Seasonal climate summary southern hemisphere (winter 1998): transition toward a cool episode (La Nina)

Southern hemisphere circulation patterns are reviewed for winter (June - Au gust) 1998, with particular emphasis given to the tropical Pacific Ocean an d the Australian region. In autumn 1998, climate indicators suggested a dec line in the strong El Nino event evident during the previous 12 months. The decline of this event continued into winter, with the climate system appar ently undergoing a rapid transition toward a cool episode. Warm surface ano malies in the western Pacific Ocean were maintained throughout the season, with cool anomalies continuing to develop in central parts. However, signif icantly cooler than normal subsurface waters in the eastern tropical Pacifi c failed to penetrate the surface in the far east, so that by the end of wi nter weak warm anomalies remained off the coast of South America. Atmospher ic indicators pointed to a dramatic resurgence of the equatorial Walker cir culation in the western and central Pacific at the end of autumn 1998. Cons equently the western and central Pacific displayed typical characteristics of a weak La Nina event during winter 1998, with the pattern yet to extend to the far east. Mean sea-level pressure over Australia was close to averag e for the season, but northwest cloudband activity resulted in a wet winter through most parts of the country. Anomalous northwesterly now and warmer than normal waters to Australia's north resulted in persistent warm tempera tures in the northern half of the continent.