Editorial Type:
Article
Article Type:
Research Article

A Radiative–Convective Equilibrium Perspective of Weakening of the Tropical Walker Circulation in Response to Global Warming

Xianglei Huang Department of Atmospheric, Oceanic, and Space Sciences, University of Michigan, Ann Arbor, Michigan

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Hui-Wen Chuang Department of Atmospheric, Oceanic, and Space Sciences, University of Michigan, Ann Arbor, Michigan

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Andrew Dessler Department of Atmospheric Sciences, Texas A&M University, College Station, Texas

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Xiuhong Chen Department of Atmospheric, Oceanic, and Space Sciences, University of Michigan, Ann Arbor, Michigan

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Kenneth Minschwaner Department of Physics, New Mexico Institute of Mining and Technology, Socorro, New Mexico

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Yi Ming NOAA/Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey

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V. Ramaswamy NOAA/Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey

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Print Publication:
01 Mar 2013
DOI:
https://doi.org/10.1175/JCLI-D-12-00288.1
Page(s):
1643–1653
Restricted access

Abstract

Both observational analysis and GCM simulations indicate that the tropical Walker circulation is becoming weaker and may continue to weaken as a consequence of climate change. Here, the authors use a conceptual radiative–convective equilibrium (RCE) framework to interpret the weakening of the Walker circulation as simulated by the GFDL coupled GCM. Based on the modeled lapse rate and clear-sky cooling rate profiles, the RCE framework can directly compute the change of vertical velocity in the descending branch of the Walker circulation, which agrees with the counterpart simulated by the GFDL model. The results show that the vertical structure of clear-sky radiative cooling rate QR will change in response to the increased water vapor as the globe warms. The authors explain why the change of QR is positive in the uppermost part of the troposphere (<300 hPa) and is negative for the rest of the troposphere. As a result, both the change of clear-sky cooling rate and the change of tropospheric lapse rate contribute to the weakening of circulation. The vertical velocity changes due to the two factors are comparable to each other from the top of the planetary boundary layer to 600 hPa. From 600 to 300 hPa lapse rate changes are the dominant cause of the weakening circulation. Above 300 hPa, the change due to QR is opposite to the change due to lapse rate, which forces a slight increase in vertical velocity that is seen in the model simulation.

Corresponding author address: Xianglei Huang, Department of Atmospheric, Oceanic, and Space Sciences, University of Michigan, 2455 Hayward St., Ann Arbor, MI 48109-2143. E-mail: xianglei@umich.edu

Abstract

Both observational analysis and GCM simulations indicate that the tropical Walker circulation is becoming weaker and may continue to weaken as a consequence of climate change. Here, the authors use a conceptual radiative–convective equilibrium (RCE) framework to interpret the weakening of the Walker circulation as simulated by the GFDL coupled GCM. Based on the modeled lapse rate and clear-sky cooling rate profiles, the RCE framework can directly compute the change of vertical velocity in the descending branch of the Walker circulation, which agrees with the counterpart simulated by the GFDL model. The results show that the vertical structure of clear-sky radiative cooling rate QR will change in response to the increased water vapor as the globe warms. The authors explain why the change of QR is positive in the uppermost part of the troposphere (<300 hPa) and is negative for the rest of the troposphere. As a result, both the change of clear-sky cooling rate and the change of tropospheric lapse rate contribute to the weakening of circulation. The vertical velocity changes due to the two factors are comparable to each other from the top of the planetary boundary layer to 600 hPa. From 600 to 300 hPa lapse rate changes are the dominant cause of the weakening circulation. Above 300 hPa, the change due to QR is opposite to the change due to lapse rate, which forces a slight increase in vertical velocity that is seen in the model simulation.

Corresponding author address: Xianglei Huang, Department of Atmospheric, Oceanic, and Space Sciences, University of Michigan, 2455 Hayward St., Ann Arbor, MI 48109-2143. E-mail: xianglei@umich.edu
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