Arctic Mediterranean Exchanges: A consistent volume budget and trends in transports from two decades of observations

Abstract

Abstract. The Arctic Mediterranean (AM) is the collective name for the Arctic Ocean, the Nordic Seas, and their adjacent shelf seas. Into this region, water enters through the Bering Strait (Pacific inflow) and through the passages across the Greenland-Scotland Ridge (Atlantic inflow) and then modified within the AM. The modified waters leave the AM in several flow branches, which are grouped into two different categories: (1) overflow of dense water through the deep passages across the Greenland-Scotland Ridge, and (2) outflow of light water – here termed surface outflow – on both sides of Greenland. These exchanges transport heat, salt, and other substances into and out of the AM and are important for conditions in the AM. They are also part of the global ocean circulation and climate system. Attempts to quantify the transports by various methods have been made for many years, but only recently, has the observational coverage become sufficiently complete to allow an integrated assessment of the AM-exchanges based solely on observations. In this study, we focus on the transport of water and have collected data on volume transport for as many AM-exchange branches as possible between 1993–2015. The total AM-import (oceanic inflows plus freshwater) is found to be 9.1 ± 0.7 Sv (1 Sv = 106 m3 s−1) and has a seasonal variation of amplitude close to 1 Sv and maximum import in October. Roughly one third of the imported water leaves the AM as surface outflow with the remaining two thirds leaving as overflow. The overflow is mainly produced from modified Atlantic inflow and around 70 % of the total Atlantic inflow is converted into overflow, indicating a strong coupling between these two exchanges. The surface outflow is fed from the Pacific inflow and freshwater, but is still ~ 2/3rds from modified Atlantic water. For the inflow branches and the two main overflow branches (Denmark Strait and Faroe Bank Channel), systematic monitoring of volume transport has been established since the mid-1990s and this allows us to estimate trends for the AM-exchanges as a whole. At the 95 % level, only the inflow of Pacific water through the Bering Strait showed a statistically significant trend, which was positive. Both the total AM-inflow and the combined transport of the two main overflow branches also showed trends consistent with strengthening, but they were not statistically significant. They do suggest, however, that any significant weakening of these flows during the last two decades is unlikely and the overall message is that the AM-exchanges remained remarkably stable in the period from the mid-1990s to the mid-2010s. The overflows are the densest source water for the deep limb of the North Atlantic part of the Meridional Overturning Circulation (AMOC), and this conclusion argues that the reported weakening of the AMOC was not due to overflow weakening or reduced overturning in the AM. Although the combined data set has made it possible to establish a consistent budget for the AM-exchanges, the observational coverage for some of the branches is limited, which introduces considerable uncertainty. This lack of coverage is especially extreme for the surface outflows through the Denmark Strait, the overflow across the Iceland-Faroe Ridge, and the inflow over the Scottish shelf. We recommend that more effort is put into observing these flows as well as maintaining the monitoring systems established for the other exchange branches.

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

“…This is readily done by reducing the flow south in the KGR column in Table a small amount from −0.64 to −0.41 Sv such that flow north and south match at 8.84 Sv. This 8.84 Sv is larger than the R18 estimate due to the way flow is estimated in the FSC and the inclusion of the FIC (Østerhus et al, ). Given the 18.4 Sv for the southern section we infer that 9.57 Sv (18.4–8.84 Sv) is lost from the upper limb of the MOC between the two key sections.…”

“…The significantly greater loss rate for the Nordic Seas is another way of noting that the temperature difference between the water flowing north and the water spilling back into the North Atlantic is much greater than for either the Labrador Sea or subpolar gyre. It should also be noted that Østerhus et al (; their Table 2) report no long‐term trends of the observed exchanges across the Greenland‐Scotland Ridge. Figure summarizes the partitioning of the MOC fluxes between the Labrador Sea, the subpolar gyre, and the Nordic Seas.…”

“…Numerous studies of volume fluxes and estimates of the MOC based on hydrographic sections across the subpolar North Atlantic have been reported (McCartney & Talley, 1984;Bacon, 1997;Sarafanov et al, 2012;Talley, 2013;Mercier et al, 2015;Daniault et al, 2016;Bringedal et al, 2018;Østerhus et al, 2018;Lozier et al, 2019). A few studies also address heat and fresh water fluxes (Bacon, 1997;Wijffels, 2001;Trenberth & Fasullo, 2008;Lozier et al, 2019).…”

Structure and Forcing of Observed Exchanges across the Greenland–Scotland Ridge

“…The OW transport through FBC is detailed by Hansen et al (2015bHansen et al ( , 2016, while the DS overflow is presented by Jochumsen et al (2017). Recent observations and estimates of the overflow across the IFR suggest a mean overflow of less than 0.4 Sv (1 Sv [ 10 6 m 3 s 21 ) (Hansen et al 2018). Observations of overflow across the Wyville-Thomson Ridge (WTR) are available, with some gaps, for 2003-13 and are on average 0.8 Sv (Sherwin et al 2008a;Sherwin 2010).…”

Sustainable Observations of the AMOC: Methodology and Technology

“…Arrows are approximately scaled proportion to transport. Gray topography is shallower than 750 m. Adapted from Østerhus et al, .…”