Generation of the North Equatorial Undercurrent Jets by Triad Baroclinic Rossby Wave Interactions

Abstract

ormation processes responsible for the North Equatorial Undercurrent (NEUC) jets that appear across the tropical North Pacific Ocean near 9°, 13°, and 18°N are explored both theoretically and using numerical models with different complexities. Analyses of an eddy-resolving global ocean general circulation model output reveal that the NEUC jets have a mode-1 baroclinic vertical structure and are spatially persistent on the interannual and longer time scales. This OGCM-simulated vertical structure prompts the authors to adopt the simpler, nonlinear -layer reduced-gravity model, as well as the baroclinic Rossby wave triad interaction theory, to unravel the essential processes underlying the NEUC jets. The seed for the NEUC jets originates in annual baroclinic Rossby waves driven by the large-scale surface wind stress forcing. Emanating from the ocean basin?s eastern boundary, these wind-forced ?primary? waves are subject to nonlinear triad interactions and break down offshore where the e-folding time scale of the most unstable triad instability matches the advective time scale of the primary waves. The breakdown boundary of the wind-forced primary waves tends to tilt northeast?southwest and, west of this boundary, finite-amplitude eddies emerge, whose meridional scales are set by the most unstable short secondary waves participating in the triad interactions along the breakdown boundary. With their meridional scales set similarly by the short secondary waves, the time-mean zonal jets of characteristics resembling the observed NEUC jets are formed by the converging potential vorticity fluxes of these finite-amplitude eddies.