Interhemispheric Mass Exchange Due to Ideal Tropical Hadley-Type Circulations Simulated by an Axially Symmetric 2-D Global Model.

Abstract

Zonal symmetric atmospheric motions are dominant in the tropical latitudes, suggesting that the meridional mass transport there depends largely on Hadley-type circulation. Simulated movements of air parcels were analyzed in low latitudes under ideal axially symmetric conditions to evaluate the contribution of this meridional circulation to interhemispheric exchanges.   A three-dimensional atmospheric general circulation model was modified into an axially symmetric model. It demonstrated that a cross-equatorial mass exchange often takes place easily as air parcels reach a simulated inter tropical convergence zone (ITCZ).   Unsteady components of atmospheric northward or southward motions are important for a cross-equatorial mass exchange, and meridional asymmetry of the Hadley circulation due to displacement of the ITCZ from the equator greatly influences the characteristics of a mass exchange, such as the amount of exchangeable air mass and the mass exchange time.   The mass exchange time in simulated ideal Hadley circulations was estimated to be 0.1 to 0.2 yr. The model in this study is ideal axially symmetric, so this exchange time scale should not be applied directly to 3-D global models or the real world. However, it is very interesting and important that this time scale is much shorter than those obtained from conventional analyses of the observed distribution of minor constituents, and that the idealized Hadley circulation induced in the model tropics performs a more efficient interhemispheric exchange.