Event definitions

• El Niño / La Niña event: ONI ≥ +0.5 °C / ≤ −0.5 °C for five or more consecutive overlapping seasons (CPC). An ENSO year runs July → June and is labelled by the July year (“1997-98”).
• Strength = peak |ONI| between the Aug–Apr seasons of that year: weak 0.5–0.9, moderate 1.0–1.4, strong ≥ 1.5 (≥ 2.0 counted with strong).
• Flavor from the Nov–Jan ERSSTv5 anomalies: East-Pacific (canonical) El Niño when Niño-3 − Niño-4 ≥ +0.5 °C; Central-Pacific (Modoki) when Niño-4 exceeds Niño-3. Mixed events appear in the strength bins only. Mirror rule for La Niña.
• Concurrent ONI ignores events and bins every season on its own ONI value.

Statistics

• Series are linearly detrended per grid cell and season over 1950 → present so the warming trend is not read as an ENSO signal. Terciles come from all base-period years.
• Chance of above / below = share of a category's years in the top / bottom third. Small bins (a strong El Niño bin has ~10 members) are noisy; read them with the p-values and the year dots.
• p-value: two-sided Welch t-test, category years vs. all other years. Map stippling marks p < 0.10. Correlation map: Pearson r between the season's ONI and the detrended field.

Reading advice

ENSO tilts the odds; it never dictates the outcome. A 60 % chance of a dry season still means four of ten such seasons were not dry. Signals are strongest in the tropics and in the classic teleconnection regions (southern US and Mexico in winter, eastern Australia in spring, the Maritime Continent, southern Africa, NE Brazil) and weakest in mid-latitude summers. Strength does not scale linearly with impact.

Data: ERA5 © ECMWF / Copernicus Climate Change Service; ONI and Niño indices © NOAA CPC.