Sea surface temperatures (SST)
Sea surface temperature (SST) refers to the measurement of the ocean's surface temperature, typically ranging from about –3° Celsius in polar regions to 30° Celsius in tropical waters. Historically, SST measurements began with simple methods like using a bucket and thermometer, but evolved to include more sophisticated techniques involving ship intake ports and buoy recordings. Data collection has expanded to include satellite observations, allowing for a broader understanding of ocean temperature variations. SST is critical for climate and weather prediction, as it influences atmospheric conditions and is a key variable in general circulation models (GCMs).
Fluctuations in SST can significantly impact weather patterns, including the development and intensity of storms and hurricanes, which require temperatures of around 27°–29° Celsius to form. Notably, while global SST has shown a rising trend, regional variations are evident, such as warming in the Atlantic and cooling in parts of the Pacific. The interaction between SST and atmospheric phenomena, like the El Niño-Southern Oscillation (ENSO), underscores the complexity of ocean-atmosphere relationships. However, obtaining consistent and reliable long-term SST data remains a challenge due to various influencing factors and measurement discrepancies. As climate models predict future trends, SST measurements will continue to play a vital role in understanding and addressing climate change impacts.
Sea surface temperatures (SST)
Definition
Sea surface temperature (SST) is a bulk measurement of the temperature of the surface of the ocean. Average SST ranges from approximately –3° Celsius in the Arctic Ocean to 30° Celsius in warm tropical waters. SST is one of the parameters recorded by ships as part of their standard atmospheric and oceanic observation practices, which are governed by international codes first established in the late nineteenth century.
The earliest sea surface temperature measurements were taken using a simple bucket and thermometer. Such measurements reported the average value for the upper 1-2 meters of the ocean. In the 1940s, it became standard practice to record the temperature of water coming through a ship’s intake ports. This measurement, however, can record a temperature of any depth up to 20 meters, depending on the ship’s buoyancy, and can be affected by heat from the ship’s engines. This remains the manner in which temperature data is recorded. As the information is gathered on research vessels, weather ships, and many commercial and military vessels, the result is that much of the data is concentrated along shipping routes.
SST data is transmitted to the World Meteorological Commission and then transferred to weather services around the globe. SST readings are also recorded using moored and drifting buoys. With buoy readings, temperature probes are placed at a standard depth of 1 meter below the surface. Data from buoys are often transmitted by satellite to a data center, such as the National Data Buoy Center. As part of the World Ocean Circulation Experiment (WOCE), over forty-eight hundred drifting buoys were released to record SST among other data. Since the 1980s, SSTs have increasingly been obtained using satellite-borne radiometers. These instruments indirectly measure the temperature of the ocean’s “skin” (the top 10 microns) based on the radiation intensity of select wavelengths, typically in the infrared spectrum.
Significance for Climate Change
Sea surface temperature is a critical parameter used in general circulation models (GCMs), as well as meteorological models, to predict future climate and weather, respectively. While air surface temperatures have increased since the 1900s, global SST records are more complicated. In fact, an examination of historical records of SSTs showed a warming in the Atlantic Ocean by up to 0.4° Celsius, while equatorial Pacific SSTs cooled by approximately 0.2° Celsius. Although not uniform, SSTs have increased an average of 0.14° Celsius per decade globally from 1901 through 2023. The warmest annual SST ever recorded occurred in 2023. Scientists predicted that these temperatures would continue to increase in the coming decade.
Because of the slow response time of oceans, as compared to the atmosphere, most scientists consider that, in most cases, observed seasonal changes in SST reflect changes in atmospheric conditions. However, there are exceptions to this assumption, most notably El Niño-Southern Oscillation (ENSO) events. ENSO events are coupled atmosphere-ocean phenomena that occur when warmer SSTs feed back into changes in atmospheric circulation patterns. El Niño phenomena are marked by warmer SSTs in the eastern equatorial Pacific Ocean, centered near the coasts of Peru and Ecuador, and they lead to a shift in the strength and direction of atmospheric circulation across the equatorial Pacific. This shift is known as the Southern Oscillation. ENSO events appear to have a periodicity of approximately three to eight years, in a highly irregular pattern.
On seasonal timescales, SSTs are important factors in the development of storms and hurricanes (tropical cyclones). A threshold SST of approximately 27°–29° Celsius is required for a hurricane to develop. The reasons for this threshold remain unknown, but, generally, higher SSTs appear to be correlated with the development of stronger hurricanes. According to observations correlated by the Intergovernmental Panel on Climate Change (IPCC), there has been an increase in intense hurricane activity in the North Atlantic since 1970. The available data does not show a clear trend in the annual number of tropical cyclones, however, and data integrity prior to 1970 remains questionable. Climate models used by the IPCC, which are based on projections of global temperature to the year 2100, indicate that an increase in hurricane activity is likely. According to the 2018 Fourth National Climate Assessment, since the early 1980s, the intensity, frequency, and duration of hurricanes in the North Atlantic have all increased.
SST measurements are crucial to the used to predict weather and climate, but obtaining long-term and reliable data sets that cover a wide geographic region remains a challenge. Because satellite-based SST measurements and ship- or buoy-based measurements are taken at different depths in the water, these data sets cannot be directly compared. This complication arises because satellite-based measurements are strongly affected by daytime heating of the thin layer they are able to measure, as well as by surface evaporation and reflected radiation. Also, because radiometers often cannot obtain readings through cloud cover, satellite SST data contain a fair-weather bias: SST readings are not obtained from these instruments on cloudy days, so short-term variations related to these meteorological conditions are not recorded. Ships, meanwhile, retain their own problems because any craft powered by a motor will necessarily alter the temperature of the water through which it travels.
Bibliography
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"Climate Indicators - Sea Surface Temperature." Copernicus, European Commission, 20 Apr. 2023, climate.copernicus.eu/climate-indicators/sea-surface-temperature. Accessed 10 Dec. 2024.
Fourth National Climate Assessment, nca2018.globalchange.gov. Accessed 10 Dec. 2024.
Intergovernmental Panel on Climate Change. Climate Change, 2007—The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Edited by Susan Solomon et al. New York: Cambridge University Press, 2007.
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