Boundary mixing and the dynamics of threedimensional thermohaline circulations

Boundary mixing is implemented in an ocean general circulation model such that the vertical mixing coefficient ky is nonzero only near side boundaries and in convection regions. The model is used in a highly idealized configuration with no wind forcing and very nearly fixed surface density to investigate the three-dimensional dynamics of the thermohaline circulation. For ky 5 20 3 1024 m2 s21 and lower, the meridional overturning strength to great accuracy is proportional to; meridional heat transport is proportional to. The circulation2/3 1/2k ky y patterns resemble those from runs with uniform vertical mixing, but vertical motion is entirely confined to the boundary regions. Near the western boundary, there is upwelling everywhere. Near the eastern boundary, there is a consistent pattern of downwelling above upwelling, with downwelling reaching deeper at high latitudes; this pattern is explained by convection and vertical advective–diffusive balance underneath. For ky 5 30 3 1024 m2 s21 and higher, no steady solutions have been found; the meridional overturning oscillates on a timescale of about 25 years. A time-averaged thermally direct overturning cell is not supported dynamically because convection extends longitudinally across the entire basin, and upwelling near the western boundary does not lead to densities higher than at the eastern boundary. Assuming uniform upwelling in the west, level isopycnals near the equator, and level isopycnals along th