What is Sea Surface Temperature?
Sea surface temperature is a key climate and weather measurement obtained by satellite microwave radiometers, infrared (IR) 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 located at about 1 meter depth, or placed 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 satellite instrument. For example, IR instruments measure a depth of about 20 micrometers, while microwave radiometers 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 a foundation SST 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 scientific 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 for IR as compared to 25 km for microwave), the IR retrieval is prevented by clouds giving microwave SSTs improved coverage since SST 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 in 1998.
Microwave Measurement of Sea Surface Temperature
At frequencies between 4 and 11 GHz, the vertically polarized microwave brightness temperature of ocean areas 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. The microwave instruments TMI, AMSR-E, AMSR2, WindSat and GMI all 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 AMSR2 but not TMI, provides improved estimates of sea-surface roughness and improved accuracy for SSTs less than 12 °C (Gentemann et al., 2010). All channels are used to simultaneously retrieve SST, wind speed, columnar water vapor, cloud liquid water, and rain rate. SST retrieval is prevented only in regions with sun-glitter, rain, or proximity to 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, or cloud liquid water can result in errors in the retrieved SSTs.
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 Optimally Interpolated (OI) SSTs
To provide users with complete maps of SST, we have developed two optimally interpolated SST products that merge multiple satellites (when available) into globally complete, daily, SST maps. A full description of the MW OI SST products is available. These data also can be viewed as browse images on our web site.
Microwave Satellite SST Observations, both Swath and Gridded, available as GHRSST netCDF Files
The SST values from the individual satellite binary files are extracted and we are in the process of producing GHRSST GDS v2.0 format netCDF files. We will update this information when the files are available.
Individual Radiometer Gridded Binary Data Files
SST is one of the ocean measurements derived from microwave data processing for the instruments that have the lower frequency channels (6-7 GHz and/or 11 GHz). We include SST in the RSS satellite gridded binary data files for TMI, AMSR-E, AMSR2, WindSat, and GMI. These SST are not diurnally corrected and represent the value at the time provided in the file. 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 – 2015 | V7.1 |
| AMSR-E | 2002 – 2011 | V7 |
| WindSat | 2003 – 2020 | V7.0.1 |
| AMSR2 | 2012 – present | V8.2 |
| GMI | 2014 – present | V8.2 |
References
Gentemann, CL, CJ Donlon, A Stuart-Menteth and others, 2003, Diurnal signals in satellite sea surface temperature measurements, Geophysical Research Letters, 30, 1140, doi:10.1029/2002GL016291.
Gentemann, CL, FJ Wentz, CA Mears and others, 2004, In situ validation of Tropical Rainfall Measuring Mission microwave sea surface temperatures, Journal of Geophysical Research, 109, C04021, doi:10.1029/2003JC002092.
Gentemann, CL, T Meissner, FJ Wentz, 2010, Accuracy of satellite sea surface temperatures at 7 and 11 GHz, IEEE Transactions on Geoscience and Remote Sensing, 48, 1009-1018.
Wentz, FJ, CL Gentemann, DK Smith and others, 2000, Satellite measurements of sea surface temperature through clouds, Science, 288, 847-850.