Observations of the subtropical mode water evolution from the Kuroshio Extension System Study

Properties and seasonal evolution of North Pacific Ocean subtropical mode water (STMW) within and south of the Kuroshio Extension recirculation gyre are analyzed from profiling float data and additional hydrographic and shipboard ADCP measurements taken during 2004. The presence of an enhanced recirculation gyre and relatively low mesoscale eddy variability rendered this year favorable for the formation of STMW. Within the recirculation gyre, STMW formed from late-winter convection that reached depths greater than 450 in near the center of the gyre. The lower boundary of STMW, corresponding to sigma(theta) similar or equal to 25.5 kg m(-3), Was set by the maximum depth of the late-winter mixed layer. Properties within the deep portions of the STMW layer remained largely unchanged as the season progressed. In contrast, the upper boundary of the STMW layer eroded steadily as the seasonal thermocline deepened from late April to August. Vertical eddy diffusivity responsible for this erosion was estimated front a budget analysis of potential vorticity to be in the range of similar to 2-5 X 10(-4) m(2) s(-1). The latitudinal extent of the STMW formation was narrow, extending from 30 degrees N to the Kuroshio Extension jet near 35 degrees N. South of 30 degrees N, STMW did not form locally but was transported from the recirculation gyre by lateral induction.