Marine Heatwaves
The marine heatwaves are the ones that form on oceans and are defined as extended period of time of extremely high temperature of the ocean. The marine heatwave lasts from days to months. They can occur in the summer or also in winter.
Temperature rise due to greenhouse gas emission is the main factor for anthropogenic climate change. The significant increase in greenhouse gas, mainly carbon dioxide, concentration in the atmosphere traps the radiation into the atmosphere, not allowing it to return back to space. This trapping of the extra energy into the atmosphere causes the surface air temperature to increase and warms the climate.
Heat absorption capacity of the atmosphere is very less, more than 90% of the extra heat that has been trapped in the climate system has been absorbed by the oceans since 1970, according to IPCC reports, consequently oceans are warming from surface to deeper depths due to this trapped extra heat in the oceans. The warming of the oceans led to an increase in intensity and frequency of extreme weather events such as marine heatwaves, rising sea levels, tropical cyclones, melting glaciers and affecting weather patterns across the globe.
Many factors are responsible for marine heatwaves but not all factors are important for each event. At local level ocean advection including by ocean currents, air-sea fluxes, wind stress and thermocline stability play dominant roles in the MHW events. Regional Climate pattern ENSO can change the possibility of MHW events occurring in certain regions. ENSO has contributed to "The Blob" , a MHW event, in the northeastern Pacific Ocean.
MHW And Coral Bleaching
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| When coral bleaches, it turn to white |
The events of marine heatwaves are linked to coral bleaching, seagrass destruction, kelp forest loss, thereby leading to the habitat destruction of marine species such as fish. MHW events accounted for coral bleaching in the Gulf of Mannar where 85% of the corals got bleached after the marine heatwave in May 2020.
91% coral of the Great Barrier Reefs of Australia have been damaged by bleaching due to a prolonged marine heatwave in May 2019 and the Great Barrier Reef has suffered three major bleaching events since 2015, said by the Australian scientist. The GBR of corals is the world's largest barrier reef system. The UN committee has recommended that the GBR should be added to the " in danger " list of UNESCO World Heritage Site because of climate change impact.
Most shallow water corals, battered and bleached white by the repeated marine heatwaves, are unlikely to last the century, said the Intergovernmental Panel on Climate Change.
MHWs and El Nino, La Nino
The marine heatwaves are caused by an increase in the heat content of oceans, especially in the upper layers of the ocean due to global warming due to climate change and decadal climate modes like strong El Nino events. El Nino is the climate pattern in the Pacific Ocean which has a warming effect on the Ocean by warming the ocean surface above the average sea surface temperature and can affect the weather pattern around the world.
The marine heatwave comes as the La Nino weather pattern causes warmer than normal temperatures in southern New Zealand. High pressure systems and climate change are also factors in heatwaves.
El Nino and La Nino are parts of the El Nino Southern Oscillation (ENSO) phenomenon which is the cycle of warm and cold sea surface temperature in the tropical central and eastern Pacific Ocean. El Nino is the warm phase of the ENSO.
It was not just the surface water temperature rising but the temperature of water as deep as 100 meters is also rising. That means the marine species that live in deeper water are getting influenced due to marine heatwaves. The marine species that are unable to migrate or can't shift their location are likely to be more impacted.
Categorization of the marine heatwaves.
The category applied to each MHW event in real-time is defined primarily by sea surface temperature anomalies (SSTA) and it also includes typology and characteristics over the time. The categorization is defined by the International Working Group, establishes naming system, typology, and characteristics for MHW events. The naming system is applied by the location and year: for example Mediterranean 2003.
Impact of Marine Heatwaves in Tropical Indian Ocean and the Southwest Monsoon.
The Indian ocean at surface is warming at a faster rate as compared to other oceans globally due to global warming. This high sea surface temperature (SST) of the Indian Ocean is getting influenced by the MHWs termed as extreme temperature conditions that persist over days to months.
The marine heatwaves are responsible for the modulation of monsoon winds which led to changes in both dry conditions and rainfall events. There is an increase in dry conditions over the central Indian subcontinent due to prevalence of marine heatwaves in the Western Indian Ocean and the Bay of Bengal and correspondingly, the rainfall over the south India peninsula has significantly increased in response to the marine heatwaves in the North Bay of Bengal. There is a close link between marine heatwaves and atmosphere circulation and rainfall as per the study published in Journal JGR Oceans.
Number of marine heatwave events has increased by around 1.5 events per decade in the Western Indian Ocean and .5 events per decade in Northern Bay of Bengal during the period of 1982 to 2018. 66 MHW events occurred in the west Indian Ocean and 94 events in the North Bay of Bengal during that period.
Climate change would lead to further increase in warming of the Indian Ocean in future which will intensify the marine heatwaves and their impacts including on monsoon rainfall. The frequency, intensity and area covered by the marine heatwaves are increasing.
Impacts of Marine Heatwaves
The marine heatwaves can have significant impacts on marine ecosystems by altering their thermal environment which in turn led to habitat degradation such as loss of seagrass beds and kelp forests and coral bleaching.
The habitats such as seagrass beds, kelp forests and coral reefs hold and conserve a significant proportion of biodiversity by providing food, shelter and breeding ground for marine species and the marine heatwaves can severely affect the biodiversity by degrading or destructing these habitats. The changing conditions also help invasive alien species to spread which can be devastating to marine food webs.
The MHW also affects the structure of marine ecosystems by supporting certain species and suppressing others. For example, fish communities in the Western Australia had become more tropical in nature and switched their habitat from kelp forests to seaweed after the marine heatwave in 2011.
The marine heatwaves can change the habitat ranges of marine species. For example, spiny sea urchin off southeastern Australia has been expanding southward into Tasmania at the expense of the kelp forests.
The marine heatwave affects the fisheries sector adversely by destructing the habitats of fish and the species on which fish rely often have to relocate in response to MHWs which in turn led to killing or reduction of productivity of fish and sometimes MHWs leads to mass mortality of fish communities.
Along with the fisheries sector it also affects the aquaculture and ecotourism industries resulting in economic loss. The intense warming of the oceans due to marine heatwaves has several socio-economic consequences.
Often MHWs occur alongside other stressors such as ocean acidification, deoxygenation and overfishing and they increase the risk of further deoxygenation and acidification besides destroying the habitats.
How are the MHWs fuelling Tropical Cyclones?
The marine heatwave has the potential to influence other extreme weather events such as tropical cyclones. A study was published in the Frontiers in Climate, which was conducted in the Indian Ocean to investigate the relation between a marine heatwave and the super cyclone Amphan in 2020.
The Bay of Bengal shows high SST (about 28 degree celsius) throughout the year and is more vulnerable to tropical cyclones. Amphan was the first super cyclone in the last 21 years and also costliest tropical cyclone on record in the North Indian Ocean, with an economic loss of approximately 14 billion dollars. Initially, it was a cyclonic storm (category 1 ) but was intensified into super cyclone (category 5) within 24 hours because there was the presence of a strong MHW beneath the cyclone track with an extremely high SST that coincided with the track of the cyclone and facilitated its rapid intensification in a short period.
The ocean stratification, acidification and warming below the surface also have an important role in this compound extreme phenomenon, the co-occurring of marine heatwave and tropical cyclone.
Fani, a tropical cyclone that occurred in the Bay of Bengal in 2019 did not turn into the super cyclone like the Amphan as it was not fuelled by the strong marine heatwave on its way as was the case in the Amphan.
Way forward
- We can raise awareness about the marine heatwave phenomenon.
- We can improve our scientific understanding of their physical properties and the ecological impacts. By doing this we can better protect the vulnerable marine habitats and resources.
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