Formation and Evolution of High-transport Denmark Strait Overflow Events

About half of the dense overflow across the Greenland-Iceland-Scotland ridge is supplied by the Denmark Strait Overflow (DSO), making Denmark Strait a critical gateway between the Arctic and the subpolar North Atlantic. Boluses and pulses are mesoscale features that cross the Denmark Strait during high-overflow transport events. They occur with the same frequency as the DSO cyclones observed downstream. However, the connection between boluses, pulses, and DSO cyclones is not well established. To investigate this connection, we designed an automated vortex detection algorithm and we applied it to a realistic, high-resolution numerical model solution. Our goal is threefold: (i) To better understand the dynamics associated with stretching of the water column in the DSO; (ii) To determine how the hydrographic properties of DSO mesoscale features evolve in space and time; (iii) To advance the understanding of the high-frequency variability characteristic of the Denmark Strait Overflow. The model shows that both boluses and pulses produce DSO cyclones, but the underlying mechanisms are different. DSO cyclones associated with boluses form in Denmark Strait and, because of potential vorticity conservation and stretching of the water column, they quickly grow as they move south. In contrast, DSO cyclones associated with pulses form downstream of the sill as slow-moving anticyclones near the East Greenland shelfbreak collapse. Regardless of their formation mechanism, DSO cyclones weaken when they encounter a rapid steepening of the bathymetric slope, and the rate of decay is slower than the earlier growth rate. The mean stratification of the cyclones decreases during their life cycle, while their initial increase in relative vorticity due to stretching is followed by a decrease. As a result, potential vorticity is only conserved during the growth phase. Last, composites of the deep cyclones show that they are preceded by downwelling and followed by upwelling during their life cycle.