Vertical propagation of extratropical Rossby waves during the 1997–1998 El Niño off the west coast of South America in a medium‐resolution OGCM simulation

Abstract

A medium‐resolution ocean general circulation model (OGCM) simulation is used to investigate the subthermocline interannual variability in the eastern South Pacific. The focus is on isotherm vertical displacement variability associated with extratropical Rossby waves (ETRW) and their connection with equatorial Kelvin waves (EKW). The WKB theory is used to interpret the modeled subsurface variability. The analysis reveals vertical propagation of energy associated with the 1997–1998 El Niño near the coast at various latitudes. Consistent with theory, WKB raypaths are steeper southward and do not extend more than ∼600 km from the coast at ∼2000 m depth. A vertical mode decomposition of model variability showed that vertical propagation mostly involves the first three baroclinic modes. The vertical isotherm displacements along the raypaths, associated with the downwelling EKW of the 1997–1998 El Niño, are negative (rising isotherms) and peak (minimum isotherm) as El Niño reverses to La Niña conditions. The relationship between vertical propagation of ETRW and equatorial variability is interpreted in the light of EKW sequence. The evolution of the vertical isotherm displacements along the raypath at extratropical latitudes is controlled by the vertical structure of the EKW. As El Niño develops, high‐order baroclinic mode contributions to the EKW progressively increase, leading to the dominance of finer vertical scales of variability along the coast, triggering the vertical propagation of ETRW. This mechanism of connection between equatorial variability and subsurface variability at the extratropical latitudes illustrates how sensitive the ETRW characteristics in the eastern South Pacific are to equatorial forcing.