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from the northern Indian Ocean the bulk of the moisture 200 lakh crore or more required to raise funds Water buckets During the summer monsoon. This means that there is a lot of evaporation from the Arabian Sea and the Bay of Bengal, which requires these seas to be warm enough to allow evaporation. This is why warm tropical oceans also give rise to a lot of cyclones.
And yet, the North Indian Ocean is the least active region in the world’s oceans in terms of the number of cyclones. There are some factors that promote cyclone development and some that suppress it, making the region unusual in terms of cyclone seasonality, number, and the response of the ocean and cyclones to global warming.
unusual indian ocean
The Indian Ocean attracts a lot of attention for its monsoon circulation and dramatic seasonal wind reversals north of the equator. But it is also unique because it has ‘ocean tunnels’ that connect it to the Pacific Ocean and the Southern Ocean. The Pacific Ocean tunnel brings a lot of warm water into the upper 500 m each year while the Southern Ocean tunnel brings cold water down about 1 km.
In the pre-monsoon season the Arabian Sea warms up rapidly as the sun penetrates the Northern Hemisphere. The Bay of Bengal is relatively warmer than the Arabian Sea, but it warms further and begins to generate atmospheric convection and rainfall. The trough that eventually leads to the onset of the monsoon in Kerala reaches over the Bay of Bengal in mid-May.
The post-monsoon season is the northeast monsoon season for India, and many states receive significant amounts of rainfall during this time.
All these wind patterns and sea surface temperatures influence cyclonic activity in the North Indian Ocean throughout the year and maintain sharp differences in cyclonic activity between the Arabian Sea and the Bay of Bengal.
Ocean response to climate change
Climate change further exacerbates the Indian Ocean’s uniqueness. More heat is now coming in from the Pacific Ocean, while the Southern Ocean is also pushing warm water in. These inputs are causing the Indian Ocean to warm faster, as well as atmospheric changes in winds and humidity.
The North Indian Ocean and monsoon are now responding to climate change drivers related to the tropical oceans as well as pole-to-pole influences.
The rapid warming of the ocean is now reported to be affecting the way the Pacific Ocean absorbs heat and sinks heavy water into the North Atlantic Ocean. The Indian Ocean is essentially acting like a clearinghouse for ocean warming during climate change.
All these region-specific characteristics play a role in cyclogenesis, the number of cyclones and their responses to climate change in the Indian Ocean.
Two cyclonic seasons
During the monsoon itself, strong southwesterly winds – or the so-called low-level jet – cause rapid evaporation and mixing of cold groundwater with surface water, so the Arabian Sea is much cooler than it is pre-monsoon. Strong convective activity over the Bay of Bengal creates many low-pressure or cyclonic systems during the monsoon, but they rarely develop into cyclones.
This is because the strong monsoon winds vary in strength and direction from the surface to the middle atmosphere. This variation is called vertical shear and it siphons off energy from any cyclone trying to grow.
The net result is that the major cyclone season is uniquely divided into pre- and post-monsoon seasons in the North Indian Ocean. All other cyclone regions have a single cyclone season throughout the year.
Weak activity in Arabian Sea
The main effect of the monsoon circulation is that warm oceans, heat content and atmospheric convection promote cyclogenesis over the northern Indian Ocean. This leads to the formation of cyclones during the pre-monsoon and post-monsoon seasons.
However, the Arabian Sea has very little convective activity before the monsoon and is relatively less prone to cyclonic events. The northeast monsoon and dry continental air cool the Arabian Sea after the monsoon. Thus the number of cyclones in the Arabian Sea is about half the number of cyclones in the Bay of Bengal in both seasons.
Thus, the cool temperatures, strong wind shear and relatively low convective activity over the Arabian Sea make the Arabian Sea a relative desert for cyclones. There has been some increase in the number of cyclones since 2010, but the Arabian Sea has been strangely quiet for the past few years.
A rare cyclone in August
There is a lot of excitement right now about a rare August cyclone – named ‘Asana’ – born from the transition of a strong land-borne depression into the warm Arabian Sea. This is the first North Indian Ocean cyclone in August since 1981.
The depression that developed into this cyclone was itself unusual in terms of its powerful development over land. Low pressure systems usually develop over the Bay of Bengal and cross the core monsoon region to bring about 60% of the total seasonal rainfall.
Ocean-borne depressions can gain nourishment from past rainfall (in the form of soil moisture) while passing through India. There is some evidence that these low-pressure systems are now contributing to extreme rainfall events, but they are not yet fully understood. Stuck in a corridor for over a century.
The unusual growth of the low-pressure system that flooded western India boosted soil moisture from excessive rainfall since the beginning of the season. It was so large that it began to collide with the warm northern Arabian Sea as it approached the western edge of land. The ocean supplied the energy needed to maintain and even enhance convection and strength.
Once its transition into the ocean is complete, it might be expected to develop into a regular cyclone, but as a remarkably unusual land-borne cyclone. The heat over the Arabian Sea has already been reported to be related to a northward shift of the low-level jet, which itself is induced by West Asia is getting hotter,
Don’t expect anything, be surprised
As English writer Daniel Defoe said, “Expect nothing and you will always be surprised.” The warm 2023-2024 resulting from a combination of global warming and El Niño, with underwater volcanic eruptions likely contributing to this, has produced plenty of surprises in terms of extreme events across the planet.
The evolution of the monsoon has also been unusual, especially since the predicted strong La Nina has not yet set in. The distribution of rainfall has been as unpredictable as ever, with peninsular India being wet and the northern part of the country experiencing a strange mix of wet and dry areas.
A low pressure system is unusual, but it moving over land is unusual. It moving from land to sea is another unexpected event. We did not expect it and were obviously surprised.
Raghu Murtugudde is a Professor at IIT Bombay and Emeritus Professor at the University of Maryland.
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