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Atmospheric dynamics and the hydrologic cycle in warm climates
Stockholm University, Faculty of Science, Department of Meteorology .ORCID iD: 0000-0003-4786-6058
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Past warm climates represent one extreme of Earth's known climate states. Here, we study warm climates in both idealized simulations and full-complexity general circulation model (GCM) simulations of the early Eocene epoch, approximately 50 million years ago.

In increasingly warmer idealized aquaplanet simulations, the amplitude of intra-seasonal tropical variability is enhanced. The anomalies propagate eastward in the tropics and resemble the observed Madden-Julian Oscillation (MJO). The strong MJO anomalies drive a momentum convergence on the equator that causes westerly winds in the troposphere, a state known as superrotation. The results in this thesis show that superrotation further enhances the MJO by affecting the penetration of midlatitude eddies into the deep tropics. An additional question is how a super-rotating atmosphere, a dramatically different general circulation regime compared to today, will affect the climate, potentially via changes in cloud distributions and ocean circulation. If the superrotation extends down to the surface near the equator, surface westerly winds will drive equatorial downwelling in the eastern equatorial Pacific Ocean, rather than upwelling as in the present climate. Here, we show that surface superrotation is unlikely in past warm climates, although this in part depends on the intensity of the vertical momentum transfer associated with cumulus convection and how this process is represented in a specific GCM. 

There is, currently, no consensus on what the specific mix of forcings was that caused the warm climates of the early Eocene. High greenhouse gases likely played a significant role, but simulations with reasonable greenhouse gas concentrations cannot reproduce the high temperatures estimated by proxy data. Here, we investigate both an early Eocene climate forced by high greenhouse gas concentrations and one forced by optically thinner clouds, with artificially increased cloud droplet radius that causes increased solar radiation at the surface. Both alternative warming scenarios produce nearly identical zonal mean temperatures, but the hydrologic cycle differs; the thinner clouds scenario has 11% larger global mean precipitation. Moreover, the results in this thesis indicate that a reasonable estimate of vegetation, based on the model simulation, is likely necessary to evaluate alternative warming scenarios with proxy data.

Place, publisher, year, edition, pages
Stockholm: Department of Meteorology, Stockholm University , 2018. , p. 30
Keywords [en]
superrotation, early Eocene, warm climates, Madden-Julian Oscillation, the hydrologic cycle, vegetation sensitivity, large-scale circulation
National Category
Meteorology and Atmospheric Sciences Climate Research
Research subject
Atmospheric Sciences and Oceanography
Identifiers
URN: urn:nbn:se:su:diva-158487ISBN: 978-91-7797-352-2 (print)ISBN: 978-91-7797-353-9 (electronic)OAI: oai:DiVA.org:su-158487DiVA, id: diva2:1237017
Public defence
2018-09-20, William-Olssonsalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Manuscript.

Available from: 2018-08-28 Created: 2018-08-07 Last updated: 2018-09-07Bibliographically approved
List of papers
1. Enhanced MJO and transition to superrotation in warm climates
Open this publication in new window or tab >>Enhanced MJO and transition to superrotation in warm climates
2016 (English)In: Journal of Advances in Modeling Earth Systems, ISSN 1942-2466, Vol. 8, no 1, p. 304-318Article in journal (Refereed) Published
Abstract [en]

Using the NCAR CAM3 model in aquaplanet configuration, we perform a suite of simulations spanning a broad range of warm climates. The simulations show a spontaneous transition to superrotation, i.e., westerly winds at upper levels above the equator. The momentum convergence leading to superrotation is driven by eastward-propagating equatorial waves with structure similar to the modern Madden-Julian Oscillation (MJO), whose amplitude increases strongly with temperature. We analyze the moist static energy (MSE) budget of the model's MJO to identify mechanisms leading to its enhanced amplitude. Two such mechanisms are identified: a rapid increase of mean low-level MSE with rising temperature, as found in previous work, and reduced damping of the MJO by synoptic-scale eddies. Both effects imply a reduced gross moist stability and enhanced MJO amplitude. The reduced eddy damping is caused by the transition to superrotation, which allows a greater penetration of extratropical eddies into the equatorial zone; the dominant effect of this greater penetration is to flatten the meridional gradient zonal-mean MSE, which effectively impedes the generation of anomalous MSE divergence by MJO-modulated eddies. This mechanism implies a positive feedback between superrotation and the MJO which may hasten the transition into a strongly superrotating state.

Keywords
MJO, superrotation
National Category
Earth and Related Environmental Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-130981 (URN)10.1002/2015MS000615 (DOI)000374770200015 ()
Available from: 2016-06-13 Created: 2016-06-09 Last updated: 2018-08-07Bibliographically approved
2. Surface superrotation
Open this publication in new window or tab >>Surface superrotation
2018 (English)In: Journal of the Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 75, no 10, p. 3671-3689Article in journal (Refereed) Published
Abstract [en]

Equatorial superrotation is commonly observed in simulations of Earth and planetary climates, but is almost without exception found to occur only at upper levels, with zero or easterly winds at the surface. Surface superrotation—a state with climatological zonal-mean westerlies at the equatorial surface—would lead to a major reorganization of the tropical ocean circulation with important consequences for global climate. Here, we examine the mechanisms that give rise to surface superrotation. We identify four theoretical scenarios under which surface superrotation may be achieved. Using an axisymmetric model forced by prescribed zonal-mean torques, we provide concrete examples of surface superrotation under all four scenarios. We also find that we can induce surface superrotation in a full-complexity atmospheric general circulation model, albeit in an extreme parameter range (in particular, convective momentum transport is artificially increased by almost an order of magnitude). We conclude that a transition to surface superrotation is unlikely in Earth-like climates, including ancient or future warm climates, though this conclusion is subject to the currently large uncertainties in the parameterization of convective momentum transport.

Keywords
Atmospheric circulation, Eddies, Hadley circulation, Madden-Julian oscillation, Planetary atmospheres, Rossby waves
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-158486 (URN)10.1175/JAS-D-18-0076.1 (DOI)000450965000002 ()
Available from: 2018-08-07 Created: 2018-08-07 Last updated: 2018-12-10Bibliographically approved
3. Atmospheric circulation and hydroclimate impacts of alternative warming scenarios for the Eocene
Open this publication in new window or tab >>Atmospheric circulation and hydroclimate impacts of alternative warming scenarios for the Eocene
2017 (English)In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 13, no 8, p. 1037-1048Article in journal (Refereed) Published
Abstract [en]

Recent work in modelling the warm climates of the early Eocene shows that it is possible to obtain a reasonable global match between model surface temperature and proxy reconstructions, but only by using extremely high atmospheric CO2 concentrations or more modest CO2 levels complemented by a reduction in global cloud albedo. Understanding the mix of radiative forcing that gave rise to Eocene warmth has important implications for constraining Earth's climate sensitivity, but progress in this direction is hampered by the lack of direct proxy constraints on cloud properties. Here, we explore the potential for distinguishing among different radiative forcing scenarios via their impact on regional climate changes. We do this by comparing climate model simulations of two end-member scenarios: one in which the climate is warmed entirely by CO2 (which we refer to as the greenhouse gas (GHG) scenario) and another in which it is warmed entirely by reduced cloud albedo (which we refer to as the low CO2-thin clouds or LCTC scenario). The two simulations have an almost identical global-mean surface temperature and equator-to-pole temperature difference, but the LCTC scenario has similar to 11% greater global-mean precipitation than the GHG scenario. The LCTC scenario also has cooler midlatitude continents and warmer oceans than the GHG scenario and a tropical climate which is significantly more El Nino-like. Extremely high warm-season temperatures in the subtropics are mitigated in the LCTC scenario, while cool-season temperatures are lower at all latitudes. These changes appear large enough to motivate further, more detailed study using other climate models and a more realistic set of modelling assumptions.

National Category
Earth and Related Environmental Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-147078 (URN)10.5194/cp-13-1037-2017 (DOI)000407957000001 ()
Available from: 2017-09-18 Created: 2017-09-18 Last updated: 2018-08-07Bibliographically approved
4. Vegetation sensitivity to alternative warming scenarios for the early Eocene
Open this publication in new window or tab >>Vegetation sensitivity to alternative warming scenarios for the early Eocene
(English)Manuscript (preprint) (Other academic)
National Category
Climate Research
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-158485 (URN)
Available from: 2018-08-07 Created: 2018-08-07 Last updated: 2018-08-09Bibliographically approved

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