Tropical Cloud Radiative Forcing and the Hadley and Walker Circulations: A Simple Model

In this paper, a simple, linear dynamic model for the tropical Hadley and Walker circulations is constructed based on Gill's (1980) linear dynamics and Neelin and Held's (1987) moist thermodynamic formulation. The present model describes the 1st baroclinic mode of the troposphere and is suitable for modeling both the climatology and short-term variations in response to the net thermodynamic forcing from the top of the atmosphere (TOA) and surface. Notwithstanding its simplicity, the model can simulate the fundamental climatological features of the Hadley and Walker circulations, such as the ascending branches in the ITCZ and the SPCZ and the descending branches in the cold tongue and subtropics as well as their annual cycle and interannual variability such as ENSO.

The present model is then used to study the role of tropical cloud radiative forcing (CRF) in the Hadley and Walker circulations. When the tropical atmospheric CRF is included in the model, the model can reproduce the Hadley and Walker circulations, however, when the tropical atmospheric CRF is removed from the model, the model Walker circulation disappears and the model Hadley cell reverses. These demonstrate the tropical atmospheric CRF play a pivotal role in maintaining the Hadley and Walker circulations in addition to the gross moist stability and latent heat flux. The present model offers a viable theoretical framework for the recent GCM studies, which have demonstrated that the Hadley and Walker circulations are very sensitive to the tropical atmospheric CRF.