A Direct Estimate of Volume, Heat, and Freshwater Exchange Across the Greenland‐Iceland‐Faroe‐Scotland Ridge

Abstract

The flow of warm salty water toward the Nordic Seas is of fundamental importance to the climate of central and northern Europe. In an effort to gain an improved quantitative assessment of these fluxes a program was started in 2008 to measure upper-ocean currents from the high-seas ferry Norröna, which operates out of the Faroes to Iceland and Denmark. The current measurements were made with an acoustic Doppler current profiler mounted in the Norröna's hull. Starting in fall of 2013 monthly deployments of Expendable BathyThermographs give comprehensive information on the temperature field. These velocity and temperature data can be combined to estimate mean volume and temperature fluxes (referenced to 0°C) for the two sections. Archived hydrographic data give us the corresponding salt transport. Thanks to an array of 12 tall moorings across the Blosseville Basin that measured currents, temperature, and salinity net transport of volume, temperature, and salt between Iceland and Greenland can also be estimated. By combining the velocity data from these three sections the strength of the Nordic Seas branch of the meridional overturning circulation is estimated to be 7.7 ± 0.8 Sv where 1 Sv = 10 6 m 3 /s. Imposing the constraint of zero net volume and salt flux, the corresponding heat and freshwater fluxes are estimated to be 264 ± 27 TW (1 TW = 10 12 W) and À0.104 ± 0.01 Sv, respectively. The uncertainties in heat and freshwater fluxes are largely governed by volume fluxes. The Norröna program is ongoing.Plain Language Summary The climate of central and northern Europe depends upon the flow of warm water into the northern North Atlantic and Nordic Seas. This study uses directly measured currents from the high-seas ferry Norröna in regular traffic between Denmark, the Faroes and Iceland to determine in great detail the flow of warm salty water into the Nordic Seas. An extensive array of current meters deployed between Iceland and Greenland covers all exchange with the Nordic Seas west of Iceland. During its transit through the Nordic Seas the warm and salty North Atlantic water loses its heat to the atmosphere, sinks, and eventually spills back across the sills between Greenland, Iceland, the Faroes, and Scotland into the deep North Atlantic. As these waters sink, they entrain cooled water from the Labrador and Irminger Seas. Without this overflow of dense water from the Nordic Seas the northern North Atlantic would become much cooler because of the cessation of warm water flow north to maintain the overturning process. Thus quantifying the flow of warm water into the Nordic Seas and tracking how it might change over time is one of the overarching questions regarding the climate of North Atlantic and surrounding landmasses.

Lagrangian Views of the Pathways of the Atlantic Meridional Overturning Circulation

“…But continuation of the trajectories northward over the Greenland‐Scotland Ridge into the Nordic Seas was not observed with these floats for several reasons: (1) many floats recirculated in the subpolar gyre and thus never reached the ridge, (2) many of the float trajectories were too short to make it to the ridge at the head of the Iceland Basin, and (3) those that did were on slightly deeper isopycnals, making it physically impossible to cross over the shallow ridge. This latter issue was addressed directly by Rossby et al () using 22 acoustically tracked floats released in the Iceland Basin just south of the Iceland‐Faroes Ridge at ~200 m. These floats drifted into the Norwegian Sea over deep channels west and east of the Faroes Islands, consistent with transport estimates from vessel‐mounted ADCP (Rossby et al, ).…”

Volume, Heat, and Freshwater Divergences in the Subpolar North Atlantic Suggest the Nordic Seas as Key to the State of the Meridional Overturning Circulation

“…The 0.88 Sv flow north in the NIIC column in (Hansen et al, 2016). Segtnan et al (2011) obtain very similar volume and temperature transport numbers for the relevant openings defined in Rossby et al (2018).…”

“…In this study, we use two recently completed sections (Rossby et al, , ) across the subpolar North Atlantic to quantify the branching of the upper limb and concomitant heat and freshwater divergences. The main finding is that out of the three MOC regions (subpolar gyre including Irminger Sea and Iceland Basin, the Labrador Sea, and the Nordic Seas), it is the Nordic Seas that the plays the key role.…”

Arctic Mediterranean exchanges: a consistent volume budget and trends in transports from two decades of observations

“…It is well known (e.g. Rudels, 2010) that the AM may be seen as a double estuary with both an estuarine and a thermohaline circulation. In Fig.…”