Wind speed effects on rainfall-induced salinity and temperature anomalies at the sea surface microlayer at mid-latitudes

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Sea surface salinity (SSS) and temperature (SST) can serve as proxies to detect freshwater fluxes at the sea surface during precipitation. Widely unknown, however, is the impact of precipitation (droplet sizes and velocities) and wind speed on the sea surface microlayer (SML), the <1 mm boundary layer between atmosphere and ocean. We used the autonomous surface vehicle HALOBATES in the German Bight, North Sea, to collect SSS and SST data with high vertical resolution at 7 depths in the top meter, spaced 10–30 cm apart, including the SML. During two rain events with similar maximum rainfall intensity, the magnitude and persistence of the resulting salinity anomaly (between SML and 100 cm depth) was dependent on wind speed. The maximum rate of change of salinity in the SML was 4 times faster with high wind speeds: 0.009 g kg min−1 at low wind speeds, 0.037 g kg min−1 at high wind speeds. A threshold of about 5 m s−1 determined whether stratification occurred in the near-surface layer or freshwater mixed with the underlying layers during precipitation. Strong winds still caused salinity changes in the near-surface layer and SML, but the water mixed rapidly with the underlying water masses. The SML salinity was affected by droplet distributions with smaller droplets, as small droplets stay at the surface. Salinity and temperature changes in the SML were >9 times higher than at 100 cm depth (−0.037 g kg min−1 vs. −0.004 g kg min−1) and still detectable during very high wind speeds. Overall, these results contribute to a better understanding of the vertical distribution of freshwater in the surface ocean and its dependence on rain intensity, wind speed, and droplet properties, helping to improve understanding of the fate of freshwater in a changing ocean due to climate warming.