Instruments aboard NASA's and NOAA's spacecrafts use their vantage point from space to collect global measurements of the ocean's surface temperature. Each day these instruments make thousands of measurements of broad swaths of the Earth - creating concurrent data sets of the entire planet. By developing global, detailed, and decades-long views of Sea Surface Temperature (SST), data obtained from NASA and NOAA satellites provide the basis for the prediction of climate change, ocean currents, and the potent El Niño-La Niña cycles.
El Niño is perhaps the best known example of the impact that changing sea surface temperature has on our climate. Every three to seven years, this warming of surface ocean waters in the eastern tropical Pacific brings winter droughts and deadly forest fires in Central America, Indonesia, Australia, and southeastern Africa, and lashing rainstorms in Ecuador and Peru. El Niño affects thousands of people worldwide, and billions of dollars in economic impact. El Niño's "sister," La Niña, occurs less frequently and has the opposite effect - the cooling of surface ocean waters.
But changing SST patterns have broader implications than just the El Niño and La Niña cycles. Changes in SST are the single most important indicator of climate change. Heat is one of the main drivers of global climate, and the ocean is a huge reservoir of heat. The top 6.5 feet of ocean has the potential to store the equivalent amount of heat contained in the atmosphere. The ocean has a high capacity and as ocean currents move tremendous amounts of water over vast distances, heat is also carried or transferred over these distances. This release of heat can play a major role in climate from the regional/basin to global scale. It is for this reason that oceans are termed the 'memory' of the Earth's climate system. Tracking SST as a variable over long periods of time, as well as operationally, is critical for developing climate models and improved weather forecasts.
Global Sea Surface Temperature. This false-color image shows a one-month composite of MODIS sea surface temperature data for May 2001. Red and yellow around the equatorial region indicates warmer temperatures, green is an intermediate value, while blues and then purples toward the poles are progressively colder values. The image reveals cold water currents that move from Antarctica northward along South America's west coast. These cold, deep waters upwell along an equatorial swath around and to the west of the Galapagos Islands. Also noticeable are warm, wide currents of the Gulf Stream moving up the United States' east coast, carrying Caribbean warmth toward Newfoundland and across the Atlantic toward Western Europe. Additionally, there is a warm tongue of water extending from Africa's east coast to well south of the Cape of Good Hope.
The distribution of temperature at the sea surface tends to be zonal, that is, it is independent of longitude. Uneven heating of the Earth by the Sun causes the warmest water to be near the equator, while the coldest water is near the poles. The deviations from these zonal measurements are small. The anomalies of sea-surface temperature, the deviation from a long term average, are also small, less than 1.5°C/34.7°F except in the equatorial Pacific where the deviations can be 3°C/37.4°F. Large deviations in the Equatorial Pacific are due primarily to the El Niño-La Niña cycle.
Most weather and climate events are the result of sea and atmospheric coupling. Heat energy released from the ocean is the dominant driver of atmospheric circulation and weather patterns. SST influences the rate of energy transfer into the atmosphere, as evaporation increases rapidly with temperature. Knowing the temperature of the ocean surface provides tremendous insight into short and long term weather and climate events.
Taking the Ocean's Temperature
The most commonly used instrument to measure sea-surface temperature from space is the Advanced Very High Resolution Radiometer AVHRR. Since 1999, the Moderate-resolution Imaging Spectroradiometer (MODIS) sensor has been collecting even more detailed measurements of surface temperature. More recently, the Advanced Microwave Scanning Radiometer for EOS (AMSR-E) has been collecting SST that includes areas covered by clouds. A key attribute of the AVHRR data is the length of the time record. An AVHRR sensor has been carried on all polar-orbiting meteorological satellites operated by NOAA since Tiros-N was launched in 1978. High quality measurements of the temperature of the ocean are now available from 1981 to the present. This unique SST data set is now the longest satellite derived oceanographic record, providing a 25-year (and continuing) record of global SST changes. Conversely, MODIS and records are much shorter.
Thermal Infrared Remote Sensing
AVHRR and MODIS instruments use radiometers to measure the amount of thermal infrared radiation given off by the surface of the ocean. Thermal infrared remote sensing is based on the fact that everything above absolute zero (-273°C/459°F) emits radiation in the thermal infrared region of the electromagnetic spectrum. The amount of thermal infrared radiation given off by an object is related to its temperature (dying embers give off less radiation than a hot fire). Thus by measuring the amount of radiation given off by the ocean we can calculate its temperature. With instruments like radiometers, it is possible to get a picture of the thermal environment that we cannot experience with our normal human sensors. The ability to record precise variations in infrared radiation has tremendous application in extending our observation of many types of phenomena where minor temperature variations are significant in understanding our environment.
Moderate-resolution Imaging Spectroradiometer (MODIS)
MODIS is sensitive to five different wavelengths, or "channels," of radiation used for measuring SST. Both night and day, the sensor measures the thermal infrared energy escaping the atmosphere at 12 microns and then compares that measurement to how much energy is escaping at 11 microns, allowing scientists to determine how much the atmosphere modifies the signal so they can "correct" the data to more accurately derive SST. The MODIS sensor, because of the increased number of channels, tells us a great deal about the influence of the atmosphere on measurements of SST. Similar to AVHRR, MODIS also takes daily measurement of the global ocean.
Advanced Microwave Scanning Radiometer for EOS (AMSR-E)
Because AVHRR and MODIS cannot observe the ocean when the atmosphere is cloudy, NASA developed a new sensor, AMSR-E, that is able to observe through the clouds. AMSR-E on the Aqua satellite is a passive microwave radiometer, modified from the Advanced Earth Observing Satellite-II (ADEOS-II). Microwaves are radio waves that are able to pass through clouds. Thus, the AMSR-E instrument can measure radiation from the ocean surface through most types of cloud cover, supplementing infrared based measurements of SST that are restricted to cloud-free areas. However, the resolution of AMSR-E is coarser than the thermal IR sensors. The addition of AMSR-E data will provide a significant improvement in our ability to monitor SST and temperature controlling phenomenon.
Sea Surface Height & Temperature
Sea surface height data can also provide clues to studying the temperature of the ocean. Warm water expands raising the sea surface height. Conversely, cold water contracts lowering the height of the sea surface. Thus, measurements of sea surface height can provide information about the heat content of the ocean. The height can tell us how much heat is stored in the ocean water column below its surface. Learn more about sea surface height.
Interpreting Sea Surface Temperature Measurements
Radiation observed by AVHRR and MODIS is modified by its passage through the atmosphere. The degree to which the signal is modified depends upon the chemistry of the overlying atmosphere. Clouds, haze, dust or smoke can interfere with a space-based remote sensor's ability to accurately measure SST, as can greenhouse gases, like water vapor. These are present in abundance in the tropics and strongly absorb infrared energy and re-radiate it back toward the surface. Scientists have created several algorithms to correct the impact of these variables creating more accurate measurements of SST.
Further, scientists analyze SST data to provide new products that have a wide variety of uses. SST data are also distributed and processed by several organizations. These data sets are then used operationally by sponsoring agency scientists and other organizations.
- The Goddard Earth Sciences Data and Information Services Center (GES DISC) at CaltTech/JPL are the key distribution point for SST data and related data sets from NASA.
- The Goddard Distributed Active Archive Center (GDAAC) is the primary distribution center for MODIS data.
- The Global Hydrology & Climate Center provides browse images and some Level 2 AMSR-E data products
- The GES DISC is the mirror site for the level 3 data sets.
SST data is also combined with other data taken in-situ by ships and bouys. This data helps calibrate the satellite data to create a more accurate measurement of SST.
SST data is used by many different organizations for regional studies, anomaly studies, climate and meteorological studies, and to provide near real - time access to the data. SST data products are also widely used by the fishing industry to track the conditions where fish are most likely to be found.
Long term averages of sea surface temperature are used to calculate the normal seas surface temperature conditions for a specific time of year and location. Deviations from the long-term mean are called anomalies. The long-term means are also used for studying climate change. Other data is made available in time intervals of less than a day - in some instances within a few hours of collection. This type of data is mostly used for detecting specific features in the ocean, such as currents and eddies.
Trends we observe
SST data is used to observe many regional phenomena around the world, including the Chesapeake Bay and the Gulf of Mexico, the Gulf Stream, Kuroshio, the Somali Current, the Brazil Current and the East Australian Current. These currents are associated with sharp changes in SST which can be detected using satellites. Coastal water studies are made off the Hawaiian and Alaskan coasts. Multiple studies are also conducted off the North Atlantic.
Sea surface temperatures in the equatorial Pacific affect precipitation (and therefore plant growth) over much of the North American continent. Warmer-than-normal water in the central and western equatorial Pacific, creates higher precipitation in southern and central North America. Conversely, cold water temperatures in the Pacific lead to a decrease in precipitation over northern North America.
SST may also affect one of the world's key large-scale atmospheric circulations - the circulation that regulates the intensity and breaking of rainfall associated with the South Asian and Australian monsoons.
Projects are underway that combine data from multiple satellite systems to produce a robust set of sea surface data for assimilation into ocean forecasting models of the waters around Europe and also the entire Atlantic Ocean. The Global Ocean Data Assimilation Experiment, GODAE, is assimilating sea-surface temperature data, altimeter data, scatterometer data, and drifter data into coupled ocean/atmosphere numerical models to produce forecasts of ocean currents and temperatures up to 30 days in advance everyehwere in the ocean.
Finally, projects are also being conducted to combine SST data from various sensors to create the highest quality SST. These projects will create a new generation of multi-sensor, high-resolution SST products. An example of such a project is the GODAE High Resolution Sea Surface Temperature Pilot Project (GHRSST-PP).
SST data are important to the development and testing of a new generation of computer models in which the interacting processes of the land, the atmosphere, and the oceans are coupled. The measurements are widely used in the creation of more accurate weather forecasts and increasingly it is seen as a key indicator of climate change. It is anticipated that projects like GHRSST will provide even higher quality data sets for such things as hurricane forecasting.
Projects like GHRSST lay the groundwork for future cooperations, between NASA and NOAA, as well as internationally. Such cooperations will lead to major innovations in how data is distributed in near real-time, searched and stored. Plans include a joint NASA/NOAA effort to provide users with an interface for accessing both near real-time and historical data for climate studies. Future technologies should allow managers, decision makers, and modelers to search and access data in near real time for specified areas of interest. Additionally, the merging of SST data from different sensors will provide high resolution SST data suitable for coastal studies and management.