The interannual variability in the tropical Indian Ocean

Using forty-year (1958-1998) ocean analyses from a global data assimilation system at the Center for Ocean-Land-Atmosphere Studies (COLA), we have investigated the evolution of the sea surface temperature (SST) and upper ocean heat content (HC) anomalies in the North and tropical South Indian Ocean. Our purpose is to determine the temporal-spatial characteristics of these oceanic anomalies and to understand their relationship to regional and global climate fluctuations. We first examine independently the roles played by oceanic dynamics and air-sea interactions within the Indian Ocean during the development of these anomalies. Then, their tele-connections to the El Niño/Southern Oscillation (ENSO) in the Pacific are studied.

An extended empirical orthogonal function (EXEOF) analysis of both the SST and HC anomalies identified basin-wide oscillatory movements with periods of 3-4 years as their leading modes. The SST anomalies initially present in the western Indian Ocean subsequently spread eastward throughout the basin. The HC anomalies, on the other hand, are characterized by westward propagating signals, especially south of the equator, that change the thermocline depth in the west. The development is somewhat similar to a mirror image of the ENSO cycles in the Pacific though the oceanic processes involved seem to be more complicated. For instance, SST anomalies usually appear first in the South Indian Ocean near the African coast, possibly in response to HC anomalies generated during the previous cycle and propagate slowly through the basin. A principal oscillation pattern (POP) analysis demonstrates that the westward propagating HC anomalies in the Indian Ocean can be decomposed into a faster component that is more symmetric about the equator and a slower component that is more concentrated in the south. These HC anomalies are closely associated with an anomalous surface wind pattern, derived from both multivariate regression and composites, that causes zonal wind anomalies near the central and eastern equatorial ocean and southeast or northwest anomalies in the south. A composite analysis based on the EXEOF and POP time series shows that the Indian Ocean variability has a certain phase relationship with the ENSO in the tropical Pacific. The relationship between the SST fluctuations and the Indian monsoon rainfall anomalies is also been investigated.