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Why Northeast Monsoon Is Quiet, Literal Drought In South Peninsula

October 26, 2023 2:30 PM |

The arrival of the northeast monsoon was announced last week, albeit with a caution of a weak phase to start with. However, the entire region nearly parched even after the formal arrival of winter monsoon. The week gone by, between 19 – 25 October, practically received no rains in most parts covered by the northeast monsoon. Most of the meteorological sub-divisions, except Kerala, have been abandoned by the seasonal rains with deficiencies as large as 95% to 100%.  The state of Karnataka, which was left marooned in parts, even during southwest monsoon season, continues to witness the spectre of dehydrated fields, more so over the northern parts.

Northeast monsoon always brings hope and relief for the South Peninsula, more so when deceived by the summer monsoon rains.  Northeast monsoon, which actually kick starts with bountiful rains over Coastal Andhra Pradesh and Tamil Nadu, still awaits a typical rainy spell vindicating the change of season. As such, more often than not, northeast monsoon has been making a late entry for the last few years. Against the normal date of 20th October,  monsoon rains commenced on 28th Oct, 25th Oct and 29th Oct in 2020, 2021 and 2022 respectively.

The onset of the northeast monsoon has to wait complete withdrawal of the southwest monsoon, as no overlapping is provisioned in the criteria.  Following the retreat of the southwest monsoon on 18th October, the stage was clear for the arrival of the northeast monsoon. However, the South India monsoon made a rather timid start, despite the formal announcement of onset on 21st October. Various meteorological factors stand to reason for such a soft beginning.

Establishing northeasterly flow over the South and Central Bay of Bengal becomes a prerequisite for this annual event. However, mere streaming of northeasterly winds may not suffice to fulfil the criteria. Unlike the Western Ghats, the Eastern Ghats are not tall enough to cause orographic lifting to set off sporadic rains. Also, the Eastern Ghats is a sub-set of discontinuous mountain ranges, from Odisha, Andhra Pradesh and Tamil Nadu, with a modest elevation of 600mtr to 900mtr.

As long as the northeasterly flow remains laminar, it will not trigger any monsoon bursts.  Some perturbations may be feeble ones, in the form of troughs or waves, moving across the southern latitudes of the Bay of Bengal to act as catalysts. Such features have been missing so far. Trough in the easterly stream have weather belts ahead of the system, while the easterly wave activity trails rear of the system.

Cyclonic storms in the Indian seas disrupt the airflow.  Storms originating over the Bay of Bengal and heading for Bangladesh or Myanmar lead to moisture depletion all along the coast and deep inland.  The sapping of moist winds and alteration of thermal configuration delay the cloud formation over the landmass. Such storms also drain out a large chunk of energy from the sea surface.  One such storm ‘Haroon’ just cleared the Bay of Bengal and Northeast India. Airflow with other captive parameters is expected to get replenished and resume normal within 72 hours.

Northeast monsoon activity will remain on mute for another 48 hours. Spread and intensity of rains will increase from 28thOctober and more authentically from 29thOctober. Tamil Nadu, Kerala, South Interior Karnataka and South Coastal Andhra Pradesh will simultaneously receive the first burst of northeast monsoon.

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