Twin Cyclone in Indian Ocean

Twin Cyclones Asani and Karim (one in the northern hemisphere and one in the southern hemisphere), on opposite sides of the equator, have formed.

Twin cyclones are formed when the wind and monsoon systems interact with the Earth system.
Equatorial Rossby waves are the primary reason.

When the vorticity in both the Northern and Southern hemispheres is positive, as it is with Rossby waves, the moist air in the boundary layer is elevated slightly. That is sufficient to initiate a feedback loop.
Water vapour condenses when the air is elevated slightly, forming clouds. The latent heat of evaporation is released as it condenses.
The atmosphere warms, the parcel of air rises, and the process triggers a positive feedback loop.
Because it is lighter than the surrounding air, the warmer parcel of air can rise higher and form deeper clouds.
Meanwhile, wetness infiltrates from all sides. If certain criteria are met, this results in the production of a cyclone.

They will normally move westward once they have formed.
They will have a slightly northward component of motion in the northern hemisphere, whereas they will have a slightly southerly component in the southern hemisphere.
As a result, the cyclone in the northern hemisphere will move north and west, while the one in the southern hemisphere will move south and west.

Rossby Waves are the name for the meandering jet streams.
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.
Rossby waves are natural phenomena that occur in the atmosphere and oceans as a result of the earth’s rotation.
They occur in planetary atmospheres due to variations in the Coriolis effect with latitude (When temperature contrast is low, the speed of the jet stream is low, and the Coriolis force is weak, causing meandering).
When polar air flows toward the Equator and tropical air moves poleward, Rossby waves form.
Low-pressure cells (cyclones) and high-pressure cells are explained by the existence of these waves (anticyclones).
Features
- Rossby waves are massive ocean waves with wavelengths of 4,000–5,000 kilometres.
- A vortex in the northern hemisphere and another in the southern hemisphere exist in this system, and one is a mirror image of the other.
- The northern hemisphere vortex spins counterclockwise and has a positive spin, whereas the southern hemisphere vortex spins clockwise and has a negative spin.
- Both have a positive vorticity value, which is a measure of rotation.
- These Rossby waves frequently produce twin cyclones.

Karim and Asani are “twin” cyclones since they originated at the same time in the same geographic location and are mostly derived from the same “parent” circulation, the Madden-Julian Oscillation, or MJO.

Madden-Julian Oscillation – The MJO is a cloud, wind, and pressure disturbance that moves eastward at a speed of 4-8 metres per second and takes 30-60 days to circle the globe. It can sometimes take up to 90 days. It’s a transient phenomenon that can be seen mostly across the Indian and Pacific Oceans.
Features
- The MJO is a huge cloud and convection cluster that spans 5000-10,000 kilometres.
- It’s made up of a Rossby wave and a Kelvin wave, both of which are types of wave structures seen in the ocean.
- The Kelvin wave is on the MJO’s eastern side, while the Rossby wave is on the MJO’s western, trailing edge, with two vortices on either side of the equator.
- The MJO does not, however, cause all tropical cyclones. It’s occasionally just a Rossby wave with two vortices on either side.

A Kelvin wave occurs when the Earth’s Coriolis force is balanced against a topographic limit, such as a coastline, or a wave guide, such as the equator, in the ocean or atmosphere.
It occurs in a nearly rectangular area of the water (e.g., the English Channel), with the tidal range increasing to the right of the progressing wave’s path and decreasing to the left.

Pic Courtesy : NASA

Content Source : The Hindu