The Impact of Wind Gusts on the Ocean Thermal Skin Layer

• Published in: Geophysical research letters 2019-10, Vol.46 (20), p.11301-11309
• Main Authors: Zappa, Christopher J., Laxague, Nathan J. M., Brumer, Sophia E., Anderson, Steven P.
• Format: Article
• Language: English
• Subjects: Air; Air temperature; Air-water interface; air‐sea heat flux; capillary‐gravity waves; Earth Sciences; Emissivity; Gusts; Heat flux; Heat transfer; Imagery; Imaging techniques; Infrared cameras; Infrared imaging; microbreaking; Micrometers; Ocean surface; Ocean temperature; Oceans; Properties; Sciences of the Universe; Sea surface temperature; Short wave radiation; Skin temperature; Surface temperature; Temperature; Thermal imaging; Water depth; Wave breaking; Wind; Wind waves
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2019GL083687

The thermodynamic and emissive properties of the ocean thermal skin layer are crucial contributors to air‐sea heat flux. In order to properly observe ocean surface temperature without disturbing any delicate fluid mechanical processes, thermal infrared imaging is often used. However, wind impacting the ocean surface complicates the extraction of meaningful information from thermal imagery; this is especially true for transient forcing phenomena such as wind gusts. Here, we describe wind gust‐water surface interaction through its impact on skin layer thermal and emissive properties. Two key physical processes are identified: (1) the growth of centimeter‐scale wind waves, which increases interfacial emissivity, and (2) microscale wave breaking and shear, which mix the cool skin layer with warmer millimeter‐depth water and increase the skin temperature. As more observations are made of air‐sea interaction under transient forcing, the full consideration of these processes becomes increasingly important. Plain Language Summary When a wind gust impacts an air‐water interface, two separate processes work to increase the temperature sensed by an infrared camera. The shortwave‐roughened surface becomes more emissive, and the skin layer (upper tens to hundreds of micrometers) becomes warmer as it is mixed by microscale wave breaking. The present paper identifies the effects of both processes in a field observational data set. This work is important to the quantification of air‐sea heat flux from thermal infrared measurements. Key Points Wind gusts produce transient ocean skin layer thermal fronts that propagate near the observed wind speed Wind gust fronts disrupt the ocean thermal skin layer due to microbreaking and increase emissivity due to capillary‐gravity wave growth Following wind gust front passage, capillary‐gravity wave relaxation reduced surface emissivity faster than the cool skin was restored