Direct observations of the upper ocean velocity in the eastern equatorial Indian Ocean by an acoustic Doppler current profiler, from November 2000 to October 2001 on the equator at 90°E, demonstrate that the dominant periods of variability in the upper layer zonal and meridional currents are in intraseasonal frequency bands with periods of 30 to 50 days and 10 to 20 days, respectively. The strong intraseasonal variability in the zonal current obscures the semiannual Wyrtki jets, which can be seen clearly in the monthly averaged field. In addition, a zone of strong vertical shear of the zonal current and a distinct Equatorial Undercurrent with semiannual period are observed. The results provide us with a new perspective on importance of the energetic intraseasonal variability in the eastern equatorial Indian Ocean, which indicates strong correlation with the wind variability near the mooring location.

Details of subsurface ocean conditions associated with the Indian Ocean Dipole event (IOD) were observed for the first time by mooring buoys in the eastern equatorial Indian Ocean. Large‐scale sea surface signals in the tropical Indian Ocean associated with the positive IOD started in August 2006, and the anomalous conditions continued until December 2006. Data from the mooring buoys, however, captured the first appearance of the negative temperature anomaly at the thermocline depth with strong westward current anomalies in May 2006, about three months earlier than the development of the surface signatures. These subsurface evolutions within the ocean would be a key factor for better understanding of IOD mechanisms and its predictability, and are providing a fundamental dataset for validation of modeling outputs.

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An anomalous climate mode, the positive Indian Ocean dipole (IOD), occurred in 2006 with the anomalous sea surface temperature (SST) distribution in the tropical Indian Ocean. Using various types of observational data, we investigated the temperature variation in the surface mixed layer in the eastern Indian Ocean to clarify the processes that produced the anomalous SST variation in 2006. Analysis was conducted at an intraseasonal time scale and focused on a location (5°S, 95°E) where in situ measurements by the Triangle Trans‐Ocean Buoy Network were available. Temporal changes in the mixed layer temperature were obtained from the buoy data. Air‐sea heat fluxes and horizontal heat advection were estimated from the buoy data, satellite‐based data, and reanalysis products. Heat balance analysis demonstrated that air‐sea heat fluxes and horizontal heat advection mainly accounted for the mixed layer temperature variation. The results indicate that the relative importance of the heat fluxes and horizontal heat advections changed remarkably with the onset of the IOD. During January to mid‐May 2006, before the onset of the IOD, the temperature variation was mainly explained by the net surface heat flux at an intraseasonal time scale. During the IOD in late August to November 2006, the northwestward horizontal temperature gradient and the surface current produced large horizontal heat advection that exceeded the contribution of surface heat fluxes. These results confirm the importance of oceanic processes in the evolution of the IOD, and the heat balance analysis would be a fundamental example in validating model outputs for the Indian Ocean.