The western equatorial Pacific warm pool is subject to strong east-west migrations on interannual time scales in phase with the Southern Oscillation Index. The dominance of surface zonal advection in this migration is demonstrated with four different current data sets and three ocean models. The eastward advection of warm and less saline water from the western Pacific together with the westward advection of cold and more saline water from the central-eastern Pacific induces a convergence of water masses at the eastern edge of the warm pool and a well-defined salinity front. The location of this convergence is zonally displaced in association with El Nino-La Nina wind-driven surface current variations. These advective processes and water-mass convergences have significant implications for understanding and simulating coupled ocean-atmosphere interactions associated with El Nino-Southern Oscillation (ENSO).

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[1] The devastating 26 December 2004 Indian Ocean tsunami stressed the need for assessing tsunami hazard in vulnerable coastal areas. Numerical modeling is but one important tool for understanding past tsunami events and simulating future ones. Here we present a robust simulation of the event, which explains the large runups and destruction observed in coastal Thailand and identifies areas vulnerable to future tsunamis, or safer for reconstruction. To do so, we use an accurate tsunami source, which was iteratively calibrated in earlier work to explain the large-scale tsunami features, and apply it over a computational domain with a finer grid and more accurate coastal bathymetry in Thailand.Computations are performed with a well-validated numerical model based on fully nonlinear and dispersive Boussinesq equations (FUNWAVE) that adequately models the physics of tsunami propagation and runup, including dissipation caused by bottom friction and wave breaking. Simulated runups in Thailand reproduce field observations with a surprising degree of accuracy, as well as their high degree of along-coast variation: a 92% correlation is found between (58) runup observations and computations, while the model explains 85% of the observed variance; overall, the RMS error is approximately 1 m or 17% of the mean observed runup value (skill of the simulation). Because we did not use runup observations to calibrate our coseismic tsunami source, these results are robust, and thus provide a uniquely accurate synoptic prediction of tsunami impact along the Andaman coast of Thailand, including those areas where no observations were made.

The December 26, 2004 tsunami was perhaps the most devastating tsunami in recorded history, causing over 200,000 fatalities and widespread destruction in countries bordering the Indian Ocean. It was generated by the third largest earthquake on record ͑M w = 9.1-9.3͒ and was a truly global event, with significant wave action felt around the world. Many measurements of this event were made with seismometers, tide gauges, global positioning system stations, and a few satellite overpasses. There were numerous eyewitness observations and video digital recordings of coastal tsunami impact, as well as subsequent coastal field surveys of runup and flooding. A few ship-based expeditions also took place in the months following the event, to measure and map seafloor disturbances in the epicenter area. Based on these various data sets, recent seismic analysis estimates of rupture propagation speed, and other seismological and geological constraints, we develop a calibrated tsunami source, in terms of coseismic seafloor displacement and rupture timing along 1,200 km of the Andaman-Sunda trench. This source is used to build a numerical model of tsunami generation, propagation, and coastal flooding for the December 26, 2004 event. Frequency dispersion effects having been identified in the deep water tsunami wavetrain, we simulate tsunami propagation and coastal impact with a fully nonlinear and dispersive Boussinesq model ͑FUNWAVE͒. The tsunami source is specified in this model as a series of discrete, properly parameterized, dislocation source segments ͓Okada, 1985, Bull. Seismol. Soc. Am., 75͑4͒, 1135-1154͔, triggered in a time sequence spanning about 1,200 s. ETOPO2's bottom bathymetry and land topography are specified in the modeled ocean basin, supplemented by more accurate and denser data in selected coastal areas ͑e.g., Thailand͒. A 1 min grid is used for tsunami simulations over the Indian Ocean basin, which is fine enough to model tsunami generation and propagation to nearshore areas. Surface elevations simulated in the model agree well, in both amplitude and timing, with measurements at tide gauges, one satellite transect, and ranges of runup values. These results validate our tsunami source and simulations of the December 26, 2004 event and indicate these can be used to conduct more detailed case studies, for specific coastal areas. In fact, part of the development of our proposed source already benefitted from such regional simulations performed on a finer grid ͑15 s͒, as part of a Thailand case study, in which higher frequency waves could be modeled ͑Ioualalen et al. 2006, J. Geophys. Res., in press͒. Finally, by running a non-dispersive version of FUNWAVE, we estimate dispersive effects on maximum deep water elevations to be more than 20% in some areas. We believe that work such as this, in which we achieve a better understanding through modeling of the catastrophic December 26, 2004 event, will help the scientific community better predict and mitigate any such future disaster. This will be achieved through a comb...