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Why the second depression in Arabian Sea in 125 years may be a damp squib

Abhishek Waghmare/ 14 Mar 18 | 05:13 PM

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A low-pressure area formed in the Indian Ocean near the equator on March 10 has transformed into a depression in the southeastern Arabian Sea, and is travelling in the northwestern direction towards Lakshadweep islands. The phenomenon gave rains in Kerala and Tamil Nadu (TN).

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The coastal port city of Tuticorin in Tamil Nadu, very close to the southern tip, received 20 cm of rain, its highest daily precipitation ever, on March 13, 2018.

The depression is about to weaken soon, according to the latest bulletin from the India Meteorological Department (IMD). A similar but powerful phenomenon in December 2017 had resulted in a deluge in the financial capital Mumbai, causing substantial damaged property. The strength of the current depression in the Arabian Sea is low, and is weakening further, with little probability of damage or extreme weather.

Low-pressure area, depression, cyclonic storm (cyclone) and severe cyclonic storm are the stages of a cyclone in increasing order of strength measured essentially in moisture content and resulting wind speeds.

(A) According to where it dissipated            

Arabian SeaBay of BengalLand431292

  

 (B) According to the month

MarchAprilMay742125

Though formation of a depression is a normal phenomenon in the pre-monsoon (March–May) and post-monsoon (October–December) periods, there has been only one such instance of formation and dissipation in the Arabian Sea in the past 125 years; and this one is only the second one after 1925 in recorded history.

There have been seven instances of depression formation in the month of March during the last 125 years (1891-2016), of which six have dissipated in the Bay of Bengal. The chances of depressions and cyclones increase as warming increases over the summer months. As against seven occurrences in March, there have been 42 cyclones above the level of depression in April, and as many as 125 in May in the 1891-2016 period.

Warmer sea surface temperature (SST) in the Bay of Bengal and about five to six other factors make the Bay the natural physical path for travelling of low pressure areas, depressions and cyclones. In between the two seas around the peninsula, the Bay has seen 130 depressions in 125 years, while the Arabian Sea has witnessed 42.

Rising oceanic heat content in Indian Ocean

YearOcean heat content in 10^22 joules20081.76820091.84520102.96120113.42320123.55820134.26820143.99820154.04620164.62620172.956

Source: National Oceanic and Atmospheric Administration, USA

While SST and oceanic heat content are parameters that decide the course of the depression, vorticity (that gives the moisture rotational ability), wind shear (variation in wind speeds at different altitudes from sea level), sea stability and the Coriolis force (the apparent force due to earth’s rotation that gives the low-pressure area its spin) are the atmospheric parameters.

But meteorologists say it is the ocean heat content, or the energy (in joules) contained in the upper layer of the ocean or sea, that is responsible for this particular depression to take the Arabian Sea route this time. Heat content in the Indian Ocean has been rising faster in the last decade than that it did in the past.

There has been an increase in extreme weather events, that of more than average daily rainfall and reduction in number of rainy days, effectively concentrating the same amount of rainfall in less number of days, a substantial number of research studies have shown recently.

Rapidly rising oceanic heat content, like never before, is a cause to worry in the light of climate change.

Ranjan Kelkar, a former Director General of the India Meteorological Department, who served between 1998 and 2003, said that this instance, though being the first in recorded history, is nothing abnormal.

“There are a set of conditions to be satisfied for the formation of a tropical cyclone in the Indian Ocean. Even when all conditions are met, a cyclone is not a certainty. On the contrary, a cyclone can manifest itself even in the situation when some pre-conditions are overbearing over others," he said.

The depression, with wind speeds of 40-50 kmph, currently stands located about 340 km west-southwest of Thiruvananthapuram. According to the latest IMD bulletin, it will weaken before it assumes a role of a deep depression, a situation where wind speeds cross 65 kmph.

“There is low probability for further intensification of the system into deep depression. It is likely to weaken gradually over southeast and adjoining east-central Arabian Sea after 24hours," says the bulletin.

Subsequent moisture incursion into the western coasts would most probably give light rains on the coast and the interiors. The regional meteorological centre at Mumbai has forecast isolated showers in the coastal and interior Maharashtra.

Kerala and southern Tamil Nadu have already witnessed rains from March 12 till date. According to some meteorologists, minor damage to small hutments and trees in Lakshadweep is possible.

Weather scientists say that though this is unusual, unprecedented weather phenomena do occur time and again in nature.

Tropical storms or cyclones are normal weather phenomena forming in the Indian Ocean in the March-May and October-November periods in any particular year, and dissipating their energy in the form of wind and rain either in the Bay of Bengal or in the Arabian Sea.

The Bay of Bengal attracts more cyclones due to its inherent characteristics. Of the 174 instances of pre-monsoon cyclones turning into a depression, 130 have dissipated in the Bay of Bengal, as against 42 in the Arabian Sea and two on land.

The water surface in the Bay of Bengal has an upper layer of fresh water due to a continuous discharge of freshwater from Ganga and Brahmaputra. This results in more warming as well as addition of moisture for cyclonogenesis—or formation of cyclone. Further, among other equally important factors, the vertical wind shear is low in Bay of Bengal, or the variation in wind speed at various altitudes over the bay is lesser than that over the Arabian Sea.

One more unusual thing about the current depression is that it was formed very close to the equator, at latitude 1.7 deg North. Though the equatorial seas get heated up more rapidly, the spin required for cyclone formation is weak near the equator.      

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