Indian Ocean SST fluctuations: Bi-product of Monsoon and ENSO

Indian Ocean SST Fluctuations: Bi-product of Monsoon and ENSO

H. Annamalai and Bin Wang IPRC/SOEST, University of Hawaii email: hanna@soest.hawaii.edu

The interannual sea surface temperature (SST) anomalies over the Indian Ocean are only of theorder of 0.5oC, unlike in the equatorial central-east Pacific, where the SSTA peak around 4oC,often during ENSO years. Albeit the weaker interannual variations, the mean SST over the Indian Ocean is high (~28oC) year around and hence climatically important. Recent modelling andobservational studies suggest that there exists a strong east-west SSTA gradient over the equatorial Indian Ocean, with peak amplitudes in the order of 1oC that might have significantimpact on the Indian Ocean climate variability. There are suggestions that this east-west SSTA gradient is independent of ENSO and may be a natural oscillation of the Indian Ocean.

In the present study, it is hypothesized that the observed east-west SSTA gradient over the Indian Ocean is a passive response to the Asian Summer Monsoon (ASM) and ENSO. The hypthesis is demonstrated by examining the 40-year NCEP/NCAR reanalyses products over the tropical Indian and Pacific Oceans. It is shown that the warm SSTA over the western Indian Ocean is a result of the weak south Asian monsoon (SAM) which is a response to El Nino in the Pacific. The weak cross-equatorial flow associated with the Somali jet results in weakened coastal upwelling and evaporation in the west leading to warm SSTA. The cold SSTA over the eastern Indian Ocean is largely due to coastal upwelling along Sumatra and also due to the advection of cold SSTA by the easterly anomalies from the Maritime continent, an impact of Indonesian throughflow. The coastal upwelling along Sumatra, induced by the strong southeasterly anomalies, is primarily due to intensified Australian anti-cyclone. The strengthening of the Australian High is a Rossby response to the heat sink over the equatorial Maritime continent which lies in the descending branch of the anomalous Walker Circulation during El Nino years. The complementary gradients in SSTA over the Indian Ocean exists when the SAM is strongerthan normal in conjunction with La Nina conditions over the Pacific.

There are other years (e.g. 1961, 67 and 94) when pronounced east-west SSTA gradient appeared over the Indian Ocean without El Nino conditions in the Pacific. Interestingly both the SAM and WPM monsoon are stronger than normal, possibly due to warmer than normal SSTA over the tropical west Pacific. The intensified local Hadley circulation has ascend over Bay of Bengal-west Pacific region and descend over the Maritime continent suppressing the precipitation over that region. Due to this reduced precipitation over the equatorial west Pacific, again as a Rossby response, the Australian High is intensified resulting in strong coastal upwelling along Sumatra. Intriguingly, the cross-equatorial flow associated with the Somali Jet is weakened in these years too, resulting in warm SSTA over the western Indian Ocean.