How marine heatwave fuelled super cyclone Amphan

A new study has suggested that warm subsurface waters in the Bay of Bengal likely helped fuel the 2020 Amphan super cyclone.

Marine heatwaves:

• First described in 2011: The term “marine heatwave” was initially used to describe an extreme surface warming event off the west coast of Australia during 2010-11 austral summer.
• Marine heatwaves like The Blob has resulted in mass mortalities in marine mammals and birds, and collapse of fisheries and aquaculture in the US, Korea, etc.
• Marine heatwaves are periods of extremely high temperatures in the ocean (above the 90th percentile).
• These events cause marine habitat destruction due to coral bleaching, seagrass destruction, and loss of kelp forests, affecting the fisheries sector adversely.
• Impact on the Monsoon
• The marine heatwaves in the western Indian Ocean and the Bay of Bengal are found to result in dry conditions over the central Indian subcontinent.
• There is an increase in rainfall over south peninsular India in response to the heatwaves in the Bay.

What causes marine heatwaves?

• Marine heatwaves can be caused by a whole range of factors, and not all factors are important for each event.
• The most common drivers of marine heatwaves include ocean currents which can build up areas of warm water and air-sea heat flux, or warming through the ocean surface from the atmosphere.
• Winds can enhance or suppress the warming in a marine heatwave, and climate models like El Niño can change the likelihood of events occurring in certain regions.
  
  

Trends:

• Bay of Bengal recorded surface temperatures of 32-34°C, before Cyclone Amphan.
• 93% of the heat from global warming goes into the oceans.
• Warm waters are an energy source for cyclones.
• As the Arabian Sea warms, the number and intensity of cyclones and heavy rains increase.
• Over the Arabian Sea,
• 150% rise in the number of very severe cyclones
• 80% rise in the total duration of cyclones
• 20%-40% rise in the intensity of cyclones Over the Bay of Bengal, rapid intensification may be occurring more.

Key findings about Marine heatwave and Cyclone:

• The heat content in the subsurface ranged between 135 and 150 kilojoules per square centimetre before the cyclone.
• Warm subsurface waters could intensify cyclones by allowing heat to flow between the sea and atmosphere. Warmer waters fuel cyclones.
• The subsurface water may have warmed because of the high sea level anomaly, which is the difference between the total and average sea level.
• An increase in sea level is brought about by anticyclonic eddies, which is a circular movement of water.