Seasonal Modulation of the Eddy Kinetic Energy and Subtropical Countercurrent Near the Western North Pacific Boundary

Seasonality of the subtropical countercurrent (STCC) and associated surface eddy kinetic energy (EKE) near the western North Pacific boundary is investigated. Unlike the interior STCC region where the EKE peaks in late spring (May) interpreted by the vertical velocity shear between the STCC and underlying North Equatorial Current in early spring (March), the high EKE near the western boundary lasts from May to August with unclear mechanism. Other studies suggest that the wind, heat flux and mode water could contribute to the EKE seasonality along the STCC band. But all these mechanisms can barely explain the observed EKE seasonality near the western boundary comparing to the interior STCC region, since there are no seasonal phase changes in the above forcings. We found that the eddies provide significant contribution to the EKE and STCC seasonal variation near the western boundary. The westward propagating eddies are intensified by the background shear instability near the western boundary and then further induce the high EKE in summer through the EKE convergence. At the same time, the strong surface current by the enhanced eddy activity changes the seasonality of the STCC. The dynamics is different near the western boundary from that in the interior ocean, with eddies playing more important role in modulating the EKE and STCC seasonal variation. The finding of this study will contribute to a better understanding of the dynamics of eddy-current interaction and ocean variability near the western boundary. Plain Language SummaryIn the Northwestern Pacific subtropical gyre, an eastward current called the Subtropical Countercurrent (STCC) flows against the broad westward North Equatorial Current (NEC). Caused by the vertical velocity shear between the STCC and underlying NEC in early spring (March), the eddy kinetic energy (EKE) peaks in late spring (May) in the open ocean (east of 135 degrees E). However, the EKE seasonality near western boundary is different, exhibiting high value from May to August with unclear mechanism. Our study shows that the westward propagating eddies modulate the seasonal phase changes of the EKE and STCC near the western boundary, comparing to the interior STCC region. When the eddies move from east to west and arrive at the western boundary, with absorbing energy from the background circulation, they intensify and induce the phase shift of the EKE and STCC seasonality through the EKE convergence. Our study shows eddies play more important role in modulating the EKE and STCC seasonal variation near the western boundary, rather than the external forcings, such as the wind, heat flux and mode water. The finding of this study will contribute to a better understanding of the dynamics of eddy-current interaction and ocean variability near the western boundary.