Several Northern Hemisphere (NH) studies have shown that midlatitude circulation characteristics play the dominant role in determining track, structure and intensity changes for tropical cyclones (TCs) undergoing extratropical transition (ET). Previous Southern Hemisphere (SH) studies have examined climatological aspects of ET behavior and looked in more detail at a few individual cases but there has yet to be a systematic attempt to establish how and why SH ET events vary greatly from case to case.

This study addresses this question by identifying how the character of the midlatitude flow influences southwest Pacific TC behavior during ET. Characteristic midlatitude circulation patterns accompanying ET are first identified using empirical orthogonal function analysis on 52 storms between 1970 and 1997. The evolution of the TC during ET was found to depend on the character of the flow. Where the primary midlatitude circulation pattern is a trough to poleward, the TC exhibits strong zonal motion toward the eastsoutheast whereas storms with a high to the south feature meridional motion. Storms undergoing ET directly beneath the upper trough are more intense than those embedded in stronger westerly flow. Finally, an upper jet approaching from the west and stronger baroclinicity were key factors in storms that regenerated in middle latitudes. One practical motivation for this study is the potential to predict gross features of future storm evolution for this basin from the nature of the midlatitude flow in which the TC is embedded. Some possible forecasting rules will be presented.