Sea Surface Temperature (SST)
Earth Observation
In Situ
Measurement Types
Sea surface temperature (SST) is defined as the subsurface bulk temperature of the top few metres of the ocean, which can be measured from space by satellite microwave and infrared (IR) radiometers, and from the ocean by buoys and ships. Each of these measurement techniques provide SST data at different depths and temporal and spatial resolutions. In situ SST measurements are generally limited to one metre in depth, while satellite measurements can support varying depths of water depending on the frequency of the instrument. IR sensors measure the skin temperature (approximately 10 µm below the surface), while microwave radiometers measure the temperature at a depth of a few millimetres. 1) 2) 3) 4)
The spatial patterns of SST displayed on global heat maps provide a visualisation of the underlying ocean dynamics, including ocean fronts, eddies, coastal upwelling, and exchanges between the coastal shelf and open ocean. Understanding these dynamics is essential for weather prediction, ocean forecasts, monitoring marine ecosystems and coral bleaching events, as well as for coastal applications such as fisheries forecasting, pollution monitoring, and tourism. SST plays a vital global role in weather systems and climate patterns, due to its influence on the exchange of energy, momentum, and gases between the ocean and atmosphere. Observing SST dynamics is important for monitoring large-scale events such as El Niño and La Niña - which are marked by changes in temperature of a few degrees within a region of the Pacific ocean along the equator (as displayed in Figure 1). On a more local scale, SST data aids weather forecasting as it influences the prediction of storm tracks, storm intensities and coastal flooding. This is particularly the case for the development of tropical cyclones as they draw energy from warm waters. 2) 3) 5) 6)
Longitudinal measurements of SST can be used to produce climate change projections. As shown in Figure 2, global mean SST rose by 0.60°C between 1980 and 2020. This shows a less dramatic trend than land surface temperature as the ocean absorbs around 90% of excess heat generated by climate change, but it still provides valuable insight. Projections assume an increase in SST of 0.86°C from 1995-2014 to 2081-2100 under the most modest greenhouse gas emissions scenarios, and up to 2.89°C under the most severe emissions scenarios. 4)
Historically, SST data collection has been made by in situ measurement techniques with sensors on ships and buoys. Moored and drifting buoys provide insight into remote regions like the Southern Ocean, where NOAA’s Argo drifting floats offer much improved coverage. The TAO-TRITON (Tropical Atmosphere Ocean) array of global tropical moored buoys monitor the emergence and evolution of El Niño events in the tropical Pacific. This in situ data also supports the calibration of satellite-based SST estimates. 7)
Since the 1980s, global SST datasets have relied on satellite observations, through microwave and IR measurements, due to their speed and coverage. Microwave retrieval of SST data became possible in 1997 with the launch of the TMI (the TRMM Microwave Imager) onboard NASA and JAXA’s TRMM (Tropical Rainfall Measuring Mission). While infrared (IR) satellite sensors provide readings with better spatial resolution (1-4 km) than microwave sensors (25 km), IR measurements are obscured by clouds as they absorb the ocean-emitted IR radiation. Both IR and microwave measurements are often combined to provide an all-weather, high-resolution SST product.
Satellite measurements of SST need to separate out the brightness temperature of the ocean, which is a measurement derived from the radiance of the ocean's infrared radiation. In addition to SST, brightness temperature consists of the heat radiation from the moist atmosphere above the ocean, which depends on sea-surface roughness and atmospheric temperature and moisture profile.
Furthermore, satellite measurements of SST, unlike in situ measurements, have a temperature deficit due to the atmosphere absorbing and re-emitting a significant fraction of the sea surface emission which must be corrected for using clear-sky atmospheric correction algorithms. Through simultaneous measurements at multiple frequencies and polarisations, SST can be separated from these other influences. For example, the microwave instruments TMI, AMSR-E, AMSR2, WindSat, and GMI all measure multiple frequencies that are sufficient to remove the contributions due to surface roughness and atmospheric effects. 2) 3) 8)
| In situ Sensor Type | Measurement Depth | Temporal Resolution |
| Ships | Up to 10 m | Hourly - daily (e.g., SOOP - 4 to 6 times per day) |
| Drifting buoys | ~0.1-2 m | Hourly - daily (e.g., Argo programme - 1 hour to 101 minutes) |
| Moored buoys | ~1 m | < 1hr - daily (e.g., TAO-TRITON - 10 minute or less) |
| Satellite Sensor Type | Measurement Depth | Spectral Range | Coverage | Spatial Resolution (km) |
| Infrared (IR) | ~10-20 µm | 3.7 μm to 10 μm | Global, impacted by cloud cover (as shown in Figure 4) | ~1-4 |
| Microwave | ~1 mm | ~1 mm to 1 m | Global, unimpacted by cloud cover | ~20-50 |
Distribution of satellite derived SST datasets has become more widespread with the establishment of the Group for High Resolution Sea Surface Temperature (GHRSST) project in 2002. The project aims to provide a new generation of global, multi-sensor, high-resolution near real time SST datasets in a common format that is easily accessible over various platforms and operating systems. Bringing together international space agencies, research institutes, universities, and government agencies, GHRSST provides SST datasets with descriptions of the associated errors which allows for accurate climate modelling. The Sea Surface Temperature Virtual Constellation (SST-VC) within the Committee on Earth Observation Satellites (CEOS) works with GHRSST to better understand the needs of CEOS Agencies and the scientific community. 2) 9) 10) 11)
Example Products
SST datasets
Global land-ocean surface temperature datasets are primarily made up of SST measurements (approximately 71% by surface area). In situ SST data from ships and buoys have been compiled into datasets like ICODAS (International Comprehensive Ocean-Atmosphere Data Set), which has recorded SST measurements from 1662 to 2014 (see Figure 6).
NOAA OI SSTv2 (NOAA Optimal Interpolation (OI) SST Analysis, version 2) is the longest satellite-based dataset spanning from 1981 to the present (see Figure 7). It is based primarily on measurements from AVHRR (Advanced Very High Resolutions Radiometer) IR measurements onboard the NOAA POES and the ESA and EUMETSAT MetOp satellites. This dataset has a spatial coverage of 1.0 degree latitude by 1.0 degree longitude global grid (180x360). There is an ongoing effort to extend this record by restoring historical datasets through the AVHRRR for Fundamental Climate Data Records project. 7)
Daily Optimum Interpolation Sea Surface Temperature (DOISST) Maps
Measurements from various satellites can be optimally interpolated into daily global SST maps. Optimum Interpolation (OI) is a data assimilation method that interpolates irregularly spaced data onto a regularly sampled grid. This is desirable in SST maps as it allows the filling in of missing data due to orbital gaps or environmental conditions precluding SST retrieval. Remote Sensing Systems have created an Optimally Interpolated (OI) SST map (as shown in Figure 8) based on the instruments TMI (on TRMM), AMSR-E (on Aqua), AMSR-2 (on GCOM-W), WindSat, GMI (on GPM), MODIS-Terra, MODIS-Aqua, VIIRS-NPP (on Suomi NPP) and VIIRS-N20 (on NOAA-20). 3) 12)
Related Missions
Copernicus: Sentinel-3
The Sentinel-3 (S3) constellation consists of two ESA-operated radar imaging satellites, S3A and S3B, which are supported by EUMETSAT. S3A and S3B were launched in February 2016 and April 2018 respectively, and are part of Copernicus, the European Union’s Earth observation program, which is managed by the European Commission (COM). The onboard Sea and Land Surface Temperature Radiometer (SLSTR) provides SST data to aid in ocean systems forecasting, environmental monitoring, and climate monitoring.
Meteosat First, Second, and Third Generation (MFG, MSG, MTG)
Meteosat is the European meteorological program in GEO (Geostationary Orbit) that was initiated in 1972 by ESRO (European Space Research Organisation), the predecessor to ESA. The three Meteosat constellations were developed by ESA and EUMETSAT for weather forecasting and climate monitoring.
The MFG Constellation operated between November 1977 and March 2017, and consisted of seven satellites with SST monitoring capabilities. This mission carried the Meteosat Visible and Infrared Imager (MVIRI), a high resolution radiometer that operated in the thermal infrared (TIR), water vapour absorption (WV), and the visible (VIS) wavelength regions. The four MSG satellites carry the Spinning Enhanced Visible and Infra-red Imager (SEVIRI), a multi-spectral radiometer that takes SST measurements. The first satellite of this constellation, MSG-1, launched in August 2002.
MTG consists of six satellites: four MTG-I (Imager) satellites and two MTG-S (Sounder) satellites, with the first MTG-I satellite launching in December 2022. The MTG-I satellites measure SST with the Flexible Combined Imager (FCI), which covers the VIS, NIR, and IR spectrum with 16 channels.
GPM (Global Precipitation Measurement) Mission
The GPM (Global Precipitation Measurement) mission is an international multi-satellite constellation, co-lead by NASA and JAXA (Japanese Aerospace Exploration Agency). NASA’s GPM Core Observatory, the primary satellite in the constellation, was launched in February 2014 to join the existing constellation, which has the aim of studying global precipitation, evaporation and the water cycle. The GPM Core Observatory carries the active microwave DPR (Dual-frequency Precipitation Radar) and the passive microwave GMI (GPM Microwave Imager). The other satellites in the constellation with significant SST capabilities are NOAA-19, JAXA’s GCOM-W (Global Change Observation Mission-Water), EUMETSAT’s MetOP A, B, and C, and Suomi-NPP (National Polar-orbiting Partnership) of NASA.
GCOM-W (Global Change Observation Mission - Water)
GCOM (Global Change Observation Mission) is a JAXA dual-satellite constellation mission consisting of GCOM-W1 (Water) and GCOM-C1 (climate change specialising). GCOM-W1, launched in May 2012, is a part of the GPM constellation. Its primary instrument is the 2nd Advanced Microwave Scanning Radiometer (AMSR2), which captures multispectral microwave emissions of the ocean surface to allow for a better understanding of the water cycle.
Aqua (EOS/PM-1) and Terra (EOS/AM-1)
The Aqua (EOS/PM-1) and Terra (EOS/AM-1), launched May 2002 and December 1999 respectively, are joint missions within NASA's ESE (Earth Science Enterprise) program. Developed by NASA’s Goddard Space Flight Centre, the MODIS (Moderate-Resolution Imaging Spectroradiometer) instrument is onboard both satellites and collects extensive data on the Earth’s atmosphere, land surface, ocean and cryosphere, including SST measurements. MODIS, onboard Terra and Aqua, orbits the Earth approximately 14 times per day, allowing it to gather more SST data in 3 months than all other combined SST measurements taken before the advent of satellites.
MetOp (Meteorological Operational Satellite Program of Europe)
The Meteorological Operational satellite program (MetOp) is a collaboration between ESA and EUMETSAT with the aim to provide satellite observation and data services for weather prediction and climate monitoring. MetOp-A, launched in 2006 and retired in 2021, was followed by the launches of MetOp-B and -C in 2012 and 2018, respectively. MetOp is a part of the GPM constellation. A primary aim of the MetOp mission is to provide global imaging, which was made possible by the onboard Advanced Very High Resolutions Radiometer (AVHRR/3) which provides day and night imaging of land, water and clouds, alongside SST measurements.
Suomi-NPP (National Polar-orbiting Partnership)
The Suomi National Polar-orbiting Partnership (Suomi NPP), operated by NASA and the National Oceanic and Atmospheric Administration (NOAA), is a weather satellite that was launched in October 2011. It is a part of the GPM constellation. The onboard VIIRS (Visible/Infrared Imager and Radiometer Suite), a multispectral radiometer with a rotating telescope, is used to observe land, ocean, and atmospheric parameters on a daily basis, providing measurements of SST.
VIIRS (Visible Infrared Imaging Radiometer Suite)
The VIIRS (Visible Infrared Imaging Radiometer Suite) contributes to improved weather forecasting by tracking long-term data on ocean surface features like SST and collecting visible and IR images of the land, atmosphere, cryosphere, and oceans. VIIRS is flying onboard NASA and NOAA’s Suomi NPP, NOAA-20, and NOAA-21 satellite missions. NOAA-20 and NOAA-21, launched November 2017 and November 2022 respectively, are part of the Joint Polar Satellite System (JPSS) program of NOAA and NASA. Both Suomi NPP and NOAA-20 are part of the GPM constellation.
Read more: JPSS-1/NOAA-20, JPSS-2/NOAA-21
AVHRR (Advanced Very High Resolution Radiometer)
The AVHRR (Advanced Very High Resolution Radiometer), provided by the National Oceanic and Atmospheric Administration (NOAA), monitors Earth’s reflected energy in the visible and infrared portions of the electromagnetic spectrum in five spectral bands, collecting multi-purpose day and night imagery of SST. The first AVHRR instrument consisted of four-channels, while AVHRR-3 (version 3), which was first launched on NOAA-15 in May 1998, is a six-channel radiometer. The instrument is carried by NOAA POES (Polar Orbiting Environmental Satellites) and ESA’s MetOp-A, B, and C satellites. NOAA-19, and NOAA-20, as well as MetOp B and C, are members of the GPM constellation.
Read more: NOAA POES 5th Generation, MetOp
TRMM (Tropical Rainfall Measuring Mission)
The Tropical Rainfall Measuring Mission (TRMM) was a research satellite developed by NASA and JAXA. It was operational from November 1997 to April 2015, and gathered information about precipitation and latent heating between the Tropics of Capricorn and Cancer, to further the understanding of global energy, water cycles, and climate. SST was measured using NASA’s TMI (the TRMM Microwave Imager), a passive multichannel, dual-polarised microwave radiometer that detected microwave energy from Earth's surface and atmosphere. The inclusion of the new 10.7 GHz channel on TMI allowed it to accurately measure SST through clouds. TRMM was the predecessor to the GPM mission.
References
1) Goni, G et al (2009), “The Ships of Opportunity Program,” URL: https://doi.org/10.5270/OceanObs09.cwp.35
2) “Ocean Temperature,” PO.DAAC JPL NASA. URL: https://podaac.jpl.nasa.gov/SeaSurfaceTemperature
3) “Sea Surface Temperature,” Remote Sensing Systems. URL: https://www.remss.com/measurements/sea-surface-temperature/
4) “Climate Change 2021 The Physical Science Basis,” IPCC. URL: https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_SPM_final.pdf
5) “Monitoring the temperature of the sea and sea ice,” EUMETSAT. URL: https://www.eumetsat.int/sea-surface-temperature-services
6) “Sea Surface Temperature,” Earth Observatory NASA. URL: https://earthobservatory.nasa.gov/global-maps/MYD28M
7) “SST Data Sets: Overview & Comparison Table,” NCAR. URL: https://climatedataguide.ucar.edu/climate-data/sst-data-sets-overview-comparison-table
8) Minnett, P. J. (2019), “Half a century of satellite remote sensing of sea-surface temperature,” URL: https://doi.org/10.1016/j.rse.2019.111366
9) “Group for High Resolution Sea Surface Temperature (GHRSST),” PO.DAAC JPL NASA, URL: https://podaac.jpl.nasa.gov/GHRSST
10) “Sea Surface Temperature,” CEOS. URL: https://ceos.org/ourwork/virtual-constellations/sst/
11) “CEOS SST Virtual Constellation,” GHRSST. URL: https://www.ghrsst.org/about-ghrsst/ceos-sst-vc/
12) “Microwave OI SST Product Description,” Remote Sensing Systems. URL: https://www.remss.com/measurements/sea-surface-temperature/oisst-description/