题 目：On the weakening of tropical Walker circulation in response to global warming:
1. A simple explanation from the radiative-convective equilibrium perspective; 2. A robust constraint on the associated ice water change
报告人：Prof. Xianglei Huang, Department of Atmospheric, Oceanic, and Space Sciences, University of Michigan
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 we 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. Our 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. We explain why the change of QR is positive in the upper most 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 planetary boundary layer to 600hPa. From 600hPa to 300hPa, lapse rate changes are the dominant cause of the weakening circulation. Above 300hPa, 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.
We further explore the changes of tropical ice clouds associated with such circulation change. By examining the interannual variability of ice water path (IWP), an important property of ice cloud, with respect to large-scale vertical velocity in observations, reanalysis and climate model simulations, and by relating the unforced variability to the human-influenced climate change, we show that a statistically significant linear relation between interannual anomalies of IWP and 500hPa vertical velocity (w500) can be identified from all data sets and, moreover, such relationship in each model is closely correlated with its simulated IWP change with respect to w500 change for the future climate. This indicates a robust constraint for the future tropical ice cloud change. Such constraint and observed IWP-w500 relation projectsa 12.8%-16.4% decrease of IWP for every 0.01Pa/s change of w500, equivalent to ~2.96%-3.80% decrease of IWP for circulation change associated with every 1K global surface warming.