Propagation of North Pacific interdecadal subsurface temperature anomalies in an ocean GCM

An ocean general circulation model (GCM) is forced with the NCEP re-analysis wind stress for 1958-1997 to understand mechanisms of ocean subsurface variability. With relatively high horizontal (1 x 1 degree) and vertical (41 levels) resolutions, the model produces mode waters on a range of density surfaces both in the northwestern 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 on the Kuroshio and its extension. In the absence of external thermohaline variability, 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 on the Kuroshio extension by wind changes in mid-1970s act cooperatively to shift the central mode waters toward the east, causing large subsurface temperature anomalies.