Interannual Variability of the North Pacific Subtropical Countercurrent and Its Associated Mesoscale Eddy Field

Interannual changes in the mesoscale eddy field along the Subtropical Countercurrent (STCC) band of 18 degrees-25 degrees N in the western North Pacific Ocean are investigated with 16 yr of satellite altimeter data. Enhanced eddy activities were observed in 1996-98 and 2003-08, whereas the eddy activities were below average in 1993-95 and 1999-2002. Analysis of repeat hydrographic data along 137 degrees E reveals that the vertical shear between the surface eastward-flowing STCC and the subsurface westward-flowing North Equatorial Current (NEC) was larger in the eddy-rich years than in the eddy-weak years. By adopting a 2 1/2-layer reduced-gravity model, it is shown that the increased eddy kinetic energy level in 1996-98 and 2003-08 is because of enhanced baroclinic instability resulting from the larger vertical shear in the STCC-NEC's background flow. The cause for the STCC-NEC's interannually varying vertical shear can be sought in the forcing by surface Ekman temperature gradient convergence within the STCC band. Rather than El Nino-Southern Oscillation signals as previously hypothesized, interannual changes in this Ekman forcing field, and hence the STCC-NEC's vertical shear, are more related to the negative western Pacific index signals.