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...

Abstract. This paper investigates the variability of sea surface salinity (SSS) in the western equatorial Pacific fresh pool. For this purpose, we processed data collected from thermosalinographs embarked on merchant ships. Two main cross-equatorial shipping lines that are representative of the oceanic conditions in the western tropical Pacific were selected: the Japan-Tarawa-Fiji line that crosses the equator near 173øE (eastern track) and the New-Caledonia-Japan line that crosses the equator near 156øE (western track). We show that there is a strong SSS variability in the region at monthly as well as interannual timescales. This high variability is attributed to the successive passages of a zonal salinity front, trapped in the (5øN-5øS) equatorial band and migrating in phase with the southern oscillation index. We also found the eastern track to be more variable in SSS because it is more exposed to these SSS front incursions. We carried out a detailed study of the mechanisms responsible for this variability; it revealed that the rainfall input acts as a source of freshwater responsible for the existence of a contrasted distribution of SSS (mainly high-salinity waters in the central Pacific and low-salinity waters in the western Pacific). However, the main mechanism responsible for the SSS variability is zonal advection that makes the two distinct masses of water converge, resulting in a salinity front which shifts back and forth in the equatorial band.

International audienceThe Ligurian sea, at the France–Italy boarder of the Mediterranean Sea, has experienced in the past numerous submarine landslides within its very near continental slope, the continental shelf being very narrow. The most recent occurred on the 1979 October 16 near Nice international airport and generated tsunami waves of order 3 m of amplitude in some specific locations. More ancient landslides are also easily identified through bathymetric surveys of the seafloor. For the 1979 event we propose two distinct tsunamigenic landslides based on identified scars observable on the seafloor. The first one corresponds to the volume Vol1 that slid at the airport (in shallow water) while the second one corresponds to the more substantial volume Vol2 that has been localized at the slope. Former studies indicate that only the combination of the two slides may explain the event. We complement these studies by asserting that when the two slides are taken separately, they already explain a significant (although not a total) part of the event: Vol1 explains partly the tsunami observations in the vicinity of the airport while Vol2 contributes to explain the ones away from the area, in particular at Antibes where the highest wave has been observed. The modelling effort is then extended to evaluate the tsunamigenesis of selected (but representative) former landslides having a clear scar signature. The vulnerability of the area to landslide-triggered tsunami is then proposed to discussion along with possible mechanisms that can be responsible for local wave amplification

A numerical procedure has been developed to study the linear stability of nonlinear three-dimensional progressive gravity waves on deep water. The three-dimensional patterns considered herein are short-crested waves which may be produced by two progressive plane waves propagating at an oblique angle, γ, to each other. It is shown that for moderate wave steepness the dominant resonances are sideband-type instabilities in the direction of propagation and, depending on the value of γ, also in the transverse direction. It is also shown that three-dimensional progressive gravity waves are less unstable than two-dimensional progressive gravity waves.

International audienceEarly in the morning of 1887 February 23, a damaging earthquake hit the towns along the Italian and French Riviera. The earthquake was followed by a tsunami with a maximum run-up of 2 m near Imperia, Italy. At least 600 people died, mainly due to collapsing buildings. This 'Ligurian earthquake' occurred at the junction between the southern French-Italian Alps and the Ligurian Basin. For such a historical event, the epicentre and the equivalent magnitude are difficult to characterize with any degree of precision, and the tectonic fault responsible for the earthquake is still under debate today. The recent MALISAR marine geophysical survey allowed the identification of a large system of active faults. We propose that the rupture of some of the segments belonging to this 80-km-long northern Ligurian Faults system connected to a shallow-dipping major thrust plane at depth was the source of the 1887 Ligurian earthquake. We investigated the macroseismic data from the SISFRANCE-08 and DBMI-04 historical databases using several models of intensity attenuation with distance and focal depth. The modelling results are consistent with the off-shore location, with an epicentre around 43.70°-43.78°N and 7.81°-8.07°E, and with a magnitude Mw in the range of 6.3-7.5. Numerous earthquake source scenarios have been tested on the tide gauge record at Genoa harbour. As a result, we present seven characteristic source earthquake scenarios for a shallow strong earthquake occurring below the northern Ligurian margin. The modelled tide gauge records were analysed with the help of basic statistical tools and a simple harmonic analysis, to extract the wave spectrum characteristics. This analysis indicates that scenarios of a magnitude Mw of 6.8-6.9 along a reverse N55°E striking fault are the best candidates to explain the known characteristics of the tsunami that followed. The best-fitting scenarios comprise a 70°-dipping southward fault plane with Mw 6.8 and a 16°-dipping northward fault plane with Mw 6.9, both with reverse kinematics. Taking into account the geometry of the active faults, the location of the macroseismic epicentre and the morphotectonic evolution of the continental slope, we propose that the 1887 Ligurian earthquake corresponded to the reverse faulting of a N55°E striking fault plane dipping to the north with a coseismic slip of 1.5