Subtropical Jet Streaks over the South Pacific

Abstract

The main objective of this study is to obtain a better understanding of the upper-tropospheric subtropical westerly wind maxima over the Australian?South Pacific region in the summer half of the year, which have been documented in previous papers to occur with a periodicity of 1?2 weeks. The focus of the study is to quantify the relative importance of tropical versus nontropical forcing during the acceleration phase of the aforementioned westerly wind maxima. Outgoing longwave radiation, wind data, and kinetic energy budgets, partitioned into rotational and divergent components, are used to examine the significance of the forcing mechanisms during the 6-month summer periods from 1985 to 1989. Criteria are developed to identify strong episodes of zonal wind accelerations. In all, 40 cases were found that met these criteria, or approximately 10 cases per year. In summary, 17 of the 40 cases suggested that tropical forcing was primarily responsible for the observed increase in the rotational kinetic energy of the jet streaks. In contrast, in 13 cases it appeared that little or no connection occurred between tropical convective heat sources and the accelerating jets. In fact, it seemed that midlatitude wave activity was the important factor during the acceleration phase of most of these 13 cases. For the remaining 10 cases, it was difficult to conclude whether tropical forcing was more important than middle latitude forcing; however, it appeared that tropical forcing, albeit weaker than the 17 aforementioned cases, did play a forcing role. An examination of the case composites in each of these three categories revealed that the energy cycle for the tropically forced cases consisted of a generation of divergent kinetic energy, a conversion of divergent to rotational kinetic energy, and a loss of rotational kinetic energy due to horizontal export and frictional dissipation. Except for the loss of rotational kinetic energy by dissipation, the main energy cycle for the nontropically forced accelerations was the reverse of that for tropically forced jets. Finally, for those 10 cases where the primary region of forcing was uncertain, the composited energy cycle generally consisted of a compromise between the tropically and nontropically forced composites, although there was a significant generation of divergent kinetic energy, as well as a conversion of divergent to rotational kinetic energy, as for all tropically forced cases.