Science & Technology

Tropical depression forms in the Arabian Sea

An interesting fact about the present depression is that it is the first-ever to form in the Arabian Sea in October after seven years

By Akshay Deoras
Published: Friday 09 October 2015

The image shows the formation of tropical depression in the Arabian Sea as can be seen in the satellite image on October 9  
Courtesy: CIMSS

Come October and India faces the threat of tropical cyclones which form in the Bay of Bengal and sometimes even in the Arabian Sea.

Last year, during the same time, weather forecasters were busy monitoring Hudhud, a powerful tropical cyclone. Two years ago, tropical cyclone Phailin lashed the eastern coast of India.

Fortunately, this year there is not much activity going on in the Bay of Bengal and the Arabian Sea. The cyclone season is quieter compared to previous years.

What is forming in the Arabian Sea?

But there is something “cooking” in the Arabian Sea at present. A tropical depression (or simply a “depression” as per the terminology of the India Meteorological Department or IMD in short) has formed in the Arabian Sea.

On Friday morning, this depression was located near 14°N and 70.3°E, about 410 km west-southwest of Goa and 630 km south-southwest of Mumbai.

The mean sea-level pressure at the centre of this low pressure is estimated to be around 1004 millibar by the Thai Meteorological Department. The maximum wind speed at the centre is estimated to be around 30 knot (56km/hour) as per the US Naval Research Laboratory.

As per the IMD, a depression is a low pressure which has an associated wind speed of 32-50km/hour. As per their scale, under favourable conditions, a low pressure in the sea intensifies into a depression, then into a deep depression and then into a cyclonic storm and so on.

Thus, a depression has the second lowest rank in terms of intensity on the scale. An interesting fact about the present depression is that it is the first-ever depression to form in the Arabian Sea in October after a gap of seven years.

As per the records, on October 19, 2008, a depression had formed in the Arabian Sea, which later intensified into a deep depression as per the IMD scale (associated with a wind speed of 51-59 km/hour).

This system is famously known as the Tropical Cyclone 3B in the western world which brought heavy rains and cause severe floods in Yemen and Oman.

Between 1891 and 2014, only 18 depressions have been recorded in the Arabian Sea. Out of these 17 strengthened to reach the category of cyclonic storms (with associated wind speed of 60-90 km/hour) but only 11 strengthened to reach the category of severe cyclonic storm or above (with associated wind speed greater than 90 km/hour). As against 149 depressions and cyclonic storms (in total) in the Bay of Bengal in October, only 35 depressions and cyclonic storms have been reported in October in the Arabian Sea between 1981 and 2014.

Why the Bay of Bengal is more vulnerable?

One of the main reasons behind the Bay of Bengal being more vulnerable to formation of low pressures in the post-monsoon season, is its proximity to places like the Gulf of Thailand.

Often, disturbances (low pressure systems) travel from areas like the Gulf of Thailand and develop in the Bay of Bengal (for instance Phailin) under favourable conditions. Such systems enjoy a lot of moist air in the Bay of Bengal unlike the Arabian Sea.

As the monsoon withdraws from Pakistan and nearby areas in early September, the air in these regions starts drying up. This, in turn creates problems for the survival of low pressure systems in the Arabian Sea. The image below clearly shows that the south-west monsoon has withdrawn from a greater part of the Arabian Sea as compared to the Bay of Bengal. Dry air acts like poison and if it enters such low pressure systems, it chokes it to death.

The image shows the withdrawal of the south-West monsoon from the Indian sub-continent 
Courtesy: IMD

Arabian Sea depression not to affect India

It is important to note that the depression in the Arabian Sea is at a safe distance from the western coast of India, particularly from the coast of Karnataka, Goa and Maharashtra.

The sea surface temperature in this region is around 29°C which is good enough for its intensification. However, it looks like it is being troubled by moderate vertical wind shears (roughly it is the change of winds with height). Such systems need relaxed vertical wind shears to develop. Phailin and Hudhud could intensify so much in their final stages because the vertical wind shears around them were completely relaxed.

The image shows the atmospheric set up on October 11.

Depression will intensify, fishermen should avoid deep sea

As per various weather models, this depression will move slightly northwards till Sunday, and in this process, it will slightly intensify due to other favourable factors.

As of now, it is not expected that it will intensify to a strength of a tropical cyclone or something equivalent to a tropical cyclone or Phailin or Hudhud.

After Sunday, this system will come under the influence of an upper air high-pressure region (having clockwise winds) which will be positioned over the Northern Arabian Sea (see image above). This high pressure will steer the depression in mostly the western direction, that is away from India. IMD believes that in the next 48 hours, this depression can intensify into a deep depression.

Thus, there is no perceived threat to the western coast or other parts of India. However, under its influence, there will be some possibility of rainfall along the western coast of Karnataka, Goa and Maharashtra till early next week.

But, the state of the Arabian Sea (off Goa and Maharashtra) will remain rough till Tuesday (October 13). It will be dangerous for fishermen to venture out into deep sea for fishing. Hence, they are advised not to venture out in the Sea till Tuesday and Wednesday.

At this stage, people in Yemen and Oman must keep an eye on this system. If this system survives and does not die in the Arabian Sea, it can bring rainfall in these two places after October 15 (Thursday).

Monsoon withdraws from north and central India

As was expected, the robust withdrawal of South-West monsoon has taken place in the last few days. Monsoon has now entirely withdrawn from north India and parts of central India.

As on October 9 afternoon, the withdrawal line of south-west monsoon passes through cities like Forbesganj, Ranchi, Raipur, Nagpur, Jalgaon and Veraval as per the IMD. The withdrawal process is expected to resume from the next week and monsoon will likely to withdraw from Maharashtra by October 15.

Mainly hot weather, with somewhat pleasant nights, will continue in north Indian plains and central India in the coming days. The maximum temperature will likely to stay above 35°C in New Delhi. But Mumbai may witness more hot days in the next week with maximum temperatures possibly touching 35°C.

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