Tamil Nadu's water security depends on a short, cyclone-prone northeast monsoon. Examine the mechanism of the NEM and its variability.

Tamil Nadu lies in the rain shadow during the southwest monsoon (SWM). It receives about 48% (442.8 mm) of its annual rainfall in October–December, during the northeast monsoon (NEM) [2]. This short season also overlaps the peak cyclone season, so its rain is both vital and volatile.

Mechanism of the NEM

• Wind reversal: After the SWM withdraws up to 15°N, the low-level winds turn northeasterly. The easterlies normally set in by 14 October, and the normal onset date is 20 October [2].
• Onset criteria: IMD's rules from 1988 (amended in 2006) require four things: persistent surface easterlies over the Tamil Nadu coast, easterlies reaching up to 850 hPa, fairly widespread coastal rain, and SWM withdrawal up to 15°N [1].
• Synoptic boost: The NEM season is the main cyclone season in the North Indian Ocean. Low-pressure systems in the Bay of Bengal that move west or northwest contribute significantly to NEM rainfall [2].

SWM withdraws to 15°N → winds reverse to NE → easterlies pick up Bay moisture
      → Coromandel coast rain + Bay depressions/cyclones → NEM rainfall

Fig: Simplified NEM mechanism

Variability of the NEM

• Onset timing: IMD's mean onset date is 22 October ± 7 days. The earliest onset was 5 October (1984) and the latest was 2 November [1].
• Season length: The average season lasts about 75 days, but it has ranged from 51 days (1992) to 98 days (2005) [1].
• Rainfall amount: The coefficient of variation is 20–35% across the five NEM subdivisions [2]. Onset date is not significantly correlated with seasonal rainfall [1].
• Teleconnections: El Niño, a positive IOD and MJO phases 2–4 are generally linked with good NEM rain. This is the opposite of the pattern for the SWM [2].
• Extremes within "normal": In 2025, seasonal rainfall was rated Normal. Yet Tamil Nadu saw 45 heavy, 29 very heavy and 7 extremely heavy rainfall days, and Cyclone Ditwah caused extensive damage around 2 December [2].

Implications for water security

• Tanks and reservoirs must fill within about ten weeks. A weak season can lead to year-long scarcity.
• When rain comes in short, intense bursts, much of it runs off as floodwater instead of being stored. Flood management and water storage therefore have to be planned together.

The NEM depends on a single wind reversal and on chance cyclones, so its timing, amount and intensity all vary widely. The way forward is to use ENSO, IOD and MJO-based seasonal outlooks for crop and reservoir planning, and to restore traditional tanks (eris) so that intense rain is captured. Building climate resilience in this way serves SDG 6 (water) and SDG 13 (climate action).