SMOS Sea Surface Salinity retrieval after 15 years of mission

In November 2009, the first L-band interferometric radiometer was launched as the second Earth Explorer mission by the European Space Agency (ESA), with the primary goal of measuring soil moisture over land and sea surface salinity over the oceans. After more than 15 years, the mission continues to provide excellent monitoring of these two essential climate variables, offering unprecedented spatial and temporal coverage and resolutions.  Over these 15 years, the data processing algorithms for the salinity retrieval have had to evolve to address significant challenges in the observation acquisition process. These challenges include: i) Land-sea  and ice-sea contamination, which results in spurious biases  near the coastlines and ice-edges due to sharp transitions between the high values of the brightness temperature over land and ice, and the low values over the ocean; ii) Degradation of the signal caused by Radio Frequency Interferences sources, which unexpectedly (and illegally) occupied the frequency reserved for Earth Observation. 

 

After huge efforts from all the expert support laboratories at level 2, the SMOS sea surface salinity maps have achieved an accuracy that has led to a wave of influential advances across many fields, especially in physical oceanography and climate change. These advancements include insights into the intensification of the water cycle, drivers of the sea-ice retreat in Antarctica, among others. As a testament to the credibility and robustness of the satellite salinity observation, the Climate Change Initiative has not only included salinity in its program, but also brings the opportunity to add new essential variables that use satellite salinity for their estimation.

 

Looking ahead, it is time to consider a successor to SMOS. Starting in 2028, the Copernicus Imaging Microwave Radiometry (CIMR) Copernicus expansion mission is expected to continuously monitor the L-band (along with other frequencies), albeit with slightly degraded spatial resolution but significantly increased signal-to-noise ratio compared to SMOS. The SMOS support laboratories are committed to be at the international forefront of the acquisition of the L-band measurement. For this reason, we are not completely satisfied with “only” continuous monitoring of the L-band; we also aim to enhance the resolution of the signal. To this end, two different proposals were submitted in the last ESA Earth Explorer 12 call for ideas. Although neither was selected, both have received excellent evaluations, encouraging the teams to mature and resubmit their proposals in the next call.