Observed equatorial Rossby waves and ENSO‐related warm water volume changes in the equatorial Pacific Ocean

Modifications of the volume of warm water above the thermocline in the equatorial Pacific are thought to be critical for ENSO (El Niño Southern Oscillation) preconditioning, development and prediction. In this paper, changes in this Warm Water Volume (WWV) are first reviewed from the literature, and then analyzed within 5°N–5°S in using 1992–2006 altimeter‐derived sea level anomalies (SLA) and in situ observations. We conclude that the 1992–2006 WWV changes in the equatorial band are a residual from the opposite effects of zonally integrated meridional Ekman and geostrophic transports of warm water, the latter being estimated from SLA and validated against in situ observations. For the first time with observations, we evidence first baroclinic first meridional mode equatorial Rossby waves to be responsible for the anomalous meridional geostrophic transports of warm water, in changing the basin‐scale zonal pressure gradient. These transports appear in phase opposition and proportional to the anomalous Sea Surface Temperature (SST) in the region NIÑO3.4 (170°W–120°W, 5°N–5°S). The links between our observational results and the recharge/discharge oscillator theory, the differences in WWV between ENSO events, and the hint for possible recent changes in the relation between WWV and NIÑO3.4 SST are discussed.