Ocean models need to parameterise the detrainment from bottom water plumes

Abstract. Because diapycnal advection in the ocean is a consequence of diapycnal mixing, there is a tendency to think that knowledge of the mixing intensity throughout the abyss would determine the ocean's density stratification. This is not the case. We show that in the abyssal ocean (deeper than ~3000 m) the magnitude of the stratification depends not only on the strength of diapycnal mixing but also on the rate of detrainment from the dense Antarctic Bottom Water plumes. Ocean models have never attempted to represent such detrainment physics so that the magnitude of the stratification in the deepest half of ocean models, if run to equilibrium, would not be the result of a correct physical balance. We show that the detrainment process causes the vertical stratification in the abyss to be approximately 2.75 as large as it would be in the absence of this detrainment process.  This means that we cannot rely on how the abyss in ocean models reacts to changes in forcing, such as global warming. Also, the detrainment process causes the density of sinking plumes to increase downstream, and this has not been observed in ocean models to date. We conclude that detrainment from dense plumes is an important physical process that needs to be incorporated into ocean models.