The influence of oceanic bottom slopes on eddy mixing in a two-layer model

Oceanic mesoscale eddy mixing plays a crucial role in the Earth’s climate system by redistributing heat, salt and carbon. Eddy mixing is impacted by various physical factors, one of which is the oceanic bottom slope. Within a barotropic framework, it can be shown analytically that bottom slopes suppress the cross-slope eddy mixing. Unfortunately, adding baroclinic effects greatly increases the complexity of the problem. To understand how bottom slopes influence eddy mixing in a baroclinic framework, we study eddy fields in a quasi-geostrophic two-layer model with a linear bottom slope. We investigate the eddy mixing by releasing and tracking virtual particles in the flow fields and analysing how they spread in the cross-slope direction. This is done for a range of bottom slope magnitudes and for prograde as well as retrograde slopes. The goal is to figure out how eddy mixing depends on the steepness and direction of the bottom slope and on the position in the water column. We find that for steep bottom slopes, the baroclinic instability is suppressed, the eddy field gets weaker, and the spreading of particles in the cross-slope direction decreases. This suppression is comparable for prograde and retrograde slopes. Moreover, the suppression is observed not only in the bottom layer, where the slope is located, but also in the upper layer. This indicates that the suppression of eddy mixing by oceanic bottom slopes can have an impact throughout the water column.