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A marine heatwave (MHW) is a period of abnormally high temperatures relative to the average seasonal temperature in a particular region of a
sea The sea, connected as the world ocean or simply the ocean, is the body of salty water that covers approximately 71% of the Earth's surface. The word sea is also used to denote second-order sections of the sea, such as the Mediterranean Sea, ...
or
ocean The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Earth's water. An ocean can also refer to any of the large bodies of water into which the wor ...
. Marine heatwaves are caused by a variety of factors, including shorter term weather phenomena such as fronts, intraseasonal, annual, or decadal modes like El Niño events, and longer term changes like climate change. Marine heatwaves can lead to severe biodiversity changes such as sea star wasting disease, harmful algal blooms, and mass mortality of benthic communities. The
Intergovernmental Panel on Climate Change The Intergovernmental Panel on Climate Change (IPCC) is an intergovernmental body of the United Nations. Its job is to advance scientific knowledge about climate change caused by human activities. The World Meteorological Organization (WMO) ...
(IPCC)
Special Report on the Ocean and Cryosphere in a Changing Climate The United Nations' Intergovernmental Panel on Climate Change's (IPCC) Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC) is a report about the effects of climate change on the world's seas, sea ice, icecaps and glaciers. ...
(SROCC) finds that it is "virtually certain" that the global ocean has absorbed more than 90% of the excess heat in our climate systems, the rate of ocean warming has doubled, and MHW events have doubled in frequency since 1982. The IPCC Sixth Assessment Report supports these findings, while also detailing new evidence that MHWs have become longer and more intense. Major marine heatwave events such as Great Barrier Reef 2002, Mediterranean 2003, Northwest Atlantic 2012, and Northeast Pacific 2013-2016 have had drastic and long-term impacts on the oceanographic and biological conditions in those areas. Unlike atmospheric heatwaves, MHWs can extend for millions of square kilometers, persist for weeks to months or even years, and occur at subsurface levels.


Definition

A marine heatwave is a discrete prolonged anomalously warm water event. The requirements for a warm water event to be described as a MHW are a duration of five or more days, temperatures greater than the 90th percentile of 30 year local measurements, no more than 3 days of cooling, and occurrence in a specific region. Recent work by the Marine Heatwaves International Working Group has proposed a categorization system to allow researchers and policy makers to define these extreme events and study the effects on biological systems. Ocean areas of carbon sinks in the mid-latitudes of both hemispheres and carbon outgassing areas in
upwelling Upwelling is an physical oceanography, oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water from deep water towards the ocean surface. It replaces the warmer and usually nutrient-depleted ...
regions of the tropical Pacific have been identified as places where persistent marine heatwaves occur; the air-sea gas exchange is being studied in these areas.


Categories

The
quantitative Quantitative may refer to: * Quantitative research, scientific investigation of quantitative properties * Quantitative analysis (disambiguation) * Quantitative verse, a metrical system in poetry * Statistics, also known as quantitative analysis ...
and qualitative categorization of MHWs, defined by the Marine Heatwaves International Working Group, establishes a naming system, typology, and characteristics for MHW events. The naming system is applied by location and year: for example Mediterranean 2003. This allows researchers to compare the drivers and characteristics of each event, geographical and historical trends of MHWs, and easily communicate MHW events as they occur in real-time. The categorization system is on a scale from 1 to 4. Category 1 is a moderate event, Category 2 is a strong event, Category 3 is a severe event, and Category 4 is an extreme event. The category applied to each event in real-time is defined primarily by
sea surface temperature Sea surface temperature (SST), or ocean surface temperature, is the ocean temperature close to the surface. The exact meaning of ''surface'' varies according to the measurement method used, but it is between and below the sea surface. Air mas ...
anomalies (SSTA), but over time it comes to include typology and characteristics. The types of MHWs are symmetric, slow onset, fast onset, low intensity, and high intensity. MHW events may have multiple categories such as slow onset, high intensity. The characteristics of MHW events include duration, intensity (max, average, cumulative), onset rate, decline rate, region, and frequency.


Drivers

The drivers for MHW events can be broken into local processes,
teleconnection Teleconnection in atmospheric science refers to climate anomalies being related to each other at large distances (typically thousands of kilometers). The most emblematic teleconnection is that linking sea-level pressure at Tahiti and Darwin, Austr ...
processes, and regional climate patterns. Two quantitative measurements of these drivers have been proposed to identify MHW, mean
sea surface temperature Sea surface temperature (SST), or ocean surface temperature, is the ocean temperature close to the surface. The exact meaning of ''surface'' varies according to the measurement method used, but it is between and below the sea surface. Air mas ...
and sea surface temperature variability. At the local level MHW events are dominated by ocean
advection In the field of physics, engineering, and earth sciences, advection is the transport of a substance or quantity by bulk motion of a fluid. The properties of that substance are carried with it. Generally the majority of the advected substance is al ...
, air-sea fluxes, thermocline stability, and wind stress. Teleconnection processes refer to climate and weather patterns that connect geographically distant areas. For MHW, the teleconnection process that play a dominant role are atmospheric blocking/
subsidence Subsidence is a general term for downward vertical movement of the Earth's surface, which can be caused by both natural processes and human activities. Subsidence involves little or no horizontal movement, which distinguishes it from slope mov ...
, jet-stream position, oceanic kelvin waves, regional wind stress, warm surface air temperature, and seasonal climate oscillations. These processes contribute to regional warming trends that disproportionately effect Western boundary currents. Regional climate patterns such as interdecadal oscillations like
El Niño Southern Oscillation EL, El or el may refer to: Religion * El (deity), a Semitic word for "God" People * EL (rapper) (born 1983), stage name of Elorm Adablah, a Ghanaian rapper and sound engineer * El DeBarge, music artist * El Franco Lee (1949–2016), American po ...
(ENSO) have contributed to MHW events such as "
The Blob ''The Blob'' is a 1958 American science fiction horror film directed by Irvin Yeaworth, and written by Kay Linaker and Theodore Simonson. It stars Steve McQueen (in his first feature film leading role) and Aneta Corsaut and co-stars Earl Rowe ...
" in the Northeastern Pacific. Drivers that operate on the scale of
biogeographical realms A biogeographic realm or ecozone is the broadest biogeographic division of Earth's land surface, based on distributional patterns of terrestrial organisms. They are subdivided into bioregions, which are further subdivided into ecoregions. De ...
or the Earth as a whole are Decadal oscillations, like Pacific Decadal Oscillations (PDO), and anthropogenic ocean warming due to
climate change In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to ...
.


Events

Sea surface temperatures have been recorded since 1904 in Port Erin, UK and measurements continue through global organizations such as the
IPCC The Intergovernmental Panel on Climate Change (IPCC) is an intergovernmental body of the United Nations. Its job is to advance scientific knowledge about climate change caused by human activities. The World Meteorological Organization (WMO) ...
, the Marine Heatwaves International Working Group,
NOAA The National Oceanic and Atmospheric Administration (abbreviated as NOAA ) is an United States scientific and regulatory agency within the United States Department of Commerce that forecasts weather, monitors oceanic and atmospheric conditio ...
,
NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeedin ...
, and many more. Events can be identified from 1925 till present day. The list below is not a complete representation of all MHW events that have been recorded. List: 1) Mediterranean 1999, 2003, 2006 2) Western Australia 1999, 2011 3) NW Atlantic 2012, 2016 4) NE Pacific 2013–2016, "The Blob" 5) Great Barrier Reef 1998, 2002, 2016 6) Tasman Sea 2015


Biological impacts

Changes in the thermal environment of terrestrial and marine organisms can have drastic effects on their health and well-being. MHW events have been shown to increase habitat degradation, change species range dispersion, complicate management of environmentally and economically important fisheries, contribute to mass mortalities of species, and in general reshape ecosystems. Habitat degradation occurs through alterations of the thermal environment and subsequent restructuring and sometimes complete loss of biogenic habitats such as
seagrass Seagrasses are the only flowering plants which grow in marine environments. There are about 60 species of fully marine seagrasses which belong to four families ( Posidoniaceae, Zosteraceae, Hydrocharitaceae and Cymodoceaceae), all in the ...
beds,
coral Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and se ...
s, and
kelp forest Kelp forests are underwater areas with a high density of kelp, which covers a large part of the world's coastlines. Smaller areas of anchored kelp are called kelp beds. They are recognized as one of the most productive and dynamic ecosystems on Ea ...
s. These habitats contain a significant proportion of the oceans biodiversity. Changes in ocean current systems and local thermal environments have shifted many tropical species' range northward while temperate species have lost their southern limits. Large range shifts along with outbreaks of toxic algal blooms has impacted many species across taxa. Management of these affected species becomes increasingly difficult as they migrate across management boundaries and the food web dynamics shift. Increases in sea surface temperature have been linked to a decline in species abundance such as the mass mortality of 25
benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning " ...
species in the Mediterranean in 2003, the Sea Star Wasting Disease, and
coral bleaching Coral bleaching is the process when corals become white due to various stressors, such as changes in temperature, light, or nutrients. Bleaching occurs when coral polyps expel the zooxanthellae ( dinoflagellates that are commonly referred to as ...
events. Climate change-related exceptional marine heatwaves in the
Mediterranean Sea The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on ...
during 2015–2019 resulted in widespread mass sealife die-offs in five consecutive years. The impact of more frequent and prolonged MHW events will have drastic implications for the distribution of species.


Atmospheric Impacts

Research on how MHWs influence atmospheric conditions is emerging. Marine heatwaves in the tropical Indian Ocean are found to result in dry conditions over the central Indian subcontinent. At the same time, there is an increase in rainfall over south peninsular India in response to MHWs in the northern Bay of Bengal. These changes are in response to the modulation of the monsoon winds by the MHWs.


Projected effects

CMIP6 projections from the Sixth Assessment Report agree with the SROCC that MHWs will very likely further increase in frequency, duration, spatial extent and intensity under future global warming in the 21st century. They project MHWs will become four times more frequent in 2081–2100 compared to 1995–2014 under SSP1-2.6, or eight times more frequent under SSP5-8.5. It is virtually certain that sea surface temperatures (SSTs) will continue to increase in the 21st  century, at a rate depending on emission scenarios. The  future global average SST increase projected by CMIP6 models for the period 1995–2014 to 2081–2100 is 0.86 –95% confidence range: 0.43–1.47°C under SSP1-2.6, 1.51 .02 to 2.19°C under SSP2-4.5, 2.19 .56 to 3.30°C under SSP3-7.0, and 2.89 .01 to 4.07°C under SSP5-8.5. Many species already experience these temperature shifts during the course of MHW events. There are many increased risk factors and health impacts to coastal and inland communities as global average temperature and extreme heat events increase. Marine heatwaves caused by the “Blob” have socioeconomic impacts including fishing closures and delays, whale entanglements in fishing gear, the substitution of fisheries, prohibition of aquaculture products, and food security issues. The aforementioned issues and other stressors caused by climate change can to some extent be mitigated by people’s willingness to follow certain restrictions to protect marine life . Lessons learned from the IPCC Sixth Assessment Report of 2022 demonstrate that there is a need for advanced seasonal forecasts, real-time predictions, monitoring responses, education, and possible fisheries impacts and adaptation.https://report.ipcc.ch/ar6wg2/pdf/IPCC_AR6_WGII_FinalDraft_Chapter03.pdf


See also

*
Effects of climate change on oceans Among the effects of climate change on oceans are: an increase in sea surface temperature as well as ocean temperatures at greater depths, more frequent marine heatwaves, a reduction in pH value, a rise in sea level from ocean warming and ic ...
*
Heat wave A heat wave, or heatwave, is a period of excessively hot weather, which may be accompanied by high humidity, especially in oceanic climate countries. While definitions vary, a heat wave is usually measured relative to the usual climate in the ...


References


External links

* Sea Surface Temperature Anomaly Maps, https://earth.nullschool.net/ * Marine Heatwave Tracker, http://www.marineheatwaves.org/tracker.html * Marine Heatwaves International Working Group, http://www.marineheatwaves.org/ *Intergovernmental Panel on Climate Change Report, https://www.ipcc.ch/sr15/ {{Climate change Climate change and the environment Physical oceanography Ocean pollution Heat waves Effects of climate change