Approaching four Years of the ESA SMOS Mission in Orbit: Progress in Measuring Salinity from Space
Banks, Chris¹; Gommenginger, Christine¹; Srokosz, Meric¹; Snaith, Helen^2
1National Oceanography Centre, UNITED KINGDOM; ²British Oceanographic Data Centre, National Oceanography Centre, UNITED KINGDOM

As the fourth anniversary of the launch of the ESA Soil Moisture and Ocean Salinity (SMOS) satellite approaches (November 2013), our study examines the temporal and spatial evolution of sea surface salinity (SSS) as seen from space during this period. We not only consider the results from SMOS but also from the NASA/CONAE Aquarius mission, which joined SMOS in orbit in June 2011.

We present comparisons of gridded satellite SSS datasets with output from the UK Met Office Forecasting Ocean Assimilation Model (FOAM), which is based on NEMO (Nucleus for European Modelling of the Ocean), both globally at 1° by 1° monthly and on shorter space and time scales for selected study regions (e.g. Gulf Stream/Agulhas Retroflection and Return Current). Whilst showing the good agreement amongst satellite products, model output and in situ measurements, we also note continuing difficulties with the retrieval algorithms.

Most notably, there are significant biases affecting the performance of SMOS related to the satellite travel direction (i.e. ascending versus descending passes), although these have decreased with revised versions of the data processor. Similar, though less significant, biases are also seen in results from Aquarius. The ascending/descending bias in SMOS SSS is likely, in part, to be linked to changes in galactic and sun glint contamination through the year. SMOS has a sun-synchronous orbit such that at ~6 a.m. (local time) SMOS is ascending (satellite moving from south to north) and at ~6 p.m. is descending (satellite moving north to south). Aquarius is in a sun-synchronous orbit too, but the direction of passes is 12-hours out of phase so that Aquarius is ascending at ~6 p.m. and descending at ~6 a.m. We review work-to-date at the UK National Oceanography Centre on a study concerned with looking at differences amongst SSS derived from SMOS and Aquarius from ascending and descending passes. The aim is to take advantage of data from both satellites and utilise the phase difference to investigate whether time of day is a factor (e.g. biases in SSS due to diurnal warming effects or the tendency for heavy precipitation to occur in the tropics mid-afternoon).