Variability of the Pacific North Equatorial Current from repeated shipboard acoustic Doppler current profiler measurements

Interannual variability and 16-year trend of the Pacific North Equatorial Current (NEC) are examined using repeated shipboard acoustic Doppler current profiler (SAD-CP) measurements in the upper 200-mlayer along 137 degrees E from 1993 until 2008 and compared with previous results inferred from hydrological and satellite data. Interannual variability of the NEC is prominent and tied to the Nino 3.4 index but lags the latter by 6 months. We find that the average NEC transport under El Nino conditions is 68 Sv and greater than that under La Nina conditions (59 Sv). Their relationship, however, involves decadal difference: the correlation coefficient is statistically significant between 2000 and 2008, but non-significant during 1993-1999. Composite analysis suggests that the change of wind response to El Nino-Southern Oscillation (ENSO) from the 1990s to 2000s might account for the difference in NEC-ENSO relationships between the past two decades. We also find that the NEC transport has intensified in the past 16 years and present the vertical structure of the NEC trend. But it is shown that the NEC enhancement is slightly canceled by the anomalous eastward Ekman current in the Ekman layer due to the strengthened northward wind stress above the NEC. Possible uncertainties related to the resolution of the data are discussed.