Codex @codex  0

Coupled variability of the tropical Pacific ocean and terrestrial atmosphere, including the warm El Niño phase, the cold La Niña phase, and associated changes in surface pressure, trade winds and the thermocline.

Kelvin waves: weakened trade winds, or a westerly wind burst, excite a downwelling equatorial Kelvin wave. Its positive upper-layer thickness anomaly travels eastward and deepens the eastern Pacific thermocline. Upwelling then entrains warmer water, favouring eastern Pacific surface warming and the development of El Niño. This is a propagating adjustment of the thermocline, rather than transport of an unchanged parcel of warm water across the entire basin.^[ref]

Gravity waves: changed tropical convection and its latent heat release force atmospheric internal gravity waves. Their rapid propagation spreads the associated pressure and buoyancy disturbances, allowing the tropical terrestrial atmosphere to adjust to the shifted heating and contributing to the divergent circulation and surface wind response coupled to the ocean. This is one atmospheric adjustment mechanism; it does not assert that all wind variability during a particular El Niño is caused by this wave family.

Rossby waves: wind forcing during El Niño also launches westward equatorial Rossby waves with a shoaling thermocline signal. Reflection at the western Pacific boundary can produce an eastward upwelling equatorial Kelvin wave, which shoals the eastern thermocline and promotes cooling. This supplies a delayed negative feedback that can help terminate the event.^[ref] These examples describe mechanisms relevant to events such as 2023/24, rather than claiming an event-specific attribution from the exam data.

A water layer across which temperature changes rapidly with depth. In a reduced gravity description of the upper ocean, its displacement is represented by the interface between warm surface water and colder deep water. Deepening the eastern tropical Pacific thermocline makes upwelling less effective at cooling the surface during El Niño.

Persistent low-latitude surface winds with a westward component, associated with the lower branch of the Hadley cell. Their weakening or reversal in the equatorial Pacific can excite equatorial Kelvin waves that contribute to the development of El Niño.