Life Cycle of the Southern Hemisphere Split Jet The Polar Front Jet

The recent availability of global reanalysis grids has made it possible to doc ument the predominance of a split-flow regime, which occurs preferentially in the region from 140°E to 130°W during the Southern Hemisphere (SH) cool season. A global scale analysis suggests that the strength and position of the subtropical jet (STJ) is quasi-steady and is largely modulated by the divergent outflow from the monsoonal circulation over Southeast Asia during the Northern Hemisphere warm season. Given the quasi-steady state of the STJ, the presence or absence of a split- flow regime in this region is largely defined by the presence or absence of a strong polar front jet (PFJ) located poleward of 55°S. In order to understand the physical mechanisms which modulate the SH split jet, a global and synoptic scale analysis is performed both objectively and subjectively through the use of composites and case studies.

The composites and case studies indicate that the onset of the split-flow regime is dependent upon the presence of a broad, positively-tilted, trough in the South Indian Ocean, centered near 100-90°E. Equatorward fractures of cold pools from this trough help anchor a baroclinic zone over northern Australia, while the broad northwesterly flow on the east side of the trough permits a broad region of poleward warm-air advection and angular-momentum flux, enhancing the high-lati tude meridional temperature gradient and the strength of the PFJ. The split is main tained as long as incipient cyclogenesis on the front side of this trough remains relatively weak, and the cyclones propagate southeastward, poleward of broad ridge located south of Australia. However vigorous cyclogenesis in association with neu trally or negatively-tilted troughs in the South Indian Ocean results in a full-latitude and large-amplitude ridge southwest of Australia. This pattern eventually results in the creation of a strong trough downstream, generally located between 160°E to 150°W. The cold outbreaks associated with these troughs act to displace the primary meridional temperature gradient zone well equatorward, resulting in the presence of one strong, broad jet at relatively low latitudes and the absence of a split-flow regime. During the SH spring, the split-jet regime ends abruptly with a late season cold surge, after which the cold-air pool over the Antarctic land mass can not be regener ated sufficiently by radiative processes.