Whitaker, J. S., G. P. Compo, X. Wei, and T. M. Hamill, 2004: Reanalysis without radiosondes using ensemble data assimilation. Mon. Wea. Rev., 132, 1190-1200.
Remote forcing of sea surface temperature (SST) variations in the Indian Ocean during the course of El Niño-Southern Oscillation (ENSO) events is investigated using NCEP reanalysis and general circulation model (GCM) experiments. Three experiments are conducted to elucidate how SST variations in the equatorial Pacific influence surface flux variations, and hence SST variations, across the Indian Ocean. A control experiment is conducted by prescribing observed SSTs globally for the period 1950-99. In the second experiment, observed SSTs are prescribed only in the tropical eastern Pacific, while climatological SSTs are used elsewhere over the global oceans. In the third experiment, observed SSTs are prescribed in the tropical eastern Pacific, while a variable-depth ocean mixed layer model is used at all other ocean grid points to predict the SST.
Composites of surface fluxes and SST over the Indian Ocean are formed based on El Niño and La Niña events during 1950-1999. The surface flux variations in the eastern Indian Ocean in all three experiments are similar and realistic, confirming that much of the surface flux variation during ENSO is remotely forced from the Pacific. Furthermore, the SST anomalies in the eastern tropical Indian Ocean are well simulated by the coupled model, which supports the notion of an "atmospheric bridge" from the Pacific. During boreal summer and fall, when climatological winds are southeasterly over the eastern Indian Ocean, remotely forced anomalous easterlies act to increase the local wind speed. SST cools in response to increased evaporative cooling, which is partially offset by increased solar radiation associated with reduced rainfall. During winter, the climatological winds become northwesterly and the anomalous easterlies then act to reduce the wind speed and evaporative cooling. Together with increased solar radiation and a shoaling mixed layer, the SST warms rapidly. The model is less successful at reproducing the ENSO-induced SST anomalies in the western Indian Ocean, suggesting that dynamical ocean processes contribute to the east-west SST dipole that is often observed in boreal fall during ENSO events.