The atmosphere
pressure · wind · convectionWarm water alone does not make an El Niño: it has to drag the atmosphere along. The trade winds weaken, rainfall shifts from Indonesia to the central Pacific, and pressure flips between Tahiti and Darwin. That coupling — the Bjerknes feedback — turns a warming into an event and spreads its effects around the world.
Period
Ember bars = El Niño phase of the index; blue = La Niña phase. All are monthly means from NOAA CPC / PSL.
SOI — Southern Oscillation Index
Standardized pressure at Tahiti minus Darwin. Persistent negative values (below −1) mean weak trade winds and an atmosphere in El Niño mode.
MEI.v2 — Multivariate ENSO Index
Combines sea level pressure, zonal and meridional winds, SST and outgoing longwave radiation. Values above +2 only appear in the biggest events.
850 hPa trade winds
Zonal wind anomaly in three sectors of the Pacific. Negative = anomalous westerlies, which push warm water east and deepen the thermocline — the engine of the event.
Convection over the central Pacific (OLR)
Standardized outgoing longwave radiation at 160°E–160°W. Negative = high, cold clouds: the rain that normally falls over Indonesia is over the central Pacific.