A radiative-convective equilibrium perspective of the weakening of tropical Walker circulation in response to global warming

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 water vapor and lapse rate feedbacks 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.