Interannual variations in low potential vorticity water and the subtropical countercurrent in an eddy-resolving OGCM

Interannual-to-decadal variations in the subtropical countercurrent (STCC) and low potential vorticity (PV) water and their relations in the North Pacific Ocean are investigated on the basis of a 60-year-long hindcast integration of an eddy-resolving ocean general circulation model. Although vertically coherent variations are dominant for STCC interannual variability, a correlation analysis shows that an intensified STCC vertical shear accompanies lower PV than usual to the north on 25.5- to 26.1-sigma(theta) isopycnal surfaces, and intensified meridional density gradient in subsurface layers, consistent with Kubokawa's theory (J Phys Oceanogr 29:1314-1333, 1999). The low-PV signals appear at least 2 years before peaks of STCC, propagating southwestward from the subduction region.