Climatology of the Southern Hemisphere split jet

Using the 41-year (1958-98) NCEP/NCAR Reanalysis grids a climatology of the planetary-scale flow has been produced. From this climatology a striking feature of the Southern Hemisphere (SH) cool season is the presence of an upper-level split-flow regime. The split-flow regime is defined by the presence of the subtropical jet (STJ) and polar front jet (PFJ) from 140°E to 130°W, with a distinct minimum in zonal velocity between the two jets.

Global scale analysis indicates that the SH STJ strength and position are tied to the divergent outflow of the boreal summer Asiatic monsoonal circulation. The cross-equatorial outflow torqued by the Coriolis force fuels the westerly STJ. Synoptic scale analysis suggests that during the austral winter the baroclinic zone across Australia created by the cooling of the continent contrasted with the western Pacific warm pool acts to modulate the strength and position of the STJ across Australia and New Zealand. Together these effects allow for the existence of a quasi-steady (in both strength and position) STJ.

By contrast, the PFJ appears and disappears across the southern Pacific Ocean. The presence or absence of the PFJ largely determines whether or not split flow occurs. Climatologically, split flow onset occurs in March, strengthens in austral winter and early spring, and disappears completely by December. A split flow index (SFI) has been developed to measure the temporal variance and relative strength of the split flow. The SFI is defined by 200 hPa area-averaged relative vorticity anomalies. The anomalies are measured across the STJ, the PFJ and the zonal velocity minimum between the two jets for the July-August-September season. The more negative the SFI, the stronger the split flow. Inspection of SFI time series reveals considerable interannual and intraseasonal variability in the split flow.