View source for Sea surface temperature

{{Short description|Water temperature close to the ocean's surface}}
{{good article}}
[[File:1979- Daily sea surface temperatures 60S-60N latitudes.png|thumb|upright=1.4|Sea surface temperature since 1979 in the extrapolar region (between 60 degrees south and 60 degrees north latitude).<ref>{{Cite web |title=Copernicus: March 2024 is the tenth month in a row to be the hottest on record {{!}} Copernicus |url=https://climate.copernicus.eu/copernicus-march-2024-tenth-month-row-be-hottest-record |access-date=2024-08-15 |website=climate.copernicus.eu}}</ref>]]
'''Sea surface temperature''' (or '''ocean surface temperature''') is the [[ocean temperature|temperature of ocean water]] close to the surface. The exact meaning of ''surface'' varies in the literature and in practice. It is usually between {{convert|1|mm|in|sigfig=1}} and {{convert|20|m|ft|sigfig=1}} below the [[sea]] surface. Sea surface temperatures greatly modify [[air mass]]es in the [[Atmosphere of Earth|Earth's atmosphere]] within a short distance of the shore. The [[thermohaline circulation]] has a major impact on average sea surface temperature throughout most of the world's oceans.<ref name="Rahmstorf2003">{{cite journal |last=Rahmstorf |first=S |year=2003 |title=The concept of the thermohaline circulation |url=http://www.pik-potsdam.de/~stefan/Publications/Nature/nature_concept_03.pdf |journal=Nature |volume=421 |issue=6924 |page=699 |bibcode=2003Natur.421..699R |doi=10.1038/421699a |pmid=12610602 |s2cid=4414604 |doi-access=free}}</ref> 

Warm sea surface temperatures can develop and [[Tropical cyclogenesis|strengthen cyclones over the ocean]]. Tropical cyclones can also cause a cool wake. This is due to turbulent mixing of the upper {{convert|30|m|ft|sigfig=1}} of the ocean. Sea surface temperature changes during the day. This is like the air above it, but to a lesser degree. There is less variation in sea surface temperature on breezy days than on calm days. 

Coastal sea surface temperatures can cause offshore winds to generate [[upwelling]], which can significantly cool or warm nearby landmasses, but shallower waters over a [[continental shelf]] are often warmer. Onshore winds can cause a considerable warm-up even in areas where upwelling is fairly constant, such as the northwest coast of [[South America]]. Coastal sea surface temperature values are important within [[numerical weather prediction]] as the sea surface temperature influences the atmosphere above, such as in the formation of [[sea breeze]]s and [[advection fog#Types|sea fog]]. 

It is very likely that global mean sea surface temperature increased by 0.88&nbsp;°C between 1850–1900 and 2011–2020 due to [[Climate change|global warming]], with most of that warming (0.60&nbsp;°C) occurring between 1980 and 2020.<ref name="AR6_WG1_Chapter9">Fox-Kemper, B., H.T. Hewitt, C. Xiao, G. Aðalgeirsdóttir, S.S. Drijfhout, T.L. Edwards, N.R. Golledge, M. Hemer, R.E. Kopp, G. Krinner, A. Mix, D. Notz, S. Nowicki, I.S. Nurhati, L. Ruiz, J.-B. Sallée, A.B.A. Slangen, and Y. Yu, 2021: [https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter09.pdf Chapter 9: Ocean, Cryosphere and Sea Level Change]. In [https://www.ipcc.ch/report/ar6/wg1/ Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L.&nbsp;Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, New York, USA, pages 1211–1362, doi:10.1017/9781009157896.011.</ref>{{rp|1228}} The temperatures over land are rising faster than [[Ocean temperature|ocean temperatures]]. This is because the [[Ocean heat content|ocean absorbs]] about 90% of [[Earth's energy budget|excess heat]] generated by [[climate change]].<ref name="ocean heat 92">{{cite web |title=The Oceans Are Heating Up Faster Than Expected |url=https://www.scientificamerican.com/article/the-oceans-are-heating-up-faster-than-expected/ |access-date=3 March 2020 |publisher=scientific american}}</ref>

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==Definitions==
{{See also|Ocean temperature}}
[[File:SST 20131220 blended Global.png|thumb|upright=1.2|Global map of sea surface temperature, showing warmer areas around the equator and colder areas around the poles (20 December 2013 at 1-km resolution). ]]
Sea surface temperature (SST), or ocean surface temperature, is the water [[temperature]] close to the [[ocean]]'s surface. The exact meaning of ''surface'' varies according to the measurement method used, but it is between {{convert|1|mm|in|sigfig=1}} and {{convert|20|m|ft|sigfig=1}} below the [[sea]] surface. 

For comparison, the [[sea surface skin temperature]] relates to the top 20 or so [[micrometres]] of the ocean's surface.

The definition proposed by [[Intergovernmental Panel on Climate Change|Intergovernmental Panel on Climate Change (IPCC)]] for ''sea surface temperature'' does not specify the number of metres but focuses more on measurement techniques: Sea surface temperature is "the subsurface bulk temperature in the top few metres of the ocean, measured by ships, buoys and drifters. [...] Satellite measurements of skin temperature (uppermost layer; a micrometre thick) in the infrared or the top centimetre or so in the microwave are also used, but must be adjusted to be compatible with the bulk temperature."<ref>IPCC, 2021: [https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_AnnexVII.pdf Annex VII: Glossary] [Matthews, J.B.R., V. Möller, R. van Diemen, J.S. Fuglestvedt, V. Masson-Delmotte, C. Méndez, S. Semenov, A. Reisinger (eds.)]. In [https://www.ipcc.ch/report/ar6/wg1/ Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 2215–2256, doi:10.1017/9781009157896.022.</ref>{{rp|2248}} 

The temperature further below that is called ''ocean temperature'' or ''deeper ocean temperature''. [[Ocean temperature|Ocean temperatures]] (more than 20 metres below the surface) also vary by region and time, and they contribute to variations in [[ocean heat content]] and [[ocean stratification]].<ref name="AR6_WG1_Chapter9" /> The increase of both ocean surface temperature and deeper ocean temperature is an important [[Effects of climate change on oceans|effect of climate change on oceans]].<ref name="AR6_WG1_Chapter9" />

=== Extent of "surface" ===
{{See also|Ocean stratification|Photic zone}}
The extent of the ''ocean surface'' down into the ocean is influenced by the amount of mixing that takes place between the surface water and the deeper water. This depends on the temperature: in the tropics the warm surface layer of about 100&nbsp;m is quite stable and does not mix much with deeper water, while near the [[Polar regions of Earth|poles]] winter cooling and storms makes the surface layer denser and it mixes to great depth and then [[Ocean stratification|stratifies]] again in summer. This is why there is no simple single depth for ''ocean surface''. The [[Photic zone|photic depth of the ocean]] is typically about 100 m and is related to this heated surface layer. It can be up to around 200 m deep in the [[open ocean]].<ref name=":11">{{Cite book |last1=Emerson |first1=Steven |url=https://www.cambridge.org/core/product/identifier/9780511793202/type/book |title=Chemical Oceanography and the Marine Carbon Cycle |last2=Hedges |first2=John |date=2008-04-24 |publisher=Cambridge University Press |isbn=978-0-521-83313-4 |edition=1 |chapter=Chapter 4: Carbonate chemistry |doi=10.1017/cbo9780511793202}}</ref><ref name=":5">{{Cite book |last1=Chester |first1=R. |url=https://www.wiley.com/en-us/Marine+Geochemistry%2C+3rd+Edition-p-9781118349090 |title=Marine geochemistry |last2=Jickells |first2=Tim |date=2012 |publisher=Wiley/Blackwell |isbn=978-1-118-34909-0 |edition=3rd |location=Chichester, West Sussex, UK |chapter=Chapter 9: Nutrients, oxygen, organic carbon and the carbon cycle in seawater |oclc=781078031}}</ref>

==Variations and changes==
[[File:ECCO2 Sea Surface Temperature and Flows.ogv|thumb|Sea surface temperature and flows]]
===Local variations===
{{See also|Upwelling}}
The sea surface temperature (SST) has a [[Diurnal cycle|diurnal range]], just like the Earth's atmosphere above, though to a lesser degree due to its greater [[thermal inertia]].<ref>{{cite book|url=https://books.google.com/books?id=WdPg_1aTtr8C&pg=PA84|page=84|author=John Siegenthaler|title=Modern hydronic heating for residential and light commercial buildings|year=2003|publisher=Cengage Learning|isbn=978-0-7668-1637-4}}</ref> On calm days, the temperature can vary by {{convert|6|C-change|F-change|sigfig=1}}.<ref name="space"/> The temperature of the ocean at depth lags the Earth's atmosphere temperature by 15 days per {{convert|10|m|ft}}, which means for locations like the [[Aral Sea]], temperatures near its bottom reach a maximum in December and a minimum in May and June.<ref>{{cite book|url=https://books.google.com/books?id=Z-LxfJVFclgC&pg=PA27|title=Physical oceanography of the dying Aral Sea|author=Peter O. Zavialov|page=27|year=2005|isbn=978-3-540-22891-2|publisher=シュプリンガー・ジャパン株式会社}}</ref> Near the coastline, some offshore and longshore winds move the warm waters near the surface offshore, and replace them with cooler water from below in the process known as [[Ekman transport]]. This pattern generally increases nutrients for marine life in the region, and can have a profound effect in some regions where the bottom waters are particularly nutrient-rich.<ref>{{cite web | title =Envisat watches for La Niña | publisher =BNSC via the Internet Wayback Machine | date =2008-04-24 | url =http://www.bnsc.gov.uk/content.aspx?nid=5989 | access-date =2011-01-09 |archive-url = https://web.archive.org/web/20080424113710/http://www.bnsc.gov.uk/content.aspx?nid=5989 |archive-date = 2008-04-24}}</ref> Offshore of [[river delta]]s, freshwater flows over the top of the denser seawater, which allows it to heat faster due to limited vertical mixing.<ref>{{cite book|url=https://books.google.com/books?id=OeZ4e5MwRigC&pg=PA258|page=258|title=State and evolution of the Baltic Sea, 1952–2005: a detailed 50-year survey of meteorology and climate, physics, chemistry, biology, and marine environment|year=2008|publisher=John Wiley and Sons|isbn=978-0-471-97968-5|author1=Rainer Feistel |author2=Günther Nausch |author3=Norbert Wasmund}}</ref>  Remotely sensed SST can be used to detect the surface temperature signature due to [[tropical cyclone]]s. In general, an SST cooling is observed after the passing of a hurricane, primarily as the result of mixed layer deepening and surface heat losses.<ref name="NASA Cooling">{{cite web|author=Earth Observatory |url=http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17164 |title=Passing of Hurricanes Cools Entire Gulf |year=2005 |access-date=2006-04-26 |publisher=[[NASA|National Aeronautics and Space Administration]] |url-status=dead |archive-url=https://web.archive.org/web/20060930235454/http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17164 |archive-date=2006-09-30}}</ref> In the wake of several day long [[Mineral dust#Saharan dust|Saharan dust]] outbreaks across the adjacent northern Atlantic Ocean, sea surface temperatures are reduced 0.2 C to 0.4&nbsp;C (0.3 to 0.7&nbsp;F).<ref>{{cite book|url=https://books.google.com/books?id=b09YTtIW3f4C&pg=PA72|title=The Impact of Saharan Dust on the North Atlantic Circulation|author=Nidia Martínez Avellaneda|page=72|year=2010|publisher=GRIN Verlag|isbn=978-3-640-55639-7}}</ref> Other sources of short-term SST fluctuation include [[extratropical cyclone]]s, rapid influxes of [[glacier|glacial]] fresh water<ref>{{cite journal|last=Boyle|first=Edward A.|author2=Lloyd Keigwin |title=North Atlantic thermohaline circulation during the past 20,000 years linked to high-latitude surface temperature|journal=Nature|date=5 November 1987|volume=330|issue=6143|pages=35–40|url=http://www.whoi.edu/cms/files/Boyle(1987)Nature330_35_52423.pdf|access-date=10 February 2011|doi=10.1038/330035a0|bibcode = 1987Natur.330...35B |s2cid=4359752}}</ref> and concentrated [[phytoplankton]] blooms<ref>{{cite journal|last=Beaugrand|first=Grégory|author2=Keith M. Brander |author3=J. Alistair Lindley |author4=Sami Souissi |author5=Philip C. Reid |title=Plankton effect on cod recruitment in the North Sea|journal=Nature|date=11 December 2003|volume=426|pages=661–664|doi=10.1038/nature02164|issue=6967|pmid=14668864|bibcode = 2003Natur.426..661B |s2cid=4420759}}</ref> due to seasonal cycles or agricultural run-off.<ref>{{cite journal|last=Beman|first=J. Michael |author2=Kevin R. Arrigo |author3=Pamela A. Matson|title=Agricultural runoff fuels large phytoplankton blooms in vulnerable areas of the ocean|journal=Nature|date=10 March 2005|volume=434|pages=211–214|doi=10.1038/nature03370|issue=7030|pmid=15758999|bibcode = 2005Natur.434..211M |s2cid=2299664}}</ref>{{clarify|What effects do algal blooms have on SST?|date=January 2022}}

The tropical ocean has been warming faster than other regions since 1950, with the greatest rates of warming in the tropical Indian Ocean, western Pacific Ocean, and western boundary currents of the [[Subtropical gyre|subtropical gyres]].<ref name="AR6_WG1_Chapter9" /> However, the eastern Pacific Ocean, subtropical North Atlantic Ocean, and Southern Ocean have warmed more slowly than the global average or have experienced cooling since the 1950s.<ref name="AR6_WG1_Chapter9" />

====Atlantic Multidecadal Oscillation====
[[Ocean current]]s, such as the [[Atlantic multidecadal oscillation|Atlantic Multidecadal Oscillation]], can affect sea surface temperatures over several decades.<ref>{{Cite journal |last1=McCarthy |first1=Gerard D. |last2=Haigh |first2=Ivan D. |last3=Hirschi |first3=Joël J.-M. |last4=Grist |first4=Jeremy P. |last5=Smeed |first5=David A. |date=2015-05-28 |title=Ocean impact on decadal Atlantic climate variability revealed by sea-level observations |url=http://mural.maynoothuniversity.ie/12187/1/McCarthy_Ocean_2015.pdf |journal=Nature |volume=521 |issue=7553 |pages=508–510 |bibcode=2015Natur.521..508M |doi=10.1038/nature14491 |issn=1476-4687 |pmid=26017453 |s2cid=4399436}}</ref> The Atlantic Multidecadal Oscillation (AMO) is an important driver of North Atlantic SST and Northern Hemisphere climate, but the mechanisms controlling AMO variability remain poorly understood.<ref>{{Cite journal |last1=Knudsen |first1=Mads Faurschou |last2=Jacobsen |first2=Bo Holm |last3=Seidenkrantz |first3=Marit-Solveig |last4=Olsen |first4=Jesper |date=2014-02-25 |title=Evidence for external forcing of the Atlantic Multidecadal Oscillation since termination of the Little Ice Age |journal=Nature Communications |volume=5 |pages=3323 |bibcode=2014NatCo...5.3323K |doi=10.1038/ncomms4323 |issn=2041-1723 |pmc=3948066 |pmid=24567051}}</ref> Atmospheric internal variability, changes in ocean circulation, or anthropogenic drivers may control the multidecadal temperature variability associated with AMO.<ref>{{Cite journal |last1=Wills |first1=R.C. |last2=Armour |first2=K.C. |last3=Battisti |first3=D.S. |last4=Hartmann |first4=D.L. |date=2019 |title=Ocean–atmosphere dynamical coupling fundamental to the Atlantic multidecadal oscillation |journal=Journal of Climate |volume=32 |issue=1 |pages=251–272|doi=10.1175/JCLI-D-18-0269.1 |bibcode=2019JCli...32..251W |s2cid=85450306 |doi-access=free}}</ref> These changes in North Atlantic SST may influence winds in the subtropical North Pacific and produce warmer SSTs in the western Pacific Ocean.<ref>{{Cite journal |last1=Wu |first1=Baolan |last2=Lin |first2=Xiaopei |last3=Yu |first3=Lisan |date=17 February 2020 |title=North Pacific subtropical mode water is controlled by the Atlantic Multidecadal Variability |url=https://www.nature.com/articles/s41558-020-0692-5 |journal=Nature Climate Change |language=en |volume=10 |issue=3 |pages=238–243 |doi=10.1038/s41558-020-0692-5 |bibcode=2020NatCC..10..238W |s2cid=211138572 |issn=1758-6798|url-access=subscription }}</ref>[[File:Weeklysst.gif|thumb|right|Weekly average sea surface temperature in the ocean during the first week of February 2011, during a period of [[El Niño-Southern Oscillation#Effects of ENSO's cool phase (La Niña)|La Niña]].]]

===Regional variations===
[[File:1997 El Nino TOPEX.jpg|thumb|200px|right|The 1997 El Niño observed by [[TOPEX/Poseidon]]. The white areas off the tropical coasts of South and North America indicate the pool of warm water.<ref>{{cite web | url = http://www.jpl.nasa.gov/news/releases/97/elninoup.html | title =Independent NASA Satellite Measurements Confirm El Niño is Back and Strong | publisher = NASA/JPL}}</ref>]]
{{Main|El Niño-Southern Oscillation}}
El Niño is defined by prolonged differences in Pacific Ocean surface temperatures when compared with the average value. The accepted definition is a warming or cooling of at least 0.5&nbsp;°C (0.9&nbsp;°F) averaged over the east-central tropical Pacific Ocean. Typically, this anomaly happens at irregular intervals of 2–7 years and lasts nine months to two years.<ref>{{cite web|url=http://www.cpc.noaa.gov/products/analysis_monitoring/ensostuff/ensofaq.shtml#HOWOFTEN|title=ENSO FAQ: How often do El Niño and La Niña typically occur?|access-date=2009-07-26|date=2005-12-19|author=Climate Prediction Center|publisher=[[National Centers for Environmental Prediction]]|author-link=Climate Prediction Center|archive-url=https://web.archive.org/web/20090827143632/http://www.cpc.noaa.gov/products/analysis_monitoring/ensostuff/ensofaq.shtml#HOWOFTEN|archive-date=2009-08-27|url-status=dead}}</ref> The average period length is 5 years. When this warming or cooling occurs for only seven to nine months, it is classified as El Niño/La Niña "conditions"; when it occurs for more than that period, it is classified as El Niño/La Niña "episodes".<ref>{{cite web|url=http://www.ncdc.noaa.gov/oa/climate/research/enso/?year=2009&month=6&submitted=true|title=El Niño / Southern Oscillation (ENSO) June 2009|author=National Climatic Data Center|publisher=National Oceanic and Atmospheric Administration|date=June 2009|access-date=2009-07-26|author-link=National Climatic Data Center}}</ref>

The sign of an El Niño in the sea surface temperature pattern is when warm water spreads from the west Pacific and the [[Indian Ocean]] to the east Pacific. It takes the rain with it, causing extensive drought in the western Pacific and rainfall in the normally dry eastern Pacific. El Niño's warm rush of nutrient-poor tropical water, heated by its eastward passage in the Equatorial Current, replaces the cold, nutrient-rich surface water of the [[Humboldt Current]]. When El Niño conditions last for many months, extensive [[ocean warming]] and the reduction in Easterly Trade winds limits upwelling of cold nutrient-rich deep water and its economic impact to local fishing for an international market can be serious.<ref name="deadfish">{{cite web|url=http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/eln/home.rxml|title=El Niño|date=1998-04-28|access-date=2009-07-17|author=WW2010|publisher=University of Illinois at Urbana-Champaign}}</ref>

Among scientists, there is medium confidence that the tropical Pacific will transition to a mean pattern resembling that of El Niño on centennial time scale, but there is still high uncertainty in tropical Pacific SST projections because it is difficult to capture El Niño variability in climate models.<ref name="AR6_WG1_Chapter9" />[[File:Land vs Ocean Temperature.svg|thumb|upright=1.35|right|Surface air temperatures over land masses have been increasing faster than the sea surface temperature.<ref>Data from [https://web.archive.org/web/20200416074510/https://data.giss.nasa.gov/gistemp/graphs_v4/ NASA GISS].</ref>]]

=== Recent increase due to climate change ===
[[File:1880- Global average sea surface temperature - global warming.svg|thumb|upright=1.2|The global average sea surface temperature has been increasing since around 1900 (graph showing annual average and 5-year smoothed average, relative to the average value for the years 1951-1980).]]
{{Further|Effects of climate change on oceans#Rising ocean temperature}}
Overall, scientists project that all regions of the oceans will warm by 2050, but models disagree for SST changes expected in the subpolar North Atlantic, the equatorial Pacific, and the Southern Ocean.<ref name="AR6_WG1_Chapter9" /> The future global mean SST increase for the period 1995-2014 to 2081-2100 is 0.86&nbsp;°C under the most modest greenhouse gas emissions scenarios, and up to 2.89&nbsp;°C under the most severe emissions scenarios.<ref name="AR6_WG1_Chapter9" />

A study published in 2025 in ''[[Environmental Research Letters]]'' reported that global mean sea surface temperature increases had more than quadrupled, from 0.06{{nbsp}}K per decade during 1985–89 to 0.27{{nbsp}}K per decade for 2019–23.<ref name=EnvRschLtrs_20250128/> The researchers projected that the increase inferred over the past 40 years would likely be exceeded within the next 20 years.<ref name=EnvRschLtrs_20250128>{{cite journal |last1=Merchant |first1=Christopher J. |last2=Allan |first2=Richard P. |last3=Embury |first3=Owen |title=Quantifying the acceleration of multidecadal global sea surface warming driven by Earth's energy imbalance |journal=Environmental Research Letters |date=28 January 2025 |volume=20 |issue=2 |page=024037 |doi=10.1088/1748-9326/adaa8a|doi-access=free |bibcode=2025ERL....20b4037M }}</ref>

==Measurement==
[[File:MODIS and AIRS SST comp fig2.i.jpg|right|thumb|Temperature profile of the surface layer of the ocean (a) at night and (b) during the day]]
There are a variety of techniques for measuring this parameter that can potentially yield different results because different things are actually being measured. Away from the immediate sea surface, general temperature measurements are accompanied by a reference to the specific depth of measurement. This is because of significant differences encountered between measurements made at different depths, especially during the daytime when low wind speed and high sunshine conditions may lead to the formation of a warm layer at the ocean's surface and strong vertical temperature gradients (a diurnal [[thermocline]]).<ref name="space"/> Sea surface temperature measurements are confined to the top portion of the ocean, known as the near-surface layer.<ref>{{cite book|url=https://books.google.com/books?id=tZary8a4HMwC|page=xi|author1=Alexander Soloviev |author2=Roger Lukas |title=The near-surface layer of the ocean: structure, dynamics and applications|journal=The Near-Surface Layer of the Ocean: Structure|isbn=978-1-4020-4052-8|year=2006|publisher=シュプリンガー・ジャパン株式会社|bibcode=2006nslo.book.....S}}</ref>

===Thermometers===
The sea surface temperature was one of the first oceanographic variables to be measured. [[Benjamin Franklin]] suspended a [[mercury thermometer]] from a ship while travelling between the United States and Europe in his survey of the [[Gulf Stream]] in the late eighteenth century. SST was later measured by dipping a [[thermometer]] into a bucket of water that was manually drawn from the sea surface. The first automated technique for determining SST was accomplished by measuring the temperature of water in the intake port of large ships, which was underway by 1963. These observations have a warm bias of around {{convert|0.6|C-change|F-change|sigfig=1}} due to the heat of the engine room.<ref>{{cite book|url=https://books.google.com/books?id=A6ew-bJDIDIC&pg=PA24|pages=24–25|title=Data analysis methods in physical oceanography|author1=William J. Emery |author2=Richard E. Thomson |year=2001|isbn=978-0-444-50757-0|publisher=Elsevier|edition=2nd Revised}}</ref>  

Fixed [[weather buoy]]s measure the water temperature at a depth of {{convert|3|m|ft}}. Measurements of SST have had inconsistencies over the last 130 years due to the way they were taken. In the nineteenth century, measurements were taken in a bucket off a ship. However, there was a slight variation in temperature because of the differences in buckets. Samples were collected in either a wood or an uninsulated canvas bucket, but the canvas bucket cooled quicker than the wood bucket. The sudden change in temperature between 1940 and 1941 was the result of an undocumented change in procedure. The samples were taken near the engine intake because it was too dangerous to use lights to take measurements over the side of the ship at night.<ref>{{cite book|last=Burroughs|first=William James|title=Climate change : a multidisciplinary approach|url=https://archive.org/details/climatechangemul0000burr_p9v1|url-access=registration|year=2007|publisher=Cambridge Univiversity Press|location=Cambridge [u.a.]|isbn=9780521690331|edition=2.}}</ref> 

Many different drifting buoys exist around the world that vary in design, and the location of reliable temperature sensors varies. These measurements are beamed to satellites for automated and immediate data distribution.<ref name="buoy">{{cite book|url=https://books.google.com/books?id=hH2NkL_318wC&pg=PA263|pages=237–238|title=Oceanography from Space: Revisited|author=Vittorio Barale|year=2010|isbn=978-90-481-8680-8|publisher=Springer}}</ref> A large network of coastal buoys in U.S. waters is maintained by the [[National Data Buoy Center]] (NDBC).<ref>{{cite book|page=[https://archive.org/details/meteorologicalbu0000unse/page/11 11]|title=The meteorological buoy and coastal marine automated network for the United States|author=Lance F. Bosart, William A. Sprigg, National Research Council|publisher=National Academies Press|year=1998|isbn=978-0-309-06088-2|url=https://archive.org/details/meteorologicalbu0000unse/page/11}}</ref> Between 1985 and 1994, an extensive array of moored and drifting buoys was deployed across the equatorial Pacific Ocean designed to help monitor and predict the [[El Niño-Southern Oscillation#Effects of ENSO's warm phase (El Niño)|El Niño]] phenomenon.<ref>{{cite book|url=https://books.google.com/books?id=DO5K1NK_ZewC&pg=PA62|title=Global energy and water cycles|author1=K. A. Browning |author2=Robert J. Gurney |page=62|year=1999|publisher=[[Cambridge University Press]]|isbn=978-0-521-56057-3}}</ref>

===Weather satellites===
{{See also|Weather satellite|Satellite temperature measurement}}
[[File:MODIS sst.png|thumb|2003–2011 SST based on [[Moderate-Resolution Imaging Spectroradiometer|MODIS]] Aqua data]]
Weather satellites have been available to determine sea surface temperature information since 1967, with the first global composites created during 1970.<ref>{{cite journal|url=http://docs.lib.noaa.gov/rescue/mwr/100/mwr-100-01-0010.pdf|title=Global Sea-Surface Temperature Distribution Determined From an Environmental Satellite|author=P. Krishna Rao, W. L. Smith, and R. Koffler|pages=10–14|journal=[[Monthly Weather Review]]|volume=100|date=January 1972|access-date=2011-01-09|issue=1|doi=10.1175/1520-0493(1972)100<0010:GSTDDF>2.3.CO;2|bibcode = 1972MWRv..100...10K}}</ref> Since 1982,<ref>{{cite book|url=https://books.google.com/books?id=qzYrAAAAYAAJ&pg=PA2|page=2|author=National Research Council (U.S.). NII 2000 Steering Committee|title=The unpredictable certainty: information infrastructure through 2000; white papers|publisher=National Academies|year=1997|isbn=9780309060363}}</ref> [[satellite]]s have been increasingly utilized to measure SST and have allowed its [[Spatial variability|spatial]] and [[time|temporal]] variation to be viewed more fully. [[Satellite temperature measurement|Satellite measurements of SST]] are in reasonable agreement with [[in situ]] temperature measurements.<ref>{{cite journal|journal=[[Journal of Geophysical Research]]|date=2001-02-15|volume=106|author1=W. J. Emery|author2=D. J. Baldwin|author3=Peter Schlüssel|author4=R. W. Reynolds|name-list-style=amp|title=Accuracy of in situ sea surface temperatures used to calibrate infrared satellite measurements|page=2387|issue=C2|bibcode=2001JGR...106.2387E|doi=10.1029/2000JC000246|doi-access=free}}</ref>  The satellite measurement is made by sensing the ocean [[radiation]] in two or more wavelengths within the [[infrared]] part of the [[electromagnetic spectrum]] or other parts of the spectrum which can then be empirically related to SST.<ref name="John">{{cite web|url=http://www2.hawaii.edu/~jmaurer/sst/|title=Infrared and microwave remote sensing of sea surface temperature (SST)|author=John Maurer|date=October 2002|publisher=[[University of Hawai{{okina}}i]]|access-date=2011-01-09}}</ref> These wavelengths are chosen because they are:

# within the peak of the [[blackbody radiation]] expected from the Earth,<ref>{{cite journal|url=http://www.wamis.org/agm/pubs/agm8/Paper-4.pdf|page=73|journal=Satellite Remote Sensing and GIS Applications in Agricultural Meteorology|title=Meteorological Satellites|author=C. M. Kishtawal|date=2005-08-06|access-date=2011-01-27|archive-date=2020-02-15|archive-url=https://web.archive.org/web/20200215044026/http://www.wamis.org/agm/pubs/agm8/Paper-4.pdf|url-status=dead}}</ref> and
# able to transmit adequately well through the [[Earth's atmosphere|atmosphere]]<ref>{{cite journal|journal=New Scientist|date=1971-09-16|title=Mapping the Atmosphere From Space|author= Robert Harwood|page=623|volume=51|issue=769}}</ref>

The satellite-measured SST provides both a [[synoptic scale|synoptic view]] of the ocean and a high frequency of repeat views,<ref>{{cite book|page=510|url=https://books.google.com/books?id=Y0iX2z48qkUC&pg=PA509|author1=David E. Alexander |author2=Rhodes Whitmore Fairbridge |title=Encyclopedia of environmental science|year=1999|publisher=Springer|isbn=978-0-412-74050-3}}</ref> allowing the examination of basin-wide upper [[ocean]] dynamics not possible with ships or buoys. <span class="plainlinks">[http://www.nasa.gov/ NASA's]</span> (National Aeronautic and Space Administration) <span class="plainlinks">[http://modis.gsfc.nasa.gov/ Moderate Resolution Imaging Spectroradiometer (MODIS)]</span> SST satellites have been providing global SST data since 2000, available with a one-day lag. NOAA's <span class="plainlinks">[http://www.goes.noaa.gov/ GOES (Geostationary Orbiting Earth Satellites)] {{Webarchive|url=https://web.archive.org/web/20200817111624/http://www.goes.noaa.gov/ |date=2020-08-17 }}</span> satellites are [[geostationary orbit|geo-stationary]] above the Western Hemisphere which enables them to deliver SST data on an hourly basis with only a few hours of lag time.

There are several difficulties with satellite-based absolute SST measurements. First, in infrared remote sensing methodology the radiation emanates from the [[Sea surface microlayer|top "skin" of the ocean]], approximately the top 0.01 [[millimetre|mm]] or less, which may not represent the [[bulk temperature]] of the upper meter of ocean due primarily to effects of solar surface heating during the daytime, reflected radiation, as well as sensible heat loss and surface evaporation. All these factors make it somewhat difficult to compare satellite data to measurements from buoys or shipboard methods, complicating ground truth <!-- clarify -->efforts.<ref>{{cite book|url=https://books.google.com/books?id=jk1fIo51uwMC&pg=PA278|page=279|author=Ian Stuart Robinson|title=Measuring the oceans from space: the principles and methods of satellite oceanography|publisher=Springer|year=2004|isbn=978-3-540-42647-9}}</ref>  Secondly, the satellite cannot look through clouds, creating a cool bias in satellite-derived SSTs within cloudy areas.<ref name="space"/>  However, passive microwave techniques can accurately measure SST and penetrate cloud cover.<ref name="John"/>  Within atmospheric sounder channels on [[weather satellite]]s, which peak just above the ocean's surface, knowledge of the sea surface temperature is important to their calibration.<ref name="space"/>

==Importance to the Earth's atmosphere==
[[File:Snow Clouds in Korea.jpg|thumb|right|Sea-effect snow bands near the [[Korean Peninsula]]]]
{{See also|Air mass|Numerical weather prediction|Precipitation (meteorology)|Effects of climate change on oceans}}

Sea surface temperature affects the behavior of the [[Earth's atmosphere]] above, so their initialization into [[atmospheric model]]s is important. While sea surface temperature is important for [[tropical cyclogenesis]], it is also important in determining the formation of sea fog and sea breezes.<ref name="space">{{cite book|url=https://books.google.com/books?id=hH2NkL_318wC&pg=PA263|page=263|title=Oceanography from Space: Revisited|author=Vittorio Barale|year=2010|isbn=978-90-481-8680-8|publisher=Springer}}</ref> Heat from underlying warmer waters can significantly modify an air mass over distances as short as {{convert|35|km|mi}} to {{convert|40|km|mi}}.<ref>{{cite journal|author=Jun Inoue, Masayuki Kawashima, Yasushi Fujiyoshi and Masaaki Wakatsuchi|title=Aircraft Observations of Air-mass Modification Over the Sea of Okhotsk during Sea-ice Growth|issue=1|date=October 2005|doi=10.1007/s10546-004-3407-y|pages=111–129|issn=0006-8314|volume=117| journal=Boundary-Layer Meteorology|bibcode = 2005BoLMe.117..111I |s2cid=121768400}}</ref>  For example, southwest of Northern Hemisphere [[extratropical cyclone]]s, curved cyclonic flow bringing cold air across relatively warm water bodies can lead to narrow [[lake-effect snow]] (or sea effect) bands. Those bands bring strong localized [[precipitation (meteorology)|precipitation]], often in the form of [[snow]], since large water bodies such as lakes efficiently store heat that results in significant temperature differences—larger than {{convert|13|C-change|F-change|sigfig=2}}—between the water surface and the air above.<ref>{{cite news|author=B. Geerts|year=1998|url=http://www-das.uwyo.edu/~geerts/cwx/notes/chap10/lake_effect_snow.html|title=Lake Effect Snow.|access-date=2008-12-24|publisher=[[University of Wyoming]]|archive-date=2020-11-06|archive-url=https://web.archive.org/web/20201106092611/http://www-das.uwyo.edu/~geerts/cwx/notes/chap10/lake_effect_snow.html|url-status=dead}}</ref> Because of this temperature difference, warmth and moisture are transported upward, condensing into vertically oriented clouds which produce snow showers. The temperature decrease with height and cloud depth are directly affected by both the water temperature and the large-scale environment. The stronger the temperature decrease with height, the taller the clouds get, and the greater the precipitation rate becomes.<ref>{{cite web|url=http://www.comet.ucar.edu/class/smfaculty/byrd/sld010.htm |publisher=[[University Corporation for Atmospheric Research]] |title=Lake Effect Snow |date=1998-06-03 |access-date=2009-07-12 |author=Greg Byrd |url-status=dead |archive-url=https://web.archive.org/web/20090617013142/http://www.comet.ucar.edu/class/smfaculty/byrd/sld010.htm |archive-date=2009-06-17}}</ref>

===Tropical cyclones===
[[File:WorldwideTCpeaks.gif|thumb|right|Seasonal peaks of tropical cyclone activity worldwide]]
[[File:Mean sst equatorial pacific.gif|thumb|right|200 px|Average equatorial Pacific temperatures]]
{{Main|Tropical cyclogenesis|Tropical cyclones and climate change}}
Ocean temperature of at least 26.5[[degrees Celsius|°C]] (79.7[[degrees Fahrenheit|°F]]) spanning through at minimum a 50-[[metre]] depth is one of the precursors needed to maintain a [[tropical cyclone]] (a type of [[mesocyclone]]).<ref name="A15">{{cite web|author=Chris Landsea|url=http://www.aoml.noaa.gov/hrd/tcfaq/A15.html|title=Subject: A15) How do tropical cyclones form?|access-date=2011-01-27|year=2011|publisher=[[Hurricane Research Division]]|author-link=Chris Landsea}}</ref><ref name=SST>{{cite journal|last=Webster|first=PJ|title=Changes in tropical cyclone number, duration, and intensity in a warming environment|journal=Science|volume=309|issue=5742|pages=1844–6|url=http://go.galegroup.com/ps/i.do?id=GALE%7CA136847986&v=2.1&it=r&p=PROF&sw=w&asid=335992f2037df3339c5901f7981d2045|publisher=Gale Group|pmid=16166514|year=2005|doi=10.1126/science.1116448|bibcode=2005Sci...309.1844W|doi-access=free|url-access=subscription}}</ref>  These warm waters are needed to maintain the [[Tropical cyclone#Mechanics|warm core]] that fuels tropical systems. This value is well above 16.1&nbsp;°C (60.9&nbsp;°F), the long term global average surface temperature of the oceans.<ref name="SSTMEAN">{{Cite web| author = Matt Menne | publisher = [[National Climatic Data Center]] | url = http://www.ncdc.noaa.gov/cmb-faq/anomalies.php#mean | title = Global Long-term Mean Land and Sea Surface Temperatures | date = March 15, 2000 | access-date = 2006-10-19}}</ref> However, this requirement can be considered only a general baseline because it assumes that the ambient atmospheric environment surrounding an area of disturbed weather presents average conditions. Tropical cyclones have intensified when SSTs were slightly below this standard temperature.

Tropical cyclones are known to form even when normal conditions are not met. For example, cooler air temperatures at a higher altitude (e.g., at the 500&nbsp;[[hPa]] level, or 5.9&nbsp;km) can lead to tropical cyclogenesis at lower water temperatures, as a certain [[lapse rate]] is required to force the atmosphere to be [[Baroclinic instability|unstable]] enough for convection. In a moist atmosphere, this lapse rate is 6.5&nbsp;°C/km, while in an atmosphere with less than 100% [[relative humidity]], the required lapse rate is 9.8&nbsp;°C/km.<ref name="TCS EESC">{{cite web|year=2000|last=Kushnir|first=Yochanan|title=The Climate System|url=http://eesc.columbia.edu/courses/ees/climate/lectures/atm_phys.html|publisher=[[Columbia University]]|access-date=24 September 2010|archive-date=20 May 2020|archive-url=https://web.archive.org/web/20200520171925/https://eesc.columbia.edu/courses/ees/climate/lectures/atm_phys.html|url-status=dead}}</ref>

At the 500&nbsp;hPa level, the air temperature averages −7&nbsp;°C (18&nbsp;°F) within the tropics, but air in the tropics is normally dry at this height, giving the air room to [[Wet-bulb temperature|wet-bulb]], or cool as it moistens, to a more favorable temperature that can then support convection. A [[wet-bulb temperature]] at 500&nbsp;hPa in a tropical atmosphere of {{convert|-13.2|C|F}} is required to initiate convection if the water temperature is {{convert|26.5|C|F}}, and this temperature requirement increases or decreases proportionally by 1&nbsp;°C in the sea surface temperature for each 1&nbsp;°C change at 500&nbsp;hpa.
Inside a [[cold-core low|cold cyclone]], 500&nbsp;hPa temperatures can fall as low as {{convert|-30|C|F}}, which can initiate convection even in the driest atmospheres. This also explains why moisture in the mid-levels of the [[troposphere]], roughly at the 500&nbsp;hPa level, is normally a requirement for development. However, when dry air is found at the same height, temperatures at 500&nbsp;hPa need to be even colder as dry atmospheres require a greater lapse rate for instability than moist atmospheres.<ref>{{cite book|title=Atmospheric Science: An Introductory Survey|author1=John M. Wallace  |author2=Peter V. Hobbs |name-list-style=amp |pages=76–77|publisher=Academic Press, Inc|year=1977}}</ref><ref name="LANDSEACLI">{{Cite web| author = Chris Landsea | url = http://www.aoml.noaa.gov/hrd/Landsea/climvari/index.html | title = Climate Variability of Tropical Cyclones: Past, Present and Future | year = 2000 | work = Storms | pages = 220–41 | access-date = 2006-10-19 | publisher = [[Atlantic Oceanographic and Meteorological Laboratory]]| author-link = Chris Landsea}}</ref> At heights near the [[tropopause]], the 30-year average temperature (as measured in the period encompassing 1961 through 1990) was −77&nbsp;°C (−132&nbsp;°F).<ref name="UAIRtropics">{{Cite journal| author = Dian J. Gaffen-Seidel, Rebecca J. Ross and James K. Angell | title = Climatological characteristics of the tropical tropopause as revealed by radiosondes | journal = Journal of Geophysical Research | volume = 106 | issue = D8 | pages = 7857–7878 | url = http://www.aero.jussieu.fr/~sparc/SPARC2000_new/OralSess2/D_Gaffen/GaffenHtml/Abs_Gaffen.html |date = November 2000| access-date = 2006-10-19 |archive-url = https://web.archive.org/web/20060508184913/http://www.aero.jussieu.fr/~sparc/SPARC2000_new/OralSess2/D_Gaffen/GaffenHtml/Abs_Gaffen.html |archive-date = May 8, 2006| bibcode = 2001JGR...106.7857S | doi = 10.1029/2000JD900837 | doi-access = free}}</ref> One example of a [[tropical cyclone]] maintaining itself over cooler waters was [[Hurricane Epsilon (2005)|Epsilon]] late in the [[2005 Atlantic hurricane season]].<ref>{{cite web|author=Lixion Avila|date=2005-12-03|title=Hurricane Epsilon Discussion Eighteen|publisher=National Hurricane Center|access-date=2010-12-14|url=http://www.nhc.noaa.gov/archive/2005/dis/al292005.discus.018.shtml?}}</ref>

==See also==
{{Portal|Oceans}}
*{{annotated link|El Niño–Southern Oscillation}}
*{{annotated link|Global surface temperature}}
*{{annotated link|Halocline}}
*{{annotated link|Instrumental temperature record}}
*{{annotated link|Marine heatwave}}
*{{annotated link|Ocean heat content}}
*{{annotated link|Pacific decadal oscillation}}
*{{annotated link|Sea level rise}}

==References==
{{Reflist|2}}

==External links==
*[https://www.star.nesdis.noaa.gov/socd/ov/?tab=settings&zoom=4&cLat=33.8555&cLon=-71.5078&crs=EPSG4326&b_o_layers=2_0_5_FFFFF&s1L=T&s1I=3&s1P=0&s1O=0.78&s1S=T&s1Lg=T&s1Va=F&s2L=F&s2I=0&s2P=0&s2O=0.75&s2S=F&s2Lg=F&s2Va=F&s1s2Sp=F&vS=T&v1L=F&v2L=T&fS=F&f1L=F&f1Ln=1.00&f1Gd=0.010&f1Va=2.00&hS=F&hL=F&hLg=F&hW=25&hP=1010&eeS=F&eqL=F&eqMM=3.00&voL=F&wfL=F&tgS=T NOAA OceanView Blended SST and animated Surface Currents]
*[https://earth.nullschool.net/#current/ocean/surface/currents/overlay=sea_surface_temp/winkel3 Global map of current sea surface temperatures]
*[https://earth.nullschool.net/#current/ocean/surface/currents/overlay=sea_surface_temp_anomaly/winkel3 Global map of current sea surface temperature anomalies]
*[http://www.star.nesdis.noaa.gov/sod/sst/squam SQUAM] {{Webarchive|url=https://web.archive.org/web/20160306055154/http://www.star.nesdis.noaa.gov/sod/sst/squam/ |date=2016-03-06 }}, SST Quality Monitor (A near real-time Global QC Tool for monitoring time-series stability & cross-platform consistency of satellite SST)
*[http://www.star.nesdis.noaa.gov/sod/sst/iquam iQuam] {{Webarchive|url=https://web.archive.org/web/20180623110254/https://www.star.nesdis.noaa.gov/sod/sst/iquam/ |date=2018-06-23 }}, in situ SST Quality Monitor (A near real-time quality control & monitoring system for in situ SST measured by ships and buoys)
*[http://www.star.nesdis.noaa.gov/sod/sst/micros MICROS] {{Webarchive|url=https://web.archive.org/web/20160305102039/http://www.star.nesdis.noaa.gov/sod/sst/micros/ |date=2016-03-05 }}, Monitoring of IR Clear-sky Radiances over Oceans for SST

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