The Australian Subtropical Jet during the Second Observing Period of the Global Weather Experiment

Abstract

The upper-level circulation of the Southern Hemisphere winter is characterized by two distinct zonal wind maxima: a subtropical jet found in the vicinity of Australia and the western Pacific Ocean, and a polar jet which maximizes in the Indian Ocean in the 45°?55°S latitudinal belt. This paper describes the global characteristics of the atmosphere for cases with strong subtropical jets during the 1979 Northern Hemisphere summer. Such cases are shown to co-exist with episodes of strong release of latent heat in the Northern Hemisphere at similar longitudes. Gridded analyses of the Global Weather Experiment produced by the Geophysical Fluid Dynamics Laboratory of the National Oceanic and Atmospheric Administration are used to obtain composites of atmospheric motions prior to the onset and during active episodes of the Asian Monsoon. Projections of these motions into the normal modes of a linearized primitive equation model about a basic state at rest are presented to isolate observed global flow characteristics. Relative contributions from internal and external modes are shown as well as those from gravity, Rossby and equatorially trapped modes. Results indicate that active hemispheric interactions take place in the longitudes of the Southern Hemisphere subtropical jet and are accomplished by motions possessing the horizontal structure of internal gravity modes. The subtropical and polar jets have very different vertical structures, projecting mostly in internal and external modes, respectively. Accelerations of the subtropical jet occur due to changes both in the internal and external Rossby modes, the latter contributing most to these accelerations.