Intraseasonal-to-Interannual Variability of the Upper-Layer Zonal Currents in the Tropical Northwest Pacific Ocean

The upper-layer circulation in the Tropical Northwest Pacific Ocean (TNWP) is characterized by the alternating zonal currents, such as North Equatorial Current (NEC) and Subtropical Countercurrent (STCC). The variabilities of the upper-layer zonal currents in the TNWP, which were obtained directly from 75 kHz acoustic Doppler current profiler (ADCP) measurements and indirectly from geostrophic calculations, were investigated on intraseasonal to interannual timescales from 2006 to 2014. Surface geostrophic current derived from sea surface height anomaly (SSHA) approximated well to the in-situ measurements with ADCP that was slightly better than the absolute geostrophic current obtained from AVISO. The interannual signals related to the El Nino-Southern Oscillation (ENSO) were quite apparent, especially near the boundary (about 17.5 degrees N-19 degrees N) between NEC and STCC. We found that the eastward-flowing southern STCC develops during the decaying phase of ENSO and weakens during La Nina. Strong easterlies during strong La Nina (e.g., 2010 La Nina event) seem to influence NEC to expand further north to the southern STCC region and this coincides with the meridional migration of the Sverdrup streamfunction. The southern STCC region is profoundly influenced by the mesoscale/sub-mesoscale eddy activities, which can intensify the eastward flow of STCC and ultimately contribute to the intraseasonal signal. We found that this intraseasonal signal weakened when La Nina happened in 2010, which is presumably associated with the northward expansion of NEC. Possible factors related to the discrepancies between surface geostrophic current and sub-surface ADCP data are discussed.