Interannual variability of the equatorial jets in the Indian Ocean from merged altimetry data

The merged ERS-1/2, TOPEX/Poseidon and Jason-1 altimeter weekly sea level anomalies (SLAs) for the period 1997- 2005 were analyzed to study the variability of sea level and computed geostrophic currents in relation to the equatorial jets in the Indian Ocean. Year-to-year-variability in SLA was large (small) at 77°E (93°E) with a pronounced (weaker) semi-annual variation. The computed geostrophic currents, using the second order differential equation, in the equatorial region (2°S-2°N) revealed the existence of spring and fall equatorial jets in each year, but with considerable variation. The 9 year mean (excluding 1997-98) SLA and the derived currents revealed that spring (fall) jet was weak (strong) with higher (lesser) magnitudes of SLA in the eastern basin. The computed surface currents agreed well with the Acoustic Doppler Current Profiler (ADCP) measured currents and OGCM for Earth Simulator (OFES) simulated currents. The interannual variability of the equatorial jets revealed that the fall jet extended for longer duration than the spring jet. The equatorial Kelvin wave with a wavelength of ~1100 km was identified. Westward propagating Rossby wave was identified at 5° N more clearly. The zonal currents at 5° N (south of Sri Lanka) revealed intense anticyclonic (cyclonic) eddy activity between 75° and 80° E during summer (winter) monsoon.