A westward-intensified decadal change in the North Pacific thermocline andgyre-scale circulation

From the early 1970s to the mid-1980s, the main thermocline of the subarcti c gyre of the North Pacific Ocean shoaled with temperatures at 200-400-m de pth cooling by 1 degrees-4 degrees C over the region. The gyre-scale struct ure of the shoaling is quasi-stationary and intensified in the western part of the basin north of 30 degrees N, suggesting concurrent changes in gyre- scale transport. A similar quasi-stationary cooling in the subtropical gyre south of 25 degrees N is also observed but lags the subpolar change by sev eral years. To explore the physics of these changes, the authors examine an ocean model forced by observed wind stress and heat flux anomalies from 19 70-88 in which they find similar changes in gyre-scale thermocline structur e. The model current fields reveal that the North Pacific subpolar and subt ropical gyres strengthened by roughly 10% from the 1970s to the 1980s. The bulk of the eastward Row of the model Kuroshio-Oyashio Extension returned w estward via the subpolar gyre circuit, while the subtropical gyre return fl ow along 20 degrees N lags the subpolar changes by several years. The autho rs demonstrate that the model thermocline cooling and increased transport o ccurred in response to decadal-scale changes in basin-scale wind stress cur l with the quasi-stationary oceanic response being in a time-dependent quas i-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 w ith a propagating temperature anomaly associated with subduction in the cen tral North Pacific that links the subpolar and subtropical gyre stationary changes and gives the appearance of circumgyre propagation. Different physi cs evidently controls the decadal subsurface temperature signal in differen t parts of the extratropical North Pacific.