Role of Indian Ocean SST on developing El Niño and tropical west Pacific convection

Prior to the 1976-77 climate shift (1950-76), SST anomalies in the tropical Indian Ocean consisted of a basin-wide warming during boreal fall of the developing phase of most El Ni�os whereas after the shift (197799) they had an east-west asymmetry, a consequence of El Ni�o being associated with the Indian Ocean Dipole/Zonal Mode (IODZM) after the shift but not before. In this study, we investigate the possible impact of these contrasting SST patterns on the on-going El Niño, using atmospheric reanalysis products and solutions to both an atmospheric general circulation model (AGCM) and a simple atmospheric model (LBM), the latter used to identify basic processes. Then, we focus on the role of the basin-wide Indian Ocean SST anomalies that occur during December through May after the mature phase of El Niño on tropical West Pacific convection. A suite of AGCM experiments, each consisting of a 10-member ensemble, is carried out to assess the relative importance of remote (Pacific) versus local (Indian Ocean) SST anomalies in determining precipitation anomalies over the Indian Ocean. Solutions indicate that during boreal fall both local and remote SST anomalies are necessary for realistic precipitation simulations. Prior to the climate shift, the basin-wide Indian-Ocean SST anomalies generate an atmospheric Kelvin wave associated with easterly flow over the equatorial western-central Pacific, thereby weakening the westerly anomalies associated with the developing El Niño. In contrast, after the shift the east-west contrast in Indian-Ocean SST anomalies does not generate a significant Kelvin-wave response, and there is little effect on the El Niño-induced westerlies. The LBM solutions confirm the AGCM's results. Model results reveal that for nearly three seasons, from boreal fall into the following spring, Indian Ocean SST anomalies modulate the convection in the tropical West Pacific, and the subsequent development and amplification of the South China Sea anticyclone. Our model results indicate that about 50% of the total precipitation anomalies over the tropical west Pacific Maritime Continent is forced by remote Indian Ocean SST anomalies, and therefore offer an additional or alternative mechanism for the maintenance of the South China Sea anticyclone. As a consequence, the East Asian Winter Monsoon strength appears to be influenced by remote Indian Ocean SST anomalies. In addition, the persistence of SST anomalies over the southwest Indian Ocean, primarily due to the active role of ocean dynamics, delays the onset of the Indian Summer Monsoon.