The response of the ocean to decadal variability in atmospheric forcing: wind versus thermal forcing

A vertical modal analysis is done of the density deviations in an ocean general circulation model of the North Pacific driven by decadal variability in atmospheric forcing. The different dynamic vertical modes show varying propagation characteristics, with the first baroclinic mode exhibiting consistent westward propagation at latitudes south of 40N, while the higher baroclinic modes show westward phase propagation at low latitudes, but propagate eastward at higher latitudes. The change in direction occurs at lower latitudes for successively higher vertical modes. The propagation characteristics of each mode can be understood by the inclusion of the mean flow in the quasi-geostrophic potential vorticity equation.

Projection of the Ekman pumping and diapycnal fluxes in the potential vorticity equation for each dynamic vertical mode shows the importance of Ekman pumping throughout the North Pacific. Diabatic pumping, or that associated with thermal forcing, is important in the Kuroshio Extension, and much less so further to the south. The spatial distribution of the forcing is consistent with the structure of the energy in the baroclinic modes. The first baroclinic mode energy increases poleward, while the second baroclinic mode has a band of positive energy emanating westward from the eastward end of the Kuroshio Extension and ends at at the western boundary at 20N.