Maintenance of short-term subtropical wind maxima in the Southern Hemisphere: Observational and modeling study

James Wilson Hurrell, Purdue University

Abstract

Observational and general circulation model (GCM) data are used to investigate the short-term relationship between tropical heating and subtropical westerly maxima in the Southern Hemisphere during SOP-1, FGGE (5 January-4 March, 1979). Enhancements of the subtropical westerlies are found to be highly correlated to episodes of maximum poleward moving divergent outflow from the tropics, and the response of the subtropical zonal flow to the tropical divergence appears to be almost instantaneous (i. e., less than 12 hours). Moreover, the divergent meridional wind accounts for much of the total ageostrophic meridional wind in the subtropics and, consequently, results in strong Coriolis accelerations in the entrance regions of the westerly maxima. This result is in contrast to the northern winter jet stream regions where divergent circulations appear to play a secondary role to the rotational ageostrophic components. The importance of the divergent circulations in supporting the rotational flow is further demonstrated by an analysis of the kinetic energy budget, partitioned into rotational and divergent components. In the latitudes of the Southern Hemisphere subtropical westerly maxima, the dominant term in the rotational budget is the conversion of divergent to rotational kinetic energy. Furthermore, nearly all of the generated divergent kinetic energy is converted. The importance of local forcings other than the Coriolis acceleration on the Southern Hemisphere subtropical westerly maxima is examined through an application of the localized E-P flux equations. The analyses reveal that the South Pacific is a unique region compared to the rest of the subtropical belt. Here, transient eddies play an important role by partially offsetting the strong acceleration by the Coriolis term in the entrance region of the westerly maximum, and they help accelerate the westerly flow in the exit region through both barotropic and baroclinic processes. The Coriolis term plays a complementary role to the E-P flux divergence in the exit region, and the sum of both terms appears to be largely balanced by frictional processes. The relationship between tropical heating and upper tropospheric subtropical westerly maxima is further examined through integrations with a state-of-the-art GCM. Results from both reduced sea surface temperature runs and reduced tropical latent heating runs strongly corroborate the observational findings, and indicate that subtropical rotational flows in the summer hemisphere are strongly influenced by direct local meridional overturning induced by tropical heating.

Degree

Ph.D.

Advisors

Vincent, Purdue University.

Subject Area

Atmosphere

COinS