GCOS Ocean Surface ECV
Sea Surface Temperature (SST)

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: Global SST fields have been produced on a monthly basis for many years, but comparisons of different analyses reveal discrepancies that are unacceptable for many climate purposes, including some of those of the UNFCCC. However, adequate global SST analysis is achievable through enhanced global deployment of existing technology and the improved calibration of satellite sensors, better validation of derived products and further advancement of methodologies.
 
The networks and satellite systems that contribute to the observation of SST and are included in the development of global integrated products comprise:

• Surface drifters;
•  Tropical moored buoy network;
•  VOS;
•  VOSClim;
•  Reference mooring network;
•  Argo profiling float network;
•  Ship of Opportunity Programme (SOOP) expendable bathythermograph (XBT) network; and the SOOP subset carrying thermosalinographs (TSGs)
•  Satellite nadir IR (polar orbiting and geostationary);
•  Satellite dual view IR (e.g., the Along-Track Scanning Radiometer (ATSR))
•  Satellite microwave.

Issues relative to the observation and analysis of SST include:

• Different sensors measure the temperature of different “surfaces.” IR systems observe radiance from a very thin skin layer, while microwave systems observe radiance from a slightly thicker (sub-skin) layer and traditional in situ methods sample water from well below these skin layers (near-surface and mixed-layer “bulk” temperatures). The relationship between IR, MW, and in situ temperatures are complicated by both the surface skin layer and diurnal thermal stratification.  Under almost all conditions, skin temperatures are expected to differ from in situ temperatures; the difference can be up to a degree under certain extremes but is typically a few tenths of a degree.  Under wind speeds less than 6 m/s, the depth of the measurement becomes increasingly important; the difference can be several degrees under certain conditions. Enhanced coverage and improved mechanisms for data exchange are needed for geostationary data. 
• Atmospheric variability (e.g., cloud, water, aerosols, sea fog, spindrift, air-sea temperature difference) affects both coverage and accuracy from satellite systems. In situ systems are limited in their spatial resolution, collect data from different depths, make use of different sensors, etc.  
• The ability to exploit historical and contemporary datasets is affected by the limited amount of metadata typically available; 
• For climate purposes integrated analysis products are needed that take advantage of the strengths of each data stream that make best use of our understanding of the limitations of each data stream, and that adjust for variations in the uncertainty from region to region.

To address these issues, ongoing support is needed for an integrated and coordinated approach to satellite SST measurements (incorporating polar and geostationary IR, and microwave measurements). Sustained support is required for microwave instruments. Continuing support is needed for efforts such as the Group for High-Resolution SST (GHRSST) Project which attempts to make optimum use of satellite and in situ observations at the highest feasible space and time resolution whilst continuing to support efforts to improve the absolute accuracy of satellite SST measurements, and improving our understanding of the characteristics of the uncertainties.

(from the Implementation Plan for the Global Observing System for Climate in Support of the UNFCCC' August 2010, GCOS-138 (GTOS-184, GTOS-76, WMO-TD/No. 1523))

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)

Additional Information:

• National Activities Summaries of Operational & Planned Observation Programs (Moorings, ARGO, Sea Level, XCTD/XBT/TSG, TS Hydrography, VOS, Sea Ice, Satellites, Black Sea, BOOS, NEAR-GOOS, Bio/Chem, Carbon, Coastal)
• Video: Interview with Rupa Kumar Kolli, Chief, WMO World Climate Applications and Services Division about El Niño.
• El Niño/La Niña Update/Outlook: WMO NOAA

References:

• Implementation Plan for the Global Observing System for Climate in Support of the UNFCCC (2010 Update) GCOS-138/GOOS-184/GTOS-76/WMO-TD/No. 1523
• The Second report on the Adequacy of the Global Observing Systems for Climate in Support of the UNFCCC - April 2003 (GCOS-82/WMO/TD Noc 1143)

Data, Product, Metadata and Information Access

[ECV Matrix Main Page] [About the ECV Matrix] [Reference Documents] [Contact] [Updated May 25, 2011]