Entrainment, diapycnal mixing and transport in three-dimensional bottom gravity current simulations using the Mellor–Yamada turbulence scheme

The diapycnal mixing, entrainment and bottom boundary layer (BBL) dynamics in simulations of dense overflows are evaluated, using a generalized coordinate ocean model that can utilize terrain-following or z-level vertical grids and uses the Mellor–Yamada (M–Y) turbulence closure scheme to provide vertical mixing coefficients. Results from idealized dense water overflow experiments at resolutions of 10 and 2.5 km using a terrain-following vertical coordinates compare very well with the results from a nonhydrostatic ocean model (MITgcm) with resolution of 0.5 km, and also with the basic observed properties of the Denmark Straits overflow. At both 10 and 2.5 km resolutions the dilution of the bottom plume is comparable to the nonhydrostatic results, indicating that the M–Y scheme represents the subgrid-scale mixing very well. The 2.5 and 0.5 km model simulations are surprisingly very similar even in the details of the eddy field structure. Strong diapycnal mixing and large entrainment result in more than doubling the plume transport within ∼100 km from the source, similar to observations, while further downstream small detrainment and reduced transport occur. However, when the diapycnal mixing associated with the mixing scheme is eliminated by setting KH=0, the terrain-following model results closely resemble the results of isopycnal models whereas the dense plume slides farther downslope while its transport continue to increase indefinitely. On the other hand, when eliminating the bottom Ekman transport associated with the mixing scheme by setting KM=0, the model results resemble the results of a stepped-topography z-level model without BBL. The bottom Ekman transport associated with the M–Y vertical mixing was found to be responsible for ∼20% of the downslope transport of the bottom plume. The vertical mixing coefficient shows a spatially variable asymmetric structure across the bottom plume indicating a stronger mixing over a thicker BBL in the upslope side of the plume, and a weaker mixing over a thinner BBL in the downslope side of the plume.