Early hints of a storm that formed over the Arabian Sea, near the Lakshadweep Islands, intensified into a deep depression on Monday, 10 June and is expected to develop into severe cyclonic storm Cyclone Vayu by morning on 12 June, Wednesday. Cyclone Vayu, which continues to gather strength from warm ocean waters, is expected to peak on 13 June, Thursday with wind speeds of over 135 kmph over waters of the Arabian sea.
The tropical depression Vayu continues to strengthen and may get stronger than 65 Kmph, which seems very likely in the Arabian Sea near the Lakshadweep Islands. The region is well known for its monsoon onset vortex, or low-pressure counter-clockwise flow that comes on just before the onset of monsoon. The result is a ‘vortex’, which lays the groundwork for a tropical depression and the genesis of the cyclone-to-be Vayu.
How the soon-to-be-Cyclone Vayu was formed
The most important ingredient for depressions to be born is the warm sea surface temperatures, which can provide the evaporation and water vapour to a ‘deep depression’ lifting air rapidly into the atmosphere. When the warm water vapour cools and condenses as it rises, it further strengthens the depression, causing a developing cyclone to pick up in intensity.
A timely onset of the monsoon tends to cool the Arabian Sea thanks to the strong southwesterly winds zooming in from across the equator to the southwest towards the Indian west coast. A delayed monsoon onset this year has allowed the Arabian Sea temperatures to remain very warm, favouring the growth of a tropical depression into a potentially full-blown cyclone in just a few days.
The second important ingredient is the so-called cyclogenesis or the seeding of the depression itself. This typically occurs because the atmosphere is unstable and is trying to convect or organise rain bands. The air that is being sucked into the growing convection begins to rotate because of earth’s rotation which increases as we move further away from the equator. Once this cyclogenesis happens, the lack of shearing winds and warm sea surface can allow it to grow.
The third ingredient is the steering winds in the 2Km to 5 Km range which navigate the depression as it wanders around the ocean looking for warm water and energy. If the depression was more towards the African/Arabian coast, the upper-level winds from the east towards the west would have steered the cyclone towards the northwest into Oman. But since the depression is much closer to the Indian coast, the steering winds will take it towards Gujarat instead.
All these ingredients are available over the Arabian Sea and the Bay of Bengal during the pre-monsoon and post-monsoon months. Most cyclones form over the Bay of Bengal. But the late onset of the monsoon and the continued warming of the Arabian Sea are making it more likely to grow cyclones over the Arabian Sea, especially during the post-monsoon seasons.
Cyclone Vayu will be less intense than Fani was
Strong southwesterly winds are now in motion as monsoon has begun, also increasing the “shearing winds” in the atmosphere. Another critical requirement for a low-pressure area to grow into a cyclone is a weak or a non-existent “vertical shear”.
This is the change in wind intensity from the surface to greater heights in the atmosphere. These forces can either make or break cyclones depending on how strong they are. Vertical shear needs to be very weak, like it was in the case of Cyclone Fani, for storms to pick up in intensity. When these forces are stronger, vertical shear can chop the head off a cyclone and decimate it. The early stage of this late-onset monsoon is thus providing the lifeline of a weak shear for this tropical depression especially since it is so Far East in the Arabian Sea.
Cyclone Vayu will not get strong as Fani did. Since it is much closer to the Indian coast than the African/Arabian coast, it is going to drift northward because of steering winds.
Now, preparedness will count for more than predications
Forecasting of cyclones including their strength and tracks are now remarkably good especially for a day or two ahead. As Vayu forms in the next two days and heads to the Gujarat coast, it will likely achieve wind speeds of greater 110 kmph as predicted by IMD. The damages are also expected to be minimum considering the generally lower population densities at the target region.
But that is hardly a consolation or a reason for complacency. Preparations for Vayu are important. But it is also important to keep an eye on how the Arabian Sea cyclones are changing with the warming of the land, atmosphere, and ocean around India.
It is already well established that the warmer Arabian Sea and the much warmer land in the northwest of India including Pakistan are enabling the humongous moisture supply for driving a threefold increase in widespread floods over India.
In other words, even if we tide through Vayu just fine, this monsoon season could bring widespread floods and the future, more such cyclones. One piece of good news, if any, is that this cyclone is far enough north and weak enough that it should not affect the monsoons and could even advance it northward a bit faster than normal.
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Updated Date: Jun 11, 2019 13:26:23 IST