The Northeast monsoon is a boon and a bane for Tamil Nadu. Discuss its mechanism and the role of Bay of Bengal cyclones in rainfall variability.

The Northeast (NE) monsoon is the main rainy season for Tamil Nadu [1]. In 2026 it set in on 15 October, the day the southwest monsoon withdrew from India [1]. IMD's normal onset date is about 20 October [2]. Because its rain depends heavily on Bay of Bengal systems, the season can be a lifeline in one year and a disaster in the next.

Mechanism

• Wind reversal: NE monsoon onset coincides with SW monsoon withdrawal. Winds over the Bay reverse and blow from the northeast [3].
• Moisture pick-up: these winds gather moisture over the Bay and drop it on the Coromandel coast. During June–September, the Western Ghats keep most SW monsoon rain away from Tamil Nadu.
• Synoptic triggers: low-pressure systems, depressions and cyclones produce most of the heavy spells. The Bay saw 168 depressions in October–December over 1971–2020 [4].

SW monsoon withdraws → NE winds over Bay → moisture + Bay lows/cyclones → rain on TN coast

Fig: Simplified NE monsoon mechanism

Boon

• Water security: the season refills reservoirs, tanks and groundwater for a state that gets little SW monsoon rain [1].
• Agriculture: it supports samba paddy and rabi crops.

Bane

• Extreme bursts: from 15–18 October 1976, Madras (Nungambakkam) recorded 344 mm, about 75% above normal [5].
• Cyclone damage: Very Severe Cyclonic Storm Thane (2011) caused extensive damage on the north Tamil Nadu coast [3].
• Urban flooding: concrete has replaced open land and drains, so the same rain now does more damage.

Bay of Bengal cyclones and rainfall variability

• Rain-givers: a depression in mid-October 2024 started the season, and Fengal followed in November [2].
• Rain-stealers: VSCS Hamoon (2023) pulled moisture away and sharply cut Tamil Nadu's rainfall [6].
• Onset-delayers: Sitrang (2022) delayed onset [7].
• Track matters more than landfall: the 1976 storm turned north towards Masulipatnam, yet Madras still flooded [5]. Rain bands reach far beyond the landfall point.

Overall, the same Bay systems both feed and threaten Tamil Nadu. Whether the state floods or faces a shortfall depends on the storm's track as well as its strength. The way forward is to use IMD's day-wise heavy-rain warnings [1] for district-level urban-flood alerts, run reservoirs based on forecasts, and build drainage that can handle extreme rain. These steps would turn the monsoon's variability into manageable risk, in line with SDG 11 and SDG 13.