Radiative-Convective Processes in Regulating Tropical Ocean-Atmosphere

We study physical processes maintaining equilibrium climate of the ascending atmosphere over tropical Pacific warm pool and the descending atmosphere over the surrounding cold oceans. The tropical climate was argued to be regulated by ocean currents connecting the warm pool with the cold tongue that are driven by atmospheric circulation (e.g. Sun and Liu 1996; Liu and Huang 1996). In this study, the role of radiative cooling in regulating the coupled tropical climate is emphasized. This is done in a cumulus ensemble model as well as an observational analysis. Idealized experiments are performed with the warm sea surface temperature (SST) specified at 28.5oC, 29.5oC, and 30.5oC, respectively while the cold pool SST is specified at 26oC. The domain averaged vertical motion in all experiments is constrained to be a profile of upward motion to allow excess energy to be exported out of the tropics as in the observed tropical climate. All experiments are carried out for 50-70 days to reach equilibrium. A "Walker" type circulation of ascending (descending) motion over the warm (cool) pool is developed in all simulations that strengthens appreciably as the warm pool SST changes from 28.5oC to 29.5oC, but remains almost unchanged as SST changes from 29.5oC to 30.5oC. The corresponding changes of model moisture distribution in the two regimes are positively correlated with those of the mean vertical motion, yet the temperature becomes warmer in both regimes as warm pool SST increases. A major factor for the above nonlinear responses is attributed to the heat and water balance in the subsidence regime. From weak to normal SST gradient, the circulation becomes stronger in the cold pool that stabilizes the atmosphere and effectively prevents the existence of deep convection. The increased subsidence warming is primarily balanced by reduced condensation heating. As SST gradient becomes stronger, a further enhanced subsidence warming in the absence of clouds is no longer possible because of the insensitivity to IR cooling to changes in relative humidity in the tropics. Thus the strength of "Walker" circulation cannot increase more than what the radiative cooling can sustain. This can act as a regulation mechanism limiting the basin-scale SST gradient and the strength of coupled system. The above coupling mechanism is further supported by an observational analysis of ISCCP clouds, SST, and water vapor amount.