Intraseasonal, Annual, and Interannual Variabilities of Subsurface Currents at 4.7°N in the Western Pacific Ocean

Strong multi-timescale variabilities of subsurface zonal velocity over 300-600 m depth are observed beneath the North Equatorial Countercurrent (NECC) by the mooring observation at 4.7 degrees N, 140 degrees E during 2014-2018. The observed subsurface velocity shows peaks at intraseasonal, semiannual, and annual periods, and is interconnected with the El Nino-Southern Oscillation (ENSO) cycle at a nearly zero lag. Aided by a reanalysis product, we reveal mechanisms for the above variabilities. Forced by wind stress curl over 150 degrees-155 degrees E and 160 degrees-170 degrees E along 4.7 degrees N, the vertical propagation of the third meridional mode long Rossby waves is primarily responsible for the observed semiannual and annual cycles, respectively. The subsurface intraseasonal variability with peak periods of 50- and 80-day are induced by short Rossby waves with the southeastward and downward group velocity. Such waves interconnect observed 50- and 80-day ISV of the subsurface current to that of surface currents at 8 degrees N, 128 degrees-130 degrees E and 6.5 degrees N, 132 degrees-134 degrees E, respectively. The ENSO variability results mainly from the interannual change of the winds that force the long Rossby waves. The wind forcing leads the Nino 3.4 index and the subsurface velocity by a similar time, resulting in the subsurface instantaneous response to ENSO.

Automated summary

This study observed strong multi-timescale variabilities of subsurface zonal velocity beneath the North Equatorial Countercurrent (NECC). The observed velocity showed peaks at intraseasonal, semiannual, and annual periods, and was interconnected with the El Nino-Southern Oscillation (ENSO) cycle. The study also revealed the mechanisms behind these variabilities, with long Rossby waves responsible for the semiannual and annual cycles, and short Rossby waves inducing the intraseasonal variability.