[1] Subsurface fronts in the subtropical North Pacific were investigated by constructing a high-resolution temperature climatology. Three distinct subtropical fronts (STFs) are identified, which are the southern, northern and eastern STFs according to their relative geographical locations. The southern STF extends along 19°-21°N west of the dateline, while the northern and eastern STFs appear along 21°-24°N and 26°N in the western and central subtropical gyre, respectively. Our analysis showed that each of these STFs is associated with large negative meridional potential vorticity (PV) gradient in the thermocline below the front. The northern STF is located on the southern edge of the Kuroshio recirculation, where the negative PV gradient occurs within a narrow density range and is maintained by the low PV core of the subtropical mode water (STMW). On the other hand, the negative PV gradient at the eastern and southern STFs spans over a wide density range. The eastern STF forms near the center axis of the subtropical gyre, north of which the upper and lower portions of the central mode water (CMW) converge, forming a thick low PV pool that maintains the negative PV gradient on the front. Similarly, at the southern STF, the negative PV gradient is due to a low PV pool to the north, with the southernmost portion of the STMW stacked above the upper CMW advected from the central subtropical gyre. It is concluded that the mode waters are essential for the North Pacific STFs.

[1] The relation between baroclinic instability of the North Pacific Subtropical Countercurrent (STCC) and the Hawaiian Lee Countercurrent (HLCC) and seasonal evolution of mesoscale sea surface height (SSH) variability was investigated by using historical hydrographic data and altimeter-derived SSH anomaly data during 1992-2000. The annual and monthly mean dynamic height climatologies show two robust eastwardflowing countercurrents. The STCC extends typically along 24°N from 130°E to 160°W and slightly shifts to the north as it flows toward the east, and the HLCC is located typically along 20°N and extends from about 150°E to just west of the Hawaiian Islands. Seasonal variations of the STCC in the western North Pacific and the HLCC west of the Hawaiian Island, where the eddy variability is relatively larger than the background, were quite different: the STCC (HLCC) is strong in winter (summer) to summer (winter) and weak in fall (spring). In contrast, seasonal evolution of mesoscale SSH variability has almost the same cycle in the two countercurrent regions. The eddy kinetic energy (EKE) is a maximum in spring and a minimum in fall. The dominant zonal scales are shortest in late winter and become longer with time. A linear quasigeostrophic stability analysis using a three-layer model indicates that instability properties are very similar between the two countercurrent regions. The STCC and the HLCC become baroclinically more unstable during winter and late fall to winter, respectively, due to strong vertical velocity shear and weak stratification between the STCC/HLCC and underlying westward flow, and the most unstable wave during this season has an e-folding timescale of about 2-3 months and a zonal wavelength of about 300-400 km. The same behavior of the seasonal evolution of the mesoscale variability in the two countercurrent regions is a manifestation of these similarities. In terms of increasing nonlinearity of the unstable waves and an inverse cascade of the eddy energy, we discussed generation processes of the mesoscale SSH disturbances.