How a straw in the northerly wind (instead of seasonally north-easterly) can prove a speck in the eye for Chennai, as well as the larger coastal Tamil Nadu!

Weather events — or the lack thereof — over regions as far apart from each other as the Arabian Sea, North-West India, and the Bay of Bengal have seemingly conspired to deliver us a fait accompli.

Here is what has happened over the past couple of days. Barren westerly winds from North-West India have taken advantage of the absence of north-easterly to easterly winds in the Bay of Bengal (which bring seasonal rains to Chennai and Tamil Nadu).

The north-westerlies blow almost as northerlies (vertically from North to South) to fill the vacuum, and bring to bear part of the wafting hazardous pollution all the way down to the South along the East Coast.

@rajbhagatt posted a sensational satellite graphic of how these plumes of polluted air are filling the air, which went viral on Twitter and other social media through the day on Sunday.

 

 

 

 

So what gives?

 

So long as the easterly to north-easterly winds from the Bay of Bengal resume their normal pattern, Chennai and Tamil Nadu would have to live with the threat of polluted air drifting in from North India.

 

Banking on a ‘low’ for rescue

This is why the developing low-pressure area over the North Andaman Sea (South-East Bay of Bengal) becomes crucial in this context. Not just for its capacity to bring us rains but also in blowing away the invading pollutants.

A pall of gloom had descended over social media, with several Twitter handles — not least Chennai bloggers with their large following — giving vent to their feelings/airing concerns in public over the evolving situation.

 

 

 

So, it is the break in the North-East monsoon — when north-easterly winds pause briefly in the season — that has allowed the hazardous pollution from farmers burning their stubble after the Kharif harvest to sneak in along the East Coast and into Chennai/Tamil Nadu.

(A delightful irony here -- it is during the break in the preceding South-West monsoon that both the East Coast and Tamil Nadu got their fair share of rain! Just goes to show how a change in wind patterns upends weather across the two back-to-back monsoons!)

The westerly winds that prevail in North-West India (after the monsoon ends and shuts out the moist easterlies-south-easterlies from the Bay of Bengal by September-end) blow horizontal along the plains, or clockwise around the prevailing anticyclone.

An anticyclone, with its strong band of westerly winds, is what shuts out the monsoon over North India in the first place. The westerlies can blow along its right side from North to South, i.e., along the East Coast and into Chennai/Tamil Nadu. Thus, it also brings alternating barren or moist westerlies into play into the autumn over North India, later heralding the winter.

The pattern of westerlies in the western disturbance, the prevailing anti-cyclone over North India, the circulation in the Bay of Bengal (expected to become a low-pressure area) are shown in the graphic below.

Stubble burning

Unfortunately, this is also the time that coincides with the annual event — and increasingly a serious health hazard — of farmers burning huge mounds of stubble that the kharif harvest leaves them with.

The farmers have to prepare their fields for the impending rabi crop — but stubble burning is now at the heart of the winter air turning toxic in North India.

The burning happens mostly in Punjab and Haryana as testified vividly by satellite pictures, and the severe pollution levels prompted authorities to declare a public health emergency a couple of days ago.

The widespread concern is that unless farmers find a better way of disposing their stubble — even if that means giving them incentives — the situation will worsen in future.

It in in this context that weathermen are literally scouring the air — for what else, straws in the wind, for a hint that portends a change in the wind direction over North-West India.

So, the focus is back again on a couple of inbound western disturbances that blow from across the border all the way from the Mediterranean-Black Sea-Iran-Afghanistan-Pakistan into North-West India.

Influence on weather

These low-pressure bands of winds frequenting the region during the winter impart a significant influence on winter weather over North-West, Central, East, and by extension Peninsular India, though the cold nip they bring into in the air gets diluted into the South.

Their arrival is associated with snow and rain in the Himalayas and parts of North-West India. When they are dry or don't carry enough moisture, they drive in the Arctic cold into the region, leading the mercury level to plummet.

They help sustain Himalayan glaciers and the local ecology and provide nourishment to winter crops. They may lack the powerful punch of the South-West monsoon but cause the occasional weather disruption, especially in the hilly regions of North India, which are prone to landslides and flash floods.

For example, the June 2013 Uttarakhand floods, which devastated the pilgrim town of Kedarnath and killed over 5,700 people across the State, are thought to have been caused by anomalous western disturbances. Experts also believe western disturbances were behind the Jammu & Kashmir deluge of September 2014 and the Leh cloudburst of August 2010.

Reverting to the current context, the latest of the incoming western disturbances will dig into Central India and should help blow away the polluting air hanging over North India all the way to East India.

The track of the building low-pressure area over the Andaman Sea (South-East Bay of Bengal) would also be watched for its capacity to favourably influence the weather over the East Coast and South India.

The evolving scenario just goes to show how the weather couldn’t care less about geographical boundaries arbitrarily dictated by man. It can affect one and all, cutting across, geography, language, class, caste or creed.

What the bloggers say

The break in the North-East monsoon caused Chennai bloggers to stress on the need to save precious water...

 

 

 

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