Sea Surface Salinity Under Rain Cells: SMOS and in-Situ Observations

The Soil Moisture and Ocean Salinity (SMOS) mission monitors sea surface salinity over the global ocean for more than 4.5 years. The objective of this presentation is 1) to analyse the signature of rainfall on SMOS salinity and 2) to validate it. This is a challenging task given that SMOS measures the salinity in the first top centimeter (S1cm) while most in situ measurements of salinity (SSS) are made at a few meters depth and that other effects than sea surface salinity may affect the radiometric signal, like sea surface roughness modified by rain drops and rain atmospheric effects. We investigate the signature of rainfall on SMOS S1cm using various methods. First, we compare SMOS S1cm with ARGO SSS measured at about 5m depth in the Intertropical Convergence Zone (ITCZ) and in the Southern Pacific Convergence Zone. Second, we study the correlation between spatial variability of SMOS S1cm and spatial variability of rainfall. In both cases, we estimate that S1cm decrease associated with rainfall occurring within less than 1hour from salinity measurement is close to -0.2 pss (mm hr-1)-1. Given the multiangular retrieval of SMOS radiometric polarized measurements and given existing atmospheric models, we estimate that rain induced roughness and atmospheric effects are responsible for no more than 20% of this value. We then study the signature of rainfall on sea surface salinity measured by surface drifters at 45cm depth. 22 rain events, as detected by satellite rain measurements, have been sampled by surface drifters. We observe a decrease of the drifters salinity associated with rainfall of -0.21 (+/-0.14) pss (mm hr-1)-1, consistent with SMOS observations. When averaged over one month, the rain associated salinity instantaneous decrease is at most -0.2 in monthly 100x100km2 pixels, and at most 40% of the difference between SMOS S1cm and interpolated in situ SSS in pixels near the ITCZ. This suggests that more than half of this difference could be related to other effects like the in-situ products obtained from optimal interpolation and therefore influenced by smoothing and relaxation to climatology, radio frequency interferences or rain history. The relative importance of each of these effects will be discussed.