How much of the interannual-to-decadal fluctuations of the Indian Ocean Sea-Level is due to atmospheric forcing and to connections with the other oceans?

Sea-level variations relative to the 1980-1996 climatology are derived from XBT/TOPEX/Poseidon/Jason data for 25 years up to 2005 in the Indian and Pacific Oceans. The variations are averaged over the width of the ocean in latitudinal bands: [5šN-20šN] for the North Pacific, [5šS-20šS] for the South Pacific, and [30šS-5šS] for the Indian Ocean. When multiplied by their respective surface areas, the Indian-Ocean variations are as strong as for the Pacific, even though the Indian surface area is only one third that of the Pacific. To quantify the wind-induced sea-level variability, sea-level variations are then estimated from the Sverdrup relationship using FSU wind stress data. Results show that, for the area-multiplied signals, the magnitudes of the wind-induced sea –levels are similar to their observed counterparts.

The observed and wind-determined variations are then integrated in time to focus on their cumulative effect at all frequencies. With opposite trends in the South and North, the Pacific Ocean is in balance with its wind until 1997/1998, while for the 25-year period the Indian Ocean sea-level variations are positively correlated to the North Pacific signal. The Indian Ocean and its winds reach balance in the early 90s, after 10 years of opposite trends. Note also that the sea-level accumulation drastically drops between 1998 and 2003.

We examine Indian Ocean sea –level, together with wind variations and internal ocean/atmosphere processes using a regional Indian Ocean model forced by FSU winds and GPCP rains since 1980. Its formulation allows us to decompose the simulated sea level in terms of mass, heat and salt changes that are only due to the Indian atmospheric forcing or to the combination of surface and lateral forcings. In particular, we focus on the part played by the rain, heat flux and nonlinear advection on the basin sea-level variations. The southern boundary and Indonesian Throughflow transport variations are also addressed, by comparing a control experiment where no flow from the Pacific is allowed and with an open boundary conditions at 30šS (with a correction applied to conserve volume) to a twin experiment in which model conditions at the southern and eastern boundaries are modified by prescribing the 1980-2005 variations of inflow/outflow obtained from applying geostrophy to the sea-level data along the boundaries. Then, using a tropical atmosphere model forced by the SSTs simulated by these different Indian Ocean experiments, we find evidence that the lateral oceanic boundary conditions play a significant role in feeding back onto inter-annual–to-decadal Indian atmosphere fluctuations, and also affect the intra-seasonal-to-seasonal variations of the Indian monsoons.