The Wintertime Southern Hemisphere Split Jet: Structure, Variability, and Evolution

A persistent feature of the Southern Hemisphere upper-level time-mean flow is the presence of a split jet across the South Pacific east of Australia during the austral winter. The split jet is composed of the subtropical jet (STJ) on its equatorward branch and the polar front jet (PFJ) on its poleward branch. The NCEP-NCAR reanalysis is used to investigate the structure and evolution of the split jet. Results show that the presence/ absence of the PFJ determines the degree of split flow, given that the STJ is a quasi-steady feature. A splitflow index (SFI) is developed to quantify the variability of the split jet, in which negative values represent strong split flow and positive values nonsplit flow. Correlations with teleconnection indices are investigated, with the SFI positively correlated to the Southern Oscillation index and negatively correlated to the Antarctic oscillation.