The impact of rainfall on the sea surface salinity: a mesocosm study

Sea surface salinity may serve as a tracer for freshwater fluxes because it is linked to evaporation and precipitation that force the freshwater balance of the ocean’s surface. The relationship between freshwater fluxes and salinity anomalies in the upper few centimeters remains widely unknown. In a mechanistic approach, we investigated how these anomalies develop by conducting experiments with artificial rain over a large basin. We measured conductivity and temperature at different depths and rain characteristics (intensity, rain temperature, droplet sizes, and velocities). In the absence of turbulence, the rain causes a strong salinity change of up to 6.02 g kg \(^{-1}\) $_{}^{-1}$ in 0–4 cm depth. At the highest rain intensity of 56 mm h \(^{-1}\) $_{}^{-1}$ , salinity changed thrice as fast as at an intensity of 18 mm h \(^{-1}\) $_{}^{-1}$ . At the sea surface microlayer (first millimeter of the surface) the anomalies are always highest and reached a maximum of 14.18 g kg \(^{-1}\) $_{}^{-1}$ . With mechanical mixing, salinity changes were less pronounced (maximum SML salinity anomaly: 6.17 g kg \(^{-1}\) $_{}^{-1}$ ), and freshwater was mixed fast with the existing seawater body. In general, our study shows that freshwater remains in the upper few centimeters, and even with induced turbulence, are not mixed below 16 cm.