Abstract
Sea-surface salinity (SSS) is an essential climate variable connected to Earth’s hydrological cycle and a dynamical component of ocean circulation, but its variability is not well-understood. Thanks to Argo floats, and the first decade of salinity remote sensing, this is changing. While satellites can retrieve salinity with some confidence, accuracy is regionally dependent and challenging within 500–1000 km offshore. The present work assesses the first four years of the National Aeronautics and Space Administration’s Soil Moisture Active Passive (SMAP) satellite in the North Indian Ocean. SMAP’s improved spatial resolution, better mitigation for radio-frequency interference, and land contamination make it particularly attractive to study coastal areas. Here, regions of interest are the Bay of Bengal, the Arabian Sea, and the extremely salty Red Sea (the last of which has not yet received attention). Six SMAP products, which include Levels 2 and 3 data, were statistically evaluated against in situ measurements collected by a variety of instruments. SMAP reproduced SSS well in both the Arabian Sea and the Bay of Bengal, and surprisingly well in the Red Sea. Correlations there were 0.81–0.93, and the root-mean-square difference was 0.38–0.67 for Level 3 data.
Highlights
The National Aeronautics and Space Administration’s (NASA) Soil Moisture Active Passive (SMAP) satellite launched on 31 January 2015 was designed as a soil-moisture mission, but expanded to retrieve sea-surface salinity (SSS) from space continuing the legacy of the Aquarius mission (2011–2015), the NASA pathfinder ocean-salinity satellite [1,2,3,4]
Because there are no in situ SSS observations in the Arabian/Persian Gulf over the SMAP era in public repositories such as CORIOLIS, Global Ocean Surface Underway Data (GOSUD), or NCEI, this study did not perform a statistical evaluation there
A comparison with the Red Sea is made in a dedicated section since the Red Sea is a challenging region for salinity remote sensing
Summary
The National Aeronautics and Space Administration’s (NASA) Soil Moisture Active Passive (SMAP) satellite launched on 31 January 2015 was designed as a soil-moisture mission, but expanded to retrieve sea-surface salinity (SSS) from space continuing the legacy of the Aquarius mission (2011–2015), the NASA pathfinder ocean-salinity satellite [1,2,3,4]. SMAP spinning antenna allows it to observe Earth in nearly full 360° scans in few seconds, and overlapping loops create a 1000 km wide swath. The SMAP satellite’s 40 km effective spatial resolution is similar to that of Soil Moisture Ocean Salinity (SMOS), the European Space Agency’s salinity mission launched in November 2009, and quite improved relative to the retired Aquarius (about 150 km). SSS retrievals are available from April 2015 to the present
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