Structure and Variability of the North Icelandic Jet From Two Years of Mooring Data

Mooring data from September 2011 to July 2013 on the Iceland slope north of Denmark Strait are analyzed to better understand the structure and variability of the North Icelandic Jet (NIJ). Three basic configurations of the flow were identified: (1) a strong separated East Greenland Current (EGC) on the mid-Iceland slope coincident with a weak NIJ on the upper slope, (2) a merged separated EGC and NIJ, and (3) a strong NIJ located at its climatological mean position, coincident with a weak signature of the separated EGC at the base of the Iceland slope. Our study reveals that the NIJ-dominant scenario was present during different times of the year for the two successive mooring deployments-appearing mainly from September to February the first year and from January to July the second year. Furthermore, when this scenario was active it varied on short timescales. An energetics analysis demonstrates that the high-frequency variability is driven by mean-to-eddy baroclinic conversion at the shoreward edge of the NIJ, consistent with previous modeling work. The seasonal timing of the NIJ dominant scenario is investigated in relation to the atmospheric forcing upstream of Denmark Strait. The resulting lagged correlations imply that strong turbulent heat fluxes in a localized region on the continental slope of Iceland, south of the Spar Fracture zone, lead to a stronger NIJ dominant state with a two-month lag. This can be explained dynamically in terms of previous modeling work addressing the circulation response to dense water formation near an island. Plain Language Summary The dense water flowing southward through Denmark Strait represents the largest contribution to the deep limb of the Atlantic Meridional Overturning Circulation, the large-scale flow pattern that helps regulate Earth's climate. Three different currents feed the overflow in the strait: the shelfbreak East Greenland Current adjacent to Greenland, the separated East Greenland Current located farther offshore, and the North Icelandic Jet (NIJ) on the Iceland continental slope. The latter, which was recently discovered, supplies the deepest and densest overflow water to the strait. To understand the structure and variability of the NIJ, we analyze two years of mooring data on the Iceland slope roughly 200 km northeast of Denmark strait. We present the spatial and temporal variability of three configurations of the flow and focus on the NIJ-dominant scenario. The rapid (less than 1 week) variability is found to arise from instability associated with the current's density structure. We then address the monthly variation, which we argue is driven by air-sea heat fluxes in a localized region on the continental slope north of Iceland. The ocean responds to the fluxes in this region by exciting waves that propagate clockwise around the island, which in turn impact the strength of the NIJ.