Re-curving cyclones

In Northern Hemisphere, recurvature of a tropical cyclone is defined as the situation when a tropical cyclone transits from a mainly westward track to a northward and sometimes even an eastward track.

A strong subtropical ridge usually steers a tropical cyclone to move along its periphery. If the ridge retreats eastward, there is a high chance for the tropical cyclone to move along the southwestern or western flank of the ridge and track northward.

Like the googly in cricket, it’s deflected right or eastwards. This is due to air currents in the local atmosphere that push cold air from the poles towards the equator and interfere with cyclone formation. That’s what make them ‘re-curving.’ In the southern hemisphere, the cyclones spin clockwise and therefore also re-curve in the opposite direction.

Re curving cyclones in India’s  neighbourhood

The ones that typically strike the Indian neighbourhood in the northern hemisphere rotate anticlockwise. Their normal behaviour is to derive strength from the moisture in waters such as the Bay of Bengal, move west, incline in a northerly direction and peter out into the sea or land, depending on their origin.

For example cyclone Mora formed over the Bay of Bengal in May 2017. It rapidly strengthened with the India Meteorological Department classifying it as a “depression” and eventually as a cyclonic storm. It kept north, almost parallel to the Myanmar coast and then made landfall in Bangladesh and blew over Nagaland. In a re-curving cyclone, the cyclone gets a sort of second wind when it is on the wane.

Impact of Re-curving cyclones on monsoon

During the monsoon months, cyclones in the Western Pacific move westwards towards India and aid the associated rain-bearing systems over the country. However, in the years of a re-curve, they do not give as much of a push to the rain as they do in the good monsoon years .

Long-term data suggest that while there has been an increase in the number of tropical cyclones in India’s neighbourhood there is no clear trend in re-curving ones. In general, cyclone activity in India peaks around November, by which time, the summer monsoon has already passed. Rarely do re-curving cyclones pose a mortal threat to Indian coasts.

Prediction of Re-curving cyclones

As climate change is projected to increase the frequency of extreme events, scientists have warned that tropical cyclones are likely to get more intense, and this could mean more scrutiny of re-curving ones. A challenge with re-curving cyclones is that it is hard for weather models to pick them early on and so they pose unique challenges in terms of hazard preparedness and disaster management.