Sea surface freshening inferred from SMOS and ARGO salinity: impact of rain

摘要

The sea surface salinity (SSS) measured from space by the Soil Moisture andOcean Salinity (SMOS) mission has recently been revisited by the EuropeanSpace Agency first campaign reprocessing. We show that, with respect to theprevious version, biases close to land and ice greatly decrease. The accuracyof SMOS SSS averaged over 10 days, 100 × 100 km2 in the openocean and estimated by comparison to ARGO (Array for Real-Time Geostrophic Oceanography) SSS is on the order of 0.3–0.4 intropical and subtropical regions and 0.5 in a cold region. The averagednegative SSS bias (−0.1) observed in the tropical Pacific Ocean between5° N and 15° N, relatively to other regions, is suppressedwhen SMOS observations concomitant with rain events, as detected from SSM/Is (Special Sensor Microwave Imager)rain rates, are removed from the SMOS–ARGO comparisons. The SMOS fresheningis linearly correlated to SSM/Is rain rate with a slope estimated to−0.14 mm?1 h, after correction for rain atmosphericcontribution. This tendency is the signature of the temporal SSS variabilitybetween the time of SMOS and ARGO measurements linked to rain variability andof the vertical salinity stratification between the first centimeter of thesea surface layer sampled by SMOS and the 5 m depth sampled by ARGO.However, given that the whole set of collocations includes situations withARGO measurements concomitant with rain events collocated with SMOSmeasurements under no rain, the mean −0.1 bias and the negative skewness ofthe statistical distribution of SMOS minus ARGO SSS difference are verylikely the mean signature of the vertical salinity stratification. In thefuture, the analysis of ongoing in situ salinity measurements in the top50 cm of the sea surface and of Aquarius satellite SSS are expected toprovide complementary information about the sea surface salinitystratification.