Explained | The science behind twin cyclones

Cyclone Asani, active now in the Bay of Bengal, has a twin — cyclone Karim — in the southern hemisphere. How do we explain this phenomenon?

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. 

According to a bulletin posted on the Indian Meteorological Department website, the severe cyclonic storm Asani moved in from its position in the Bay of Bengal, reaching a point 210 km south-southeast of Kakinada in Andhra Pradesh around 11.30 a.m. on May 10. It is expected to move nearly northwestwards and reach the coast near Kakinada-Vishakhapatnam around the morning of May 11. Then it is predicted to curve and move along the Andhra Pradesh coast before moving back over the Bay of Bengal, where it will weaken to a cyclonic storm and then a depression by the morning of May 12. 

It is fascinating that cyclone Asani is not alone, but has a twin in the southern hemisphere – cyclone Karim. According to NASA’s Earth Observatory website, cyclone Karim has created a path in the open seas west of Australia.

A much stronger wind speed has been observed in this member of the pair, which has led to its classification as a category one hurricane. According to the website, the cyclone will weaken quickly due to wind shear in the coming days and will pose no problem for heavily populated areas, although it may affect the Cocos Islands with a population of 600.

Twin cyclones are not really rare. The interplay of the wind and the monsoon system combined with the Earth system produces these synchronous cyclones.

Debasis Sengupta, professor at the Centre for Atmospheric and Oceanic Sciences at the Indian Institute of Science, Bengaluru, explains that 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.