On the Maintenance of Short-Term Subtropical Wind Maxima in the Southern Hemisphere during SOP-1, FGGF

Abstract

The extent to which divergent circulations, induced by tropical heating, help to maintain westerly maxima in the Southern Hemisphere subtropics during the SOP-1 of FGGE is explored using Level Bib analyses from the Goddard Laboratory for Atmospheres (GLA). The contribution of the divergent wind component to the total ageostrophic flow in the subtropics is examined, as are the roles of other forcing terms in the localized Eliassen-Palm flux zonal momentum equation. In addition, the interaction of divergent and rotational flows in the subtropics is analyzed using the complete kinetic energy budget, partitioned into rotational and divergent components. Throughout the summertime subtropics, it is generally found that the dominant term in the zonal momentum budget is the Coriolis force applied to the diabatically driven meridional circulation. The 1argest positive tendencies due to this term are found in the entrance regions of the subtropical westerly maxima, and divergent circulations account for nearly all of the total ageostrophic flow. In the SPCZ region, however, it is found that transient eddies play an important role by partially offsetting the strong Coriolis acceleration in the entrance region of the local jet and they help accelerate the westerly flow in the exit region through both barotropic and baroclinic processes. Energetically, the dominant term in the rotational kinetic energy budget throughout the subtropical belt is the conversion of divergent to rotational kinetic energy. Furthermore, nearly all of the generated divergent kinetic energy is converted. The evidence from all of these approaches supports the view that tropical beating in transient events drives or enhances local meridional overturning in the atmosphere, which, in turn, strengthens the summer subtropical westerly jet stream.