There is increasing evidence that while ENSO warm events can strongly modulate the extratropical circulation of the South Pacific through Rossby wave propagation (just as in the North Pacific) this does not happen in all events. In part, this may be due to the ‘unforced’ part of the extratropical circulation dominating over the ‘forced’ part. An alternative hypothesis is explored in this paper that variations in the extratropical circulation of the South Pacific are due to the differing seasonal development of individual warm events that then modulates the occurrence of deep tropical convection. Early findings will be discussed based on analysis of tropical SST (for the Nino 3.4 region), OLR and wind fields and extratropical MSLP data for ENSO events in the period 1973-93.

It is found that SST data for the central tropical Pacific can be used to reliably determine when the expected high latitude response during warm events occurs with increased MSLP in the South Pacific. In particular, this response only occurs when seasonal cooling in the central tropical Pacific is below-normal in austral winter, or, in the case of the austral spring, warming takes place. In contrast, high latitude MSLP anomalies reverse sign when cooling intensifies (events weaken). In fact, SLP in the southeast Pacific systematically decreases as seasonal cooling intensifies in the central tropical Pacific. Using the other tropical datasets a physical explanation for the breakdown of the expected high latitude ‘response’ is being sought. A case study will focus on possible changes in the trade winds and tropical convection during ‘weakening’ warm events to explain why ‘breakdowns’ in the typical high latitude circulation may occur