Etude diagnostique de la variabilité de la salinité de surface de l'Océan Pacifique. Apport des données SMOS

La salinité est un paramètre essentiel de l'océan car elle impacte les processus océaniques de la sous-meso échelle à l'échelle du bassin et interannuelle. Son rôle a été souligné dans la dynamique du phénomène El Nino ainsi que dans le déplacement de masses d'eaux telles que les eaux intermédiaires subtropicales et les eaux profondes. Elle est considérée comme une Variable Climatique Essentielle par l'Organisation Météorologique Mondiale. La distribution du sel dans l'océan est le résultat d'un équilibre subtil entre le forçage de surface (évaporation, précipitation et ruissellement), l'advection horizontale de sel et les échanges avec la sub-surface (entrainement et mélange), chacun de ces termes étant d'égale importance. Même si ces processus sont connus de façon qualitative, quantifier l'effet de chacun d'entre eux est toujours une question ouverte. Cette thèse a pour but de : a) quantifier les mécanismes responsables de la variabilité de la salinité de surface dans l'Océan Pacifique tropical (principalement aux échelles saisonnières et interannuelles), b) décrire et évaluer les processus à l'origine des variations de salinité de surface pendant l'évènement La Nina de 2010-2011 et c) analyser la formation et la variabilité du noyau de maximum de sel de l'Océan Pacifique subtropical (aux mêmes échelles de temps). Différents jeux de données sont utilisés conjointement : des observations de salinité in situ (bateaux marchands, profileurs Argo ...), des données de salinité de surface dérivées du nouveau satellite SMOS et d'autres produits issus de mesures satellitaires (précipitations, évaporation et courants de surface) ainsi qu'une simulation spécifique d'un modèle forcé.Salinity is one of the key parameters of the ocean impacting its dynamics through density. It is considered as an Essential Climate Variable. The salinity patterns result from a subtle balance between surface forcing (E-P, Evaporation minus Precipitation), horizontal salt advection (at low and high frequencies) and subsurface forcing (entrainment and mixing), all terms being of analogous importance. While processes responsible for sea surface salinity (SSS) changes are qualitatively well known, quantifying those mechanisms is very challenging and hence still under debate. My Ph.D. research work aims at: a) quantifying mechanisms responsible for the tropical Pacific Ocean SSS variability (mainly at seasonal and ENSO time scale), b) describing and assessing mechanisms behind the 2010-2011 La Niña SSS changes, and c) analysing the formation and variability of the south Pacific subtropical high SSS core (at the same time scales). In order to do so, various datasets are used conjointly: in-situ salinity observations mainly from voluntary observing ships and Argo profilers, satellite based surface salinity (from SMOS), precipitation, evaporation and near-surface currents as well as a specific forced model simulation

Satellite Salinity Observing System: Recent Discoveries and the Way Forward

Advances in L-band microwave satellite radiometry in the past decade, pioneered by ESA’s SMOS and NASA’s Aquarius and SMAP missions, have demonstrated an unprecedented capability to observe global sea surface salinity (SSS) from space. Measurements from these missions are the only means to probe the very-near surface salinity (top cm), providing a unique monitoring capability for the interfacial exchanges of water between the atmosphere and the upper-ocean, and delivering a wealth of information on various salinity processes in the ocean, linkages with the climate and water cycle, including land-sea connections, and providing constraints for ocean prediction models. The satellite SSS data are complimentary to the existing in situ systems such as Argo that provide accurate depiction of large-scale salinity variability in the open ocean but under-sample mesoscale variability, coastal oceans and marginal seas, and energetic regions such as boundary currents and fronts. In particular, salinity remote sensing has proven valuable to systematically monitor the open oceans as well as coastal regions up to approximately 40 km from the coasts. This is critical to addressing societally relevant topics, such as land-sea linkages, coastal-open ocean exchanges, research in the carbon cycle, near-surface mixing, and air-sea exchange of gas and mass. In this paper, we provide a community perspective on the major achievements of satellite SSS for the aforementioned topics, the unique capability of satellite salinity observing system and its complementarity with other platforms, uncertainty characteristics of satellite SSS, and measurement versus sampling errors in relation to in situ salinity measurements. We also discuss the need for technological innovations to improve the accuracy, resolution, and coverage of satellite SSS, and the way forward to both continue and enhance salinity remote sensing as part of the integrated Earth Observing System in order to address societal needs

Formation and variability of the South Pacific Sea Surface Salinity maximum in recent decades

International audienceThis study investigates causes for the formation and variability of the Sea Surface Salinity maximum (SSS > 36) centered near 18°S-124°W in the South Pacific Ocean over the 1990-2011 period at the seasonal time scale and above. We use two monthly gridded products of SSS based on in situ measurements, high-resolution along-track Voluntary Observing Ships thermo-salinograph data, new SMOS satellite data, and a validated ocean general circulation model with no direct SSS relaxation. All products reveal a seasonal cycle of the location of the 36-isohaline barycenter of about ±400 km in longitude in response to changes in the South Pacific Convergence Zone location and Easterly winds intensity. They also show a low frequency westward shift of the barycenter of 1400 km from the mid 1990s to the early 2010s that could not be linked to the El Nino Southern Oscillation phenomena. In the model, the processes maintaining the 22 year equilibrium of the high salinity in the mixed layer are the surface forcing (˜+0.73 pss/yr), the horizontal salinity advection (˜-0.37 pss/yr), and processes occurring at the mixed layer base (˜-0.35 pss/yr)

Tropical Instability Waves in the Atlantic Ocean: investigating the relative role of sea surface salinity and temperature from 2010 to 2018

International audienc

Northward Pathway Across the Tropical North Pacific Ocean Revealed by Surface Salinity: How do El Niño Anomalies Reach Hawaii?

International audienceUsing the unprecedented 7 year monitoring of sea surface salinity (SSS) from the Soil Moisture Ocean Salinity (SMOS) satellite mission, an unexpected large‐scale anomaly at 20°N is studied in the tropical Pacific Ocean following the 2015‐2016 extreme El Niño event. This basin‐wide negative anomaly (below −0.3) is present in October 2015 between 15 and 25°N, reaching the Hawaiian archipelago. It has not been previously observed during El Niño events. It is accompanied by a negative equatorial SSS anomaly at the dateline (below −0.5) which has been previously described as an El Niño‐associated SSS anomaly. A wide range of observations (in situ and space‐borne) and a state‐of‐the‐art ocean model simulation are used together to characterize and understand the mechanisms leading to this singular SSS signal. The extra‐equatorial negative SSS anomaly is found to be a superposition of a persisting SSS anomaly due to the 2014 weak El Niño and of the larger 2015‐2016 El Niño SSS anomaly. Both were advected northward in the tropical current system by the mean Ekman currents and hypothetically by instabilities in the zonal currents patterns. An analysis of analogous structures in the past 20 years shows that this northward displacement of SSS anomalies is not El Niño specific, even if their advection is enhanced during El Niño events. This study shows that when surface freshwater fluxes are weak SSS, unlike sea surface temperature, can be used to trace water mass displacement for up to 20 months

Spatiotemporal Variability of the South Pacific Convergence Zone Fresh Pool Eastern Front from Coral-Derived Surface Salinity Data

International audienceDirect observations indicate a southeastward expansion of the South Pacific convergence zone (SPCZ) fresh pool and a freshening trend since the 1970s. Understanding decadal and longer-term variability of the SPCZ fresh pool and of the salinity front located at its southeastern margin has been limited by the scarcity of instrumental sea surface salinity (SSS) measurements. This study uses coral δ18O as a proxy for SSS to extend the salinity record back to the 1880s, from three different locations across the SSS front: Fiji, Tonga, and Rarotonga (FTR region). High percentages of observed SSS variance are explained by multicoral δ18O mean composite at each site. At the interannual time scale, the salinity front displacement over the last 200 years follows the El Niño–Southern Oscillation (ENSO) index. The different El Niño flavors are observable in the amplitude of the salinity front interannual displacement. However, no significant changes in either the frequency or the amplitude of its displacements were observed. At longer time scales, the timing and magnitude of the freshening trend vary among sites. The earliest freshening onset of about −0.06 psu decade−1 is detected in Fiji (around 1865), then Rarotonga (around 1939), and Tonga (around 1982). The role of atmospheric freshwater fluxes on SSS variability is evaluated by comparing coral SSS to historical precipitation data. The results suggest that, despite the known influence of the interdecadal Pacific oscillation (IPO) negative phases on increasing atmospheric freshwater fluxes and lowering SSS in the FTR region, ocean dynamics has a dominant influence at decadal time scale and in the onset of freshening trends

Analyzing the 2010-2011 La Niña signature in the tropical Pacific sea surface salinity using in situ data, SMOS observations, and a numerical simulation

International audienceThe tropical Pacific Ocean remained in a La Niña phase from mid-2010 to mid-2012. In this study, the 2010–2011 near-surface salinity signature of ENSO (El Niño-Southern Oscillation) is described and analyzed using a combination of numerical model output, in situ data, and SMOS satellite salinity products. Comparisons of all salinity products show a good agreement between them, with a RMS error of 0.2–0.3 between the thermosalinograph (TSG) and SMOS data and between the TSG and model data. The last 6 months of 2010 are characterized by an unusually strong tripolar anomaly captured by the three salinity products in the western half of the tropical Pacific. A positive SSS anomaly sits north of 10°S (>0.5), a negative tilted anomaly lies between 10°S and 20°S and a positive one south of 20°S. In 2011, anomalies shift south and amplify up to 0.8, except for the one south of 20°S. Equatorial SSS changes are mainly the result of anomalous zonal advection, resulting in negative anomalies during El Niño (early 2010), and positive ones thereafter during La Niña. The mean seasonal and interannual poleward drift exports those anomalies toward the south in the southern hemisphere, resulting in the aforementioned tripolar anomaly. The vertical salinity flux at the bottom of the mixed layer tends to resist the surface salinity changes. The observed basin-scale La Niña SSS signal is then compared with the historical 1998–1999 La Niña event using both observations and modeling

Intraseasonal variability of surface salinity in the Eastern Tropical Pacific associated with mesoscale eddies

International audienceStrong variability in sea surface salinity (SSS) in the Eastern Tropical Pacific (ETPac) on intraseasonal to interannual timescales was studied using data from the Soil Moisture and Ocean Salinity, Soil Moisture Active Passive, and Aquarius satellite missions. A zonal wave number-frequency spectral analysis of SSS reveals a dominant timescale of 50-180days and spatial scale of 8 degrees-20 degrees of longitude with a distinct seasonal cycle and interannual variability. This intraseasonal SSS signal is detailed in the study of 19 individual ETPac eddies over 2010-2016 identified by their sea level anomalies, propagating westward at a speed of about 17cm/s. ETPac eddies trap and advect water in their core westward up to 40 degrees of longitude away from the coast. The SSS signatures of these eddies, with an average anomaly of 0.5-pss magnitude difference from ambient values, enable the study of their dynamics and the mixing of their core waters with the surroundings. Three categories of eddies were identified according to the location where they were first tracked: (1) in the Gulf of Tehuantepec, (2) in the Gulf of Papagayo, and (3) in the open ocean near 100 degrees W-12 degrees N. They all traveled westward near 10 degrees N latitude. Category 3 is of particular interest, as eddies seeded in the Gulf of Tehuantepec grew substantially in the vicinity of the Clipperton Fracture Zone rise and in a region where the mean zonal currents have anticyclonic shear. The evolution of the SSS signature associated with the eddies indicates the importance of mixing to their dissipation