Why is not the equatorial thermocline shallow at the eastern Indian Ocean ?
Saji N H
Institute for Global Change Research of FRSGC, Tokyo
The equatorial thermocline in the eastern Indian Ocean is deep and shallows towards the west. The winds overlying it are westerlies. Both these features are consistent with each other. At times the westerlies are replaced by easterlies and the thermocline shallows in the east, deepening towards the west. This abnormal situation in thermocline slope manifests a dipole structure in sea surface temperature (SST) anomalies with cool SST anomalies in the east and warm SST anomalies in the west. The corresponding abnormality in surface circulation results in a westward migration of convection from the southeast equatorial Indian Ocean. This phenomenon referred to as Dipole Mode (DM) events introduces a see-saw in rainfall between the countries at the eastern and western sides of the equatorial Indian Ocean basin.
With this background, the following questions are raised and explored:
1. Does the monsoonal circulation demand that westerlies lie over the equatorial eastern Indian Ocean ?
2. If so, are DM events a result of fluctuations of the equatorial windstress forced by fluctuations of the monsoonal circulation?
3. Or are DM events a consequence of modulations of equatorial Indian Ocean windstress in response to the ENSO?
