Sea surface skin temperature explained

The sea surface skin temperature (SSTskin), or ocean skin temperature, is the temperature of the sea surface as determined through its infrared spectrum (3.7–12 μm) and represents the temperature of the sublayer of water at a depth of 10–20 μm.[1] High-resolution data of skin temperature gained by satellites in passive infrared measurements is a crucial constituent in determining the sea surface temperature (SST).

Since the skin layer is in radiative equilibrium with the atmosphere and the sun, its temperature underlies a daily cycle. Even small changes in the skin temperature can lead to large changes in atmospheric circulation. This makes skin temperature a widely used quantity in weather forecasting and climate science.

Remote Sensing

Large-scale sea surface skin temperature measurements started with the use of satellites in remote sensing. The underlying principle of this kind of measurement is to determine the surface temperature via its black body spectrum. Different measurement devices are installed where each device measures a different wavelength. Every wavelength corresponds to different sublayers in the upper 500 μm of the ocean water column. Since this layer shows a strong temperature gradient, the observed temperature depends on the wavelength used.[2] Therefore, the measurements are often indicated with their wavelength band instead of their depths.[3]

History

First satellite measurements of the sea surface were conducted as early as 1964 by Nimbus-I.[4] Further satellites were deployed in 1966 and the early 1970s. Early measurements suffered from contamination by atmospheric disturbances. The first satellite to carry a sensor operating on multiple infrared bands was launched late in 1978, which enabled atmospheric correction. This class of sensors is called Advanced very-high-resolution radiometers (AVHRR) and provides information that is also relevant for the tracking of clouds. The current, third-generation features six channels at wavelength ranges important for cloud observation, cloud/snow differentiation, surface temperature observation and atmospheric correction. The modern satellite array is able to give a global coverage with a resolution of 10 km every ~6 h.[5]

Conversion to SST

Sea surface skin temperature measurements are completed with SSTsubskin measurements in the microwave regime to estimate the sea surface temperature. These measurements have the advantage of being independent of cloud cover and underlie less variation. The conversion to SST is done via elaborate retrieval algorithms.[6] These algorithms take additional information like the current wind, cloud cover, precipitation and water vapor content into account and model the heat transfer between the layers. The determined SST is validated by in-situ measurements from ships, buoys and profilers. On average, the skin temperature is estimated to be systematically cooler by 0.15 ± 0.1 K compared to the temperature at 5m depth.[7]

Vertical temperature profile of the sea surface

The vertical temperature profile of the surface layer of the ocean is determined by different heat transport processes. At the very interface, the ocean is in thermal equilibrium with the atmosphere which is dominated by conductive and diffusive heat transfer. Also, evaporation takes place at the interface and thus cools the skin layer. Below the skin layer lies the subskin layer, this layer is defined as the layer where molecular and viscous heat transfer dominates. At larger scales, as the much bigger foundation layer, turbulent heat transport through eddies contributes most to the vertical heat transfer.

During the day, there is additional heating by the sun. The solar radiation entering the ocean gets heats the surface following the Beer-Lambert law. Here, approximately five percent of the incoming radiation is absorbed in the upper 1 mm of the ocean.[8] Since the heating from above leads to a stable stratification, other processes dominate the heat transport, depending on the considered scale.

Notes and References

  1. Web site: NASA Sea Level Change Portal . NASAs/ghrsst . 2022-03-20.
  2. Donlon . C. J. . Minnett . P. J. . Gentemann . C. . Nightingale . T. J. . Barton . I. J. . Ward . B. . Murray . M. J. . February 2002 . Toward Improved Validation of Satellite Sea Surface Skin Temperature Measurements for Climate Research . Journal of Climate . 15 . 4 . 353–369 . 10.1175/1520-0442(2002)015<0353:tivoss>2.0.co;2 . 2002JCli...15..353D . 0894-8755. free . 10.1.1.509.7822 .
  3. Book: J., Minnett, P.J. Alvera-Azcárate, A. Chin, T.M. Corlett, G.K. Gentemann, C.L. Karagali, Ioanna Li, X. Marsouin, A. Marullo, S. Maturi, E. Santoleri, R. Saux Picart, S. Steele, M. Vazquez-Cuervo . Half a century of satellite remote sensing of sea-surface temperature . 2019 . 1137572851.
  4. Book: Allison, Lewis J. . An evaluation of sea surface temperature as measured by the nimbus I high resolution infrared radiometer. . 1967 . NATIONAL AERONAUTICS AND SPACE ADMINISTRATION NOV . 1074065169.
  5. Donlon . C. . 1 August 2007 . The Global Ocean Data Assimilation Experiment High-resolution Sea Surface Temperature Pilot Project . American Meteorological Society. 88 . 8 . 1197 . 10.1175/BAMS-88-8-1197 . 2007BAMS...88.1197D . 20.500.11820/01c9ad56-38d8-4058-911e-4e2ba3db1fbc . 46982596 . free . free .
  6. Web site: Admin . 2021-06-14 . A new ocean skin temperature analysis . 2022-03-24 . ECMWF . en.
  7. Book: M, Donlon, C Minnett, P Gentemann, C Nightingale, T Barton, I Ward, B Murray . Toward improved validation of satellite sea surface skin temperature measurements for climate research . 779490675.
  8. Fairall . C. W. . Bradley . E. F. . Godfrey . J. S. . Wick . G. A. . Edson . J. B. . Young . G. S. . 1996-01-15 . Cool-skin and warm-layer effects on sea surface temperature . Journal of Geophysical Research: Oceans . 101 . C1 . 1295–1308 . 10.1029/95jc03190 . 1996JGR...101.1295F . 0148-0227. subscription .
  9. GHRSST Science Team (2010), The Recommended GHRSST Data Specification (GDS) 2.0, document revision 4, available from the GHRSST International Project Office, 2011, pp 123.
  10. Zeng . Xubin . Beljaars . Anton . 2005-07-19 . A prognostic scheme of sea surface skin temperature for modeling and data assimilation . Geophysical Research Letters . 32 . 14 . n/a . 10.1029/2005gl023030 . 2005GeoRL..3214605Z . 55071249 . 0094-8276. subscription .
  11. Liu . W. Timothy . Businger . Joost A. . 1975 . Temperature profile in the molecular sublayer near the interface of a fluid in turbulent motion . Geophysical Research Letters . 2 . 9 . 403–404 . 10.1029/gl002i009p00403. 1975GeoRL...2..403L . subscription .
  12. Kawai . Yoshimi . Wada . Akiyoshi. 2007. Diurnal sea surface temperature variation and its impact on the atmosphere and ocean: A review. Journal of Oceanography . 63 . 5. 721–744. 10.1007/s10872-007-0063-0 . 53637504 .
  13. Jessup . A.T. . Hesany . V. . 1996 . Modulation of ocean skin temperature by swell waves. Journal of Geophysical Research . 101 . C3 . 6501–6511 . 10.1029/95JC03618. 1996JGR...101.6501J .
  14. Trombetta . Thomas . Vidussi . Francesca . Mas . Sébastien . Parin . David . Simier . Monique . Mostajir . Behzad . 2019-04-05 . Ianora . Adrianna . Water temperature drives phytoplankton blooms in coastal waters . PLOS ONE . en . 14 . 4 . e0214933 . 10.1371/journal.pone.0214933 . 1932-6203 . 6450617 . 30951553. 2019PLoSO..1414933T . free .
  15. Kelly . Paige . Clementson . Lesley . Davies . Claire . Corney . Stuart . Swadling . Kerrie . 2016 . Zooplankton responses to increasing sea surface temperatures in the southeastern Australia global marine hotspot . Estuarine, Coastal and Shelf Science . en . 180 . 242–257 . 10.1016/j.ecss.2016.07.019. 2016ECSS..180..242K .
  16. Kahru . Mati . Leppanen . Juha-Markku . 1993 . Cyanobacterial blooms cause heating of the sea surface . Marine Ecology Progress Series . 101. 1–7. 10.3354/meps101001 . 1993MEPS..101....1K . free .

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