85th AMS Annual Meeting

Tuesday, 11 January 2005: 2:15 PM
Low frequency variability of the Southern Hemisphere winter split jet
Xiaosong Yang, SUNY at Stony Brook, Stony Brook, NY; and E. K. M. Chang
Previous studies suggest that the split jet is one of the most important low-frequency variabilities in Southern Hemisphere (SH) winter, and the split jet paradigm is not only centered in the longitudes between Australia/New Zealand and the date line, also inherently hemispheric. The local split jet is associated with the variability of the whole SH hemispheric circulation, or the so-called SH annular mode, so understanding of the split jet may help us to investigate the dynamics of the SH annular mode.

A new split jet index is defined in this study, and the composites based on this index show that the cold-warm-cold tripole temperature anomaly starting from the South pole occurs in the South Pacific in association with the split-flow regimes and the non-split flow exists when the phase of the tripole temperature anomaly is reverse. The heat budget reveals that the temperature anomaly associated with the split/non-split jet is the response to the large-scale wave, not the forcing from the local diabatic heating. The localized E-P flux diagnostics and the stationary wave model diagnostics both illustrate that the split/non-split jet is maintained by the vorticity flux, and the vorticity flux is dominant over the heat flux, which has the opposite effect to the maintenance of the split/non-split jet.

The organization of high-frequency eddies by the low-frequency split/non-split jet has been studied. Two sets of linear runs of a simple GCM with random initial conditions for the split and non-split jet basic state respectively, have been conducted to establish the statistics of the storm tracks. The storm track anomalies that are caused by the split/non-split jet match the storm track anomalies that are associated with the split/non-split jet observed in the ECMWF reanalysis data, thus demonstrating that the low-frequency split/non-split jet does organize the high-frequency eddies. Our results suggest that there is a two-way feedback between the eddies and the mean flow in the low frequency variability of the SH winter split jet.

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