The Subtropical Jet Stream (STJ) is a prominent feature of the upper-level circulation at Australian longitudes and the western Pacific Ocean during Southern Hemisphere (SH) winter. The core zonal wind speed typically exceeds 45 ms-1 in this season when the STJ is well-separated from the Polar Front Jet (PFJ). Despite its important role for the SH climate, few studies have directly examined the nature and causes of the STJ’s variability on intraseasonal to interannual timescales.
The interannual variability of the SH winter STJ exhibits a bimodal distribution with maxima at 25°S and 35°S, and the largest variability occurs between 20-40°S and 400-100 hPa. The magnitude of its interannual variability is small (>2.5 ms-1) relative to the mean.
We define a dynamically based index that describes the wintertime location and intensity of the STJ so to quantify variations of the STJ. A positive shift in the wintertime STJ is associated with stronger westerlies north of the STJ’s climatological latitude and anomalous easterlies on the poleward edge, as well as a strengthening of the PFJ. In comparison, a strong STJ is associated with stronger westerlies at approximately 27°S that extend to the surface, but there is no distinct PFJ. We explore possible mechanisms that influence these variations in strength and location, as well as the accompanying variations of boundary forcing (SST), tropical convection and large-scale circulation.