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).

International audienceTrends in observed sea surface salinity (SSS) and temperature are analyzed for the tropical Pacific during 1955–2003. Since 1955, the western Pacific Warm Pool has significantly warmed and freshened, whereas SSS has been increasing in the western Coral Sea and part of the subtropical ocean. Waters warmer than 28.5°C warmed on average by 0.29°C, and freshened by 0.34 pss per 50 years. Our study also indicates a significant horizontal extension of the warm and fresh surface waters, an expansion of the warm waters volume, and a notable eastward extension of the SSS fronts located on the equator and under the South Pacific Convergence Zone. Mixed layer depth changes examined along 137°E and 165°E are complex, but suggest an increase in the equatorial barrier layer thickness. Our study also reveals consistency between observed SSS trends and a mean hydrological cycle increase inferred from Clausius–Clapeyron scaling, as predicted under global warming scenarios. Possible implications of these changes for ocean–atmosphere interactions and El Niño events are discussed

International audienceRemote sensing of salinity using satellite-mounted microwave radiometers provides new perspectives for studying ocean dynamics and the global hydrological cycle. Calibration and validation of these measurements is challenging because satellite and in situ methods measure salinity differently. Microwave radiometers measure the salinity in the top few centimeters of the ocean, whereas most in situ observations are reported below a depth of a few meters. Additionally, satellites measure salinity as a spatial average over an area of about 100 × 100 km². In contrast, in situ sensors provide pointwise measurements at the location of the sensor. Thus, the presence of vertical gradients in, and horizontal variability of, sea surface salinity complicates comparison of satellite and in situ measurements. This paper synthesizes present knowledge of the magnitude and the processes that contribute to the formation and evolution of vertical and horizontal variability in near-surface salinity. Rainfall, freshwater plumes, and evaporation can generate vertical gradients of salinity, and in some cases these gradients can be large enough to affect validation of satellite measurements. Similarly, mesoscale to submesoscale processes can lead to horizontal variability that can also affect comparisons of satellite data to in situ data. Comparisons between satellite and in situ salinity measurements must take into account both vertical stratification and horizontal variability

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The recent detection of a central Pacific type of El Niño has added a new dimension to the El Niño‐Southern Oscillation climatic puzzle. Sea surface salinity (SSS) observations collected during 1977–2008 in the tropical Pacific are used to contrast the three eastern Pacific (EP) (1982–1983, 1991–1992, 1997–1998) and seven central Pacific (CP) (1977–1978, 1986–1988, 1990–1991, 1992–1995, 2002–2003, 2004–2005, 2006–2007) types of El Niño events, as well as the six EP (1985–1986, 1988–1989, 1995–1996, 1999–2001, 2005–2006, 2007–2008) and two CP (1983–1984, 1998–1999) types of La Niña events. The EP El Niño events result in large (∼30° longitude) eastward displacements of the eastern edge of the low‐salinity warm pool waters in the equatorial band, a resulting well‐marked SSS freshening (∼−1) near the dateline, and a SSS increase (∼+1) below the mean position of the South Pacific Convergence Zone (SPCZ). The CP El Niño events are characterized by smaller (50%) eastward displacements of the eastern edge, a ∼15° longitude westward shift of the equatorial SSS freshening, and a comparatively reduced (∼50%) SSS increase in the SPCZ. A qualitative analysis indicates that changes in zonal currents and precipitation can account for the observed contrasted signature in SSS. Eastward current anomalies appear over most of the equatorial band during EP El Niño events. In contrast, there is a tendency for zonal current convergence slightly west of the dateline during CP El Niño events, consistent with the confinement of the warm/fresh pool in the western central equatorial basin, the related quasi‐inexistent northeastward migration of the SPCZ, and associated heavy precipitation regime.

Sea surface bucket measurements, obtained through a ship‐of‐opportunity program, are used to describe the sea surface salinity (SSS) field for the tropical Pacific during the period 1969–1988. Emphasis is placed upon the mean SSS distribution and the seasonal and interannual SSS variability occurring along four well‐sampled shipping tracks. These tracks extend from New Zealand to Japan, from New Zealand to Hawaii, from Tahiti to California, and from Tahiti to Panama. They cross the equator at 155°E, 160°W, 140°W, and 100°W, respectively. Along each track, the mean SSS distribution is characterized by SSS minima which are 4°–6° further poleward than the axes of maximum precipitation associated with the Intertropical Convergence Zone (ITCZ) and South Pacific Convergence Zone (SPCZ). It is suggested that these SSS minima owe their existence mainly to heavy rainfall and poleward Ekman salt transport associated with the trade winds. The role of zonal salt advection was found negligible for these SSS minima. Except along the eastern track, maximum seasonal SSS variations are located in the ITCZ and SPCZ regions, with minimum SSS in September‐October and March‐April, respectively. On the basis of precipitation island stations, it is demonstrated that the maximum seasonal SSS variations are closely related to the rainfall regimes of the ITCZ and SPCZ (rainfall maximum 3 months before SSS minimum; rainfall amount sufficient to account for SSS changes). Along the eastern track, a strong annual SSS cycle is found from about 4°S (110°W) to the Panama coast (minimum SSS in February–March), reflecting the combined effects of rainfall, salt advection, and vertical mixing. Notable interannual SSS variability concerns only the western half of the tropical Pacific Ocean where El Niño‐Southern Oscillation (ENSO) related SSS changes are strongly related to ENSO‐related precipitation changes. During ENSO periods, the SSS field west of about 150°W is characterized by fresher‐than‐average SSS within about 8°N to 8°S, and conversely saltier‐than‐average SSS poleward of 8° latitudes. These modifications in the SSS field are thought to result mainly from an eastward displacement in the ascending branch of the Walker and Hadley cells which induces unusually high rainfall over the western and central equatorial Pacific region bordered on all sides by rainfall deficits. Reproducing the actual SSS changes at seasonal and interannual time scales would be a very stringent test for model capability.

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Sea surface salinity (SSS) and temperature (SST) data collected from voluntary observing ships over 25 years (1976–2000) are analyzed in the Southwestern Tropical Pacific (10°S–24°S/160°E–140°W). This region lies under the South Pacific Convergence Zone (SPCZ), at the southern edge of the western Pacific warm pool between Tahiti and Darwin, the two places whose atmospheric sea level pressure difference is used to define the Southern Oscillation Index (SOI). Complementary data such as precipitation are used to assist in the analysis. The mean and seasonal variations of these parameters are described. An empirical orthogonal function (EOF) analysis of low‐pass filtered time series is then performed to extract the interannual variability. All parameters show an interannual signal which correlates well with the SOI. The Southwestern Tropical Pacific Ocean is saltier and colder during El Niño than during La Niña events. In the southwestern part, there is a shortage (excess) in precipitation during El Niño (La Niña) events. The greatest anomalies appeared during the last La Niña, in 1996 and 1999 as regards SST and in 1999 and 2000 as regards SSS. SST and precipitation El Niño–Southern Oscillation (ENSO)‐related anomalies are an order of magnitude smaller than seasonal anomalies, while the SSS ENSO‐related signal is twice as strong as the seasonal signal. These facts reflect the northeastward (southwestward) shift of the SPCZ during El Niño (La Niña) events. While consistent with precipitation changes, the ENSO‐related variability in SSS can also be partly explained by the displacement of the salinity front that separates fresh, warm pool waters from salty subtropical waters. Computation of surface geostrophic current anomalies from Geosat (1987–1988) and TOPEX/Poseidon (1993–2000) indicates that westward current anomalies developed during the 1987/88 and 1997/98 El Niño and are linked to the displacement of the salinity front. The Southwestern Tropical Pacific salinity front moves westward (eastward) in contrast to the equatorial salinity front which moves eastward (westward) during an El Niño (La Niña) event.