Propagation of wind and buoyancy forced density anomalies in the North Pacific: Dependence on ocean model resolution

The response of the thermocline to changes in atmospheric forcing are explored in two 50-year hindcast North Pacific model runs. The model runs only differ in their resolution and horizontal viscosity values. The thermocline response is explored through a modal decomposition. The first baroclinic mode response is qualitatively similar in both model runs, with a somewhat smaller response in the lower resolution model. This mode is primarily wind driven. The second baroclinic mode shows a larger response at midlatitudes in the low-resolution model than in the higher resolution model. This is consistent with the presence of very large-scale baroclinic instability in the return flow of the subtropical gyre at low-resolution, and represents a spurious response of the ocean model to large horizontal viscosity. This spurious mode of decadal variability in the thermocline is maintained even when there is variability in atmospheric forcing. This result suggests that care must be taken in interpretation of the realism of mid-latitude modes of variability centered in western boundary current extensions of coupled ocean–atmosphere models.