Flow Splitting in Numerical Simulations of Oceanic Dense-Water Outflows

Flow splitting, previously observed in laboratory studies, occurs when part of a gravity current becomes neutrally buoyant and separates from the bottom-trapped plume as an intrusion. If flow-splitting occurs in the ocean, it would allow dense-water outflows, e.g., from Antarctic continental shelves, to simultaneously ventilate the abyssal ocean and intermediate water layers. In this study, high resolution (dx=dz=5 m) numerical simulations are used to investigate whether this phenomenon could occur in oceanic gravity currents flowing into linearly stratified environments. The model is configured to solve the nonhydrostatic Boussinesq equations without rotation. First, a set of 2-D experiments were conducted by varying the horizontal turbulent Prandtl number (Pr_H), the ambient stratification frequency (N) and the bottom slope (α). When Pr_H=1, splitting does not occur, regardless of N and α. However, when Pr_H>=10, splitting may occur depending on α and the buoyancy number B=QN³/g'² (where Q is the volume flux of the dense water flow per unit width and g' is the reduced gravity), and the flow always splits when B~10^-2 and α=0.1. When Pr_H is increased to 10³, the parameter space under which flow splitting occurs is expanded (B~5x10^-3 to 10^-2 and α>=0.01). Results from a 3-D simulation where Pr_H=10³, B~10^-2 and α=0.1 also show flow splitting, consistent with the equivalent 2-D experiment. An important characteristic of simulations that result in flow splitting is the flow transition from a supercritical condition, where the Froude number (Fr) is greater than one, to a slower and more uniform subcritical condition (Fr<1). This transition is associated with an internal hydraulic jump and consequent mixing enhancement. A comparison between numerical results where Pr_H>=10 and oceanic observations suggests that flow splitting may occur in dense-water outflows on steep slopes and with weak ambient stratification, such as the Antarctic outflows.