2.4.10 (HL)—ENSO cycle

ENSO links tropical Pacific sea-surface temperature with atmospheric circulation; El Niño and La Niña are opposite irregular extremes.

El Niño warms the central/eastern Pacific and weakens trade winds; La Niña cools it and strengthens them. Neutral conditions lie between, and events do not follow a fixed clock.

If eastern-Pacific water warms while trade winds weaken, the evidence points toward El Niño, not simply ‘a warm year’.

Coupled feedbacks vary, so timing and strength are irregular rather than exactly periodic.

ENSO is not just ocean temperature; the atmosphere and ocean state must be considered together.