Variability of the Pacific North Equatorial Current from 1993 to 2012 based on a 1/8° Pacific model simulation

Abstract

AbstractBased on a multidecadal Pacific basin model simulation, interannual variations of the North Equatorial Current (NEC) are investigated. The model reproduces well the characteristics of the NEC and its interannual variability. The NEC transport is calculated as an integral of the westward velocity from 6°N to 21°N and from the surface to the 1200 m depth. The magnitude and standard deviation of the NEC transport increase from 46.5 and 3.9 Sv at 175°E to 66.7 and 6.5 Sv at 130°E, respectively, and both peak around 132°E prior to entering the bifurcation region. The NEC transport tends to be higher during positive Oceanic Niño Index (ONI) years but lower during negative ONI years with the maximum difference of more than 20 Sv. The interannual variability of the NEC transport is closely related to changes of the sea surface height in the tropical Pacific Ocean, and the increase of the NEC is mostly balanced by the increase in the North Equatorial Counter Current (NECC) on the tropical gyre side. The present study further suggests a long‐term decline of the NEC transport from 1993 to 2012, which is consistent with the patterns in the trend of wind stress curl. Transport anomalies reconstructed from the normal modes of zonal velocity suggest that the first baroclinic mode captures about 95% of the variance in the NEC transport, while the second mode adds only additional 3–4%. A 1.5‐layer reduced gravity model further reveals that the first (second) baroclinic mode is driven primarily by the wind (thermal) forcing. The wind forcing plays a predominant role in determining the interannual variability in the NEC transport while the effect of the thermal forcing is rather limited.