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Recently the Bay of Bengal region saw a cyclone named Cyclone Asani that gave rain to parts of the eastern coast of India. This cyclone had a twin by name Cyclone Karim in the southern hemisphere. Such twin cyclones are said to be cause by the Madden-Julian Oscillation.
What is the Madden-Julian Oscillation or MJO as it is called?
The MJO is a large cluster of clouds and convection, perhaps 5,000-10,000 kilometres in size. Twin cyclones are born sometimes because of the MJO.
Not all tropical cyclones are born from the MJO. Sometimes they care caused by a simple Rossby wave with two vortices on either side of the equator; the MJO is a little more complicated. Often, it is said the MJO is a coherent structure with a Kelvin wave to the East and a Rossby wave on the trailing Western side. This is called the trailing side because the whole structure is moving Eastwards at a low speed of about 5 metres per second. This was discovered in early 1970s.
The Madden Julian Oscillation is created in the Indian Ocean and decays near the International Date Line. But this is just the part of the wave that is moist. That is, the part in regions where the ocean is warm, so there are lots of clouds. There is also the dry part of the MJO that goes right around the Earth and comes back again. Therefore, it is called an oscillation. This repeats every 30 to 50 days. It is aperiodic but it has a preferred recurrence time. Thus, in the winter season alone you will have three of the MJOs going past. “They are born in the Western side in the Indian Ocean and then they move to the East. You can see them going all the way to the centre of the Pacific, to the dateline. They go all the way from 60 or 65 degrees East to about 150 – 180 degrees East, where the Ocean is warm,” says Prof. Debasis Sengupta of Centre for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bengaluru, who is an expert on climate studies in the Bay of Bengal region. The warm pool in the ocean, that’s the region associated with clouds, convection and heavy rain. Beyond that, the ocean is cool and the structure moves around the earth along the equator as a dry disturbance in the atmosphere. There is not enough moisture to sustain the clouds in these regions, where the ocean is cool. The dry MJO wave can actually be traced around the world.
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