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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?

Serena Illig(1) , Claire Perigaud(1) , and Julian Mc Creary(2)

(1) JPL, 4800 Oak Grove Drive, Pasadena, CA, 91109, United States
(2) IPRC/SOEST, University of Hawaii, POST Bldg. 401, Honolulu, Hawaii 96822, United States

Abstract

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: [5ºN-20ºN] for the North Pacific, [5ºS-20ºS] for the South Pacific, and [30ºS-5ºS] 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, we apply the Sverdrup relationship to the FSU wind stress data. 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 its 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. The model formulation allows for the decomposition 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. In our control experiment, no flow from the Pacific is allowed, while the flow simulated across the 30ºS boundary is controlled by a correction applied to conserve volume. Twin experiments are carried out, 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 XBT/TOPEX/Poseidon/Jason 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 XBT/TOPEX/Poseidon/Jason prescribed at the lateral oceanic boundaries 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.

 

Workshop presentation

 

                 Last modified: 07.10.03