The Ocean's Skin Layer in the Tropics

We provide a large data set on salinity anomalies in the ocean's skin layer together with temperature anomalies and meteorological forcing. We observed an average salinity anomaly of 0.40 ± 0.41 practical salinity unity (n = 23,743), and in 83% of the observations the salinity anomaly was positive; that is, the skin layer was more saline. Temperature anomalies determined by an infrared camera were -0.23 ± 0.28 °C (upper 20-μm layer in reference to nominal 1-mm depth) and slightly warmer with -0.19 ± 0.25 °C in an upper 80-μm layer in reference to 1-m depth. In 75% of the observations, our data confirmed the presence of a cooler skin layer. Light rain rates (<4 mm/hr) induced an immediate freshening by 0.25 practical salinity unit in the skin layer without any effect in the mixed layer at 1-m depth. Vertical mixing by strong winds (12 m/s) masked freshening during a heavy rain fall (47 mm/hr) by the intrusion of saltier deeper waters, but a freshening was observed after the wind and rain calmed down. We computed density anomalies, which suggest that denser skin layers can remain afloat up to a density anomaly of 1.3 g/L, likely due to the interfacial tension between the skin layer and underlying bulk water. It implies that salinization by evaporation regulates buoyancy fluxes, a key process for the exchange of climate-relevant gases and heat between the ocean and atmosphere.

Warming and Inhibition of Salinization at the Ocean's Surface by Cyanobacteria

This paper describes high-resolution in situ observations of temperature and, for the first time, of salinity in the uppermost skin layer of the ocean, including the influence of large surface blooms of cyanobacteria on those skin properties. In the presence of the blooms, large anomalies of skin temperature and salinity of 0.95°C and -0.49 practical salinity unit were found, but a substantially cooler (-0.22°C) and saltier skin layer (0.19 practical salinity unit) was found in the absence of surface blooms. The results suggest that biologically controlled warming and inhibition of salinization of the ocean's surface occur. Less saline skin layers form during precipitation, but our observations also show that surface blooms of Trichodesmium sp. inhibit evaporation decreasing the salinity at the ocean's surface. This study has important implications in the assessment of precipitation over the ocean using remotely sensed salinity, but also for a better understanding of heat exchange and the hydrologic cycle on a regional scale.