The Response of the North Pacific Ocean to Decadal Variability in Atmospheric Forcing: Wind versus Buoyancy Forcing

Abstract

Both wind and buoyancy forcing result in variability in the North Pacific Ocean thermocline. A vertical modal analysis of the density deviations in a 30-yr run of an ocean general circulation model of the North Pacific forced by atmospheric variability is used to identify the spatial and temporal patterns of the different baroclinic modes. The different dynamic vertical modes show distinct propagation characteristics, with the first baroclinic mode exhibiting consistent westward propagation at all latitudes. The higher baroclinic modes show westward phase propagation at low latitudes but propagate eastward at higher latitudes. The propagation characteristics of each mode can be understood by the inclusion of the zonal mean flow in the vertical structure equation. Evaluation of the Ekman pumping and diapycnal fluxes in the quasigeostrophic potential vorticity equation for each dynamic vertical mode distinguishes their effects on the thermocline variability. Wind variability dominantly forces the first baroclinic mode response while buoyancy forcing results in a higher baroclinic mode response. Two additional numerical model runs, one with climatological wind stress and one with climatological heat fluxes and SST, demonstrate the relative roles of Ekman pumping and diabatic pumping on the forcing of each vertical mode. Ekman pumping is important throughout the North Pacific for forcing first-mode variability. Diabatic pumping, or that associated with thermal forcing, is important in the Kuroshio Extension and much less so farther to the south and can act to suppress the first-baroclinic-mode Ekman pumping response. The second baroclinic mode has a band of positive energy emanating westward from the eastward end of the Kuroshio Extension and ending at the western boundary at 20°N, reflecting the strong effect of the mean flow on wave propagation of the higher baroclinic modes. Mode coupling also occurs, especially in the westward return flow of the subtropical gyre. The results are shown to be consistent with the one-dimensional wave equation and show the importance of inclusion of the first several baroclinic modes in studies of the decadal variability in the ocean.