Viscosity-Dependent Internal Variability in a Model of the North Pacific

摘要

A 2°-resolution isopycnal model of the North Pacific Ocean is shown to produce anomalies that propagate around the subtropical gyre on the decadal time scale that do not appear in a Irresolution version of the same model. A principal oscillation pattern (POP) analysis of the isopycnal interface anomaly is performed to examine the dynamics responsible for the anomaly generation. The POPs show a coherent oscillation around the entire subtropical gyre with two centers of action, one in the Central Mode Water (CMW) region, the other in the Subtropical Countercurrent (STCC). Lead-lag covariances between the subduction rate in the CMW and the layer thickness along the oscillation path indicate that anomalous subduction events are not the driving mechanism for the oscillation. A linearized quasigeostrophic mode analysis shows that the anomalies are generated by flow instability in the region of the STCC. The instability disappears in the 1° model because of changes in the horizontal viscosity, which is set in each model to the minimum value necessary to resolve the western boundary current and preserve numerical stability. A criterion for model resolution of an instability of a given length and time scale damped by biharmonic viscosity is derived. The enhancement of the large-scale instabilities in the low-resolution model emphasizes the importance of achieving mesoscale resolution in ocean models used for climate studies.