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  • 1.
    Brodeau, Laurent
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Barnier, Bernard
    Gulev, Sergey K.
    Woods, Cian
    Stockholm University, Faculty of Science, Department of Meteorology .
    Climatologically significant effects of some approximations in the bulk parameterizations of turbulent air-sea fluxes2017In: Journal of Physical Oceanography, ISSN 0022-3670, E-ISSN 1520-0485, Vol. 47, no 1, p. 5-28Article in journal (Refereed)
    Abstract [en]

    This paper quantifies the impacts of approximations and assumptions in the parameterization of bulk formulae on the exchange of momentum, heat, and freshwater computed between the ocean and atmosphere. An ensemble of sensitivity experiments are examined. Climatologies of wind stress, turbulent heat flux, and evaporation for the 1982-2014 period are computed using SST and surface meteorological state variables from the ERA-Interim reanalysis. Each experiment differs from the defined control experiment in only one aspect of the parameterization of the bulk formulae. The wind stress is most sensitive to the closure used to relate the neutral drag coefficient to the wind speed in the bulk algorithm, which mainly involves the value of the Charnock parameter. The disagreement between the state-of-the-art algorithms examined is typically of the order of 10%. The largest uncertainties in turbulent heat flux and evaporation are also related to the choice of the algorithm (typically 15%), but also emerge in experiments examining approximations related to the surface temperature and saturation humidity. Thus, approximations for the skin temperature and the salt-related reduction of saturation humidity have a substantial impact on the heat flux and evaporation (typically 10%). Approximations such as the use of a fixed air density, sea level pressure, or simplified formula for the saturation humidity, lead to errors no larger than 4% when tested individually. The impacts of these approximations combine linearly when implemented together, yielding errors up to 20% over mid- and subpolar latitudes.

  • 2. Lee, Sukyoung
    et al.
    Woods, Cian
    Stockholm University, Faculty of Science, Department of Meteorology .
    Caballero, Rodrigo
    Stockholm University, Faculty of Science, Department of Meteorology .
    Relation Between Arctic Moisture Flux and Tropical Temperature Biases in CMIP5 Simulations and Its Fingerprint in RCP8.5 Projections2019In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 46, no 2, p. 1088-1096Article in journal (Refereed)
    Abstract [en]

    Arctic moisture intrusions have played an important role in warming the Arctic over the past few decades. A prior study found that Coupled Model Intercomparison Project Phase 5 (CMIP5) models exhibit large regional biases in the moisture flux across 70 degrees N. It is shown here that the systematic misrepresentation of the moisture flux is related to the models' overprediction of zonal wavenumber k = 2 contribution and underprediction of k = 1 contribution to the flux. Models with a warmer tropical upper troposphere and El-Nino-like tropical surface temperature tend to simulate stronger k = 2 flux, while k = 1 flux is uncorrelated with tropical upper tropospheric temperature and is associated with La-Nina-like surface temperature. The models also overpredict the transient eddy moisture flux while underpredicting the stationary eddy flux. Moreover, future projections in Representative Concentration Pathway 8.5 (RCP8.5) simulations show trends in moisture flux that is consistent with biases in historical simulations, suggesting that these CMIP5 projections reflect the same error(s) that cause the model biases. Plain Language Summary The Arctic is the region where climate change has been most rapid. A growing body of work indicates that moisture intrusions into Arctic have played an important role in warming the Arctic over the past decades. Coupled Model Intercomparison Project Phase 5 (CMIP5) models have served as a critical tool for projecting future climate changes. Therefore, it is imperative to evaluate whether the physical processes governing moisture intrusions are accurately represented by the models. It is shown here that there is a systematic misrepresentation of the moisture flux into the Arctic related to the models' biased representation of tropical temperatures. Moreover, future projections in Representative Concentration Pathway 8.5 (RCP8.5) simulations show moisture flux trends that are consistent with biases in historical simulations, suggesting that these CMIP5 projections reflect the same error(s) that cause the model biases. It is common practice to regard averages across climate model as being the true response to climate forcing. The findings here question this widespread assumption and underscore the need to pay close attention to model biases and their causes.

  • 3. Liu, Yinghui
    et al.
    Key, Jeffrey R.
    Vavrus, Steve
    Woods, Cian
    Stockholm University, Faculty of Science, Department of Meteorology .
    Time Evolution of the Cloud Response to Moisture Intrusions into the Arctic during Winter2018In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 31, no 22, p. 9389-9405Article in journal (Refereed)
    Abstract [en]

    Northward fluxes of moisture and sensible heat into the Arctic affect the atmospheric stability, sea ice and snow cover, clouds, and surface energy budget. Intense moisture fluxes into the Arctic are called moisture intrusions; some can lead to basinwide increases in downward longwave radiation (DLR) at the surface, called downward infrared (IR) events. Using the ERA-Interim reanalysis from 1990 to 2016, this study investigated the time evolution of cloud amount and cloud properties and their impact on the surface radiation fluxes in response to Arctic moisture intrusions and downward IR events during winter for better understanding of the Arctic moisture intrusions. A composite analysis revealed several key features: moisture intrusions produce more clouds and higher cloud liquid and ice water content; positive cloud amount anomalies can persist for over 10 days over the Arctic Ocean during downward IR events; positive high-level and middle-level cloud anomalies are evident in the early stage, and positive low-level cloud anomalies are evident in the late stage. Greater clear-sky DLR and longwave cloud radiative forcing (CRF) over the Arctic Ocean accompany the greater all-sky DLR during the downward IR events. Greater clear-sky DLR can be attributed to higher air temperatures and higher total column water vapor, while greater longwave CRF is the result of larger cloud amount and cloud water content. Longwave CRF anomalies account for approximately 40% of the all-sky DLR anomalies.

  • 4.
    Messori, Gabriele
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Woods, Cian
    Stockholm University, Faculty of Science, Department of Meteorology .
    Caballero, Rodrigo
    Stockholm University, Faculty of Science, Department of Meteorology .
    On the Drivers of Wintertime Temperature Extremes in the High Arctic2018In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 31, no 4, p. 1597-1618Article in journal (Refereed)
    Abstract [en]

    The salient features and drivers of wintertime warm and cold spells in the high Arctic are investigated. The analysis is based on the European Centre for Medium-Range Weather Forecasts interim reanalysis dataset. It is found that the warm spells are systematically associated with an intense sea level pressure and geopotential height anomaly dipole, displaying a low over the Arctic basin and a high over northern Eurasia. This configuration creates a natural pathway for extreme moisture influx episodes from the Atlantic sector into the Arctic (herein termed moisture intrusions). Anomalous cyclone frequency at the pole (largely attributable to local cyclogenesis) then favors a deep penetration of these intrusions across the Arctic basin. The large-scale circulation pattern associated with the warm spells further favors the advection of cold air across Siberia, leading to the so-called warm Arctic-cold Eurasia pattern previously discussed in the literature. On the contrary, cold Arctic extremes are associated with a severely reduced frequency of moisture intrusions and a persistent low pressure system over the pole. This effectively isolates the high latitudes from midlatitude air masses, favoring an intense radiative cooling of the polar region.

  • 5.
    Woods, Cian
    Stockholm University, Faculty of Science, Department of Meteorology .
    The role of high-latitude circulation and moisture transport in Arctic climate variability and change during winter2016Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis examines the connections between atmospheric circulation in the high-latitudes, northward moisture transport, and Arctic climate variability and change during winter. An event based approach is taken by objectively defining phenomena termed “moisture intrusions” -- filamentary flows of anomalously moist air which originate at 70°N and cross the entire Arctic basin. They typically emanate from within the poleward advecting branches of mid-latitude cyclones held in place by blocking patterns to the east. Moisture intrusions contribute only a minority of the total northward moisture transport at 70°N, yet drive a significant proportion of the inter annual variability in surface temperature and downward longwave radiation over the entire polar cap. A positive trend in the frequency of these events, in response to a moistening of the atmosphere, is shown to have driven approximately 45% and 35% of the observed warming and sea ice decline in the Barents Sea during Dec-Jan over the past two decades. Moisture intrusions act to erode the temperature inversion and thus contribute to bottom amplified warming even in the absence of sea ice loss. Negative sea ice anomalies induced by intrusions persist for up to weeks at a time -- promoting upward turbulent heat fluxes and further bottom amplified warming. Systematic biases in the statistics of moisture intrusions are discovered in the CMIP5 models. The biases are predominantly a result of misrepresentation of the intense moisture fluxes and are almost entirely due to biases in the meridional velocity. Moisture intrusion biases explain only about 17% of the temperature bias in the Atlantic sector. The predicted biases, while small in amplitude, are very highly correlated with the true biases in the models however, suggesting that the temperature bias directly induced by misrepresented intrusion statistics may be strongly amplified by sea ice feedback. A analysis of the uncertainties in computed turbulent air-sea flux (TASF) climatologies arising due to the parameterisation of bulk formulae is also presented. TASF climatologies are computed over a series of sensitivity experiments using surface state variables from ERA-Interim. The largest source of uncertainty is related to the computation of the transfer coefficients and hence the choice of bulk algorithm itself. The majority of parameter approximations have small impacts when tested individually, but can lead to large disagreements when implemented in tandem.

  • 6.
    Woods, Cian
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Caballero, Rodrigo
    Stockholm University, Faculty of Science, Department of Meteorology .
    The Role of Moist Intrusions in Winter Arctic Warming and Sea Ice Decline2016In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 29, p. 4473-4485Article in journal (Refereed)
    Abstract [en]

    This paper examines the trajectories followed by intense intrusions of moist air into the Arctic polar region during autumn and winter and their impact on local temperature and sea ice concentration. It is found that the vertical structure of the warming associated with moist intrusions is bottom amplified, corresponding to a transition of local conditions from a “cold clear” state with a strong inversion to a “warm opaque” state with a weaker inversion. In the marginal sea ice zone of the Barents Sea, the passage of an intrusion also causes a retreat of the ice margin, which persists for many days after the intrusion has passed. The authors find that there is a positive trend in the number of intrusion events crossing 70°N during December and January that can explain roughly 45% of the surface air temperature and 30% of the sea ice concentration trends observed in the Barents Sea during the past two decades.

  • 7.
    Woods, Cian
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Caballero, Rodrigo
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Large-scale circulation associated with moisture intrusions into the Arctic during winter2013In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 40, no 17, p. 4717-4721Article in journal (Refereed)
    Abstract [en]

    We examine the poleward transport of water vapor across 70 degrees N during boreal winter in the ERA-Interim reanalysis product, focusing on intense moisture intrusion events. We analyze the large-scale circulation patterns associated with these intrusions and the impacts they have at the surface. A total of 298 events are identified between 1990 and 2010, an average of 14 per season, accounting for 28% of the total poleward transport of moisture across 70 degrees N. They are concentrated over the main ocean basins at that latitude in the Labrador Sea, North Atlantic, Barents/Kara Sea, and Pacific. Composites of sea level pressure and potential temperature on the 2 potential vorticity unit surface during intrusions show a large-scale blocking pattern to the east of each basin, deflecting midlatitude cyclones and their associated moisture poleward. The interannual variability of intrusions is strongly correlated with variability in winter-mean surface downward longwave radiation and skin temperature averaged over the Arctic.

  • 8.
    Woods, Cian
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Caballero, Rodrigo
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Representation of Arctic Moist Intrusions in CMIP5 Models and Implications for Winter Climate Biases2017In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 30, no 11, p. 4083-4102Article in journal (Refereed)
    Abstract [en]

    This paper examines the wintertime northward moisture flux at 70 degrees N from 1981- 2005 in 31 of the CMIP5 models compared with the ERA- Interim reanalysis product. The models' total zonally integrated northward moisture flux is found to agree reasonably well with the reanalysis, but with large compensating regional biases. Specifically, the models systematically underpredict the moisture flux in the Atlantic sector and overpredict it in the Pacific sector. The biases are predominantly due to misrepresentation of extreme moisture flux events, which are known to exert a significant control on Arctic climate. Biases in these highintensity fluxes are almost entirely contributed by biases in the meridional velocity, suggesting a link with biases in storm-track activity at lower latitudes. The extent to which the deficit of moisture intrusions in the Atlantic sector and excess in the Pacific sector may account for biases in the climate of the respective sectors is assessed. Biases in the frequency of moisture intrusions explain roughly 17% of surface temperature and 24% of surface downward longwave radiation biases in the Atlantic sector, and about 14% and 16% of the gradient in these respective biases between the two sectors. The predicted bias gradients, while small in amplitude, are very highly correlated with the true bias gradients in the models, suggesting that the temperature bias directly induced by misrepresented intrusion statistics may be strongly amplified by sea ice feedback.

  • 9.
    Woods, Cian
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Caballero, Rodrigo
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Representation of Arctic moist intrusions in CMIP5 models and implications for winter climate biasesManuscript (preprint) (Other academic)
1 - 9 of 9
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