A Westward-Intensified Decadal Change in the North Pacific Thermocline and Gyre-Scale Circulation

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Abstract
From the early 1970s to the mid-1980s, the main thermocline of the subarctic gyre of the North Pacific Ocean shoaled with temperatures at 200–400-m depth cooling by 1°–4°C over the region. The gyre-scale structure of the shoaling is quasi-stationary and intensified in the western part of the basin north of 30°N, suggesting concurrent changes in gyre-scale transport. A similar quasi-stationary cooling in the subtropical gyre south of 25°N is also observed but lags the subpolar change by several years. To explore the physics of these changes, the authors examine an ocean model forced by observed wind stress and heat flux anomalies from 1970–88 in which they find similar changes in gyre-scale thermocline structure. The model current fields reveal that the North Pacific subpolar and subtropical gyres strengthened by roughly 10% from the 1970s to the 1980s. The bulk of the eastward flow of the model Kuroshio–Oyashio Extension returned westward via the subpolar gyre circuit, while the subtropical gyre ... Abstract From the early 1970s to the mid-1980s, the main thermocline of the subarctic gyre of the North Pacific Ocean shoaled with temperatures at 200–400-m depth cooling by 1°–4°C over the region. The gyre-scale structure of the shoaling is quasi-stationary and intensified in the western part of the basin north of 30°N, suggesting concurrent changes in gyre-scale transport. A similar quasi-stationary cooling in the subtropical gyre south of 25°N is also observed but lags the subpolar change by several years. To explore the physics of these changes, the authors examine an ocean model forced by observed wind stress and heat flux anomalies from 1970–88 in which they find similar changes in gyre-scale thermocline structure. The model current fields reveal that the North Pacific subpolar and subtropical gyres strengthened by roughly 10% from the 1970s to the 1980s. The bulk of the eastward flow of the model Kuroshio–Oyashio Extension returned westward via the subpolar gyre circuit, while the subtropical gyre ...