Chapter 18 Recent progress toward satellite measurements of the global sea surface salinity field

Abstract With present and incipient technology, it appears to be possible to map the global surface salinity field on climatological scales from satellites. Measurements would be made with passive microwave radiometry at 1-3 GHz (L-and S-bands), which are frequencies also used for soil moisture measurements. There may be additional scientific value for sea ice observations. Studies are being done to assess the scientific importance and applications of global surface salinity measurements, and to evaluate the potential accuracy and spatio-temporal resolution with present and near-future technologies. Three principal scientific objectives have been identified as: (1) Improving seasonal to interannual (e.g., El Nino and La Nina) climate predictions; (2) Improving ocean rainfall estimates and global hydrologie budgets; and (3) Monitoring large-scale salinity events. The assimilation of surface salinity data into ocean and climate models provides an important tool for addressing these scientific issues. Technical problems include radiometer systems and calibration, ambient corrections, algorithms, satellite systems, and orbit mechanics. Several radiometer designs are being considered. Retrieval simulations for one sensor, including the major sensor and geophysical error sources, predict errors of about 0.1 practical salinity units (psu, equivalent to grams of salt per kilogram seawater or parts per thousand, 0/00) in equatorial areas and about 0.2 psu at high latitudes, for 100-km and monthly averages. This assumes mostly uncorrelated error and could be further reduced by larger area averages. Such observations represent a significant advance in global ocean remote-sensing capability for climate research.