Interdecadal Thermocline Variability in the North Pacific for 1958–97: A GCM Simulation*

An ocean general circulation model is forced with the NCEP reanalysis wind stress for 1958–97 to understand mechanisms of ocean subsurface variability. With relatively high horizontal (1° × 1°) and vertical (41 levels) resolutions, the model produces mode waters on a range of density surfaces in the western, central, and eastern North Pacific, in qualitative agreement with observations. These mode waters appear as a thermostad or a region of weak stratification in the upper thermocline as they flow southward from their formation regions in the Kuroshio and its extension. In the model, subsurface temperature variability in the central subtropical gyre reaches a maximum within the thermostad, in contrast to what might be expected from the linear baroclinic Rossby wave theory. This variance maximum is associated with the longitudinal shift in the path of mode waters. In particular, deepened mixed layer and accelerated eastward currents in the Kuroshio Extension by wind changes in the mid-1970s act cooperatively to move the central mode waters toward the east, causing large subsurface temperature anomalies. Besides the local maximum in the central North Pacific subtropical gyre, two additional maxima of the subsurface anomaly are identified in the northwestern and southern parts of the gyre, respectively. Among these subsurface anomaly centers, the one in the northwestern North Pacific has a strong effect on the model sea surface temperature, suggesting that the Kuroshio and its extension are a key region to decadal/interdecadal ocean–atmosphere interaction. Finally, possible effects of atmospheric thermal forcing are discussed.