Inter El NiÑo variations in the winter atmospheric circulation of the South Pacific part I: observations

Evidence now exists that while the winter extratropical circulation of the South Pacific during El Ninos can be strongly modulated by seasonal Rossby wave trains this has failed to happen in many events in the last 25 years. Seasonal wave trains lead to cold conditions and extensive sea ice around the Antarctic Peninsula but these conditions fail to develop when the wave trains weaken or disappear. In this paper several 25 year observational datasets are used to test the hypothesis that the inter El Nino variations in seasonal wave trains closely follow variations in the tropical Pacific climate state. Attention is focussed on changes in deep tropical convection inferred from satellite outgoing longwave radiation data.

Early results suggest variations in seasonal wave trains are attended by changes in convection in the Pacific Intertropical Convergence Zone near the dateline. The ITCZ appears to be crucial for wave train generation in austral winter because it is the area of deepest convection. Results will be described in detail in the paper and reasons for the varying strength of the ITCZ will be discussed.