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Christophe Maes, Joël Picaut, and Sophie Belamari

Abstract

Several studies using sea level observations and coupled models have shown that heat buildup in the western equatorial Pacific is a necessary condition for a major El Niño to develop. However, none of these studies has considered the potential influence of the vertical salinity stratification on the heat buildup and thus on El Niño. In the warm pool, this stratification results in the presence of a barrier layer that controls the base of the ocean mixed layer. Analyses of in situ and TOPEX/Poseidon data, associated with indirect estimates of the vertical salinity stratification, reveal the concomitant presence of heat buildup and a significant barrier layer in the western equatorial Pacific. This relationship occurs during periods of about one year prior to the mature phase of El Niño events over the period 1993–2002. Analyses from a coupled ocean–atmosphere general circulation model suggest that this relationship is statistically robust. The ability of the coupled model to reproduce a realistic El Niño together with heat buildup, westerly wind bursts, and a salinity barrier layer suggests further investigations of the nature of this relationship. In order to remove the barrier layer, modifications to the vertical ocean mixing scheme are applied in the equatorial warm pool and during the 1-yr period of the heat buildup. At the bottom of the ocean mixed layer, the heat buildup is locally attenuated, as expected from switching on the entrainment cooling. At the surface, the coupled response over the warm pool increases the fetch of westerly winds and favors the displacement of the atmospheric deep convection toward the central equatorial Pacific. These westerly winds generate a series of downwelling equatorial Kelvin waves whose associated eastward currents drain the heat buildup toward the eastern Pacific Ocean. The overall reduction of the heat buildup before the onset of El Niño results in the failure of El Niño. These coupled model analyses confirm that the buildup is a necessary condition for El Niño development and show that the barrier layer in the western equatorial Pacific is important for maintaining the heat buildup.

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Boris Dewitte, Gilles Reverdin, and Christophe Maes

Abstract

The vertical structure of the variability in the equatorial Pacific in a high-resolution ocean general circulation model (OGCM) simulation for 1985–94 is investigated. Near the equator the linear vertical modes are estimated at each grid point and time step of the OGCM simulation. The characteristics of the vertical modes are found to vary more in space than in time. The contribution of baroclinic modes to surface zonal current and sea level anomalies is analyzed. The first two modes contribute with comparable amplitude but with different spatial distribution in the equatorial waveguide. The third and fourth modes exhibit peaks in variability in the east and in the westernmost part of the basin where the largest zonal gradients in the density field and in the vertical mode characteristics are found. Higher-order mode (sum of third to the eighth mode) variability is the largest near the date line close to the maximum in zonal wind stress variability. Kelvin and first-meridional Rossby components are derived for each of the first three baroclinic contributions by projection onto the associated meridional structures. They are compared to equivalent ones in multimode linear simulations done with the projection coefficients and phase speeds derived from the OGCM simulation. This suggests that in addition to the first-mode-forced equatorial Kelvin and Rossby waves earlier found in the data, forced waves of higher vertical modes should also be observable. For the first two vertical modes, the anomalies in the linear and the OGCM simulations have a similar magnitude and usually present similar propagation characteristics. Phase speed characteristics are however different in the eastern Pacific with larger values for the OGCM. The effect of zonal changes in the stratification is tested in the linear model for a stratification change located either in the eastern or in the western Pacific. This results in a significant redistribution of energy to higher modes via modal dispersion. In particular the third mode increases to a magnitude closer to the one in the OGCM simulation. Gravest modes are also affected. This suggests that modal dispersion plays an important role and should be considered when interpreting data as a combination of linear long equatorial waves.

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Christophe Maes, David Behringer, Richard W. Reynolds, and Ming Ji

Abstract

Empirical orthogonal functions of the combined variability of temperature and salinity have been used as basis functions for the indirect reconstruction of salinity from observations of temperature alone. The method employs a weighted least squares procedure that minimizes the misfit between the reconstructed temperature and the observed temperature, but also constrains the variability of the reconstructed salinity to remain within specified bounds.

The method has been tested by fitting to temperature profiles from the Tropical Atmosphere Ocean array along 165°E in the western equatorial Pacific Ocean (8°N–8°S) for the 1986–97 period. Comparisons of the reconstructed salinity field with sea surface salinity and conductivity–temperature–depth data and of the reconstructed dynamic height with TOPEX/Poseidon observations of sea level demonstrate the reliability of the method. The reconstructed data successfully capture the upper-ocean variability at annual to ENSO timescales. The impact of neglecting salinity variability on the dynamic height anomaly in the western tropical Pacific Ocean is addressed.

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Esther Portela, Nicolas Kolodziejczyk, Christophe Maes, and Virginie Thierry

Abstract

Using an Argo dataset and the ECCOv4 reanalysis, a volume budget was performed to address the main mechanisms driving the volume change of the interior water masses in the Southern Hemisphere oceans between 2006 and 2015. The subduction rates and the isopycnal and diapycnal water-mass transformation were estimated in a density–spiciness (στ) framework. Spiciness, defined as thermohaline variations along isopycnals, was added to the potential density coordinates to discriminate between water masses spreading on isopycnal layers. The main positive volume trends were found to be associated with the Subantarctic Mode Waters (SAMW) in the South Pacific and South Indian Ocean basins, revealing a lightening of the upper waters in the Southern Hemisphere. The SAMW exhibits a two-layer density structure in which subduction and diapycnal transformation from the lower to the upper layers accounted for most of the upper-layer volume gain and lower-layer volume loss, respectively. The Antarctic Intermediate Waters, defined here between the 27.2 and 27.5 kg m−3 isopycnals, showed the strongest negative volume trends. This volume loss can be explained by their negative isopyncal transformation southward of the Antarctic Circumpolar Current into the fresher and colder Antarctic Winter Waters (AAWW) and northward into spicier tropical/subtropical Intermediate Waters. The AAWW is destroyed by obduction back into the mixed layer so that its net volume change remains nearly zero. The proposed mechanisms to explain the transformation within the Intermediate Waters are discussed in the context of Southern Ocean dynamics. The στ decomposition provided new insight on the spatial and temporal water-mass variability and driving mechanisms over the last decade.

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Lionel Gourdeau, William S. Kessler, Russ E. Davis, Jeff Sherman, Christophe Maes, and Elodie Kestenare

Abstract

The South Equatorial Current (SEC) entering the Coral Sea through the gap between New Caledonia and the Solomon Islands was observed by an autonomous underwater vehicle (Spray glider) and an overlapping oceanographic cruise during July–October 2005. The measurements of temperature, salinity, and absolute velocity included high-horizontal-resolution profiles to 600-m depth by the glider, and sparser, 2000-m-deep profiles from the cruise. These observations confirm the splitting of the SEC into a North Vanuatu Jet (NVJ) and North Caledonian Jet (NCJ), with transport above 600 m of about 20 and 12 Sv, respectively. While the 300-km-wide NVJ is associated with the slope of the main thermocline and is thus found primarily above 300 m, the NCJ is a narrow jet about 100 km wide just at the edge of the New Caledonian reef. It extends to at least a 1500-m depth with very little shear above 600 m and has speeds of more than 20 cm s−1 to at least 1000 m. An Argo float launched east of New Caledonia with a parking depth fixed at 1000 m became embedded in the NCJ and crossed the glider/cruise section at high speed about 3 months before the glider, suggesting that the jet is the continuation of a western boundary current along the east side of the island and extends across the Coral Sea to the coast of Australia. In the lee of New Caledonia, the glider passed through a region of eddies whose characteristics are poorly understood.

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Fabrice Ardhuin, Bertrand Chapron, Christophe Maes, Roland Romeiser, Christine Gommenginger, Sophie Cravatte, Rosemary Morrow, Craig Donlon, and Mark Bourassa
Open access