Influence of mean circulation on large-scale decadal basin modes

The effect of large-scale mean circulation on the generic property of intrinsic basin modes of decadal variability is investigated through linear stability analysis of a two-layer shallow water model over a flat- and a variable-bottom topography. The mean circulation is forced through either surface wind stress or vertical velocities at the layers interface representing surface heat flux. Regardless of the type of forcing, the large-scale mean circulation reduces the damping of the decadal basin modes. The wind forcing mostly affects the mode damping, by up to 30% for a climatological amplitude, whereas the heat flux forcing mostly increases the oscillation period, by several years (up to 30%). The oscillation period, characteristic of the adjustment process to the steady mean flow, displays, however, different behavior depending on the meridional shift of the eastward barotropic advection in the region of maximum basin mode amplitude: The period gets shortened (lengthened) in the wind-forced (thermally forced) experiment with respect to the unforced one. The results are rationalized through the analysis of long Rossby waves propagation and underscore the key role of two processes in setting the oscillation period of the basin adjustment: (1) changes in Rossby wave speed due to changes in isopycnals depth and (2) changes in the mean barotropic zonal flow. These processes reinforce each other in the thermal-forcing case, resulting in large modifications of the mode period, but almost compensate in the wind-forced case, resulting in slight changes of the mode period, compared with the reference rest state.