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Satellite and In Situ Salinity: Understanding Near-Surface Stratification and Subfootprint Variability

J. BoutinLOCEAN Laboratory, Sorbonne Universités (University Pierre and Marie Curie, University of Paris 6)-CNRS-IRD-MNHN, Paris, France

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Y. ChaoRemote Sensing Solutions, Pasadena, California

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W. E. AsherApplied Physics Laboratory, University of Washington, Seattle, Washington

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T. DelcroixLaboratoire d’Etudes en Géophysique et Océanographie Spatiale, Toulouse, France

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R. DruckerSchool of Oceanography, University of Washington, Seattle, Washington

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K. DrushkaApplied Physics Laboratory, University of Washington, Seattle, Washington

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N. KolodziejczykLOCEAN Laboratory, Sorbonne Universités (University Pierre and Marie Curie, University of Paris 6)-CNRS-IRD-MNHN, Paris, France

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T. LeeJet Propulsion Laboratory, Pasadena, California

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N. ReulLaboratory of Oceanography from Space, IFREMER, Toulon, France

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G. ReverdinLOCEAN Laboratory, Sorbonne Universités (University Pierre and Marie Curie, University of Paris 6)-CNRS-IRD-MNHN, Paris, France

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J. SchanzeEarth and Space Research, Seattle, Washington

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A. SolovievNova Southeastern University, Dania Beach, Florida

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L. YuWoods Hole Oceanographic Institution, Woods Hole, Massachusetts

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J. AndersonSchool of Oceanography, University of Washington, Seattle, Washington

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L. BruckerUniversities Space Research Association, and National Aeronautics and Space Administration Goddard Space Flight Center, Greenbelt, Maryland

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E. DinnatCryospheric Sciences Laboratory, National Aeronautics and Space Administration Goddard Space Flight Center, Greenbelt, Maryland, and Center of Excellence in Earth Systems Modeling and Observations, Chapman University, Orange, California

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A. Santos-GarciaElectrical and Computer Engineering Department, University of Central Florida, Orlando, Florida

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W. L. JonesElectrical and Computer Engineering Department, University of Central Florida, Orlando, Florida

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C. MaesLaboratoire de Physique des Océans, CNRS-Ifremer-IRD-UBO, Plouzané, France

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B. WardAirSea Laboratory, School of Physics, and Ryan Institute, National University of Ireland, Galway, Galway, Ireland

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Abstract

Remote 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.

CURRENT AFFILIATION: Laboratoire de Physique des Océan (CNRS-IRD-UBO-Ifremer), Plouzané, France

CORRESPONDING AUTHOR: Jacqueline Boutin, Senior Researcher, LOCEAN Laboratory, 4 place Jussieu, F-75005 Paris, France, E-mail: jb@locean-ipsl.upmc.fr

Abstract

Remote 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.

CURRENT AFFILIATION: Laboratoire de Physique des Océan (CNRS-IRD-UBO-Ifremer), Plouzané, France

CORRESPONDING AUTHOR: Jacqueline Boutin, Senior Researcher, LOCEAN Laboratory, 4 place Jussieu, F-75005 Paris, France, E-mail: jb@locean-ipsl.upmc.fr
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