Thermally-Forced Mean Mass Circulations in the Northern Hemisphere

Abstract

A diagnostic approach to infer three-dimensional distribution of the thermally-forced, time-averaged horizontal mass and energy transport (Johnson and Townsend, 1981), which was previously applied in the Southern Hemisphere (Zillman, 1972), is used to determine the corresponding Northern Hemisphere circulations. The method is based on the steady form of the time-averaged isentropic continuity equation and allows calculation of the irrotational components of the mass circulations which are consistent with modeled diabatic heating fields determined from climatic information. The three-dimensional distributions of atmospheric heating in the Northern Hemisphere are estimated from summer and winter climatic data of precipitation, turbulent exchange of sensible heat and radiative fluxes. The results highlight the large-scale coupling of the Northern Hemisphere heat source and heat sink regions by Hadley-type and Walker-type circulations. The zonally-averaged mass circulation exhibits a thermally direct, meridional cell spanning the entire hemisphere in winter. This circulation shifts northward and weakens in summer with the corresponding Southern Hemisphere winter Hadley-type circulation now extending to northern latitudes. The composite three-dimensional mass circulations also reveal prominent mass transports associated with the Asian monsoon in summer and winter. Zonal asymmetries in the heating lead to longitudinal variations of the meridional circulation and pronounced east-west overturnings. A complementary study (Johnson and Townsend, 1981) in which similar mass circulations were derived from the FGGE observational global data set validates the large-scale patterns established in this study.