The January–April rainfall at Port Blair (Sri Vijaya Puram), though only about five per cent of the annual total, is the sole inflow to the Dhanikhari reservoir during its continuous drawdown season and therefore governs summer water security for a population exceeding 1,50,000. We compile a continuous 77-year station series of January–April totals (1950–2026), together with November–December totals and annual totals, and relate them to the state of the tropical Pacific. Four results emerge. (i) There is a graded, continuous relationship between the intensity of the preceding winter's Oceanic Niño Index (ONI) and spring rainfall (Pearson r = −0.465, P < 0.0001): mean spring rainfall falls from 221 mm after La Niña winters to 164 mm in neutral years to 99 mm after El Niño winters. (ii) The relationship is a suppression, not a forcing: strong El Niño winters (ONI ≥ 1.0) impose a hard ceiling on spring rainfall, none of fifteen exceeded 177 mm, whereas weak warm winters impose none (maximum 280 mm). El Niño removes the possibility of a wet spring without dictating how dry the spring becomes. (iii) The signature is therefore a necessary but not sufficient condition for scarcity: every one of the twenty springs below 60 mm in 77 years was El Niño-associated (P(El Niño | collapse) = 1.0), yet only about 40 per cent of strong El Niño winters produced such a collapse (P(collapse | strong El Niño) ≈ 0.4). (iv) For water management, the decisive quantity is the cumulative dry-season inflow, the preceding November–December rainfall plus the January–April rainfall, which separates the documented scarcity years from the safe years without overlap (crisis years 207–482 mm; safe years 651–745 mm). The signature is decoupled from the annual total (seven of the collapses fell in above-normal years), is stationary across both halves of the record, and is corroborated at an independent catchment gauge (r = 0.79) and by archipelago-wide IMD deficits in 2026. With an El Niño declared in June 2026 and forecast to strengthen through the coming winter, spring 2027 falls in the suppressed regime, and its dry-season inflow is already tracking towards the crisis range. Three zero-cost operational triggers are proposed, and implications for compound groundwater stress and seawater intrusion are discussed.