Lagrangian pathways and transformation of Denmark Strait Overflow Water in the Irminger Basin

The Denmark Strait Overflow (DSO) supplies one third of the North Atlantic Deep Water and is a key component of the global thermohaline circulation. Knowledge of the pathways of DSO through the Irminger Basin and its transformation there is still incomplete however.

The Lagrangian framework is ideal for addressing these questions, but no Lagrangian observations of DSO exist. Instead, its evolution has been derived solely from sparse Eulerian (fixed-point) measurements. We deploy over 10,000 Lagrangian particles at Denmark Strait in a high resolution ocean model to study these issues. The particle trajectories show that: First, the mean-position and potential density of dense waters cascading over the Denmark Strait sill evolve consistently with hydrographic observations. These sill particles transit the Irminger basin to the Spill Jet section (65.25N) in 5-7 days and to the Angmagssalik section (63.5N) in two-three weeks. Second, the dense water pathways on the continental shelf are consistent with observations and shelf particles contribute significantly to the dense water at the Angmagssalik section (approx. 25%). Some particles circulate on the shelf for several weeks before they spill off the shelf break and join the overflow from the sill. Third, there are two places where the water density following particle trajectories decreases rapidly due to intense mixing with waters of Atlantic origin: southwest of the sill and southwest of the Kangerdlugssuaq Trough on the continental slope. Intense mixing occurs also on the shelf, involving the Polar Waters. Following the transformation in these places, the overflow particles exhibit a wide range of densities downstream. We quantify the horizontal and vertical mixing processes with eddy diffusivities estimated from particle velocities by using the recently proposed clustering method. Finally, we find that the densest waters in the Irminger Basin originate from the shelf adjacent to the sill, not from the sill itself as previously believed.

Our study extends the conceptual view of the DSO in the Irminger Basin, and urges extensive observational campaigns to verify these results and monitor different pathways of DSO components.