Remote-controlled research catamaran finds subtleties
During the voyage of the research vessel FALKOR of the US Schmidt Ocean Institute in October and November 2016, the remote-controlled catamaran "Sea Surface Scanner" (S³) of Wurl's team was used to investigate the surface film in the Timor Sea north of Australia and in the western Pacific. Among other things, it was used to record changes in the salinity of the surface film and the underlying water layer in long-term measurement series. For this purpose, S³ is equipped with six parallel rotating, semi-submerged glass discs. Due to the surface tension, the wafer-thin boundary layer sticks to the glass discs, is wiped off by rubber lips and guided past the sensors. During its deployment, S³ also collected water samples from a depth of one metre, whose temperature and conductivity, and therefore salinity, were automatically determined. The catamaran also recorded meteorological data at a height of three metres. The FALKOR also recorded corresponding data at a height of eleven metres and infrared radiometers on board also measured the temperature of the surface film. The scientists used the meteorological data to mathematically determine the evaporation rates.

Important surface layer
The upper layer of the oceans, which extends to a depth of around five metres, plays an important role in the exchange of heat, particles and climate-relevant gases. Nevertheless, it has been largely ignored in studies to date: measurements directly from the research vessel are difficult as the hull and propulsion mix the water layers. Under calm conditions, a film less than a millimetre thick forms at the boundary of the surface layer between the air and the sea surface, which plays a key role in the exchange with the atmosphere. It has been known for some time that this film is often tenths of a degree cooler than the well-mixed water masses directly below it. In addition, several attempts have been made to describe its salinity in models. Both salinity and temperature regulate the density and thus the continued existence of the film. They are therefore important influencing variables for the exchange between the ocean and the atmosphere and thus ultimately for the water balance and climate on a global scale.

Mostly cooler and saltier film almost everywhere
The current study confirms the ubiquitous distribution of a surface film in the Indo-Pacific. In 83 per cent of the measurements, this film was saltier and in three quarters of the observations it was on average almost 0.2 degrees cooler compared to the measurements at a water depth of one metre. This is despite the fact that precipitation in the region brings more freshwater into the sea than is lost through evaporation. During two tropical rainstorms, the scientists observed that the wind mixed in saltier water in the upper layer to a depth of one metre below. However, the water in the surface film evaporated so quickly due to the strong air movement during the storms that the salt content increased even during the rain. The evaporation rate was twice as high as the dilution by rainwater. On days with little rain and moderate winds, the surface film became temporarily cloudy due to the lack of mixing. After the rain front had passed, however, its salt content increased again. "Our measurements showed that a dense surface film can float on less dense surface water up to a certain threshold," says Wurl, whose work was funded by the European Research Council (ERC). According to the scientists, a sufficient interfacial tension presumably ensures this. It is also what significantly influences the renewal rates of the surface membrane and thus the exchange. A density cycle of the surface film was observed at measuring stations without rain: Within minutes, the density increased due to evaporation until the interfacial tension was no longer sufficient to separate the film from the underlying water: It collapsed, and the underlying water with a lower density forced its way to the surface to equalise the volume. Within a very short time, the density rose again due to evaporation and the cycle started all over again.

More research technology for a deeper understanding
"In general, the renewal of the surface film is a complex process. Ultimately, we need a mechanistic understanding of where the water that enters the ocean via rainfall remains in order to gain a more complete picture of the changing oceanic water cycle due to climate change," says Wurl. After all, 80 per cent of precipitation and evaporation takes place above the oceans. According to the researchers, this requires autonomous research technology on a larger scale, such as drifting buoys or the catamaran used by Wurl.

Original publication
O. Wurl, W. M. Landing; N. I. Hamizah Mustaffa; M. Ribas-Ribas; C. Riggs Witte; C. J. Zappa (2018) The Ocean's Skin Layer in the Tropics. JGR Oceans, Early View.
doi.org/10.1029/2018JC014021