Rain-induced variability of near sea-surface T and S from drifter data

The effect of rain on sea surface temperature, salinity and density is examined using data of surface drifters in regions of the tropical oceans with large rainfall. In a few off-equatorial areas, there are sufficient drifter data to composite average daily cycles. There, the period of the day with largest salinity changes is associated with the largest rainfall rates and the lowest salinity. For a one-yearlong trajectory in the southwest Pacific, this results in an early morning salinity minimum, whereas the opposite is found close to equatorial West Africa. We then consider individual freshening events larger than 0.1 psu (averaging 0.56 psu at 50 cm), and find that they are often related with local rainfall, are associated with a surface cooling, and relax in a time inversely proportional to wind intensity. The temperature cooling is dependent on the time of day, but the freshening presents less daily cycle and the largest fast changes in salinity tend to be associated with the largest rainfall rates. When two measurement levels are available, the initial salinity signal is larger by more than 20% at the shallow depth (15 cm) compared with the deeper measurement level (near 50 cm), and the temperature and salinity gradients between the two levels are proportional (0.22 degrees C for a 1% dilution).