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GCOS Essential Climate Variables (ECV) Definition: Sea Surface Temperature (SST) - The temperature of the ocean surface. The term sea surface temperature is generally meant to be representative of the upper few meters of the ocean as opposed to the skin temperature, which is the temperature of the upper few centimeters. Introduction: Ocean surface temperature (often known as ‘sea surface temperature’ or SST) is one of the most important boundary conditions for the general circulation of the atmosphere. The ocean exchanges vast amounts of heat and energy with the atmosphere and these air/sea interactions have a profound influence on the Earth’s weather and climate patterns. SST is linked closely with the ocean circulation, as demonstrated time and again by the El Niño-Southern Oscillation (ENSO) cycle. A major research goal is to enable seasonal and longer time scale forecasting by development of coupled atmosphere and ocean models that correctly link the many processes. Progress towards this goal depends on a more precise and comprehensive set of SST measurements for use in initialising and verifying such models. Satellite Observations: Satellite remote sensing provides the only practical means of developing such a dataset. In situ data, predominantly from ships of opportunity and from networks of moored and drifting buoys, are limited in coverage, whereas satellites offer the potential for surveying the complete ocean surface in just a few days. The in situ data have a key role to play in calibrating the satellite data and in providing information needed for deriving bulk temperatures. Instruments on polar satellites provide information for short to medium-range NWP with global coverage, good horizontal and temporal resolution and accuracy, except in areas that are persistently cloud-covered. Accurate SST determinations, especially in the tropics, are important for seasonal to inter-annual forecasts. The advent of high spectral resolution infrared sounders will enable separation of surface emissivity and temperature, and the accuracy of the SST product is expected to improve to an acceptable level. Geostationary imagers with split window measurements are also helping to expand the temporal coverage by making hourly measurements, thus creating more opportunities for finding cloud-free areas and characterising any diurnal variations (known to be up to 4K in cloud-free regions with relatively calm seas). For regional NWP, sea-surface temperature is inferred with acceptable horizontal resolution from polar satellites, while geostationary satellites complement information with better temporal resolution. A range of instruments with thermal bands are being used for SST measurements. Visible/infrared imagers such as AVHRR, AATSR, and MODIS currently provide the main source of SST data, with AATSR and MODIS providing better accuracy (0.25–0.3K). AVHRR, however, gives greater coverage, enabling it to track ocean currents and monitor ENSO phenomena through its larger swath width. The Aqua mission, which includes MODIS along with AIRS+ and AMSR, provides oceanographers with further precise information and the ability to remove atmospheric effects. NOAA’s VIIRS and CMIS instrument on the planned NPOESS missions will provide capabilities to produce higher resolution and more accurate measurements of SST than currently available from AVHRR. Other sources of SST data include: AMSR-E on Aqua; the SEVIRI and IASI instruments on the Meteosat-8/9 (MSG-1/2) and MetOp missions respectively.The GHRSST Pilot Project provides a new generation of global, high resolution (<10 km) SST products, combining complementary satellite and in situ data (www.ghrsst-pp.org/). GCOS is concerned that the continuity of the 4 km resolution global data be maintained through adequate instruments onboard operational weather satellites and its quality must be enhanced through high-precision sensors on board other Earth observation missions. CEOS has defined four actions in support: — an ATSR-like instrument is planned on ESA’s Sentinel-3, presently scheduled for launch in 2012. JAXA will lead planning for the Global Change Observation Mission to maintain continuity of the sea surface temperature ECV; — CEOS agencies will examine their respective plans to maintain provision of microwave brightness temperatures for the sea surface temperature ECV; — relevant CEOS agencies will examine their respective plans to maintain continuity of a 10 km resolution sea surface temperature data sets global product; — CEOS agencies will cooperate to support the combination of all existing sea surface temperature data sets into a global FCDR. (Satellite Missions) El Niño/La Niña Update/Outlook: WMO NOAA Data, Product, Metadata and Information Access
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