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Mesoscale features present at the Denmark Strait sill regularly enhance the volume transport of the Denmark Strait overflow (DSO). They are important for the Atlantic Meridional Overturning Circulation and ultimately, for the global climate system. Using a realistic numerical model, we find new evidence of the causal relationship between overflow surges (i.e., mesoscale features associated with high-transport events) and DSO cyclones observed downstream. Most of the cyclones form at the Denmark Strait sill during overflow surges and, because of potential vorticity conservation and stretching of the water column, grow as they move equatorward. A fraction of the cyclones form downstream of the sill, when anticyclonic vortices formed during high-transport events start collapsing. Regardless of their formation mechanism, DSO cyclones weaken starting roughly 150 km downstream of the sill, and potential vorticity is only materially conserved during the growth phase.

Plain Language Summary Ocean currents affecting the global climate are sustained by cold and dense water that sinks in the North Atlantic Ocean. A large portion of this water passes through Denmark Strait, the ocean channel located between Greenland and Iceland. The amount of water entering the strait varies from day to day and is controlled by ocean vortices. Knowing the mechanisms associated with these vortices is of key importance for understanding and predicting Earth's climate. Using a realistic numerical model, we find that the vortices are generated in the strait during dense water surges. In one scenario, the ocean vortices cross Denmark Strait rotating in the same direction as the Earth (counterclockwise). These vortices strengthen as they move toward the south. If the vortices initially rotate clockwise, they move slowly and quickly collapse. The water converging south of them triggers the formation of new vortices rotating counterclockwise. All of these energetic vortices move toward the equator. First they quickly grow, then they lose energy starting roughly 150 km south of Denmark Strait.