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.