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Post at: Apr 02 2022

Marine Heatwaves

Recent Context

  • According to a study appearing in journal JGR Oceans, Marine Heatwaves have been on the rise in the waters around India.
    • Emerging studies have reported their occurrence and impacts in the global oceans, but are little understood in the tropical Indian Ocean.

Highlights of the study

  • The Western Indian Ocean region experienced the largest increase in marine heatwaves at a rate of about 1.5 events per decade, followed by the north Bay of Bengal at a rate of 0.5 events per decade.
    • From 1982 to 2018, the Western Indian Ocean had a total of 66 events, while the Bay of Bengal had 94 events, the study notes.
  • The marine heatwaves in the Western Indian Ocean and the Bay of Bengal increased drying conditions over the central Indian subcontinent. 
  • Correspondingly, there is a significant increase in the rainfall over south peninsular India in response to the heatwaves in the north Bay of Bengal.

What are Marine Heatwaves?

  • Marine heatwaves are periods of extremely high temperatures in the ocean.
  • Such heat waves are caused by an increase in the heat content of oceans, especially in the Upper Layers.
  • These waves can persists for days to months.

  • Worldwide, they are one of the major results of human-induced global warming. 

  • During an MHW, the average temperatures of the ocean surface (up to a depth of 300 feet) goes 5-7 degrees Celsius above normal.

What Causes Marine Heatwaves
(a)    Ocean currents can build up areas of warm water.
(b)    Air - sea heat flowx (from the atmosphere).
(c)    Ocean winds (El-nino).

Impacts of Marine Heatwaves

  • Ecosystem structure
    • Marine Heatwaves affect ecosystem structure, by supporting certain species and suppressing others. 
    • For example, after the 2011 marine heatwave in Western Australia the fish communities had a much more “tropical” nature than previously and switched from kelp forests to seaweed turfs.
    • These events are linked to coral bleaching, seagrass destruction, and loss of kelp forests, affecting the fisheries sector adversely. 
    • An underwater survey showed that 85% of the corals in the Gulf of Mannar near the Tamil Nadu coast got bleached after the Marine Heatwave in May 2020.
  • Change in the Habitat Ranges
    • Marine heatwaves can change the Habitat Ranges of certain species, such as the spiny sea urchin off southeastern Australia which has been expanding southward into Tasmania at the expense of kelp forests which it feeds upon. 
    • Rogue animals can also find their way well outside their normal range, following the warm waters of a marine heatwave, such as this tropical fish found off Tasmania during the 2015/16 marine heatwave.
  • Economic losses 
    • Marine heatwaves can cause economic losses through impacts on Fisheries and Aquaculture.

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