Sea Surface Temperature

What is Sea Surface Temperature?

Sea surface temperature is a key climate and weather measurement obtained by satellite infrared (IR) and microwave radiometers, in situ moored and drifting buoys, and ships of opportunity.  Different instruments measure the temperature at different depths.  For instance, most buoys have sensors at about 1 meter depth, or at regular intervals along a tether line.  Sea surface temperatures when measured from space represent a depth that is related to the frequency of the instrument.  For example, IR instruments measure a depth of about 20 micrometers, while microwave instruments measure a depth of a few millimeters.  See Characterizing Sea Surface Temperature on the GHRSST-PP site for a good discussion of thermal variability in the upper ocean. The microwave (MW) optimally-interpolated (OI) SSTs are designed to represent pre-dawn temperatures at a depth of ~1 meter, or temperatures just below the diurnal layer.

These measurements can be combined in various ways to create daily spatially-complete global SST maps used for weather prediction, ocean forecasts, and in coastal applications such as fisheries forecasts, pollution monitoring, and tourism.  SST maps are  also widely used by oceanographers, meteorologists, and climate scientists for research.  Prior to 1997, SSTs were only available globally from IR satellite retrievals, but with the launch of TMI, microwave retrievals became possible. While IR SSTs have a higher resolution than microwave SSTs (1 – 4 km as compared to 25 km), their retrieval is prevented by clouds giving microwave SSTs improved coverage since they can be measured through clouds.  This has proven especially important in tropical cyclone forecasting as the clouds surrounding a cyclone prevented adequate SST measurements until microwave instruments became available. 

Microwave Measurement of Sea Surface Temperature

At frequencies between 4 and 11 GHz, the vertically polarized microwave brightness temperature of the ocean has an appreciable sensitivity to SST. In addition to SST, brightness temperature depends on the sea-surface roughness and on the atmospheric temperature and moisture profile.  Fortunately, the spectral and polarimetric signatures of the surface-roughness and the atmosphere are quite distinct from the SST signature, and the influence of these effects can be removed given simultaneous measurements at multiple frequencies and polarizations. TMI, AMSR-E, and WindSat measure multiple frequencies that are more than sufficient to remove the surface-roughness and atmospheric effects. Sea-surface roughness, which is tightly correlated with the local wind, is usually parameterized in terms of the near-surface wind speed and direction. The additional 7 GHz channel present on AMSR-E and not TMI, provides improved estimates of sea-surface roughness and improved  accuracy for SSTs less than 12°C (Gentemann et al., in press). All channels are used to simultaneously retrieve SST, wind speed, columnar water vapor, cloud liquid water, and rain rate (Wentz and Meissner, 2000). SST retrieval is prevented only in regions with sun-glitter, rain, and near land. Since only a small number of retrievals are unsuccessful, almost complete global coverage is achieved daily. Any errors in retrieved wind speed, water vapor, cloud liquid water can result in errors in retrieved SST.

The through-cloud capabilities of microwave radiometers provide a valuable picture of global sea surface temperature (SST). To utilize this, scientists at RSS have calculated a daily, Optimally Interpolated (OI) SST product at quarter degree (~25 kilometer) resolution. This product is ideal for research activities in which a complete, daily SST map is more desirable than one with missing data due to orbital gaps or environmental conditions precluding SST retrieval.

RSS Sea Surface Temperature Products

Daily Global Optimal Interpolated SSTs

To provide users with a complete map of SST measurements, we developed an optimally interpolated SST product that merges multiple satellites (when available) into a globally complete, daily, SST maps.  A full description of the MW OI SST product is available.  These data also can be viewed as browse images on our web site.

Microwave Satellite SST Observations, both Swath and Gridded as GHRSST netCDF Files

The SST values from the individual satellite binary files have been extracted and placed in GHRSST GDS v1.6 format files.  This has been done for TMI (swath and gridded), AMSRE (swath and gridded), and WindSAT (gridded).  These files are in netCDF format complete with metadata.  We are in the process of producing GHRSST GDS V2.0 format files.

Individual Radiometer Gridded Binary Data Files

SST is one of the ocean measurements derived during passive microwave satellite data processing for instruments with lower frequency channels (6-7 GHz and/or 11 GHz).  We include SST in the satellite gridded binary data files for TMI, AMSRE and WindSat.  More data access information is available on the mission description pages.  The table below shows the period of operation for each instrument.

Instrument Period of Operation Version
TMI 1997 - present V4
AMSR-E 2002 - 2011 V7
WindSat 2003 - present V7.0.1