An Idealized Model for the Emergence of the North Icelandic Jet

The North Icelandic Jet (NIJ) is a significant contributor to the lower limb of the Atlantic Meridional Overturning Circulation, transporting dense overflow waters banked up along the slope north of Iceland equatorward and supplying up to half of the Denmark Strait overflow water, including the densest portion. Major uncertainties remain regarding what mechanisms contribute to the emergence of the NIJ northeast of Iceland. This study investigates previously proposed mechanisms using a novel setup with a high-resolution idealized model for the north Icelandic slope. We set up a channel model along the slope north of Iceland with differing slope geometry, no external forcing, and horizontally uniform initial and boundary conditions based on observations. We impose highly idealized inflows and outflows as boundary conditions in the west, emulating the North Icelandic Irminger Current (NIIC) inflow and dense NIJ outflow through Denmark Strait. The model consistently replicates key features of the NIJ, such as its mid-depth intensified core associated with diverging isopycnals away from the slope. Our results corroborate that a steeper slope, a stronger NIIC-like current, and stronger cross-slope density gradients promote instabilities closely related to the emerging NIJ-like current. Moreover, the simulated gradual eastward weakening of the NIJ-like current combined with enhanced eddy kinetic energy in the east is sufficient to facilitate an emergence. The variability of the model's NIJ-like current is largely associated with passing eddies, which might explain some of the observed occupations where the NIJ features a double-core structure.