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The Unusual Dynamics of Cyclones in the North Indian Ocean
Context:
The North Indian Ocean contributes significantly to the moisture needed for the summer monsoon, which involves substantial evaporation from the Arabian Sea and the Bay of Bengal.
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- Despite the high moisture supply and warm temperatures, the North Indian Ocean is the least active region globally in terms of cyclone frequency.
- This region’s unique characteristics lead to a complex interplay of factors that both promote and inhibit cyclogenesis making its cyclone activity and response to global warming particularly unusual.
Cyclogenesis
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Unique Oceanic Dynamics:
- The Indian Ocean is distinctive due to its unique ‘oceanic tunnels’ connecting it to the Pacific and Southern Oceans.
- The Pacific tunnel introduces warm waters into the upper 500 metres of the Indian Ocean, while the Southern Ocean brings cooler waters below about 1 kilometre.
Seasonal Temperature Variations:
- During the pre-monsoon season, the Arabian Sea heats up rapidly as the Sun shifts to the Northern Hemisphere. The Bay of Bengal, already warmer, further warm up, leading to atmospheric convection and rainfall.
- The onset of the monsoon over Kerala typically begins in mid-May, driven by these temperature variations.
- During the post-monsoon period, the northeast monsoon brings significant rainfall to several states. These factors significantly influence cyclogenesis throughout the year.
- Ocean water is constantly circulated by various currents. Tidal currents near the shore are influenced by the sun and moon, while surface currents are driven by the wind.
- Additionally, slower, deep currents, which move from the surface to the seafloor, are driven by changes in salinity and temperature through a process known as thermohaline circulation.
Atlantic Meridional Overturning Circulation (AMOC)
- The AMOC is a system of ocean currents in the Atlantic Ocean that moves warm water toward the north and cold water toward the south.
- The circulation process begins when warm surface water, such as the Gulf Stream in the North Atlantic, travels toward the poles. There, it cools and forms sea ice, which leaves salt behind, increasing the water’s density. This denser water then sinks and flows southward at deeper levels.
- Eventually, the water rises back to the surface through upwelling, warms up, and continues the cycle.
- The AMOC and global conveyor belt are slow systems, taking around 1,000 years for a cubic metre of water to complete its journey. There is evidence that the AMOC’s pace is slowing even further.
- If the AMOC continues to slow, it could lead to significant climate impacts, such as altering rainfall patterns in regions like South Africa and causing sea level rise along the U.S. East Coast.
Cyclone Seasons and Ocean Response:
- The Indian Ocean experiences two distinct cyclone seasons: pre-monsoon and post-monsoon.
- During the monsoon season, strong southwesterly winds (low-level jets) lead to significant evaporation and mixing of cold subsurface waters with surface waters in the Arabian Sea.
- The Bay of Bengal experiences high convective activity during the monsoon, producing many low-pressure systems. However, these systems rarely intensify into cyclones.
The Impact of Climate Change:
- Increased heat inflow from the Pacific Ocean and warmer waters from the Southern Ocean are accelerating the warming of the Indian Ocean.
- This rapid warming influences the heat uptake by the Pacific Ocean and affects the sinking of heavy waters in the North Atlantic. Essentially, the Indian Ocean is acting as a clearinghouse for ocean warming.
- The specific regional effects including warming and altered atmospheric conditions, are affecting cyclone formation (cyclogenesis), the number of cyclones, and their behaviour in the Indian Ocean.
The Cyclone-Prone Bay of Bengal
- The Bay of Bengal, the largest bay in the world at 2,600,000 square kilometres, is one of the most cyclone-prone areas globally.
- According to researchers, hurricane scientists, and meteorologists, 22 of the 30 deadliest tropical cyclones in recorded history have occurred in the region over the past two centuries.
- Topography and Storm Surge: Its funnel-like shape and shallow waters close to the coast contribute to large and destructive storm surges.
- The northern part of the Bay, with its shallow waters and complex river systems, is particularly vulnerable to flooding when cyclones make landfall.
- Sea surface temperatures usually range from 26 to 30 degrees Celsius, but recent measurements show a significant rise to 31 to 32 degrees Celsius.
- The region’s elevation, with islands typically just 4 to 5 metres above sea level, and the presence of numerous rivers and rivulets, compound the severity of cyclones.
- The coast along West Bengal and Bangladesh is particularly susceptible due to its low elevation and insufficiently robust sea-dykes and embankments.