The Indian Ocean region recently witnessed the formation of twin cyclones – Cyclone Karim and Cyclone Asani. Cyclone Karim, categorised as a category II hurricane, exhibited wind speeds reaching up to 112 kmph. On the other hand, Cyclone Asani fell under the category of a Severe Cyclonic Storm, with wind speeds between 100-110 kmph, gusting to 120 kmph.
Both these cyclones originated from the same longitude in the Indian Ocean region and have now started moving apart. Cyclone Karim has formed a trajectory in the open seas west of Australia. South African country Seychelles gave the name “Karim,” while Sri Lanka proposed the name “Asani” for the cyclones.
The Phenomenon of Twin Cyclones
Twin cyclones are the result of a collective interplay of wind, the monsoon system, and the Earth’s rotational system, causing the formation of simultaneous cyclones. These twin cyclones often occur due to equatorial Rossby waves, massive ocean waves with wavelengths ranging between 4,000–5,000 kilometres. The term “Rossby Waves” honors famous meteorologist Carl-Gustaf Rossby, who first explained their formation due to Earth’s rotation.
Each Rossby wave system consists of two vortices – one in the northern hemisphere spinning counterclockwise, and the other in the southern hemisphere spinning clockwise. Despite their differing directions of spin, both vortices exhibit positive vorticity, indicating their rotational motion.
The Birth of Cyclones
The formation of cyclones is a complex process that begins when moist air in the boundary layer gets uplifted slightly due to positive vorticity in both hemispheres. This lifting initiates a feedback process: the rising moist air leads to water vapour condensation, resulting in cloud formation. The process further releases the latent heat of evaporation, warming the atmosphere and causing the air parcel to rise higher, forming deeper clouds.
The success of cyclone formation depends on specific conditions. Firstly, the ocean surface temperature should be 27 degrees or warmer. Secondly, the wind shear in the atmosphere must not be excessively high. If these conditions are met, the cyclone can form a tall vortex with various kinds of clouds.
Movements of Twin Cyclones
Once formed, the twin cyclones typically move westward. The cyclone in the Northern Hemisphere will exhibit a slightly northerly motion component, while the counterpart in the Southern Hemisphere will have a marginal southern component. This pattern means that the northern cyclone moves north-west, while the southern one moves south-west.
Role of Madden-Julian Oscillation (MJO) in Cyclone Formation
MJO, a large cluster of clouds and convection spanning 5,000-10,000 kilometers, plays a crucial part in the formation of some twin cyclones. Comprising a Rossby wave and a Kelvin wave, MJO carries two vortices on either side of the equator. However, it is essential to note that not all tropical cyclones originate from the MJO. Sometimes, a solitary Rossby wave with two opposing eddies can give rise to cyclones.
UPSC Civil Services Examination: Previous Questions Discussed
In the 2020 UPSC Civil Services Examination, statement 1 was incorrect as Jet Streams occur in both hemispheres and extend from 20° latitude to the poles. Statement 2 was accurate as only tropical cyclones develop an ‘eye.’ Lastly, statement 3 was incorrect because the warmest temperatures are found inside the eye of a cyclone rather than its surroundings. The erroneous statement suggested the opposite — that the temperature inside the eye of a cyclone is nearly 10ºC lower than that of the surroundings.