What are Marine Heatwaves?

• Marine Heatwaves (MHWs) are prolonged periods of abnormally high sea surface temperatures (SST) in a particular ocean region.
• An event is classified as a marine heatwave when sea surface temperatures exceed the 90th percentile of the regional climatological threshold for at least five consecutive days.

Key Data and Statistics

• Regional Coverage: INCOIS’s Marine Heatwave Advisory System (MAHAS) monitors marine heatwaves across six ocean basins and 14 Indian coastal sectors, including the Red Sea, Persian Gulf and South China Sea.
• Intensity Classification: Watch and Alert categories indicate sustained sea temperature anomalies, with thresholds reaching 1°C above normal in affected regions.
• Indian Ocean Warming: Marine heatwaves in the western Indian Ocean have increased nearly fourfold over recent decades, reflecting rapid warming of the Tropical Indian Ocean.
• Swell Surge Waves: Swell waves exceeding 1 metre have been forecast across 11 coastal States and Union Territories, including Gujarat, Kerala, Odisha, Tamil Nadu and Andaman and Nicobar Islands

Formation of Marine Heatwaves

• Increased Atmospheric Heating (Air–Sea Heat Flux): Persistent high-pressure systems, clear skies and reduced cloud cover allow greater solar radiation to reach the ocean surface, increasing sea surface temperatures.
• Reduced Wind Speeds and Vertical Mixing: Weak surface winds reduce ocean turbulence and mixing, preventing warm surface water from mixing with cooler water at greater depths.
• Shallow Ocean Mixed Layer: A shallow mixed layer concentrates heat in the upper ocean, causing surface temperatures to rise rapidly.
• Ocean Current Heat Advection: Changes in ocean circulation transport warm equatorial waters towards coastal and mid-latitude regions, increasing local sea surface temperatures.
• Influence of Climate Drivers (ENSO and IOD): Global warming raises baseline ocean temperatures, while climate phenomena such as El Niño and a positive Indian Ocean Dipole (IOD) can alter ocean circulation and upwelling, favouring marine heatwave development.

Implications of Marine Heatwaves

• Coral Bleaching and Disruption of Calcification: Elevated sea temperatures disrupt the symbiotic relationship between corals and zooxanthellae (photosynthetic algae), causing corals to expel these algae, resulting in bleaching and reduced growth.
  • Example: Threatens coral reef ecosystems in the Gulf of Mannar, Palk Bay, Lakshadweep and Andaman Islands.
• Fish Migration and Decline in Fisheries: Marine species migrate towards cooler waters or move to greater depths to escape thermal stress, disrupting marine food chains and reducing fish availability for coastal fishing communities.
  • Example: Declining sardine and mackerel catches along the Kerala and Karnataka coasts during prolonged warm-water events.
• Proliferation of Harmful Algal Blooms (HABs): Elevated temperatures and favourable conditions can promote harmful algal blooms, including toxic phytoplankton and dinoflagellates. These may release toxins and cause oxygen depletion, threatening marine organisms.
  • Example: Red tides can cause fish mortality and damage cage aquaculture and shellfish populations.
• Degradation of Seagrass Meadows and Kelp Ecosystems: Prolonged thermal stress damages seagrass beds and kelp forests, reducing marine biodiversity, carbon sequestration and ecosystem productivity.
  • Example: Dieback of Halophila and Enhalus seagrass meadows threatens the feeding grounds of endangered dugongs and green sea turtles.
• Atmospheric Feedback and Cyclone Intensification: Elevated ocean temperatures increase the heat available to tropical weather systems. Marine heatwaves can contribute to rapid cyclone intensification, particularly when warm water extends to greater ocean depths.
  • Example: Warm waters in the Arabian Sea and Bay of Bengal can favour the intensification of tropical cyclones and increase associated coastal rainfall.