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 degrees x 1 degrees) and vertical (41 l evels) resolutions, the model produces mode waters on a range of density su rfaces in the western, central, and eastern North Pacific, in qualitative a greement 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 regio ns in the Kuroshio and its extension. In the model, subsurface temperature variability in the central subtropical gyre reaches a maximum within the th ermostad, in contrast to what might be expected from the linear baroclinic Rossby wave theory. This variance maximum is associated with the longitudin al 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, tw o additional maxima of the subsurface anomaly are identified in the northwe stern and southern parts of the gyre, respectively. Among these subsurface anomaly centers, the one in the northwestern North Pacific has a strong eff ect on the model sea surface temperature, suggesting that the Kuroshio and its extension are a key region to decadal/interdecadal ocean-atmosphere int eraction. Finally, possible effects of atmospheric thermal forcing are disc ussed.