The structure of anomalous oceanic circulation in the Indian Ocean Dipole

Abstract

Using an Indian-Pacific ocean circulation model with high resolution a simulation study on the Indian Ocean Dipole (IOD) has been done. Forcing the model with monthly observational wind stress in 1990-1999 the main characteristics of sea temperature variations in the two IODs (in 1997 and 1994) have been reproduced well. The patterns and center positions of sea temperature anomalies in the tropical Indian Ocean surface and in the section of equator-depth during the IOD from the simulation are basically consistent with that from observation. The physical image of anomalous circulation during IOD is revealed from the simulation. We find that an anomalous easterly current along the equator in the upper layer of the eastern Indian Ocean during the IOD period. It is very strong, narrow band and is divergent from the equator to both sides. It represents a Rossby wave propagated westwards. During IOD phase there is a significant anomalous current cell in the section of equator-depth: the easterly current in the upper layer; westerly compensated current below it; a strong upwelling to the east of 80/spl deg/E; a weak downwelling to the west of 55/spl deg/E. Meanwhile two anomalous meridian cells are on both sides of the equator in the eastern Indian Ocean. The common upwelling of them is near the equator. The patterns of anomalous current in the out-of-IOD phase are basically opposite to that in the IOD phase; besides, the absolute value of the anomalous current is weaker. Therefore the anomalous sea temperature in the tropical Indian Ocean during IOD could be interpreted with anomalous horizontal and vertical current, especially large-scale upwelling and downwelling.