Transient Response of the Southern Hemisphere Subtropical Jet to Tropical Forcing

Abstract

The effect of tropical latent heat release in accelerations of the Southern Hemisphere subtropical jet is discussed based on a case study for 6?8 August 1979 and general circulation model simulations. This jet is a main feature of the winter time circulation extending over Australia and the western Pacific Ocean. An intensification of this jet was observed for 8 August, with peak values of 93 m s?1. Satellite pictures and daily precipitation amounts revealed that the 6?8 August period was characterized by extensive rainfall in the tropics, with precipitation diminishing from that day on. Level III-b data for the Global Weather Experiment produced by the Geophysical Fluid Dynamics Laboratory for these dates was projected into the normal modes of a primitive equation model linearized about a basic state at rest. The analysis shows that the jet accelerations were due both to external and internal Rossby modes, with somewhat stronger contributions from the external mode. An enhancement of the local meridional circulation is due to these extensive precipitation areas which project mostly in internal gravity modes with maximum values upstream of the jet accelerations. Conjecture that the jet stream is reacting in this short time scale to tropical latent heat release is tested with the Goddard Laboratory for the Atmospheres fourth order general circulation model. A 15 day integration starting on 1 August 1979 is used as the control case. This run produced peak values of 82 m s?1 for the zonal wind at 200 mb during August 8 centered over Australia at about 26°S in conjunction with increased divergent circulations at about 6°N about 50° longitude upstream from the jet maximum. A simulation was started on 4 August and run for ten days suppressing tropical Pacific heating from 90°E through 120°W. Normal mode contributions were also obtained for the control and ?no tropical heating? experiment. The case study and GCM simulations suggest the following time scale of response of subtropical latitudes to tropical latent heat release: divergent circulations which project mostly into inertia-gravity waves react to changes in latent heat release in 1?2 days, its impact in subtropical latitudes is felt in 2?4 days; and becomes fully established after about 6 days.