Dynamics of the Southern Hemisphere spiral jet and storm track (formerly paper 6.5)

The formation of the Southern Hemisphere spiral jet and storm track is investigated using observations over a 40-year period. It is found that between late March and early April, the upper tropospheric westerly jet in the Southern Hemisphere undergoes a transition from an annular structure in the midlatitudes, to a spiral structure that extends from 20S to 60S. The transition to the spiral structure is initiated by the formation of a subtropical jet, localized in the central Pacific. During this transition, the midlatitude storm tracks gradually shift equatorward. Initially, the storm track is too far poleward to influence the jet structure in the subtropics; however, as the storm track moves equatorward, vorticity fluxes associated with the storms drive a northwesterly jet that links the subtropical and midlatitude jets, thereby forming a spiral jet.

Further observations of the seasonal transition demonstrate that the formation of the spiral structure in the storm track lags significantly behind (about 50 days) that of the jet spiral. This large time lag evidently arises because the subtropical jet is not capable of organizing storms until the beginning of the austral winter when the subtropical jet becomes significantly strong.

A set of calculations with an idealized model is performed to test the above dynamical processes, and to gain additional insight. The results of these calculations will also be presented.