A Westward - intensi ed Decadal Change in the North Paci cThermocline and Gyre - Scale Circulation Arthur

From the early 1970's to the mid-1980's, the main thermocline of the subarctic gyre of the North Paci c Ocean shoaled with temperatures at 200-400m depth cooling by 1-4 C over the region. The gyre-scale structure of the shoaling is quasi-stationary and intensi ed in the western part of the basin north of 30N, suggesting concurrent changes in gyre-scale transport. A similar quasi-stationary cooling in the subtropical gyre south of 25N is also observed but lags the subpolar change by several years. To explore the physics of these changes, we examine an ocean model forced by observed wind stress and heat ux anomalies from 1970-1988 in which we nd similar changes in gyre-scale thermocline structure. The model current elds reveal that the North Paci c subpolar and subtropical gyres strengthened by roughly 10% from the 1970s to the 1980s. The bulk of the eastward ow of the model Kuroshio/Oyashio Extension returned westward via the subpolar gyre circuit, while the subtropical gyre return ow along 20N lags the subpolar changes by several years. We 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 dateline. 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 Paci c that links the subpolar and subtropical gyre stationary changes and gives the appearance of circumgyre propagation. Di erent physics evidently controls the decadal subsurface temperature signal in di erent parts of the extratropical North Paci c. 2 A Westward-intensi ed Decadal Change in the North Paci c Thermocline and Gyre-Scale Circulation