A numerical study of the 1997-98 El Niño off Peru

F. Colas, X. Capet, J.C. McWilliams and A. Shchepetkin
IGPP, UCLA, Los Angeles, USA

A ROMS numerical simulation for the Peru-Chile System over most of the 1990s is analyzed, with a focus on the 1997-98 El Niño. Atmospheric and lateral oceanic forcings are realistic and contain a wide range of scales from days to interannual. The solution is validated against altimetric observations and the few in-situ observations available. Simple dynamical analyses are performed to explain the 1997-1998 evolution of the upwelling in the model. The intensity of the upwelling appears to be determined by an interplay between alongshore, poleward advection (related to coastal-trapped waves) and wind intensity, but also by the cross-shore geostrophic flow and distribution of the water masses on a scale of 1000 km or more (involving Rossby waves westward propagation and advection from equatorial currents). In particular, the delay of upwelling recovery until fall 1998 is partly due to the persistent advection of offshore stratified water toward the coast of Peru. Altimetry data suggest that these interpretations of the numerical solution also apply to the real ocean.