spatial patterns of the leading modes are changed in response to enhanced forcing. The first and second modes of interweekly variability are the Pacific-South American modes, zonal wavenumber-3 wave trains from the central Pacific to the southern Atlantic. In response to the enhanced forcing, the spatial patterns of these modes change, and the wave train extends along a circumpolar path with amplitude also in the Eastern Hemisphere. This change in the spatial patterns is associated with a strengthening of the waveguide for barotropic Rossby waves.

  • 12.
    Brandefelt, Jenny
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence.
    Kornich, Heiner
    Northern Hemisphere Stationary Waves in Future Climate Projections2008In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 21, no 23, p. 6341-6353Article in journal (Refereed)
    Abstract [en]

    The response of the atmospheric large-scale circulation to an enhanced greenhouse gas (GHG) forcing varies among coupled global climate model (CGCM) simulations. In this study, 16 CGCM simulations of the response of the climate system to a 1% yr(-1) increase in the atmospheric CO2 concentration to quadrupling are analyzed with focus on Northern Hemisphere winter. A common signal in 14 out of the 16 simulations is an increased or unchanged stationary wave amplitude. A majority of the simulations may be categorized into one of three groups based on the GHG-induced changes in the atmospheric stationary waves. The response of the zonal mean barotropic wind is similar within each group. Fifty percent of the simulations belong to the first group, which is categorized by a stationary wave with five waves encompassing the entire NH and a strengthening of the zonal mean barotropic wind. The second and third groups, respectively consisting of three and two simulations, are characterized by a broadening and a northward shift of the zonal mean barotropic wind, respectively. A linear model of barotropic vorticity is employed to study the importance of these mean flow changes to the stationary wave response. The linear calculations indicate that the GHG-induced mean wind changes explain 50%, 4%, and 37% of the stationary wave changes in each group, respectively. Thus, for the majority of simulations the zonal mean wind changes do significantly explain the stationary wave response.

  • 13. Büntgen, Ulf
    et al.
    Krusic, Paul J.
    Stockholm University, Faculty of Science, Department of Physical Geography. University of Cambridge, United Kingdom.
    Verstege, Anne
    Sanguesa-Barreda, Gabriel
    Wagner, Sebastian
    Julio Camarero, J.
    Ljungqvist, Fredrik Charpentier
    Stockholm University, Faculty of Humanities, Department of History.
    Zorita, Eduardo
    Oppenheimer, Clive
    Konter, Oliver
    Tegel, Willy
    Gärtner, Holger
    Cherubini, Paolo
    Reinig, Frederick
    Esper, Jan
    New Tree-Ring Evidence from the Pyrenees Reveals Western Mediterranean Climate Variability since Medieval Times2017In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 30, no 14, p. 5295-5318Article in journal (Refereed)
    Abstract [en]

    Paleoclimatic evidence is necessary to place the current warming and drying of the western Mediterranean basin in a long-term perspective of natural climate variability. Annually resolved and absolutely dated temperature proxies south of the European Alps that extend back into medieval times are, however, mainly limited to measurements of maximum latewood density (MXD) from high-elevation conifers. Here, the authors present the world's best replicated MXD site chronology of 414 living and relict Pinus uncinata trees found >2200 m above mean sea level (MSL) in the Spanish central Pyrenees. This composite record correlates significantly (p <= 0.01) with May-June and August-September mean temperatures over most of the Iberian Peninsula and northern Africa (r = 0.72; 1950-2014). Spanning the period 1186-2014 of the Common Era (CE), the new reconstruction reveals overall warmer conditions around 1200 and 1400, and again after around 1850. The coldest reconstructed summer in 1258 (-4.4 degrees C compared to 1961-90) followed the largest known volcanic eruption of the CE. The twentieth century is characterized by pronounced summer cooling in the 1970s, subsequently rising temperatures until 2003, and a slowdown of warming afterward. Little agreement is found with climate model simulations that consistently overestimate recent summer warming and underestimate preindustrial temperature changes. Interannual-multidecadal covariability with regional hydroclimate includes summer pluvials after large volcanic eruptions. This study demonstrates the relevance of updating MXD-based temperature reconstructions, not only back in time but also toward the present, and emphasizes the importance of comparing temperature and hydroclimatic proxies, as well as model simulations for understanding regional climate dynamics.

  • 14. Büntgen, Ulf
    et al.
    Trnka, Miroslav
    Krusic, Paul J.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Kyncl, Tomáš
    Kyncl, Josef
    Luterbacher, Jürg
    Zorita, Eduardo
    Charpentier Ljungqvist, Fredrik
    Stockholm University, Faculty of Humanities, Department of History.
    Auer, Ingeborg
    Konter, Oliver
    Schneider, Lea
    Tegel, Willy
    Štěpánek, Petr
    Brönnimann, Stefan
    Hellmann, Lena
    Nievergelt, Daniel
    Esper, Jan
    Tree-Ring Amplification of the Early Nineteenth-Century Summer Cooling in Central Europe2015In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 28, no 13, p. 5272-5288Article in journal (Refereed)
    Abstract [en]

    Annually resolved and absolutely dated tree-ring chronologies are the most important proxy archives to reconstruct climate variability over centuries to millennia. However, the suitability of tree-ring chronologies to reflect the “true” spectral properties of past changes in temperature and hydroclimate has recently been debated. At issue is the accurate quantification of temperature differences between early nineteenth-century cooling and recent warming. In this regard, central Europe (CEU) offers the unique opportunity to compare evidence from instrumental measurements, paleomodel simulations, and proxy reconstructions covering both the exceptionally hot summer of 2003 and the year without summer in 1816. This study uses 565 Swiss stone pine (Pinus cembra) ring width samples from high-elevation sites in the Slovakian Tatra Mountains and Austrian Alps to reconstruct CEU summer temperatures over the past three centuries. This new temperature history is compared to different sets of instrumental measurements and state-of-the-art climate model simulations. All records independently reveal the coolest conditions in the 1810s and warmest after 1996, but the ring width–based reconstruction overestimates the intensity and duration of the early nineteenth-century summer cooling by approximately 1.5°C at decadal scales. This proxy-specific deviation is most likely triggered by inflated biological memory in response to reduced warm season temperature, together with changes in radiation and precipitation following the Tambora eruption in April 1815. While suggesting there exists a specific limitation in ring width chronologies to capture abrupt climate perturbations with increased climate system inertia, the results underline the importance of alternative dendrochronological and wood anatomical parameters, including stable isotopes and maximum density, to assess the frequency and severity of climatic extremes.

  • 15.
    Charpentier Ljungqvist, Fredrik
    et al.
    Stockholm University, Faculty of Humanities, Department of History. University of Cambridge, United Kingdom.
    Zhang, Qiong
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Brattström, Gudrun
    Stockholm University, Faculty of Science, Department of Mathematics.
    Krusic, Paul J.
    Stockholm University, Faculty of Science, Department of Physical Geography. University of Cambridge, United Kingdom.
    Seim, Andrea
    Li, Qiang
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Zhang, Qiang
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Moberg, Anders
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Centennial-Scale Temperature Change in Last Millennium Simulations and Proxy-Based Reconstructions2019In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 32, no 9, p. 2441-2482Article in journal (Refereed)
    Abstract [en]

    Systematic comparisons of proxy-based reconstructions and climate model simulations of past millennium temperature variability offer insights into climate sensitivity and feedback mechanisms, besides allowing model evaluation independently from the period covered by instrumental data. Such simulation-reconstruction comparisons can help to distinguish more skillful models from less skillful ones, which may subsequently help to develop more reliable future projections. This study evaluates the low-frequency simulation-reconstruction agreement within the past millennium through assessing the amplitude of temperature change between the Medieval Climate Anomaly (here, 950-1250 CE) and the Little Ice Age (here, 1450-1850 CE) in PMIP3 model simulations compared to proxy-based local and continental-scale reconstructions. The simulations consistently show a smaller temperature change than the reconstructions for most regions in the Northern Hemisphere, but not in the Southern Hemisphere, as well as a partly different spatial pattern. A cost function analysis assesses how well the various simulations agree with reconstructions. Disregarding spatial correlation, significant differences are seen in the agreement with the local temperature reconstructions between groups of models, but insignificant differences are noted when compared to continental-scale reconstructions. This result points toward a limited possibility to rank models by means of their low-frequency temperature variability alone. The systematically lower amplitude of simulated versus reconstructed temperature change indicates either too-small simulated internal variability or that the analyzed models lack some critical forcing or have missing or too-weak feedback mechanisms. We hypothesize that too-cold initial ocean conditions in the models-in combination with too-weak internal variability and slow feedbacks over longer time scales-could account for much of the simulation-reconstruction disagreement.

  • 16. Christiansen, Bo
    et al.
    Charpentier Ljungqvist, Fredrik
    Stockholm University, Faculty of Humanities, Department of History.
    Reconstruction of the Extratropical NH Mean Temperature over the Last Millennium with a Method that Preserves Low-Frequency Variability2011In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 24, no 23, p. 6013-6034Article in journal (Refereed)
    Abstract [en]

    A new multiproxy reconstruction of the Northern Hemisphere extratropical mean temperature over the last millennium is presented. The reconstruction is performed with a novel method designed to avoid the underestimation of low-frequency variability that has been a general problem for regression-based reconstruction methods. The disadvantage of this method is an exaggerated high-frequency variability. The reconstruction is based on a set of 40 proxies of annual to decadal resolution that have been shown to relate to the local temperature. The new reconstruction shows a very cold Little Ice Age centered around the 17th century with a cold extremum (for 50-yr smoothing) of about 1.1 K below the temperature of the calibration period, AD 1880–1960. This cooling is about twice as large as corresponding numbers reported by most other reconstructions. In the beginning of the millennium the new reconstruction shows small anomalies in agreement with previous studies. However, the new temperature reconstruction decreases faster than previous reconstructions in the first 600 years of the millennium and has a stronger variability. The salient features of the new reconstruction are shown to be robust to changes in the calibration period, the source of the local temperatures, the spatial averaging procedure, and the screening process applied to the proxies. An ensemble pseudoproxy approach is applied to estimate the confidence intervals of the 50-yr smoothed reconstruction showing that the period AD 1500–1850 is significantly colder than the calibration period.

  • 17. Christiansen, Bo
    et al.
    Charpentier Ljungqvist, Fredrik
    Stockholm University, Faculty of Humanities, Department of History.
    Reply to “Comments on ‘Reconstruction of the Extratropical NH Mean Temperature over the Last Millennium with a Method That Preserves Low-Frequency Variability’”2012In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 25, no 22, p. 7998-8003Article in journal (Refereed)
  • 18. Dangendorf, Soenke
    et al.
    Müller-Navarra, Sylvin
    Jensen, Juergen
    Schenk, Frederik
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Wahl, Thomas
    Weisse, Ralf
    North Sea Storminess from a Novel Storm Surge Record since AD 18432014In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 27, no 10, p. 3582-3595Article in journal (Refereed)
    Abstract [en]

    The detection of potential long-term changes in historical storm statistics and storm surges plays a vitally important role for protecting coastal communities. In the absence of long homogeneous wind records, the authors present a novel, independent, and homogeneous storm surge record based on water level observations in the North Sea since 1843. Storm surges are characterized by considerable interannual-to-decadal variability linked to large-scale atmospheric circulation patterns. Time periods of increased storm surge levels prevailed in the late nineteenth and twentieth centuries without any evidence for significant long-term trends. This contradicts with recent findings based on reanalysis data, which suggest increasing storminess in the region since the late nineteenth century. The authors compare the wind and pressure fields from the Twentieth-Century Reanalysis (20CRv2) with the storm surge record by applying state-of-the-art empirical wind surge formulas. The comparison reveals that the reanalysis is a valuable tool that leads to good results over the past 100 yr; previously the statistical relationship fails, leaving significantly lower values in the upper percentiles of the predicted surge time series. These low values lead to significant upward trends over the entire investigation period, which are in turn supported by neither the storm surge record nor an independent circulation index based on homogeneous pressure readings. The authors therefore suggest that these differences are related to higher uncertainties in the earlier years of the 20CRv2 over the North Sea region.

  • 19.
    Dekker, Evelien
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Bintanja, Richard
    Severijns, Camiel
    Nudging the Arctic Ocean to Quantify Sea Ice Feedbacks2019In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 32, no 8, p. 2381-2395Article in journal (Refereed)
    Abstract [en]

    With Arctic summer sea ice potentially disappearing halfway through this century, the surface albedo and insulating effects of Arctic sea ice will decrease considerably. The ongoing Arctic sea ice retreat also affects the strength of the Planck, lapse rate, cloud, and surface albedo feedbacks together with changes in the heat exchange between the ocean and the atmosphere, but their combined effect on climate sensitivity has not been quantified. This study presents an estimate of all Arctic sea ice related climate feedbacks combined. We use a new method to keep Arctic sea ice at its present-day (PD) distribution under a changing climate in a 50-yr CO2 doubling simulation, using a fully coupled global climate model (EC-Earth, version 2.3). We nudge the Arctic Ocean to the (monthly dependent) year 2000 mean temperature and minimum salinity fields on a mask representing PD sea ice cover. We are able to preserve about 95% of the PD mean March and 77% of the September PD Arctic sea ice extent by applying this method. Using simulations with and without nudging, we estimate the climate response associated with Arctic sea ice changes. The Arctic sea ice feedback globally equals 0.28 +/- 0.15 W m(-2) K-1. The total sea ice feedback thus amplifies the climate response for a doubling of CO2, in line with earlier findings. Our estimate of the Arctic sea ice feedback agrees reasonably well with earlier CMIP5 global climate feedback estimates and shows that the Arctic sea ice exerts a considerable effect on the Arctic and global climate sensitivity.

  • 20.
    Devasthale, Abhay
    et al.
    SMHI, Research Department, Atmospheric remote sensing.
    Thomas, Manu Anna
    SMHI, Research Department, Air quality.
    Sensitivity of Cloud Liquid Water Content Estimates to the Temperature-Dependent Thermodynamic Phase: A Global Study Using CloudSat Data2012In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 25, no 20, p. 7297-7307Article in journal (Refereed)
    Abstract [en]

    The main purpose of this study is to underline the sensitivity of cloud liquid water content (LWC) estimates purely to 1) the shape of computationally simplified temperature-dependent thermodynamic phase and 2) the range of subzero temperatures covered to partition total cloud condensate into liquid and ice fractions. Linear, quadratic, or sigmoid-shaped functions for subfreezing temperatures (down to -20 degrees or -40 degrees C) are often used in climate models and reanalysis datasets for partitioning total condensate. The global vertical profiles of clouds obtained from CloudSat for the 4-yr period June 2006-May 2010 are used for sensitivity analysis and the quantitative estimates of sensitivities based on these realistic cloud profiles are provided. It is found that three cloud regimes in particular-convective clouds in the tropics, low-level clouds in the northern high latitudes, and middle-level clouds over the midlatitudes and Southern Ocean-are most sensitive to assumptions on thermodynamic phase. In these clouds, the LWC estimates based purely on quadratic or sigmoid-shaped functions with a temperature range down to -20 degrees C can differ by up to 20%-40% over the tropics (in seasonal means). 10%-30% over the midlatitudes, and up to 50% over high latitudes compared to a linear assumption. When the temperature range is extended down to -40 degrees C. LWC estimates in the sigmoid case can be much higher than the above values over high-latitude regions compared to the commonly used case with quadratic dependency down to -20 C. This sensitivity study emphasizes the need to critically investigate radiative impacts of cloud thermodynamic phase assumptions in simplified climate models and reanalysis datasets.

  • 21.
    Döös, Kristofer
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Kjellsson, Joakim
    Zika, Jan
    Laliberte, Frederic
    Brodeau, Laurent
    Stockholm University, Faculty of Science, Department of Meteorology .
    Aldama Campino, Aitor
    Stockholm University, Faculty of Science, Department of Meteorology .
    The Coupled Ocean-Atmosphere Hydrothermohaline Circulation2017In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 30, no 2, p. 631-647Article in journal (Refereed)
    Abstract [en]

    The thermohaline circulation of the ocean is compared to the hydrothermal circulation of the atmosphere. The oceanic thermohaline circulation is expressed in potential temperature-absolute salinity space and comprises a tropical cell, a conveyor belt cell, and a polar cell, whereas the atmospheric hydrothermal circulation is expressed in potential temperature-specific humidity space and unifies the tropical Hadley and Walker cells as well as the midlatitude eddies into a single, global circulation. The oceanic thermohaline streamfunction makes it possible to analyze and quantify the entire World Ocean conversion rate between cold-warm and fresh-saline waters in one single representation. Its atmospheric analog, the hydrothermal streamfunction, instead captures the conversion rate between cold-warm and dry-humid air in one single representation. It is shown that the ocean thermohaline and the atmospheric hydrothermal cells are connected by the exchange of heat and freshwater through the sea surface. The two circulations are compared on the same diagramby scaling the axes such that the latent heat energy required to move an air parcel on the moisture axis is equivalent to that needed to move a water parcel on the salinity axis. Such a comparison leads the authors to propose that the Clausius-Clapeyron relationship guides both the moist branch of the atmospheric hydrothermal circulation and the warming branches of the tropical and conveyor belt cells of the oceanic thermohaline circulation.

  • 22. Endris, Hussen Seid
    et al.
    Omondi, Philip
    Jain, Suman
    Lennard, Christopher
    Hewitson, Bruce
    Chang'a, Ladislaus
    Awange, J. L.
    Dosio, Alessandro
    Ketiem, Patrick
    Nikulin, Grigory
    SMHI, Research Department, Climate research - Rossby Centre.
    Panitz, Hans-Juergen
    Buechner, Matthias
    Stordal, Frode
    Tazalika, Lukiya
    Assessment of the Performance of CORDEX Regional Climate Models in Simulating East African Rainfall2013In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 26, no 21, p. 8453-8475Article, review/survey (Refereed)
    Abstract [en]

    This study evaluates the ability of 10 regional climate models (RCMs) from the Coordinated Regional Climate Downscaling Experiment (CORDEX) in simulating the characteristics of rainfall patterns over eastern Africa. The seasonal climatology, annual rainfall cycles, and interannual variability of RCM output have been assessed over three homogeneous subregions against a number of observational datasets. The ability of the RCMs in simulating large-scale global climate forcing signals is further assessed by compositing the El Nino-Southern Oscillation (ENSO) and Indian Ocean dipole (IOD) events. It is found that most RCMs reasonably simulate the main features of the rainfall climatology over the three subregions and also reproduce the majority of the documented regional responses to ENSO and IOD forcings. At the same time the analysis shows significant biases in individual models depending on subregion and season; however, the ensemble mean has better agreement with observation than individual models. In general, the analysis herein demonstrates that the multimodel ensemble mean simulates eastern Africa rainfall adequately and can therefore be used for the assessment of future climate projections for the region.

  • 23. England, Matthew H.
    et al.
    Hutchinson, David K.
    University of New South Wales, Australia.
    Santoso, Agus
    Sijp, Willem P.
    Ice-Atmosphere Feedbacks Dominate the Response of the Climate System to Drake Passage Closure2017In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 30, no 15, p. 5775-5790Article in journal (Refereed)
    Abstract [en]

    The response of the global climate system to Drake Passage (DP) closure is examined using a fully coupled ocean-atmosphere-ice model. Unlike most previous studies, a full three-dimensional atmospheric general circulation model is included with a complete hydrological cycle and a freely evolving wind field, as well as a coupled dynamic-thermodynamic sea ice module. Upon DP closure the initial response is found to be consistent with previous ocean-only and intermediate-complexity climate model studies, with an expansion and invigoration of the Antarctic meridional overturning, along with a slowdown in North Atlantic Deep Water (NADW) production. This results in a dominance of Southern Ocean poleward geostrophic flow and Antarctic sinking when DP is closed. However, within just a decade of DP closure, the increased southward heat transport has melted back a substantial fraction of Antarctic sea ice. At the same time the polar oceans warm by 4 degrees-6 degrees C on the zonal mean, and the maximum strength of the Southern Hemisphere westerlies weakens by similar or equal to 10%. These effects, not captured in models without ice and atmosphere feedbacks, combine to force Antarctic Bottom Water (AABW) to warm and freshen, to the point that this water mass becomes less dense than NADW. This leads to a marked contraction of the Antarctic overturning, allowing NADW to ventilate the abyssal ocean once more. Poleward heat transport settles back to very similar values as seen in the unperturbed DP open case. Yet remarkably, the equilibrium climate in the closed DP configuration retains a strong Southern Hemisphere warming, similar to past studies with no dynamic atmosphere. However, here it is ocean-atmosphere-ice feedbacks, primarily the ice-albedo feedback and partly the weakened midlatitude jet, not a vigorous southern sinking, which maintain the warm polar oceans. This demonstrates that DP closure can drive a hemisphere-scale warming with polar amplification, without the presence of any vigorous Southern Hemisphere overturning circulation. Indeed, DP closure leads to warming that is sufficient over the West Antarctic Ice Sheet region to inhibit ice-sheet growth. This highlights the importance of the DP gap, Antarctic sea ice, and the associated ice-albedo feedback in maintaining the present-day glacial state over Antarctica.

  • 24. Engström, Anders
    et al.
    Bender, Frida A-M.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Karlsson, Johannes
    Stockholm University, Faculty of Science, Department of Meteorology .
    Improved Representation of Marine Stratocumulus Cloud Shortwave Radiative Properties in the CMIP5 Climate Models2014In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 27, no 16, p. 6175-6188Article in journal (Refereed)
    Abstract [en]

    The radiative properties of subtropical marine stratocumulus clouds are investigated in an ensemble of current-generation global climate models from phase 5 of the Climate Model Intercomparison Project (CMIP5). Using a previously documented method for determining regional mean cloud albedo, the authors find a closer agreement with observations in the CMIP5 models as compared to the previous generation of models (phase 3 of CMIP). The multimodel average indicates regional mean, monthly mean cloud albedos ranging from 0.32 to 0.5 among 26 models and five regions, to be compared with satellite observations that indicate a range from 0.32 to 0.39 for the same five regions. The intermodel spread in cloud fraction gives rise to a spread in albedo. Within models, there is a tendency for large cloud fraction to be related to low cloud albedo and vice versa, a relationship that dampens the intermodel variability in total albedo. The intramodel variability in albedo, for a given cloud fraction, is found to be up to twice as large in magnitude in models as in satellite observations. The reason for this larger variability in models is not settled, but possible contributing factors may be imperfect representation in the models of cloud type distribution or of sensitivity to meteorological variability or aerosols. Changes in aerosol loading are found to be the likely cause of an increase in cloud albedo over time. The radiative effect of such a scene brightening in marine stratocumulus cloud regions, from preindustrial times to present day, is estimated to be up to -1W m(-2) for the global ocean, but there are no observations to verify this number.

  • 25.
    Engström, Anders
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Karlsson, Johannes
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    The Importance of Representing Mixed-Phase Clouds for Simulating Distinctive Atmospheric States in the Arctic2014In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 27, no 1, p. 265-272Article in journal (Refereed)
    Abstract [en]

    Observations from the Surface Heat Budget of the Arctic Ocean experiment (SHEBA) suggest that the Arctic Basin is characterized by two distinctly different preferred atmospheric states during wintertime. These states appear as two peaks in the frequency distribution of surface downwelling longwave radiation (LWD), representing radiatively clear and opaque conditions. Here, the authors have investigated the occurrence and representation of these states in the widely used ECMWF Interim Re-Analysis (ERA-Interim) dataset. An interannually recurring bimodal distribution of LWD values is not a clearly observable feature in the reanalysis data. However, large differences in the simulated liquid water content of clouds in ERA-Interim compared to observations are identified and these are linked to the lack of a radiatively opaque peak in the reanalysis. Using a single-column model, dynamically controlled by data from ERA-Interim, the authors show that, by tuning the glaciation speed of supercooled liquid clouds, it is possible to reach a very good agreement between the model and observations from the SHEBA campaign in terms of LWD. The results suggest that the presence of two preferred Arctic states, as observed during SHEBA, is a recurring feature of the Arctic climate system during winter [December–March (DJFM)]. The mean increase in LWD during the Arctic winter compared to ERA-Interim is 15 W m−2. This has a substantial bearing on climate model evaluation in the Arctic as it indicates the importance of representing Arctic states in climate models and reanalysis data and that doing so could have a significant impact on winter ice thickness and surface temperatures in the Arctic.

  • 26.
    Fitch, Anna
    SMHI, Research Department, Climate research - Rossby Centre.
    Climate Impacts of Large-Scale Wind Farms as Parameterized in a Global Climate Model2015In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 28, no 15, p. 6160-6180Article in journal (Refereed)
    Abstract [en]

    The local, regional, and global climate impacts of a large-scale global deployment of wind power in regionally high densities over land are investigated for a 60-yr period. Wind farms are represented as elevated momentum sinks as well as enhanced turbulence to represent turbine blade mixing in the Community Atmosphere Model, version 5 (CAM5), a global climate model. For a total installed capacity of 2.5 TW, to provide 16% of the world's projected electricity demand in 2050, minimal impacts are found both regionally and globally on temperature, sensible and latent heat fluxes, cloud, and precipitation. A mean near-surface warming of 0.12 +/- 0.07 K is seen within the wind farms, with a global-mean temperature change of -0.013 +/- 0.015 K. Impacts on wind speed and turbulence are more pronounced but largely confined within the wind farm areas. Increasing the wind farm areas to provide an installed capacity of 10 TW, or 65% of the 2050 electricity demand, causes further impacts; however, they remain slight overall. Maximum temperature changes are less than 0.5 K in the wind farm areas. To provide 20 TW of installed capacity, or 130% of the 2050 electricity demand, impacts both within the wind farms and beyond become more pronounced, with a doubling in turbine density. However, maximum temperature changes remain less than 0.7 K. Representing wind farms instead as an increase in surface roughness generally produces similar mean results; however, maximum changes increase, and influences on wind and turbulence are exaggerated. Overall, wind farm impacts are much weaker than those expected from greenhouse gas emissions, with very slight global-mean climate impacts.

  • 27. Fomichev, V. I.
    et al.
    Jonsson, A. I.
    Stockholm University, Faculty of Science, Department of Meteorology .
    de Grandpré, J.
    Beagley, S. R.
    McLandress, C.
    Semeniuk, K.
    Shepherd, T. G
    Response of the middle atmosphere to CO2 doubling: Results from the Canadian Middle Atmosphere Model2007In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 20, no 7Article in journal (Refereed)
    Abstract [en]

    The Canadian Middle Atmosphere Model (CMAM) has been used to examine the middle atmosphere response to CO2 doubling. The radiative-photochemical response induced by doubling CO2 alone and the response produced by changes in prescribed SSTs are found to be approximately additive, with the former effect dominating throughout the middle atmosphere. The paper discusses the overall response, with emphasis on the effects of SST changes, which allow a tropospheric response to the CO2 forcing. The overall response is a cooling of the middle atmosphere accompanied by significant increases in the ozone and water vapor abundances. The ozone radiative feedback occurs through both an increase in solar heating and a decrease in infrared cooling, with the latter accounting for up to 15% of the total effect. Changes in global mean water vapor cooling are negligible above 30 hPa. Near the polar summer mesopause, the temperature response is weak and not statistically significant. The main effects of SST changes are a warmer troposphere, a warmer and higher tropopause, cell-like structures of heating and cooling at low and middlelatitudes in the middle atmosphere, warming in the summer mesosphere, water vapor increase throughout the domain, and O3 decrease in the lower tropical stratosphere. No noticeable change in upward-propagating planetary wave activity in the extratropical winter–spring stratosphere and no significant temperature response in the polar winter–spring stratosphere have been detected. Increased upwelling in the tropical stratosphere has been found to be linked to changed wave driving at low latitudes.

  • 28. Gaetani, Marco
    et al.
    Messori, Gabriele
    Stockholm University, Faculty of Science, Department of Meteorology .
    Zhang, Qiong
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Flamant, Cyrille
    Pausata, Francesco S. R.
    Stockholm University, Faculty of Science, Department of Meteorology . University of Quebec in Montreal, Canada.
    Understanding the Mechanisms behind the Northward Extension of the West African Monsoon during the Mid-Holocene2017In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 30, no 19, p. 7621-7642Article in journal (Refereed)
    Abstract [en]

    Understanding the West African monsoon (WAM) dynamics in the mid-Holocene (MH) is a crucial issue in climate modeling, because numerical models typically fail to reproduce the extensive precipitation suggested by proxy evidence. This discrepancy may be largely due to the assumption of both unrealistic land surface cover and atmospheric aerosol concentration. In this study, the MH environment is simulated in numerical experiments by imposing extensive vegetation over the Sahara and the consequent reduction in airborne dust concentration. A dramatic increase in precipitation is simulated across the whole of West Africa, up to the Mediterranean coast. This precipitation response is in better agreement with proxy data, in comparison with the case in which only changes in orbital forcing are considered. Results show a substantial modification of the monsoonal circulation, characterized by an intensification of large-scale deep convection through the entire Sahara, and a weakening and northward shift (similar to 6.5 degrees) of the African easterly jet. The greening of the Sahara also leads to a substantial reduction in the African easterly wave activity and associated precipitation. The reorganization of the regional atmospheric circulation is driven by the vegetation effect on radiative forcing and associated heat fluxes, with the reduction in dust concentration to enhance this response. The results for the WAM in the MH present important implications for understanding future climate scenarios in the region and in teleconnected areas, in the context of projected wetter conditions in West Africa.

  • 29. Garfinkel, Chaim I.
    et al.
    Harnik, Nili
    Stockholm University, Faculty of Science, Department of Meteorology . Tel Aviv University, Israel.
    The Non-Gaussianity and Spatial Asymmetry of Temperature Extremes Relative to the Storm Track: The Role of Horizontal Advection2017In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 30, no 2, p. 445-464Article in journal (Refereed)
    Abstract [en]

    The distribution of near-surface and tropospheric temperature variability in midlatitudes is distinguishable from a Gaussian in meteorological reanalysis data; consistent with this, warm extremes occur preferentially poleward of the location of cold extremes. To understand the factors that drive this non-Gaussianity, a dry general circulation model and a simple model of Lagrangian temperature advection are used to investigate the connections between dynamical processes and the occurrence of extreme temperature events near the surface. The non-Gaussianity evident in reanalysis data is evident in the dry model experiments, and the location of extremes is influenced by the location of the jet stream and storm track. The cause of this in the model can be traced back to the synoptic evolution within the storm track leading up to cold and warm extreme events: negative temperature extremes occur when an equatorward propagating high-low couplet (high to the west) strongly advects isotherms equatorward over a large meridional fetch over more than two days. Positive temperature anomalies occur when a poleward propagating low-high couplet (low to the west) advects isotherms poleward over a large meridional fetch over more than two days. The magnitude of the extremes is enhanced by the meridional movement of the systems. Overall, horizontal temperature advection by storm track systems can account for the warm/cold asymmetry in the latitudinal distribution of the temperature extremes.

  • 30.
    Graversen, Rune G.
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Langen, Peter L.
    Mauritsen, Thorsten
    Polar Amplification in CCSM4: Contributions from the Lapse Rate and Surface Albedo Feedbacks2014In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 27, no 12, p. 4433-4450Article in journal (Refereed)
    Abstract [en]

    A vertically nonuniform warming of the troposphere yields a lapse rate feedback by altering the infrared irradiance to space relative to that of a vertically uniform tropospheric warming. The lapse rate feedback is negative at low latitudes, as a result of moist convective processes, and positive at high latitudes, due to stable stratification conditions that effectively trap warming near the surface. It is shown that this feedback pattern leads to polar amplification of the temperature response induced by a radiative forcing. The results are obtained by suppressing the lapse rate feedback in the Community Climate System Model, version 4 (CCSM4). The lapse rate feedback accounts for 15% of the Arctic amplification and 20% of the amplification in the Antarctic region. The fraction of the amplification that can be attributed to the surface albedo feedback, associated with melting of snow and ice, is 40% in the Arctic and 65% in Antarctica. It is further found that the surface albedo and lapse rate feedbacks interact considerably at high latitudes to the extent that they cannot be considered independent feedback mechanisms at the global scale.

  • 31.
    Hannachi, Abdelwaheb
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Trendafilov, N.
    Archetypal Analysis: Mining Weather and Climate Extremes2017In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 30, no 17, p. 6927-6944Article in journal (Refereed)
    Abstract [en]

    Conventional analysis methods in weather and climate science (e.g., EOF analysis) exhibit a number of drawbacks including scaling and mixing. These methods focus mostly on the bulk of the probability distribution of the system in state space and overlook its tail. This paper explores a different method, the archetypal analysis (AA), which focuses precisely on the extremes. AA seeks to approximate the convex hull of the data in state space by finding corners'' that represent pure'' types or archetypes through computing mixture weight matrices. The method is quite new in climate science, although it has been around for about two decades in pattern recognition. It encompasses, in particular, the virtues of EOFs and clustering. The method is presented along with a new manifold-based optimization algorithm that optimizes for the weights simultaneously, unlike the conventional multistep algorithm based on the alternating constrained least squares. The paper discusses the numerical solution and then applies it to the monthly sea surface temperature (SST) from HadISST and to the Asian summer monsoon (ASM) using sea level pressure (SLP) from ERA-40 over the Asian monsoon region. The application to SST reveals, in particular, three archetypes, namely, El Nino, La Nina, and a third pattern representing the western boundary currents. The latter archetype shows a particular trend in the last few decades. The application to the ASM SLP anomalies yields archetypes that are consistent with the ASM regimes found in the literature. Merits and weaknesses of the method along with possible future development are also discussed.

  • 32.
    Hieronymus, Magnus
    et al.
    SMHI, Research Department, Oceanography.
    Nycander, Jonas
    Interannual Variability of the Overturning and Energy Transport in the Atmosphere and Ocean During the Late Twentieth Century with Implications for Precipitation and Sea Level2020In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 33, no 1, p. 317-338Article in journal (Refereed)
  • 33.
    Hieronymus, Magnus
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish Meteorological and Hydrological Institute, Sweden.
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Interannual Variability of the Overturning and Energy Transport in the Atmosphere and Ocean During the Late Twentieth Century with Implications for Precipitation and Sea Level2020In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 33, no 1, p. 317-338Article in journal (Refereed)
    Abstract [en]

    The overturning circulations in the atmosphere and ocean transport energy from the tropics to higher latitudes and thereby modulate Earth's climate. The interannual variability in the overturning over the last 40 years is found to be dominated by two coupled atmosphere-ocean modes. The first is related to the meridional motion of the intertropical convergence zone and the second to El Nino. Both modes have a strong influence on the sea level variability in the tropical Indo-Pacific Ocean. The interannual variability of the cross-equatorial energy transport is dominated by the first mode, and the variability is larger in the Indo-Pacific Ocean than in the Atlantic Ocean or the atmosphere. Our results suggest an important role of oceanic energy transport in setting precipitation patterns in the tropics and a key role of the Indo-Pacific Ocean as a climate modulator.

  • 34.
    Hieronymus, Magnus
    et al.
    SMHI, Research Department, Oceanography.
    Nycander, Jonas
    Nilsson, Johan
    Doos, Kristofer
    Hallberg, Robert
    Oceanic Overturning and Heat Transport: The Role of Background Diffusivity2019In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 32, no 3, p. 701-716Article in journal (Refereed)
  • 35.
    Hieronymus, Magnus
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish Meteorological and Hydrological Institute, Sweden.
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nilsson, Johan
    Stockholm University, Faculty of Science, Department of Meteorology .
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    Hallberg, Robert
    Oceanic Overturning and Heat Transport: The Role of Background Diffusivity2019In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 32, no 3, p. 701-716Article in journal (Refereed)
    Abstract [en]

    The role of oceanic background diapycnal diffusion for the equilibrium climate state is investigated in the global coupled climate model CM2G. Special emphasis is put on the oceanic meridional overturning and heat transport. Six runs with the model, differing only by their value of the background diffusivity, are run to steady state and the statistically steady integrations are compared. The diffusivity changes have large-scale impacts on many aspects of the climate system. Two examples are the volume-mean potential temperature, which increases by 3.6 degrees C between the least and most diffusive runs, and the Antarctic sea ice extent, which decreases rapidly as the diffusivity increases. The overturning scaling with diffusivity is found to agree rather well with classical theoretical results for the upper but not for the lower cell. An alternative empirical scaling with the mixing energy is found to give good results for both cells. The oceanic meridional heat transport increases strongly with the diffusivity, an increase that can only partly be explained by increases in the meridional overturning. The increasing poleward oceanic heat transport is accompanied by a decrease in its atmospheric counterpart, which keeps the increase in the planetary energy transport small compared to that in the ocean.

  • 36.
    Hong, Gang
    et al.
    Department of Atmospheric Sciences, Texas A&M University, College Station, Texas.
    Heygster, Georg
    Institute of Environmental Physics, University of Bremen.
    Notholt, Justus
    Institute of Environmental Physics, University of Bremen.
    Buehler, Stefan
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Space Technology.
    Interannual to diurnal variations in tropical and subtropical deep convective clouds and convective overshooting from seven years of AMSU-B measurements2008In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 21, no 17, p. 4168-4189Article in journal (Refereed)
    Abstract [en]

    This study surveys interannual to diurnal variations of tropical deep convective clouds and convective overshooting using the Advanced Microwave Sounding Unit B (AMSU-B) aboard the NOAA polar orbiting satellites from 1999 to 2005. The methodology used to detect tropical deep convective clouds is based on the advantage of microwave radiances to penetrate clouds. The major concentrations of tropical deep convective clouds are found over the intertropical convergence zone (ITCZ), the South Pacific convergence zone (SPCZ), tropical Africa, the Indian Ocean, the Indonesia maritime region, and tropical and South America. The geographical distributions are consistent with previous results from infrared-based measurements, but the cloud fractions present in this study are lower. Land-ocean and Northern-Southern Hemisphere (NH-SH) contrasts are found for tropical deep convective clouds. The mean tropical deep convective clouds have a slightly decreasing trend with -0.016% decade(-1) in 1999-2005 while the mean convective overshooting has a distinct decreasing trend with -0.142% decade(-1). The trends vary with the underlying surface (ocean or land) and with latitude. A secondary ITCZ occurring over the eastern Pacific between 2 degrees and 8 degrees S and only in boreal spring is predominantly found to be associated with cold sea surface temperatures in La Nina years. The seasonal cycles of deep convective cloud and convective overshooting are stronger over land than over ocean. The seasonal migration is pronounced and moves south with the sun from summer to winter and is particularly dramatic over land. The diurnal cycles of deep convective clouds and convective overshooting peak in the early evening and have their minima in the late morning over the tropical land. Over the tropical ocean the diurnal cycles peak in the morning and have their minima in the afternoon to early evening. The diurnal cycles over the NH and SH subtropical regions vary with the seasons. The local times of the maximum and minimum fractions also vary with the seasons. As the detected deep convective cloud fractions are sensitive to the algorithms and satellite sensors used and are influenced by the life cycles of deep convective clouds, the results presented in this study provide information complementary to present tropical deep convective cloud climatologies.

  • 37. Hood, Lon
    et al.
    Schimanke, Semjon
    SMHI, Research Department, Oceanography.
    Spangehl, Thomas
    Bal, Sourabh
    Cubasch, Ulrich
    The Surface Climate Response to 11-Yr Solar Forcing during Northern Winter: Observational Analyses and Comparisons with GCM Simulations2013In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 26, no 19, p. 7489-7506Article in journal (Refereed)
    Abstract [en]

    The surface climate response to 11-yr solar forcing during northern winter is first reestimated by applying a multiple linear regression (MLR) statistical model to Hadley Centre sea level pressure (SLP) and sea surface temperature (SST) data over the 1880-2009 period. In addition to a significant positive SLP response in the North Pacific found in previous studies, a positive SST response is obtained across the midlatitude North Pacific. Negative but insignificant SLP responses are obtained in the Arctic. The derived SLP response at zero lag therefore resembles a positive phase of the Arctic Oscillation (AO). Evaluation of the SLP and SST responses as a function of phase lag indicates that the response evolves from a negative AO-like mode a few years before solar maximum to a positive AO-like mode at and following solar maximum. For comparison, a similar MLR analysis is applied to model SLP and SST data from a series of simulations using an atmosphere-ocean general circulation model with a well-resolved stratosphere. The simulations differed only in the assumed solar cycle variation of stratospheric ozone. It is found that the simulation that assumed an ozone variation estimated from satellite data produces solar SLP and SST responses that are most consistent with the observational results, especially during a selected centennial period. In particular, a positive SLP response anomaly is obtained in the northeastern Pacific and a corresponding positive SST response anomaly extends across the midlatitude North Pacific. The model response versus phase lag also evolves from a mainly negative AO-like response before solar maximum to a mainly positive AO response at and following solar maximum.

  • 38.
    Jaramillo, Fernando
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Destouni, Georgia
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Hydroclimatic changes worldwide: distinguishing freshwater signals of flow regulation and irrigation effectsIn: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442Article in journal (Refereed)
    Abstract [en]

    This study analyzes worldwide hydroclimatic changes over the period 1900-2009 based on observation data for 99 large hydrological basins across all continents. Worldwide, the observed atmospheric changes in temperature and (uncorrected or bias-corrected) precipitation over land cannot alone explain corresponding changes in evapotranspiration and runoff on land. Additional landscape drivers are needed to explain the latter. Possible effects of river system fragmentation and flow regulation (FFR) as such drivers are here investigated based on independent categorization and parameterization of FFR impact in the studied basins. Consistent signals of FFR-driven change are distinguished and include decrease in the long-term average runoff and the coefficient of short-term variation of runoff; these decreases are greater for basins with higher flow regulation factor. The signals also include increase in evapotranspiration relative to precipitation for strongly FFR-affected basins; this increase is greater for basins with higher flow regulation factor and higher irrigation index. These FFR-related change signals are distinguished consistently and directly from worldwide observation data, against the background of large change variability among basins and several coexisting drivers of water change for each basin. These findings should be used and accounted for in further quantification and projection of global freshwater change.

  • 39.
    Jonsson, Karin
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
    Nilsson, Christer
    Scots Pine (pinus sylvestris L.) on Shingle Fields: A Dendrochronologic Reconstruction of Early Summer Precipitation in Mideast Sweden2009In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 22, no 17, p. 4710-4722Article in journal (Refereed)
    Abstract [en]

    Scots pine (Pinus sylvestris L.) trees growing on shingle fields offer a unique possibility to reconstruct precipitation and study climate variability in the fairly humid eastern part of central Sweden. Tree-ring characteristics were compared with monthly (1890-2001) and daily (1961-2001) climate data from an adjacent meteorological station. Chronologies for latewood (LW), earlywood (EW), and tree-ring widths (RW) were constructed from 73 living and dead trees. Correlation analyses show that tree growth is most sensitive to early summer precipitation. EW shows the strongest correlation with precipitation in May and June while LW is best correlated with June and July precipitation. A reconstruction model for May-June precipitation was calculated using principal component analysis (PCA) regression (regular regression) including EW, LW, and RW for present and previous years. The model explained 46% of the variation in May-June precipitation and allowed a reconstruction back to 1560. Information about wet and dry years was collected from historical documents and was used to validate the result. Periods with precipitation above and below the mean show agreement with previous reconstructions of spring precipitation from tree rings in Finland and of spring floods from estuary sediments in the region. Analyses of correlations between meteorological stations and reconstructed precipitation show that the model is valid for the coastal part of central Sweden. The authors conclude that Scots pine trees on shingle fields are well suited for precipitation reconstruction, and the separate analyses of LW and EW improve the reconstruction.

  • 40.
    Jönsson, Karin
    et al.
    Department of Natural Sciences, Mid Sweden University, Sundsvall, Sweden.
    Nilsson, Christer
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Scots pine (pinus sylvestris L.) on shingle fields: a dendrochronologic reconstruction of early summer precipitation in mideast Sweden2009In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 22, no 17, p. 4710-4722Article in journal (Refereed)
    Abstract [en]

    Scots pine (Pinus sylvestris L.) trees growing on shingle fields offer a unique possibility to reconstruct precipitation and study climate variability in the fairly humid eastern part of central Sweden. Tree-ring characteristics were compared with monthly (1890–2001) and daily (1961–2001) climate data from an adjacent meteorological station. Chronologies for latewood (LW), earlywood (EW), and tree-ring widths (RW) were constructed from 73 living and dead trees. Correlation analyses show that tree growth is most sensitive to early summer precipitation. EW shows the strongest correlation with precipitation in May and June while LW is best correlated with June and July precipitation. A reconstruction model for May–June precipitation was calculated using principal component analysis (PCA) regression (regular regression) including EW, LW, and RW for present and previous years. The model explained 46% of the variation in May–June precipitation and allowed a reconstruction back to 1560. Information about wet and dry years was collected from historical documents and was used to validate the result. Periods with precipitation above and below the mean show agreement with previous reconstructions of spring precipitation from tree rings in Finland and of spring floods from estuary sediments in the region. Analyses of correlations between meteorological stations and reconstructed precipitation show that the model is valid for the coastal part of central Sweden. The authors conclude that Scots pine trees on shingle fields are well suited for precipitation reconstruction, and the separate analyses of LW and EW improve the reconstruction.

  • 41. Kalognomou, Evangelia-Anna
    et al.
    Lennard, Christopher
    Shongwe, Mxolisi
    Pinto, Izidine
    Favre, Alice
    Kent, Michael
    Hewitson, Bruce
    Dosio, Alessandro
    Nikulin, Grigory
    SMHI, Research Department, Climate research - Rossby Centre.
    Panitz, Hans-Juergen
    Buechner, Matthias
    A Diagnostic Evaluation of Precipitation in CORDEX Models over Southern Africa2013In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 26, no 23, p. 9477-9506Article, review/survey (Refereed)
    Abstract [en]

    The authors evaluate the ability of 10 regional climate models (RCMs) to simulate precipitation over Southern Africa within the Coordinated Regional Climate Downscaling Experiment (CORDEX) framework. An ensemble of 10 regional climate simulations and the ensemble average is analyzed to evaluate the models' ability to reproduce seasonal and interannual regional climatic features over regions of the subcontinent. All the RCMs use a similar domain, have a spatial resolution of 50 km, and are driven by the Interim ECMWF Re-Analysis (ERA-Interim; 1989-2008). Results are compared against a number of observational datasets.In general, the spatial and temporal nature of rainfall over the region is captured by all RCMs, although individual models exhibit wet or dry biases over particular regions of the domain. Models generally produce lower seasonal variability of precipitation compared to observations and the magnitude of the variability varies in space and time. Model biases are related to model setup, simulated circulation anomalies, and moisture transport. The multimodel ensemble mean generally outperforms individual models, with bias magnitudes similar to differences across the observational datasets. In the northern parts of the domain, some of the RCMs and the ensemble average improve the precipitation climate compared to that of ERA-Interim. The models are generally able to capture the dry (wet) precipitation anomaly associated with El Nino (La Nina) events across the region. Based on this analysis, the authors suggest that the present set of RCMs can be used to provide useful information on climate projections of rainfall over Southern Africa.

  • 42.
    Kapsch, Marie-Luise
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Graversen, Rune Grand
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Bintanja, Richard
    The Effect of Downwelling Longwave and Shortwave Radiation on Arctic Summer Sea Ice2016In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 29, no 3, p. 1143-1159Article in journal (Refereed)
    Abstract [en]

    The Arctic summer sea ice has diminished fast in recent decades. A strong year-to-year variability on top of this trend indicates that sea ice is sensitive to short-term climate fluctuations. Previous studies show that anomalous atmospheric conditions over the Arctic during spring and summer affect ice melt and the September sea-ice extent (SIE). These conditions are characterized by clouds, humidity and heat anomalies which all affect shortwave (SWD) and longwave (LWD) radiation to the surface. In general, positive LWD anomalies are associated with cloudy and humid conditions, whereas positive anomalies of SWD appear under clear-sky conditions. Here we investigate the effect of realistic anomalies of LWD and SWD on summer sea ice, by performing experiments with the Community Earth System Model. The SWD and LWD anomalies are studied separately and in combination for different seasons. It is found that positive LWD anomalies in spring and early summer have significant impact on the September SIE, whereas winter anomalies show only little effect. Positive anomalies in spring and early summer initiate an earlier melt onset, hereby triggering several feedback mechanisms that amplify melt during the succeeding months. Realistic positive SWD anomalies appear only important if they occur after the melt has started and the albedo is significantly reduced relative to winter conditions. Simulations where both positive LWD and negative SWD anomalies are implemented simultaneously, mimicking cloudy conditions, reveal that clouds during spring have a significant impact on summer sea ice while summer clouds have almost no effect.

  • 43.
    Karlsson, Bodil
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Becker, Erich
    How Does Interhemispheric Coupling Contribute to Cool Down the Summer Polar Mesosphere?2016In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 29, no 24, p. 8807-8821Article in journal (Refereed)
    Abstract [en]

    Interhemispheric coupling is commonly associated with events of high planetary wave activity in the winter stratosphere triggering a heating of the polar mesopause region in the opposite hemisphere. Here, a more fundamental role that this mechanism plays in the absence of planetary wave variability is highlighted. This study focuses directly on the mesospheric part of the coupling chain, which is induced by the gravity wave drag in the winter mesosphere. To investigate the effect that the winter residual flow has on the summertime high-latitude upwelling, the Kuhlungsborn Mechanistic General Circulation Model (KMCM) is used to compare a control simulation to runs where the parameterized gravity waves are removed from the winter hemisphere. The model response in the summer mesosphere reveals that the winter mesospheric residual circulation fosters a net (and substantial) cooling of the summer polar mesopause. These results offer an extension of the current view of interhemispheric coupling: from a mode of internal variability to a constant, gravity wave-driven phenomenon that is modulated by planetary wave activity.

  • 44.
    Karlsson, Bodil
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Kuilman, Maartje
    Stockholm University, Faculty of Science, Department of Meteorology .
    On How the Middle Atmospheric Residual Circulation Responds to the Solar Cycle Close to the Solstices2018In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 31, no 1, p. 401-421Article in journal (Refereed)
    Abstract [en]

    During high solar activity, the atmosphere receives more energy from the sun, particularly in the form of shortwave radiation. Most notable is the effect in the middle and upper atmosphere, which in general shows a positive temperature response due to physical and chemical processes that are intensified at high solar activity. It is thus surprising that a clear solar cycle signal is absent in the summer polar mesosphere region in spite of it being illuminated around the clock. In this study, it is investigated how the circulation in the summer mesosphere is affected by changes in the solar flux using a 30-yr run from the nudged version of the Canadian Middle Atmosphere Model (CMAM30). It is found that-in July-the solar cycle signal from direct solar heating is counteracted by an enhanced residual circulation, which adiabatically cools the region at a higher rate when the solar activity is above average. The dynamical cooling is partly initiated in the Southern Hemisphere winter stratosphere.

  • 45.
    Karlsson, Johannes
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Teixeira, Joao
    A Simple Model of the Northeast Pacific Stratocumulus to Cumulus Transition Based on the Climatological Surface Energy Budget2014In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 27, no 11, p. 4111-4121Article in journal (Refereed)
    Abstract [en]

    Air advected equatorward by the trade winds off the coast of California is associated with decreasing cloud cover and is subjected to increasingly warmer sea surface temperatures. These gradients imply large gradients in the surface energy fluxes. Based on the surface energy balance and on the assumption of a small net surface energy flux, which is supported by reanalysis data, a cloud cover model of the climatological stratocumulus to cumulus transition in the northeastern subtropical Pacific Ocean is developed. Using climatological meteorological surface variables, the model, despite its simplicity, is able to describe the transition from stratocumulus to cumulus reasonably well in terms of cloud cover.

  • 46. Kendon, Elizabeth J.
    et al.
    Jones, Richard G.
    Kjellström, Erik
    SMHI, Research Department, Climate research - Rossby Centre.
    Murphy, James M.
    Using and Designing GCM-RCM Ensemble Regional Climate Projections2010In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 23, no 24, p. 6485-6503Article in journal (Refereed)
    Abstract [en]

    Multimodel ensembles, whereby different global climate models (GCMs) and regional climate models (RCMs) are combined, have been widely used to explore uncertainties in regional climate projections. In this study, the extent to which information can be enhanced from sparsely filled GCM RCM ensemble matrices and the way in which simulations should be prioritized to sample uncertainties most effectively are examined. A simple scaling technique, whereby the local climate response in an RCM is predicted from the large-scale change in the GCM, is found to often show skill in estimating local changes for missing GCM RCM combinations. In particular, scaling shows skill for precipitation indices (including mean, variance, and extremes) across Europe in winter and mean and extreme temperature in summer and winter, except for hot extremes over central/northern Europe in summer. However, internal variability significantly impacts the ability to determine scaling skill for precipitation indices, with a three-member ensemble found to be insufficient for identifying robust local scaling relationships in many cases. This study suggests that, given limited computer resources, ensembles should be designed to prioritize the sampling of GCM uncertainty, using a reduced set of RCMs. Exceptions are found over the Alps and northeastern Europe in winter and central Europe in summer, where sampling multiple RCMs may be equally or more important for capturing uncertainty in local temperature or precipitation change. This reflects the significant role of local processes in these regions. Also, to determine the ensemble strategy in some cases, notably precipitation extremes in summer, better sampling of internal variability is needed.

  • 47. Langen, Peter L.
    et al.
    Graversen, Rune Grand
    Stockholm University, Faculty of Science, Department of Meteorology .
    Mauritsen, Thorsten
    Separation of Contributions from Radiative Feedbacks to Polar Amplification on an Aquaplanet2012In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 25, no 8, p. 3010-3024Article in journal (Refereed)
    Abstract [en]

    When climate is forced by a doubling of CO2, a number of feedback processes are induced, such as changes of water vapor, clouds, and surface albedo. Here the CO2 forcing and concomitant feedbacks are studied individually using a general circulation model coupled to an aquaplanet mixed layer ocean. A technique for fixing the radiative effects of moisture and clouds by reusing these variables from 1 x CO2 and 2 x CO2 equilibrium climates in the model's radiation code allows for a detailed decomposition of forcings, feedbacks, and responses. The cloud feedback in this model is found to have a weak global average effect and surface albedo feedbacks have been eliminated. As in previous studies, the water vapor feedback is found to approximately double climate sensitivity, but while its radiative effect is strongly amplified at low latitudes, the resulting response displays about the same degree of polar amplification as the full all-feedbacks experiment. In fact, atmospheric energy transports are found to change in a way that yields the same meridional pattern of response as when the water vapor feedback is turned off. The authors conclude that while the water vapor feedback does not in itself lead to polar amplification by increasing the ratio of high-to low-latitude warming, it does double climate sensitivity both at low and high latitudes. A polar amplification induced by other feedbacks in the system, such as the Planck and lapse rate feedbacks here, is thus strengthened in the sense of increasing the difference in high-and low-latitude warming.

  • 48.
    Lind, Petter
    et al.
    SMHI, Research Department, Climate research - Rossby Centre.
    Lindstedt, David
    SMHI, Research Department, Climate research - Rossby Centre.
    Kjellström, Erik
    SMHI, Research Department, Climate research - Rossby Centre.
    Jones, Colin
    SMHI, Research Department, Climate research - Rossby Centre.
    Spatial and Temporal Characteristics of Summer Precipitation over Central Europe in a Suite of High-Resolution Climate Models2016In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 29, no 10, p. 3501-3518Article in journal (Refereed)
  • 49.
    Lind, Petter
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish Meteorological and Hydrological Institute, Sweden.
    Lindstedt, David
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish Meteorological and Hydrological Institute, Sweden.
    Kjellström, Erik
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish Meteorological and Hydrological Institute, Sweden.
    Jones, Colin
    University of Leeds, National Centre for Atmospheric Science, United Kingdom.
    Spatial and temporal characteristics of summer precipitation over Central Europe in a suite of high-resolution climate models2016In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 29, no 10, p. 3501-3518Article in journal (Refereed)
    Abstract [en]

    High-impact, localized intense rainfall episodes represent a major socio-economic problem for societies worldwide, and at the same time these events are notoriously difficult to simulate properly in climate models. Here, the authors investigate how horizontal resolution and model formulation influence this issue by applying the HARMONIE regional climate model (HCLIM) with three different setups; two using convection parameterization at 15 and 6.25 km horizontal resolution (the latter within the “grey-zone” scale), with lateral boundary conditions provided by ERA-Interim reanalysis and integrated over a pan-European domain, and one with explicit convection at 2 km resolution (HCLIM2) over the Alpine region driven by the 15 km model. Seven summer seasons were sampled and validated against two high-resolution observational data sets. All HCLIM versions underestimate the number of dry days and hours by 20-40%, and overestimate precipitation over the Alpine ridge. Also, only modest added value were found of “grey-zone” resolution. However, the single most important outcome is the substantial added value in HCLIM2 compared to the coarser model versions at sub-daily time scales. It better captures the local-to-regional spatial patterns of precipitation reflecting a more realistic representation of the local and meso-scale dynamics. Further, the duration and spatial frequency of precipitation events, as well as extremes, are closer to observations. These characteristics are key ingredients in heavy rainfall events and associated flash floods, and the outstanding results using HCLIM in convection-permitting setting are convincing and encourage further use of the model to study changes in such events in changing climates.

  • 50.
    Lindvall, Jenny
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Hannay, Cecile
    Evaluation of near surface parameters in the two versions of the atmospheric model in cesm1 using flux station observations2013In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 26, no 1, p. 26-44Article in journal (Refereed)
    Abstract [en]

    This paper describes the performance of the Community Atmosphere Model (CAM) versions 4 and 5 in simulating near-surface parameters. CAM is the atmospheric component of the Community Earth System Model (CESM). Most of the parameterizations in the two versions are substantially different, and that is also true for the boundary layer scheme: CAM4 employs a nonlocal K-profile scheme, whereas CAM5 uses a turbulent kinetic energy (TKE) scheme. The evaluation focuses on the diurnal cycle and global observational and reanalysis datasets are used together with multiyear observations from 35 flux tower sites, providing high-frequency measurements in a range of different climate zones. It is found that both model versions capture the timing of the diurnal cycle but considerably overestimate the diurnal amplitude of net radiation, temperature, wind, and turbulent heat fluxes. The seasonal temperature range at mid-and high latitudes is also overestimated with too warm summer temperatures and too cold winter temperatures. The diagnosed boundary layer is deeper in CAM5 over ocean in regions with low-level marine clouds as a result of the turbulence generated by cloud-top cooling. Elsewhere, the boundary layer is in general shallower in CAM5. The two model versions differ substantially in their representation of near-surface wind speeds over land. The low-level wind speed in CAM5 is about half as strong as in CAM4, and the difference is even larger in areas where the subgrid-scale terrain is significant. The reason is the turbulent mountain stress parameterization, only applied in CAM5, which acts to increase the surface stress and thereby reduce the wind speed.

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