A marine heatwave is a short period of abnormally high temperatures in a sea or ocean. Marine heatwaves are caused by a variety of factors and have been associated with severe biodiversity changes such as sea star wasting disease, toxic algal blooms, and mass mortality of benthic communities. 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. The Intergovernmental Panel on Climate Change report on Global Warming of 1.5 °C 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. Under RCP 4.5 and RCP 6.0 scenarios, there will be serious impacts on terrestrial and oceanic ecosystems as the Ocean continues to warm.
Definition
A marine heatwave is a discrete prolonged anomalously warm water event. The requirements for warm water events to be described as MHW are a duration of 5 or more days, temperatures greater than the 90th percentile of 30 year local measurements, no more than 3 days of cooling, and occurring 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.
Categories
The quantitative and qualitative categorization of MHW, 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 drivers and characteristics of each event, geographical and historical trends of MHW, and easily communicate MHW events as they occur in real-time. The categorization system is 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 a extreme event. The Category applied to each event in real-time is defined primarily sea surface temperature anomalies, but in the long-term include typology and characteristics. The types of MHW 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, onset rate, decline rate, regions, and frequency.
Drivers
The drivers for MHW events can be broken into local processes, teleconnection processes, and regional climate patterns. Two quantitative measurements of these drivers have been proposed to identify MHW, mean sea surface temperature and sea surface temperature variability. At the local level MHW 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 distant areas. 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. Regional climate patterns such as interdecadal oscillations like El Niño Southern Oscillation have contributed to MHW 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, like Pacific Decadal Oscillations, and anthropogenic ocean warming.
Events
Sea surface temperatures have been recorded since 1904 in Port Erin, UK and continue through global organizations such as the IPCC, Marine Heatwaves International Working Group, NOAA, NASA, 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
Name
Category
Duration
Intensity
Area
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
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 moralities 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 beds, corals, and kelp forests. These habitats contain a significant proportion of the oceans biodiversity. Changes in ocean current systems and local thermal environments has 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. 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 have been linked to increase in sea surface temperature. The impact of more frequent and prolonged MHW events will have drastic implications for the distribution of species.
Projected effects
Ocean atmosphere changes between 1925–1954 & 1987–2016 show a 34% increase in frequency, 17% increase in duration, and 54% increase in total annual days under MHW using IPCC RCP simulation MHW global average prediction RCP 4.5 and RCP 6.0. On September 24th, 2019 the IPCC released its report on "The Ocean and Cryosphere in a Changing Climate", and this report MHW are referred to 72 times. RCP scenarios 2.6–8.5 show a global mean surface temperatures rising 1.6–2.0 °C between 2031–2050 and continuing to rise to 1.6–4.3 °C between 2081–2100. For sea surface temperatures this results in an average increase of 0.9–1.3 °C between 2031–2050 and an increase of 1.0–3.7 °C between 2081–2020. Many species already experience these temperature shifts during the course of MHW events. There are many increased risk factors and health impacts to many coastal and inland communities as global average temperature and extreme heat events rise.
