The Role of El Niño in Regulating the Long-term Mean Strength of the Walker and Hadley Circulation

A coupled model for the Pacific region has been used to study the response of the zonal SST contrast and the strength of Walker circulation to changes in the meridional differential heating over the Pacific ocean. Different from the previous studies that use an intermediate ocean model, the present model employs a primitive equation model for the Pacific ocean—the NCAR Pacific basin model— and therefore has an explicit heat budget for the subsurface ocean. By calculating subsurface ocean temperature explicitly from the first principles, the model also allows feedbacks of El Niños upon the thermal structure of the equatorial upper ocean and thereby upon the stability of the equatorial ocean. Different from full-blown GCM studies that use either a low resolution ocean GCM or questionable treatment of clouds and water vapor that lead to an unrealistic El Niño in the coupled model, the present coupled model has a fine resolution for the equatorial region and employs an empirical atmosphere in which the clouds and water vapor feedbacks have observed values. Consequently, the model simulates ENSO well. The amplitude, the dominant frequency, as well as the evolution of the subsurface temperature in the present model agree well with observations (Fig. 1; Also see Sun 2002)