Twin Cyclones

Why in news?

• Recent satellite images have shown a pair of cyclones in the Indian Ocean region, one in the northern hemisphere and one in the southern hemisphere.
• Named cyclone Asani and cyclone Karim respectively, these are twin cyclones originating in the same longitude and now drifting apart.

What are Twin Cyclones?

• Twin cyclones are not really rare. The interplay of the wind and the monsoon system combined with the Earth system produces these synchronous cyclones.
• Twin tropical cyclones are caused by what are called equatorial Rossby waves.
• Rossby waves are huge waves in the ocean with wavelengths of around 4,000–5,000 kilometres.
• 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 positive value of the vorticity which is a measure of the rotation. 
• Very often twin cyclones are formed from these 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.

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.

PRACTICE QUESTION:

Consider the following statements regarding ‘Rossby waves’:

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

2. These waves arose due to the rotation of the Earth

Which of the statements given above is/are correct?

(a) 1 only

(b) 2 only

(c) Both 1 and 2

(d) Neither 1 nor 2

Answer