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 eastward-flowing countercurrents. The STCC extends typically along 24°N from 130°E to 160°W and slightly shifts to the north as it flows 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 a inverse cascade of the eddy energy, we discussed generation processes of the mesoscale SSH disturbances.
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