A 41-year (1958-1998) data set of monthly upper ocean heat content (HC) is used to study the interannual variability in the tropical Indian Ocean and its connections to the Pacific Ocean. The HC data is produced by an ocean data assimilation system that assimilates a comprehensive set of in situ measurements of sea surface temperatures (SST) and vertical profiles the upper ocean temperature into a global ocean general circulation model. Our analysis shows an interannual oscillation with time scales of 2-4 years in the tropical Indian Ocean. It is associated with equatorial HC anomalies propagating from eastern-central Indian Ocean and then maintaining in the western Indian Ocean south of the equator, in response to wind anomalies along the equator and in the southern ocean. It has been further demonstrated that this Indian Ocean oscillation is connected to the Pacific El Niño/Southern Oscillation (ENSO) through the global shifts of the Walker Circulation cells.

Our further examination, however, shows that there is a long-term modulation to the Indian Ocean oscillation. Specifically, the period of the oscillation was relatively short from early 1960s to late 1970s, demonstrating stronger biennial features, while it grew longer, reaching 3-4 years from 1980s to 1990s. Moreover, the connection to ENSO as described above is much stronger during 1980s-1990s. Using the NCEP reanalysis data, we have analyzed the atmospheric circulation patterns associated with the different regimes of the Indian Ocean oscillation. We have also noticed that, although the leading modes of the surface wind and precipitation are consistent with that of the heat content, the SST pattern is somewhat different. In fact, the change of thermocline depth mainly affects SST fluctuations near the equator. The SST variations in the subtropics are mainly by surface heat flux changes through the atmosphere. This effect is also being evaluated using the NCEP data.