= Solution
\b[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.<El Niño and La Niña: Causes and Global Consequences by Michael J. McPhaden>[Box 2, “Equatorial Waves”, and the adjoining discussion of eastern thermocline depression, printed page 358]{ref}
\b[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.
\b[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.<El Niño and La Niña: Causes and Global Consequences by Michael J. McPhaden>[Printed pages 358–359, reflection of the western thermocline-elevation signal and the return of surface cooling]{ref} These examples describe mechanisms relevant to events such as 2023/24, rather than claiming an event-specific attribution from the exam data.
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