An impact of SST anomalies in the Indian ocean in acceleration of the El Nino to La Nina transition

One possible impact of sea surface temperature (SST) anomalies in the Indian Ocean (IO) on the ongoing El Nino is investigated using an air-sea coupled general circulation model (CGCM). A suite of CGCM experiments by imposing the IO basin-wide warming (BW) and IO dipole/zonal mode (IODZM) are conducted to assess the feedback effects of the El Nino-related SST anomalies on the Pacific. While the IODZM during boreal fall does not have a significant impact on the Pacific sector, the BW during boreal winter enhances the surface easterlies over the equatorial Western Pacific (WP) during the mature-decay phase of El Nino. The strengthened easterlies act to enhance the SST cooling over the WP, thereby the zonal gradient of SST between the IO and WP is greater than the climatology. These enhanced WP easterlies induce an advanced transition to the La Nina phase, which is caused by the upwelling ocean Kelvin waves. These results imply that the BW in the IO, to some extent, can be hastening the El Nino to La Nina transition.