Ross W. Griffiths

The relative roles of advective processes and mixing on the temporal adjustment of the meridional overturning circulation are examined, in particular the effects of mixing in either the abyssal or upper ocean. Laboratory experiments with convectively driven overturning and imposed stirring rates show that the circulation adjusts toward an equilibrium state on time scales governed by mixing in the upper boundary layer region but independent of the mixing rate in the bulk of the interior. The equilibrium state of the stratification is dependent only on the rate of mixing in the boundary layer. An idealized high-resolution ocean model shows adjustment (of a two-cell circulation) dominated primarily by the advective ventilation time scale, consistent with a view of the circulation determined by water mass transformation occurring primarily near the surface. Both the experiments and the model results indicate that adjustments of the circulation are controlled by surface buoyancy uptake (or rejection) and that the nonequilibrium circulation is dominated by advective processes, especially if the average abyssal ocean diffusivity is less than 3 × 10⁻⁵ m² s⁻¹.

A large-scale circulation, a turbulent boundary layer, and a turbulent plume are noted features of convection at large Rayleigh numbers under differential heating on a single horizontal boundary. These might be attributed to the forcing, which in all studies has been limited to a unidirectional gradient over the domain scale. We instead apply forcing on a length scale smaller than the domain, and with variation in both horizontal directions. Direct numerical simulations show turbulence throughout the domain, a regime transition to a dominant domain-scale circulation, and a region of logarithmic velocity in the boundary layer, despite zero net heat flux. The results show significant similarities to Rayleigh-Bénard convection, demonstrate the significance of plume merging, support the hypothesis that the key driver of convection is the production of available potential energy without necessarily supplying total potential energy, and imply that contributions to domain-scale circulation in the oceans need not be solely from the large-scale gradients of forcing.

A small range (∼ 1°C) of under‐ice water temperature is shown to result in remarkably different circulation regimes under spring ice in a deep, oligotrophic boreal lake. With the water column at < 4°C, melting of snow led to deepening vertical convection before ice break and a final depth of convection inversely correlated with earlier deep‐water temperature. We attribute that to the nonlinear dependence of water density on temperature, albeit further affected by stochastic weather factors. In four of nine study years, convection led to complete under‐ice overturn of the lake, indicating that this may not be uncommon in similar lakes with steep topography. River inflow and more intensive warming of water in the littoral zone also created a horizontal density differential, convection that involved flow down the sloping bottom and a lateral intrusion of this sinking water at a depth between the vertical convection and the quiescent deep‐water layers. The vertical and horizontal convection together produced a profile of temperature slightly increasing from the surface to the bottom of the convection layer. The contribution of horizontal convection to under‐ice mixing was interannually variable, and in one of the study years it eventually dominated under‐ice mixing. A thermal bar circulation regime developed occasionally and only in the open water between ice and shoreline. We identified five different under‐ice mixing regimes that form an interannually variable continuum of behavior during the ice melting period. The dependence on a narrow temperature range likely makes the circulation regime sensitive to a warming climate.