Marine heatwave

A marine heatwave is a period of abnormally high sea surface temperatures (SST) compared to typical temperatures for a particular season and locale. Marine heatwaves are caused by a variety of drivers. These include short term weather events such as fronts, intraseasonal events (30 to 90 days), annual, and decadal (10-year) modes like El Niño events, and human-caused climate change. Such heatwaves affect marine ecosystems. For example, heatwaves can lead to events such as coral bleaching, sea star wasting disease, harmful algal blooms, and mass mortality of benthic communities. Unlike heatwaves on land, marine heatwaves can extend over vast areas, persist for weeks to months to years, and extend to subsurface levels. Major marine heatwaves affected the Great Barrier Reef in 2002, the Mediterranean Sea in 2003, the Northwest Atlantic in 2012, and the Northeast Pacific during 2013–2016. These events had drastic, long-term impacts. Scientists predict that the frequency, duration, scale (area), and intensity of marine heatwaves will increase. This is because sea surface temperatures will continue to increase with global warming. The IPCC Sixth Assessment Report (SAR6) in 2022 stated that "marine heatwaves are more frequent [...], more intense and longer [...] since the 1980s, and since at least 2006 very likely attributable to anthropogenic climate change". This confirmed earlier findings in a 2019 IPCC report that "Marine heatwaves [...] have doubled in frequency and have become longer lasting, more intense and more extensive (very likely)." The 2022 report predicted that marine heatwaves will become "four times more frequent in 2081–2100 compared to 1995–2014" under the lower greenhouse gas emissions scenario, or eight times more frequent under the higher emissions scenario.

Definition

SAR6 defined marine heatwave as: "A period during which water temperature is abnormally warm for the time of the year relative to historical temperatures, with that extreme warmth persisting for days to months. The phenomenon can manifest in any place in the ocean and at scales of up to thousands of kilometres." A 2016 study defined it as: an anomalously warm event that "lasts for five or more days, with temperatures warmer than the 90th percentile based on a 30-year historical baseline period". The term was coined following an unprecedented warming event off the west coast of Australia in summer 2011 that led to a rapid dieback of kelp forests and associated ecosystem shifts along hundreds of kilometers of coastline.

Categories Categorization allows researchers to compare event drivers and characteristics, geographical and historical trends, and communicate about them consistently. Marine heatwaves are classified via a naming system, typology, and characteristics. The naming system involves locale and year: for example Mediterranean 2003. Events are rated on a scale from 1 to 4. Category 1 is a moderate event, Category 2 strong, Category 3 severe, and Category 4 extreme. The category is defined primarily by sea surface temperature anomalies (SSTA), later adjusted to include typology and characteristics. They are further classified according to the degree of symmetry, duration, intensity (max, average, cumulative), onset rate (slow/fast), decline rate, locale, and frequency. Various combinations have been observed. While marine heatwaves have mostly been studied at the sea surface, they can also occur at depth, including at the sea floor.

Drivers The drivers for marine heatwave events can be broken into local processes, teleconnection processes, and regional climate patterns. Two quantitative measurements of these drivers have been proposed to characterize marine heatwaves, mean sea surface temperature and sea surface temperature variability. At the local level marine heatwave events are dominated by ocean advection, air-sea fluxes, thermocline stability, and wind stress. Teleconnection processes refer to climate and weather patterns that connect geographically separated areas. The teleconnection processes that play a dominant role are atmospheric blocking/subsidence, jet-stream position, oceanic kelvin waves, regional wind stress, warm surface air temperature, seasonal climate oscillations, and nearby cyclones. These processes contribute to regional warming trends that disproportionately affect Western boundary currents. Regional climate patterns including interdecadal oscillations like El Niño Southern Oscillation (ENSO) have contributed to marine heatwave events such as "The Blob" in the Northeastern Pacific. Drivers that operate on the scale of biogeographical realms or the Earth as a whole are decadal oscillations, including Pacific decadal oscillations (PDO), and climate change. Ocean carbon sinks in the mid-latitudes of both hemispheres and carbon outgassing areas in upwelling regions of the tropical Pacific experience persistent marine heatwaves.

Climate change

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