SST changes over the subtropical south central Pacific (SSCP) region are associated with large variability among El Niņo (EN) or warm ENSO events. The analysis of the inter-El Niņo differences show that EN events associated with cold conditions in the SSCP exhibit enhanced convection not only in the equatorial central Pacific but also in subtropical regions of the southeastern Pacific Ocean. The resulting heating forcing intensifies a localized Hadley cell, which in turn produces an anomalous Rossby wave source that triggers a well-defined Rossby wave pattern in the Central South Pacific extratropical region. Neither the Rossby wave source nor the resulting wave pattern is evident during EN events associated with warm conditions in the SSCP.

The processes responsible for the generation of SST anomalies in the SSCP region and the feedback of SST anomalies on the atmosphere circulation are not clear yet. Whether the SST anomalies in the SSCP region result from an atmosphere-ocean coupling at the South Pacific extratropics or they are merely forced by the atmospheric circulation response to ENSO, certainly deserve further attention. Therefore, we examine the air-sea interaction over south central Pacific during EN austral springs using two sets of model experiments. In both set of experiments, simulations are conducted using the GFDL R30 AGCM in which observed SSTs are specified in the eastern tropical Pacific (15S-15N, 172.5E-South America) over the period 1950-1999. The two experiments only differ in their treatment of the ocean outside of this region. In the "Control" experiment, climatological SSTs, which repeat the same seasonal cycle each year, are specified at all remaining ocean grid points. (This experiment design is often referred to as "Tropical Ocean Global Atmosphere" or "TOGA" in the literature.) In the mixed layer model ("MLM") experiment, a grid of column ocean models is coupled to the atmosphere at each AGCM grid point over the ocean outside the tropical Pacific region. The atmospheric response to prescribed boundary conditions can drive SST anomalies outside the tropical Pacific in the MLM but not in the Control experiment.

Differences between the two experiments are used to study whether air-sea interaction in the south central Pacific extratropics affects the atmospheric response to EN. To enhance the signal-to-noise ratio, experiments are performed using an ensemble of simulations where each simulation is initiated with a different atmospheric state. There are 8 Control and 16 MLM simulations. Recent works show that the model is able to get the basic structure of the ENSO induced SSTs although they are somewhat west of where they are observed. The discussion of different composites based on EN events of the atmospheric circulation and the oceanic conditions over the SSCP for both types of model experiments as well as their comparison with observations will be presented at the Conference.