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

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 return flow along 20°N lags the subpolar changes by several years. The authors demonstrate that the model thermocline cooling and increased transport occurred in response to decadal-scale changes in basin-scale wind stress curl with the quasi-stationary oceanic response being in a time-dependent quasi-Sverdrup balance over much of the basin east of the date line. This wind stress curl driven response is quasi-stationary but occurs in conjunction with a propagating temperature anomaly associated with subduction in the central North Pacific that links the subpolar and subtropical gyre stationary changes and gives the appearance of circumgyre propagation. Different physics evidently controls the decadal subsurface temperature signal in different parts of the extratropical North Pacific.