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

Abstract

The upper-layer circulation of the tropical northwest Pacific Ocean (TNWP) is characterized by 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 the ADCP measurements and indirectly from the geostrophic calculation, were investigated in the intraseasonal to interannual time scales for the period of 2006-2014. Surface geostrophic current derived from sea surface height anomaly (SSHA) was well approximated to the in-situ measurements with Acoustic Doppler Current Profilers (ADCP) that was slightly better than the absolute geostrophic current obtained from AVISO. The interannual signals related with the El Ni&ntilde o-Southern Oscillation (ENSO) quite strongly appeared, especially near the boundary (about 17.5°N &#8211 19°N) between NEC and STCC. The eastward-flowing southern STCC seeminglybecomes strong and the border appears clearly when the ENSO index is weak. During strong La Ni&ntilde a (e.g. 2010 La Ni&ntilde aevent), the shallow eastward current turns into the opposite (westward) direction with the speed up to 0.25 ms-1. The strong easterlies during La Ni&ntilde a seem to influence NEC to become wider and to expand further north to the southern STCC region.onal currents in the TNWP, which were obtained directly from the ADCP measurements and indirectly from the geostrophic calculation, were investigated in the intraseasonal to interannual time scales for the period of 2006-2014. Surface geostrophic current derived from sea surface height anomaly (SSHA) was well approximated to the in-situ measurements with Acoustic Doppler Current Profilers (ADCP) that was slightly better than the absolute geostrophic current obtained from AVISO. The interannual signals related with the El Ni&ntilde o-Southern Oscillation (ENSO) quite strongly appeared, especially near the boundary (about 17.5°N &#8211 19°N) between NEC and STCC. The eastward-flowing southern STCC seeminglybecomes strong and the border appears clearly when the ENSO index is weak. During strong La Ni&ntilde a (e.g. 2010 La Ni&ntilde aevent), the shallow eastward current turns into the opposite (westward) direction with the speed up to 0.25 ms-1. The strong easterlies during La Ni&ntilde a seem to influence NEC to become wider and to expand further north to the southern STCC region.