Water, heat and freshwater flux out of the northern Indian Ocean in September–October 1995

Abstract

Abstract World Ocean Circulation Experiment (WOCE) Transindian Hydrographic Section I1 (I1) is the northernmost of the zonal sections carried out during the WOCE Indian Ocean Expedition of 1994–1995. It crosses the southern boundaries of both the Bay of Bengal (I1e) in the east and the Arabian Sea (I1w) in the west. From I1, heat, freshwater and water-mass budgets are computed for the Arabian Sea and Bay of Bengal. Unfortunately, unlike the flow in the Atlantic and Pacific, the flow through I1 experiences considerable seasonal variability due to the annual reversal of the monsoonal winds. Therefore, at best we can expect to compute a “snapshot” of the heat and freshwater flux at the end of the SW Monsoon. But at least the timing of this section was chosen to coincide in the period where the mean circulation is most like the “normal” subtropical gyres found at mid-latitudes in the other oceans. During WOCE I1 both the Arabian Sea and the Bay of Bengal acted as heat sources. The mechanisms of the heat exportation in these two basins differed slightly from each other with the deep-ocean flow playing an important role in exporting heat from the Arabian Sea. The total heat transport out of the Arabian Sea was 0.60±0.27 PW. Of the 0.60 PW heat transport, a total of 0.28 PW was exported below 2000 m. The monsoonally driven southward surface flow accounted for the remaining 50% of the total heat export. Meanwhile, the Bay of Bengal was exporting heat at a rate of 0.63±0.16 PW, with half of the heat export due to surface flow and the other half due to meridional overturning at mid-depths. Meanwhile, the Arabian Sea was importing freshwater at a rate of 0.38±0.09×106 m3 s−1 while the Bay of Bengal was exporting freshwater at a rate of 0.38±0.08×106 m3 s−1. The mechanisms for the freshwater transport from the two basins were fundamentally different. In the Arabian Sea, vertical recirculation cells in the upper and deep ocean contributed to the freshwater import across I1w with the deep cell accounting for ∼25% of the total freshwater transport. In the Bay of Bengal, most of the freshwater export occurred in the surface layer because of strong southward Ekman surface flow and fresh surface waters from river runoff and monsoon rainfall. The role the horizontal circulation plays in the heat and freshwater transport across I1 was different in the Arabian Sea and Bay of Bengal. The horizontal circulation contributed 0.06 PW of the total heat transport in contrast to −0.60 PW of the total heat transport crossing I1w and ∼30% of the freshwater transport across I1w in the Arabian Sea. In the Bay of Bengal, the horizontal circulation contributed ∼20% heat transport and ∼45% of the freshwater transport across I1e. The difference in horizontal circulation between the two basins is predominately due to the role of the Somali Current in the Arabian Sea.