The merged and superposed sub-tropical jet and polar-front jet in the southwest Pacific: A case study

In the southwest Pacific, a meandering jet-stream in the upper troposphere is sometimes found at ~30° S during austral winters and is usually treated as a sub-tropical jet (STJ) due to its low latitude. For two contrasting cases, we have conducted analyses from two perspectives to identify the STJ and PFJ: first, using previously published qualitative criteria to identify jet-cores and second, investigating the jet-stream axes of STJ and PFJ identified using 2-PVU curves. The results showed that the chosen meandering jet-stream case at ~30° S was a merged, and for a time, a superposed STJ and PFJ. Downstream of the jet-streak, the PFJ split to the south and the STJ to the east. This is in significant contrast to the horizontally well-separated jet-stream case chosen in this study. Some processes likely contributing to the superposition of the STJ and PFJ were analyzed and discussed. The movement of PFJ that was closely associated with the movement of the low over the Tasman Sea and the convection in and near the tropical region may have played dominant roles.