Surface and subsurface dipole variability in the Indian Ocean and its relation with ENSO

An ocean general circulation model, forced with observed winds for the period 1958–97, was used to examine surface and subsurface temperature variability in the Indian Ocean and its relation to ENSO. Empirical orthogonal function (EOF) analysis of temperature anomalies in the equatorial longitude-depth plane indicates that the leading mode of variability has a dipole character, with strongest loadings in the thermocline and more modest loadings at the surface. The subsurface dipole is weakly correlated with Nino3.4 SST. However, the subsurface dipole is well correlated with dipole variability at the surface, which itself is well correlated with ENSO. These results indicate that “dipole” variability that is independent of ENSO is more prominent at depth than in the SST. While the influence of ENSO on subsurface variability is detectable, the ENSO-induced surface dipole is primarily controlled by surface heat fluxes. On the other hand, subsurface variations play an important role for surface dipole events that are independent of ENSO. This is especially true in the eastern Indian Ocean where the strong surface cooling in late summer is generated by up-welling and horizontal heat advection in response to basin-wide surface easterlies.