Heat storage variability in the Indian Ocean using Topex/ Poseidon Altimeter Data

Sea surface height anomalies (SSHA) derived from the Topex/Poseidon (T/P) satellite are used for computing heat storage anomalies (HSA) and heat storage rates (HSR) over the north Indian Ocean [20 o S – 25 o N and 35 o E – 115 o E] for a period of 10 years (1993-2002). In normal years during September to November positive HSA and HSR were observed in the region 10 o S - Equator, 90 o E 110 o E. But during the years 1994 and 1997 negative HSA and HSR were observed in this region, this interannual variability has recently been addressed as Indian Ocean Dipole (IOD). The heat content anomaly clearly showed the existence of the dipole like structure in the equatorial Indian Ocean (IO) in 1994 and 1997. The T/P measurement showed large SSHA in the western equatorial Indian Ocean during 1994-1995 and 1997 -1998 IOD events that represent the oceanic response to the surface wind forcing. These anomalies in turn played an important role in forming the sea surface temperature anomalies (SSTA). The 1997 Dipole mode structure was observed to be stronger than 1994 and that can be clearly seen in calculated HSA, HSR, T/P SSH anomalies, thermocline depth (D20) anomaly derived from Simple Ocean Data Assimilation (SODA) and in HADISST anomaly. The Rossby wave propagation is found to have a good correlation with the heat content anomaly derived from Topex/Poseidon sea surface height anomalies. During the dipole years 199495 and 1997-98 the anomalous westward propagation of SSHA and HSA were clearly observed especially in the region south of 7 o S and strengthened in 80 - 90 o E belt. Wind stress curl anomalies play an important role in strengthening this propagation in 80-90 o E and hence warming the west Indian Ocean in the early months of 1998. It was seen that positive and negative dipole years are inversely correlated in the southeastern equatorial Indian Ocean (10 o S - Equator, 90 o E -110 o E). To understand the interannual variability of upper ocean SSHA, Complex Empirical Orthogonal Function (CEOF) has been applied to T/P SSHA and HSA. IOD has been shown to be the leading mode of the interannual variability of the upper ocean SSHA and HSA. The westward propagation of the phase is in agreement with the sea saw thermocline variability observed in the equatorial Indian Ocean.