4.7 Article

The Ocean's Skin Layer in the Tropics

期刊

JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS
卷 124, 期 1, 页码 59-74

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AMER GEOPHYSICAL UNION
DOI: 10.1029/2018JC014021

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资金

  1. European Research Council (ERC) project [GA336408]
  2. Schmidt Ocean Institute [AABM9767]

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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 degrees C (upper 20-m layer in reference to nominal 1-mm depth) and slightly warmer with -0.19 +/- 0.25 degrees C in an upper 80-m 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. Plain Language Summary We provide a very large data set of high-resolution in situ observations of the saline skin layer. The major finding is that the ocean's skin layer is more saline than the underlying bulk water in the tropical Pacific, despite the fact that in this region the freshwater flux in and out of the ocean is dominated by precipitation rather than evaporation. We observed that salinity anomalies in the skin layer are very dynamic and that freshwater input by precipitation can be accompanied by wind-driven mixing. During a rain event, we observed that salting by evaporation and vertical mixing is twofold faster than freshening by dilution with rainwater. This knowledge is essential to interpret salinity data from satellites with respect to tracing freshwater fluxes over the ocean.

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