Cyclone Fani: Indian Ocean helps us gauge global warming's impact on cyclones

It is beyond doubt that tropical cyclones are responding to global warming and we can tell by the number of high-intensity cyclones which are intensifying more rapidly after forming and moving more slowly than before. The impact of global warming on cyclones obviously results in greater risk to life, property and infrastructure, and these cyclones are also dumping much more rain even for the same intensity of the storm. The warming of the ocean, especially deeper below the surface, means that cyclones are also receiving more energy in terms of moisture, which packs a punch in these cyclones.

Atlantic hurricanes are now arriving earlier than the official hurricane season which starts on 1 June. From the 1980s to the 2010s, the number in each decade has gone from one pre-season storm to seven pre-season storms. In our part of the world, Fani is now the strongest April cyclone in the Indian Ocean.

The northern Indian Ocean is no exception. It typically accounts only for about 5 percent of the global tropical cyclones with an average of four cyclones per year, three occurring in the Bay of Bengal and one over the Arabian Sea. Over the last few decades, high-intensity storms with a greater than 95 knot mean sustained winds have shown a clear increase while those above mean sustained winds of 63 knots do not show much of an increase. This is consistent with global warming – the total number of cyclones will not necessarily increase but the number of high-intensity storms will increase. This is because of a warmer atmosphere with a higher moisture-holding capacity as well as the warmer upper ocean with an increased capacity of the fuel tank.

🚨 #CycloneFani is expected to cross Odisha, Andhra Pradesh, West Bengal, Assam, Arunachal Pradesh, Meghalaya, Kerala, and Tamil Nadu. Spread the word. Stay safe.
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The Indian Ocean is unique for many reasons. Firstly, it is a warm ocean, especially during the pre- and post-monsoon seasons. This also means that the impact of increased upper ocean heat content or the hurricane heat potential is not the most dominant factor in the cyclone trends. That points to the second unique feature of the Indian Ocean. The strong monsoonal circulation creates a vertical shear, i.e., a change in the wind speeds or wind direction from the surface to the upper atmosphere. Such a vertical shear can prevent the cyclone from intensifying by shearing off its energy from the top. This is why the cyclone season over the Northern Indian Ocean is split into a pre- and a post-monsoon season.

Thirdly, the Northern Indian Ocean is also split by peninsular India into the Arabian Sea and the Bay of Bengal. The Bay remains a hot bathtub all year long whereas the Arabian Sea cools down significantly during the monsoon due to the strong monsoon southwesterly winds causing one of the strongest upwellings off Somalia and in the central Arabian Sea. The Arabian Sea also cools down considerably due to the dry continental winds driving evaporation during the winter or Northeast Monsoon season.

Human activity is playing a complicated role in the Northern Indian Ocean as well. While the increased greenhouse gases are causing ocean and land warming, the land has not warmed as fast as it should due to pollution and the related solar dimming. This has generally weakened the monsoon but also altered the vertical shear which inhibits cyclone intensification. Black carbon and sulfates have reduced the vertical shear in the pre-monsoon season to produce more intense cyclones before the monsoon. Aerosols are also blamed for an increase of extremely severe cyclonic storms over the Arabian Sea during late post-monsoon cyclone season.

Considering that the reduced mean monsoon has come with a threefold increase in widespread floods with a boost in moisture supply from the warming Arabian Sea, it is not surprising that the cyclone story has only cons and no pros either. The relatively smaller extents of the Arabian Sea and the Bay of Bengal prevent the cyclones from gaining super-cyclone categories. But this is no solace since the smaller basins also mean that landfalls are all but guaranteed.

For example, the warming of the Northern Indian Ocean has intensified cyclogensis during both pre- and post-monsoon seasons. But the tracks have become more focused on Andhra Pradesh and Tamil Nadu. From 1891 till 2002, the two states experienced a cyclone hit of 77 and 55, respectively. But since 2003, there have already been 8 and 13 cyclone impacts on these states already.

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As seen for Hurricane Harvey, coastal warming can also lead to the intensification of cyclones all the way to land instead of the usual weakening in a counterfactual world where no global warming occurs. And this also means extreme amounts of rainfall, storm surges and inundation as well as wind damage.

The Indian Ocean has warmed faster and more monotonically than other tropical oceans due to its unique circulation features. It is also expected to emerge as the canary in the coal mine when it comes to tropical cyclone response to global warming. This just ups the ante for the Indian Meteorological Department (IMD) and (Indian National Centre for Ocean Information Services (INCOIS) to continue to enhance their cyclone and ocean state monitoring and forecasts. All indications are that they are up to the challenge. But continued investments in satellites, drones, ships, buoys, etc., along with human and computation resources will be required.

The author is a Professor of Atmospheric & Oceanic Science and Earth System Science at the University of Maryland, currently a Visiting Professor at IIT Bombay.