Two case studies are presented of winter hemisphere cold surges affecting the summer hemisphere tropics. One case has been chosen from each of the Northern and Southern Hemispheres. In both instances, the subtropical rise in pressure is tracked equatorward. This leads to a pressure rise at the equator and the establishment of a west–east pressure gradient at low latitudes in the opposite hemisphere. These effects lead in turn to an enhanced cross-equatorial component of flow and enhanced monsoon westerly flow in the summer hemisphere. Both enhanced wind flow effects are observed through a deep layer from the surface up to at least 500 mb. In both cases, the sequence of events also includes the development of a tropical cyclone in the summer hemisphere monsoon trough.
The generality of the above sequence of events is investigated with time series data for several seasons. It is shown that day-to-day changes in equatorial pressure are significantly correlated to pressure changes in the winter hemisphere subtropics. It is also shown that the strength of the low-latitude westerly winds is well correlated with the synoptic scale west–east pressure difference near the equator.