Explained | The science behind twin cyclones

What is the News?

A pair of tropical cyclones, Cyclone Asani & Karim (one in the northern hemisphere and one in the southern hemisphere) , one on each side of the equator have been formed.

These are twin cyclones originating in the same longitude and now drifting apart. 

What are Twin Cyclones?

The interplay of the wind and the monsoon system combined with the Earth system produces twin cyclones.

Formation of twin cyclones: They are basically formed by equatorial Rossby waves.

Rossby waves are named for famous meteorologist Carl-Gustaf Rossby who was the first to explain that these waves arose due to the rotation of the Earth.

They are huge waves in the ocean with wavelengths of around 4,000–5,000 kilometers.

This system has a vortex in the Northern Hemisphere and another in the Southern Hemisphere, and each of these is a mirror image of the other.

The vortex in the north spins counterclockwise and has a positive spin, while the one in the Southern Hemisphere spins in the clockwise direction and therefore has a negative spin.

Both have a positive value of the vorticity which is a measure of the rotation.

What is the process behind the formation of Twin Cyclones?

When the vorticity is positive in both Northern and Southern hemispheres, as is the case with Rossby waves, the air in the boundary layer, which is moist, is lifted slightly. That is enough to set off a feedback process.

When the air is lifted slightly, the water vapour condenses to make clouds. As it condenses, it lets out the latent heat of evaporation.

The atmosphere warms, this parcel of air rises, and positive feedback is set off by this process. The warmer parcel of air can rise further because it is lighter than the surrounding air, and it can form deeper clouds.

Meanwhile, moisture comes in from both sides. This leads to the formation of a cyclone if certain conditions are present.

What is Madden-Julian Oscillation or MJO? 

Karim and Asani are “twin” cyclones not only because they formed at the same time in the same general area but also because they were formed primarily from the same “parent” circulation: the Madden-Julian Oscillation or MJO.

The MJO is a large cluster of clouds and convection, around 5000-10,000 kilometres in size.

It is composed of a Rossby wave and a Kelvin wave, which is a type of wave structure that was seen in the ocean.

On the eastern side of the MJO is the Kelvin wave, while on the western, trailing edge of the MJO, is the Rossby wave, once again with two vortices on either side of the equator.

Source: The post is based on the article “Explained | The science behind twin cyclones” published in The Hindu on 13th May 2022.