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Marine heatwave

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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 or ocean.[1] 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.[2][3][4] Marine heatwaves can lead to severe biodiversity changes such as sea star wasting disease,[5][6] harmful algal blooms,[7] and mass mortality of benthic communities.[8] The Intergovernmental Panel on Climate Change (IPCC) Special Report on the Ocean and Cryosphere in a Changing Climate (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.[9] The IPCC Sixth Assessment Report supports these findings, while also detailing new evidence that MHWs have become longer and more intense.[10]

Major marine heatwave events such as Great Barrier Reef 2002,[11] Mediterranean 2003,[8] Northwest Atlantic 2012,[2][12] and Northeast Pacific 2013-2016[13][14] have had drastic and long-term impacts on the oceanographic and biological conditions in those areas.[8][15][7]

Unlike atmospheric heatwaves, MHWs can extend for millions of square kilometers, persist for weeks to months or even years, and occur at subsurface levels.[16][17][18][19]

Definition

A marine heatwave is a discrete prolonged anomalously warm water event.[20] 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.[20]

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.[21] Ocean areas of carbon sinks in the mid-latitudes of both hemispheres and carbon outgassing areas in upwelling 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.[22]


Categories

Categories of marine heatwaves as defined in Hobday et al. (2018)

The quantitative and qualitative categorization of MHWs, defined by the Marine Heatwaves International Working Group, establishes a naming system, typology, and characteristics for MHW events.[20][21] The naming system is applied by location and year: for example Mediterranean 2003.[21][8] 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.[21] 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 anomalies (SSTA), but over time it comes to include typology and characteristics.[21] The types of MHWs are symmetric, slow onset, fast onset, low intensity, and high intensity.[20] 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.[20]

Drivers

Examples of regional oscillations ENSO, ASO, and NAO taken from NOAA.org[23]

The drivers for MHW events can be broken into local processes, teleconnection processes, and regional climate patterns.[2][3][4] Two quantitative measurements of these drivers have been proposed to identify MHW, mean sea surface temperature and sea surface temperature variability.[21][2][4] At the local level MHW events are dominated by ocean advection, air-sea fluxes, thermocline stability, and wind stress.[2] Teleconnection processes refer to climate and weather patterns that connect geographically distant areas.[24] For MHW, the teleconnection process that play a dominant role are atmospheric blocking/subsidence, 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.[2] Regional climate patterns such as interdecadal oscillations like El Niño Southern Oscillation (ENSO) have contributed to MHW events such as "The Blob" in the Northeastern Pacific.[25] Drivers that operate on the scale of biogeographical realms or the Earth as a whole are Decadal oscillations, like Pacific Decadal Oscillations (PDO), and anthropogenic ocean warming due to climate change.[2][4][9]

Events

The MHW termed "The Blob" that occurred in the Northeastern Pacific from 2013 to 2016.[26]

Sea surface temperatures have been recorded since 1904 in Port Erin, UK[4] and measurements continue through global organizations such as the IPCC, the Marine Heatwaves International Working Group, NOAA, NASA, and many more. Events can be identified from 1925 till present day.[4] The list below is not a complete representation of all MHW events that have been recorded.

List: 1) Mediterranean 1999, 2003, 2006 [21][2][8] 2) Western Australia 1999, 2011 [21][2][27] 3) NW Atlantic 2012, 2016 [21][2][12][28] 4) NE Pacific 2013–2016, "The Blob" [13][14] 5) Great Barrier Reef 1998, 2002, 2016 [21][2][11] 6) Tasman Sea 2015[21][2]


Breakdown of MHWs 1999–2019
Name Category Duration (days) Intensity (°C) Area(Mkm2)
Mediterranean 1999 1 8 1.9 NA
Mediterranean 2003 2 10 5.5 0.5
Mediterranean 2003 2 28 4.6 1.2
Mediterranean 2006 2 33 4.0 NA
Western Australia 1999 3 132 2.1 NA
Western Australia 2011 4 66 4.9 0.95
Great Barrier Reef 2016 2 55 4.0 2.6
Tasman Sea 2015 2 252 2.7 NA
Northwest Atlantic 2012 3 132 4.3 0.1–0.3
Northeast Pacific 2015 3 711 2.6 4.5–11.7
Santa Barbara 2015 3 93 5.1 NA
Southern California Bight 2018[29] 3 44 3.9 NA

Biological impacts

Changes in the thermal environment of terrestrial and marine organisms can have drastic effects on their health and well-being.[15][30] MHW events have been shown to increase habitat degradation,[31][32] change species range dispersion,[15] complicate management of environmentally and economically important fisheries,[13] contribute to mass mortalities of species,[8][7][5] and in general reshape ecosystems.[11][33]

Habitat degradation occurs through alterations of the thermal environment and subsequent restructuring and sometimes complete loss of biogenic habitats such as seagrass beds, corals, and kelp forests.[31][32] These habitats contain a significant proportion of the oceans biodiversity.[15] 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.[7] 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 species in the Mediterranean in 2003, the Sea Star Wasting Disease, and coral bleaching events.[8][15][5] Climate change-related exceptional marine heatwaves in the Mediterranean Sea during 2015–2019 resulted in widespread mass sealife die-offs in five consecutive years.[34] The impact of more frequent and prolonged MHW events will have drastic implications for the distribution of species.[9]

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.[35] 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.[10]

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 [5–95% confidence range: 0.43–1.47] °C under SSP1-2.6, 1.51 [1.02 to 2.19] °C under SSP2-4.5, 2.19 [1.56 to 3.30] °C under SSP3-7.0, and 2.89 [2.01 to 4.07] °C under SSP5-8.5.[10]

Many species already experience these temperature shifts during the course of MHW events.[20][21] There are many increased risk factors and health impacts to coastal and inland communities as global average temperature and extreme heat events increase.[30] 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 [citation needed].

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.[36]

See also

References

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