Upstream sources of the Denmark Strait Overflow: Observations from a high-resolution mooring array

Abstract

We present the first results from a densely instrumented mooring array upstream of the Denmark Strait sill, extending from the Iceland shelfbreak to the Greenland shelf. The array was deployed from September 2011 to July 2012, and captured the vast majority of overflow water denser than 27.8 kg m −3 approaching the sill. The mean transport of overflow water over the length of the deployment was 3.54 ± 0.16 Sv. Of this, 0.58 Sv originated from below sill depth, revealing that aspiration takes place in Denmark Strait. We confirm the presence of two main sources of overflow water: one approaching the sill in the East Greenland Current and the other via the North Icelandic Jet. Using an objective technique based on the hydrographic properties of the water, the transports of these two sources are found to be 2.54 ± 0.17 Sv and 1.00 ± 0.17 Sv, respectively. We further partition the East Greenland Current source into that carried by the shelfbreak jet (1.50 ± 0.16 Sv) versus that transported by a separated branch of the current on the Iceland slope (1.04 ± 0.15 Sv). Over the course of the year the total overflow transport * Corresponding author. is more consistent than the transport in either branch; compensation takes place among the pathways that maintains a stable total overflow transport. This is especially true for the two East Greenland Current branches whose transports vary out of phase with each other on weekly and longer time scales. We argue that wind forcing plays a role in this partitioning.

The North Icelandic Jet and its relationship to the North Icelandic Irminger Current

“…Intriguingly, the transports of both the NIJ and NIIC diminish as one progresses around the north slope of Iceland (Våge et al 2011), such that both currents tend to "disintegrate" near the northeast corner of the island. The volume flux of each current also appears to be comparable (O(1 Sv); Jónsson and Valdimarsson 2012;Harden et al 2016). Hence, from a mass budget perspective, the inflow of warm, light water roughly balances the outflow of cold, dense water (although this is not required for such an overturning loop).…”

“…Four of these occupations (all carried out in the summer) were used by Våge et al (2013) in their investigation of the circulation north of Denmark Strait. Also, Harden et al (2016) created yearlong mean vertical sections from the mooring data along the Kögur line, which are obviously more robust than those presented here. However, the spatial coverage of the moorings was not as complete as that provided by our CTD/velocity transects.…”

“…Notably, the degree of partitioning between these branches varies over the course of the year such that the total amount of dense water delivered to Denmark Strait remains approximately the same. Harden et al (2016) argued that much of this partitioning is attributable to wind forcing. Using more than two decades' worth of hydrographic data from the region of the sill, Mastropole et al (2017) revealed that the Arctic-origin overflow water from the NIJ is found in the deepest part of the strait, whereas the Atlantic-origin overflow water resides above this and to the west, in the vicinity of the East Greenland shelf break (the constriction of the strait forces the shelf break and separated EGC branches to move closer to each other).…”

“…Using data from a yearlong mooring array deployed across the northern part of Denmark Strait, Harden et al (2016) determined that, on average, the shelf break EGC accounts for 42% of the overflow transport (1.50 Sv), the separated EGC accounts for 30% (1.04 Sv), and the NIJ accounts for 28% (1.00 Sv). Notably, the degree of partitioning between these branches varies over the course of the year such that the total amount of dense water delivered to Denmark Strait remains approximately the same.…”

“…Only then can we begin to determine the sensitivity of the AMOC to high-latitude climate change, including the effects of increased freshwater input to the Nordic Seas (Gierz, Lohmann, and Wei 2015;Dukhovskoy et al 2016), changes in the air-sea heat fluxes (Moore, Renfrew, and Pickart 2012), and trends in sea-ice concentration (e.g., Moore et al 2015). The mean transport of the overflow water exiting Denmark Strait is between 3 and 3.5 Sverdrup (Sv) (Macrander et al 2005;Jochumsen et al 2012Jochumsen et al , 2017Harden et al 2016). This represents about half of the input of dense water to the North Atlantic Deep Western Boundary Current (Dickson and Brown 1994), making Denmark Strait a critical passageway in the AMOC.…”