Satellite Sea-surface Density and Spiciness

Density drives the global ocean’s thermohaline circulation. In the past, observations of density were limited to sporadic and sparse in situ observations. At the ocean’s surface, density is principally a function of temperature. Satellites routinely have made global sea-surface temperature (SST) observations. NASA’s Aquarius mission, launched in 2011, made global sea-surface salinity (SSS) observations at approximately 150-km resolution. This study employs satellite SST observations (NOAA’s GOES-POES blended SST) and SSS observations (Aquarius) to explore the ocean’s surface density at greater temporal/spatial resolution than previously possible. An advantage of using both satellite SST and satellite SSS is that both observations are skin observations, with satellite salinity observations being observed in the top 1 centimeter and satellite SST observations being observed in approximately the top 10 micrometers, permitting the evaluation of the skin density. Using both satellite SSS and SST provides consistency when computing sea-surface density (SSD), avoiding the mixing of a skin observation with an in situ near-surface observation, which can be as deep as 5-10 m, e.g., Argo float observations. This study explores the differences between satellite and in situ SSD. An additional factor when considering SSD is spiciness, which indicates the relative weighting of temperature and salinity in determining the density, i.e., warm-salty water can have the same density as cool-fresh water. Examining the ocean’s spiciness in the context of thermal expansion (alpha) and salinity contraction (beta) helps highlight the significance of satellite SSS observations for ocean modeling, along with regional sensitivities.