Scientific objectives in the eastern Indian Ocean

The Indian Ocean is known as a data sparse region among the tropical oceans. At the same time, the Indian Ocean is believed to play a significant role in climate variability in the Asian-Australian Monsoon region. In order to better understand the role of the Indian Ocean, more in situ measurements for analyses of the ocean conditions and for validation of the satellite data are required.

The Indian Ocean is characterized by its land ocean asymmetry and seasonally reversing forcing by monsoons. The maintenance mechanisms of the Indian ocean warm pool are not well understood in terms of E-P forcing, heat and salt balance, etc. The warm pool in the eastern Indian ocean may have impacts on climate changes through inter-ocean interaction by shifting of the Walker circulation, by controlling MJO activities which may influence ENSO onset, and by changing the heat and fresh water transports from the Pacific to Indian Oceans via the Indonesian Throughflow. Thus, it is important to observe intraseasonal, seasonal and interannual oceanic changes in the eastern Indian Ocean to elucidate such inter-basin interaction and the region's impact on climate.

As one of the major phenomena of interannual variability, it is known that a dipole structure of sea surface temperature (SST) anomalies in the Indian Ocean has significant impacts on regional as well as global climate variability (Nicholls, 1989; Meyers et al., 1999). Recent comprehensive analyses of the available data and results from numerical models further increase our knowledge of the SST anomaly variations of the ocean-atmosphere-land system in the Indian Ocean sector. Saji et al. (1999) developed a simple index, called the Dipole Mode Index (DMI), to represent SST variability in the tropical Indian Ocean. The DMI, the difference of SST anomaly between the western tropical Indian Ocean (10S-10N, 50E-70E) and the southeastern tropical Indian Ocean (10S-Eq., 90E-110E), shows clear interannual variability with strong positive events in 1961, 1963, 1967, 1972, 1982, 1994, and 1997. Comparison with the NINO3 index indicates very weak correlation (r < 0.35) between the two indices. The DMI is associated with strong surface wind variability over the entire tropical Indian Ocean, showing the maximum correlation with the zonal wind anomaly time series along the equator (|r| > 0.6). This suggests that the dipole structure of the SST anomaly is a manifestation of a system inherent to the Indian Ocean sector, in which the ocean, the atmosphere, and possibly the land interact.

Model simulations and satellite altimetry data highlight several important processes that affect the evolution of the DM in 1994 (e.g. Vinayachandran et al., 1999; Behera et al., 1999). Behera et al. (1999) indicates the importance of the evaporative cooling due to the strong southeasterly wind anomaly over the eastern Indian Ocean, together with the coastal upwelling induced by the same wind anomaly along the coast of Sumatra and Java. Vinayachandran et al. (1999) shows anomalously weak Wyrtki jets along the equator in 1994, and they suggest that this anomalous condition also contributes to the decrease in the SST in the eastern tropical Indian Ocean.

In order to contribute to the accumulation of necessary in situ data of the upper-layer conditions, and to investigate mechanisms involved in the seasonal variations and the above dipole mode (DM) variability in the tropical Indian Ocean, the following TRITON observations are proposed. Initially, a particular focus will be put on the mechanisms that determine the SST variability in the eastern part of the DM.