Indian aerosol pollution stimulates tropical cyclones over Arabian Sea – because of monsoon disturbance

cyclones Arabian Sea India monsoonWhite sulfur aerosols cool the climate; black carbon soot warms the climate. So when you mix the two kinds of aerosol pollution up in the Asian brown cloud, one would expect climate effects to even out.

Unfortunately in our physical world things are never that simple.

That brown haze is the result of a sixfold increase in aerosol emissions over the Indian subcontinent since the 1930s – caused by inefficient combustion of fossil fuels and biomass on an ever larger scale*.

[*) Currently 20 percent of children are born in India and in absolute numbers the Indian population will overtake China within a decade from now. Meanwhile the population growth in Pakistan is almost 30 percent higher than in India (whereas growth in Bangladesh is much closer to replacement level). These demographics are of course just background to rising energy consumption per capita.]

Cooling on ground, warming in air

The reason the brownish mix of reflective and absorbing aerosols does not even out, is they both block sunlight – so they both lead to cooling on the surface directly beneath the haze, which is thickest over the north of India, including the Ganges Basin. As darker coloured soot aerosols are somewhat dominant in the mix higher up in the atmosphere energy absorption outweighs solar reflection – so the brown haze leads to net atmospheric warming.

When you have warm air up high and cooler temperatures on the ground, you create what meteorologists call a stable atmosphere, with suppressed convection, and little precipitation. Higher air pressure at the surface makes the brown haze block the monsoon – we learned from a Science publication last month. As a result monsoon rains in northern India have decreased by 10 percent, whereas the south and the southwest has become wetter – so more droughts and more floods for the country as a whole, but not thanks to global GHG-induced climate change, but to local aerosol pollution.

Temperature was never limiting for cyclones

Now an international group of scientists led by the University of Virginia write in Nature they have discovered another climate consequence of this air pollution: an increase in damaging hurricanes – or tropical cyclones as they are referred to in this part of the world.

Over much of the northern Indian Ocean, including the Arabian Sea and the Bay of Bengal sea water temperatures are high enough to support tropical cyclone formation year-round. Meteorological conditions are however a limiting factor, especially during the months of July and August (which are the hottest, so with the largest amount of climatic energy) as during summer the ITCZ sweeps north and India experiences the monsoon rains. Cyclogenesis is therefore confined to the period leading up to the monsoon, or the period thereafter, the researchers write.

Wind shear though was – now that it’s weakened there are more and stronger cyclones

But as the monsoon changes, so does its influence on the tropical cyclones over the Arabian Sea. The same atmospheric pattern that blocks the monsoon, leads to a decrease of easterlies higher up in the atmosphere. These high-altitude winds blow from the almost opposite direction of the winds over the surface of the Arabian Sea (which come predominantly from the SW), creating what’s called a high vertical wind shear, which halts cyclone formation by blowing off the tops of tropical storms, removing the stratospheric cold air engine that creates the circular winds of real cyclones.

Now that as a consequence of the brown haze the high altitude winds have decreased, more storms can grow into full-blown cyclones, the researchers say – and these tend to develop much closer to the rainy season, when water temperatures are highest.

Tropical cyclones over Oman and Iran

The results are derived from theoretical analyses and model data, but the researchers also have statistical evidence to show cyclones have in fact increased between 1979 and 2010, both in frequency and strength. There used to be two or three weak cyclones per year, now there are extra cyclones in the weeks just prior to the arrival of the monsoon and their wind speeds can now reach hurricane category 5. The affected area has also increased – since 2007 cyclones have for the first time reached as far west as the Gulf of Oman and Iran.

monsoon cyclones wind shear
Aerosol pollution over the Indian subcontinent has led to monsoon disturbance and thereby a decrease of wind shear (b) and increase of cyclone intensity (a) over the Arabian Sea. Effects are most pronounced directly prior to the onset of the monsoon.

Tropical cyclones that form over the Arabian Sea have a high chance of making landfall – as the stretch of ocean is relatively small compared to Atlantic hurricanes and Pacific Asian typhoons – and (densely inhabited) coastlines are to the west, north and east. Depending on course there is a real chance the new cyclones can strike megacities like Mumbai and Karachi, home to almost 20 million people each.

© Rolf Schuttenhelm |

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