While Tamil Nadu was bracing for strong winds ahead of Nivar, it’s impact was rather mellow. Weather blogger Pradeep John who is fondly called as Tamil Nadu Weatherman tells SUSHMITHA RAMAKRISHNAN why Nivar was not as violent as expected and speaks about factors that influence the weather.
Before Nivar reached Tamil Nadu, there was great fear of property damage by high-speed winds. But the cyclone seemed quite harmless. What happened?
Nivar crossed the land in an area which is not densely populated. In the last minute, there could have been an increase in wind shear on one side. The eye of the cyclone also did not form properly. This could have been the reason it wasn’t as windy. But the conditions were explosive; which is why IMD gave a strong warning and I too expected the same. We expected a sustained wind speed of up to 120-130 kmph. We do not have devices to measure winds north of Marakkanam to surely know how fast the winds were around landfall. During Nivar, Meenambakkam recorded gusts of 70 kmph, Tambaram recorded 50-60 kmph winds and in Nungambakkam and inland Vellore we saw gust speed of over 50 kmph. We could’ve had faster winds in the location it crossed the land.
When does a cyclone become windy and when is it rainy? Why did Nivar bring more rain?
The last three intense cyclonic storms, Gajah, Vardah or Thane, didn’t give widespread rain, but we had a lot of winds. All these had less wind shear and other ideal conditions and intensified till the last moment. We will have a cyclone with less rains and less winds, if the cyclone is filled with dry air or if the wind shear is high it will blow away the top clouds of cyclone, leading to its collapse. It will become rainless in a few hours. If the shear is less it will be a rain-bearing system and a strong one. The cyclone should also be a slow moving one for good rains. Traditionally the cyclones which move West-Northwest will be more rain bearing ones.
We have a few more weather systems brewing right now. Should we be concerned about any of them?
The first one coming up will give some rains to Tamil Nadu from December 2. There will be heavy rains in the South Tamil Nadu, Southwest Tamil Nadu and Delta regions. It will go through the Gulf of Mannar and might interact with rugged Sri Lanka landmass, go via Kanniyakumari sea and go to Arabian Sea. Southern districts such as Theni, Dindigul, Kanyakumari, Tenkasi, Virudhunagar, Thoothukudi, Tirunelveli and Ramanathapuram are areas we need to watch in the next rains. Coonoor, Kodaikanal, Papanasam and Manjolai areas need closer watch for heavy rains. Winds will not be much of a problem, maximum speed will be around 50-60 kmph in southern districts.
What are the different factors that affect the weather pattern around the State?
When a low pressure moves North-Northwest direction from above Sri Lanka, North Tamil Nadu will get rains. Nivar is an example of this. It formed in southwest bay, so it moved up. The location where the cyclone forms is also important. The upcoming weather system has formed on lower latitude and as the conditions are not ideal, it will not intensify into a stronger cyclone. It will move pure west pushed by the easterlies and will not travel northward. If the same cyclone had formed in October it would have gone up. Its formation in November-December, is favourable to Tamil Nadu with shifting down of Inter Tropical Convergence Zone. Other than that, there is the Equatorial wave - Madden-Julian Oscillation (MJO). The low pressure systems this season will keep coming in pulses along MJO via Andaman.
Usually when there are good rains, it is often followed by droughts. should we expect the same in a year or two?
The North-East monsoon is so erratic to define a clear pattern. It has often gone in series of good years, or series of bad years or one bad year followed by one good year. Hence, a drought year is not far away. So conserve all this precious water.
Do you think the low pollution levels due to the lock down has anything to do with the monsoon being active this year?
Not at all. Heavy rains have happened in the past when pollution levels were high too.
1 Wind shear, sometimes referred to as wind gradient, is a difference in wind speed or direction over a relatively short distance in the atmosphere. Pradeep John suggested that a last-minute increase in the vertical windshear before landfall could have softened the winds during Nivar.
2 MJO is a major fluctuation in tropical weather on weekly to monthly time scales. The MJO can be characterised as an eastward moving ‘pulse’ of cloud and rainfall near the equator that typically recurs every 30 to 60 days.