Sea Surface Temperature
Physical Ocean | Topography | Temperature | Salinity | Winds | Currents | Ice
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.
Every day the Moderate-resolution Imaging Spectroradiometer (MODIS) measures sea surface temperature over the
entire globe with high accuracy. This false-color image shows a one-month composite for May 2001. Red and yellow
indicates warmer temperatures, green is an intermediate value, while blues and then purples are progressively colder
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.
Temperature as measured by the NASA Aqua satellite's Advanced Microwave Scanning Radiometer
(AMSR-E) instrument. Temperature is represented by the colors in the ocean. Orange and red indicate the necessary
27.7°C/82°F and warmer sea surface temperatures for a hurricane to form. This visualization was match-frame
rendered to another visualization showing clouds. This visualization was created in support of the Recipe for a Hurricane feature story.
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.
Sea surface temperature data are used to help us predict
weather patterns, to track ocean currents, and to monitor El Niño and La Niña. Sea surface temperature influences the growth of phytoplankton, as
well as precipitation patterns across continents, thus indirectly influencing land vegetation. (Data from Advanced Very
High Resolution Radiometer [AVHRR])
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.
The temperature of the world's ocean
surface provides a clear indication of the regions where hurricanes and typhoons form, since they can only form when the
sea surface temperature exceeds 27.8°C (82°F). In this visualization of AMSR-E data covering the period from
June, 2002 to September, 2003, areas with surface temperatures greater than 82°F are shown in yellow and orange,
while sea surface temperatures below 82°F are shown in blue. The region in the Atlantic from the Caribbean to the
equator only exceeds the critical temperature during late summer and early fall in the Northern Hemisphere, the period
known as "Hurricane Season." It is also possible to see the Gulf Stream, the warm river of water that parallels the east
coast of the United States before heading towards northern Europe, in this data. Around January 1, 2003, a cooler-than-
normal region of the ocean appears just to the west of Peru as part of an La
Niña and flows westward, driven by the trade winds. The waves that appear on the edges of this cooler area
are called tropical instability waves and can also be seen in the equatorial Atlantic Ocean at about the same
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
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
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.
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.
The Gulf Stream, one of the ocean's most significant and fastest currents moves at four miles per hour. This
current of warm water, which is called the North Atlantic Drift after it turns offshore at Cape Hatteras, travels from
the Gulf of Mexico to northern European waters. The current's warm waters are responsible for the more temperate
climates experienced by Ireland, England, Scotland, and the Isles of Scily. At the beginning of the Current, in the Gulf
of Mexico, its temperature is around 27°C (80°F), but by the time it reaches northern Europe it has cooled down to
a few degrees Fahrenheit. As it cools, it gives up heat to the atmosphere, which carries the warmth to Europe.
This false-color Terra/MODIS image produced from Direct Broadcast data by the Space Science and Engineering Center at
the University of Wisconsin-Madison is made from data on Band 31. Its bright colors represent temperature data of the
water: the Gulf Stream is shown in shades of orange and yellow, which puts it near 20°C (68°F) on the scale
provided in the image. Land temperature data are not shown in this image and appear black, while white represents areas
where no data were collected (because clouds obscured the water). The colder waters off of the coasts of Virginia,
Maryland, Delaware, and New Jersey contrast sharply with the warmer waters of the Gulf Stream in this image from March
15, 2005. Credit: SSEC University of Wisconsin - Madison
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
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.