The Meridional Overturning Circulation in the western Nordic seas

Warm Gulf Stream-origin waters flow northward across the Greenland-Scotland Ridge into the Nordic Seas where they are subject to intense heat loss. The resulting cold and dense water mass returns southward by flowing through gaps in the ridge and descending the continental slope as overflow plumes. These overflows are the headwaters of the lower limb of the Atlantic Meridional Overturning Circulation (AMOC), and, as such, are crucial components of the Earth's climate system. The largest contributor is the Denmark Strait overflow plume, which passes southward between Greenland and Iceland. There are two major sources of dense water to this overflow plume. The North Icelandic Jet transports dense water into Denmark Strait along the slope off northern Iceland. It has been hypothesized that the North Icelandic Jet is the deep limb of a local overturning loop that involves the boundary current system north of Iceland and water mass transformation in the Iceland Sea. Recent high-resolution numerical simulations provide some support for this hypothesis. Sparse wintertime hydrographic data confirm that dense waters are formed in the Iceland Sea, but indicate that the densest component of this local overturning loop may be formed primarily in the Greenland Sea and only to a limited extent in the Iceland Sea. Measurements from three gliders that were deployed through winter 2015-16 suggest that the densest waters from the Iceland Sea are formed near the ice edge off east Greenland. The primary source of dense water to the Denmark Strait overflow plume is the East Greenland Current. The current also transports substantial amounts of cold, fresh surface water from the Arctic Ocean, and is composed of a complex system of branches that reflect the different origins of the water masses they transport. North of Denmark Strait the East Greenland Current is, particularly in winter, unstable and a source of eddy kinetic energy that may be one cause of the substantial short-term variability in the overflow plume observed at the Denmark Strait sill. Frequent wind-driven upwelling and downwelling events modulate the volume transport and redistribute the water masses that the current transports. A wintertime retreat of sea ice from the western margin of the Nordic Seas combined with different rates of warming for the atmosphere and sea surface in the Iceland and Greenland Seas has resulted in diminished air-sea heat fluxes in this region. Further decreases in atmospheric forcing could result in reduced ventilation of the mid-depth waters in the Nordic Seas and a decreased supply of the densest waters to the AMOC. However, the retreating ice edge has also partially uncovered the ice-laden East Greenland Current. Severe heat loss from the ocean offshore of the ice edge triggers convection, which further modifies the current?s dense waters. This additional cooling of one of the dense water masses that supplies the Denmark Strait overflow plume is a counterintuitive occurrence in a warming climate.

Blant virkninger og potensielle virkninger er økt internasjonalt samarbeid mellom forskere i Norge, Island og USA. Dette prosjektet har blant annet bidratt til å legge faglig grunnlag for en multinasjonal hav-atmosfære feltkampanje i Islands- og Grønlandshavene som ble gjennomført vinteren 2018. Resultatene fra dette eksperimentet vil sannsynligvis prege videre forskning på dette feltet i lang tid. Som direkte følge av prosjektet ble en PhD grad og en MSc grad oppnådd. Prosjektet har til nå resultert i 13 fagfelle-vurderte artikler. Disse artiklene bidrar til å øke vår grunnleggende forståelse av det marine klimasystemet i den vestlige delen av De nordiske hav. Blant annet demonstrerer resultatene at en solid grunnleggende forståelse av hav-sirkulasjonen i dette området er en forutsetning for å kunne forutsi hvordan det atlantiske marine klimasystemet vil endres under global oppvarming. Vi forventer at disse resultatene vil ha en effekt på klimaforskningen.

The Nordic seas' overflow waters constitute the lower limb of the Atlantic Meridional Overturning Circulation (AMOC), and, as such, are crucial components of the Earth's climate system. Dense water formed north of the Greenland-Scotland Ridge returns sout hward by flowing through gaps in the ridge as overflow plumes. The largest of these is the Denmark Strait Overflow Water (DSOW) which forms the densest component of the AMOC. The purpose of this proposed basic research project is to enhance our understa nding of the climate system and its variability upstream of the Denmark Strait and in the Iceland Sea. In particular, we seek to study the processes by which the DSOW is formed and transported to the Denmark Strait. This project will contribute both to re medy a gap in our current knowledge about a central part of the AMOC system and to clarify how a changing climate will impact the AMOC.