Circulation and dynamics of the Irminger Basin

The Irminger Basin is located south of the Denmark Strait, between East Greenland and Iceland, where cold and fresh Arctic waters meet warm and salty Atlantic waters.
It is named after the Danish Admiral Carl Ludvig Christian Irminger, one of the first scientists to determine that Arctic water flowed southward through Denmark Strait.
The dynamics within the basin are important for climate change scenarios because the mixing between Arctic and Atlantic waters controls the transformation and the sinking rates of the dense waters.

Our research is aimed at understanding and investigating the small scale physical processes at the base of this exchange process via numerical simulations.

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A new small scale process was discovered in summer 2001, when researchers from Woods Hole Oceanographic Institution carried out a high-resolution survey in the western Irminger Basin. They found out a new component of the basin circulation, especially evident in a single ship transect (magenta-dashed line in the figures below). The feature results from the spilling of dense waters over the shelf and is characterized by a narrow (10 - 15 km) and strong (0.6 m/s) equatorward flow, banked against the upper continental slope. It was therefore named as East Greenland Spill Jet.

Our high-resolution numerical simulation of the basin compares well with observations of surface fields, Denmark Strait Overflow (DSO) and the hydrographic structure of typical sections in the area. In particular, the model shows surface cyclones which are observed in the satellite thermal imagery and related to DSO pulses present at depth. A train of model cyclonic eddies is shown in the figures above. Model eddy diameters are approximately 35 - 40 km and compare well with estimates from satellite.
We show that the southward propagation of these cyclones strongly influences the dynamics at the Spill Jet (magenta) section.

The simulation extends the synoptic view provided by the observations: The model Spill Jet is a persistent feature but exhibits large variations on periods of O(0.1 - 10) days. Furthermore, model results suggest two different types of spilling events (see schematic below). In the first case (Type I, upper panels), a local perturbation results in dense waters descending over the shelfbreak which may interact with other dense structures at depth. In the second case (Type II, lower panels), the spilling is caused by the passage of a cyclonic feature, linked to DSO boluses. It is the leading edge of the cyclone which draws dense waters off the shelf. In the schematic below, the cyan and violet surfaces represent the idealized positions of the σ = 27.75 and σ = 27.80 kg/m³ isopycnals, respectively. The dark blue disks at depth are indicative of σ ≥ 27.85 kg/m³ waters and denote the idealized DSO dense domes.

Type II spilling events are particularly evident in the following three-dimensional movie which shows the evolution of the densest isopycnal surfaces with different levels of transparency. The animation shows that spilling of dense waters over the shelf can occur in various locations southwest of Denmark Strait and it is present throughout the simulation.

The time series below is for the model Spill Jet transport in summer 2003. Spill Jet transport maxima are distinguished by the two types of spilling. During the simulated period, more than half of the largest Spill Jet transport values are of Type II.
The average model Spill Jet transport is 4.9 ± 1.7 Sv equatorward, close to the transport of the DSO waters at the same location. Hence, model results show that the Spill Jet may be one of the main processes by which Arctic and Atlantic waters mix in the Irminger Basin.