Nonlinear Interaction between the Drivers of the Monsoon and Summertime Stationary Waves

An intermediate complexity moist General Circulation Model is used to investigate the forcing of the Asian monsoon and the associated upper level anticyclone by land-sea contrast, net horizontal heat transport by the ocean, and topography. The monsoonal pattern is not simply the linear additive sum of the response to each forcing; only when all three forcings are included simultaneously does the monsoonal circulation extend westward to India. This nonadditivity impacts the location of the upper level anticyclone, which is shifted eastward and weaker if the forcings are imposed individually. Sahelian precipitation, and also austral summer precipitation over Australia, southern Africa, and South America, are likewise stronger if all forcings are imposed simultaneously. The source of the nonlinearity can be diagnosed using gross moist stability, but cannot be accounted for using the land-sea breeze paradigm. This non-additivity implies that the question of which forcing is most important is ill-posed.