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1. How Much Arctic Fresh Water Participates in the Subpolar Overturning Circulation?

   https://doi.org/10.1175/JPO-D-20-0240.1  

   Le Bras, Isabela; Straneo, Fiamma; Muilwijk, Morven; Smedsrud, Lars H.; Li, Feili; Lozier, M. Susan; Holliday, N. Penny (March 2021, Journal of Physical Oceanography)

   null (Ed.)

   Abstract Fresh Arctic waters flowing into the Atlantic are thought to have two primary fates. They may be mixed into the deep ocean as part of the overturning circulation, or flow alongside regions of deep water formation without impacting overturning. Climate models suggest that as increasing amounts of freshwater enter the Atlantic, the overturning circulation will be disrupted, yet we lack an understanding of how much freshwater is mixed into the overturning circulation’s deep limb in the present day. To constrain these freshwater pathways, we build steady-state volume, salt, and heat budgets east of Greenland that are initialized with observations and closed using inverse methods. Freshwater sources are split into oceanic Polar Waters from the Arctic and surface freshwater fluxes, which include net precipitation, runoff, and ice melt, to examine how they imprint the circulation differently. We find that 65 mSv (1 Sv ≡ 10 6 m 3 s −1 ) of the total 110 mSv of surface freshwater fluxes that enter our domain participate in the overturning circulation, as do 0.6 Sv of the total 1.2 Sv of Polar Waters that flow through Fram Strait. Based on these results, we hypothesize that the overturning circulation is more sensitive to future changes in Arctic freshwater outflow and precipitation, while Greenland runoff and iceberg melt are more likely to stay along the coast of Greenland. 

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2. The Bering Strait Throughflow Component of the Global Mass, Heat and Freshwater Transport

   https://doi.org/10.1029/2024JC021463  

   Yang, Xiaoting; Cessi, Paola (October 2024, Journal of Geophysical Research: Oceans)

   Abstract As the only oceanic connection between the Pacific and Arctic‐Atlantic Oceans, Bering Strait throughflow carries a climatological northward transport of about 1 Sv, contributing to the Atlantic Meridional Overturning Circulation (AMOC). Here, Lagrangian analysis quantifies the global distributions of volume transport, transit‐times, thermohaline properties, diapycnal transformation, heat and freshwater transports associated with Bering Strait throughflow. Virtual Lagrangian parcels, released at Bering Strait, are advected by the velocity of Estimating the Circulation and Climate of the Ocean, backward and forward in time. Backward trajectories reveal that Bering Strait throughflow enters the Pacific basin on the southeast side, as part of fresh Antarctic Intermediate Water, then follows the wind‐driven circulation to Bering Strait. Median transit time from S in Indo‐Pacific to Bering Strait is 175 years. Sixty‐four percent of Bering Strait throughflow enters the North Atlantic through the Labrador Sea. The remaining 36% flows through the Greenland Sea, warmed and salinified by the northward flowing Atlantic waters. Deep water formation of water flowing through Bering Strait occurs predominantly in the Labrador Sea. Subsequently, this water joins the lower branch of AMOC, flowing southward in the deep western boundary current as North Atlantic Deep Water. Median transit time from Bering Strait to S in South Atlantic is 160 years. The net heat transport of Bering Strait throughflow is northward everywhere, and net freshwater transport by Bering Strait throughflow is mostly northward. The freshwater transport is largest in the subpolar region of basin sectors: northward in the Pacific and Arctic and southward in the Atlantic. 

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3. A Review of Arctic–Subarctic Ocean Linkages: Past Changes, Mechanisms, and Future Projections

   https://doi.org/10.34133/olar.0013  

   Wang, Qiang; Shu, Qi; Wang, Shizhu; Beszczynska-Moeller, Agnieszka; Danilov, Sergey; Steur, Laura; Haine, Thomas W.; Karcher, Michael; Lee, Craig M.; Myers, Paul G.; et al (January 2023, Ocean-Land-Atmosphere Research)

   Arctic Ocean gateway fluxes play a crucial role in linking the Arctic with the global ocean and affecting climate and marine ecosystems. We reviewed past studies on Arctic–Subarctic ocean linkages and examined their changes and driving mechanisms. Our review highlights that radical changes occurred in the inflows and outflows of the Arctic Ocean during the 2010s. Specifically, the Pacific inflow temperature in the Bering Strait and Atlantic inflow temperature in the Fram Strait hit record highs, while the Pacific inflow salinity in the Bering Strait and Arctic outflow salinity in the Davis and Fram straits hit record lows. Both the ocean heat convergence from lower latitudes to the Arctic and the hydrological cycle connecting the Arctic with Subarctic seas were stronger in 2000–2020 than in 1980–2000. CMIP6 models project a continuing increase in poleward ocean heat convergence in the 21st century, mainly due to warming of inflow waters. They also predict an increase in freshwater input to the Arctic Ocean, with the largest increase in freshwater export expected to occur in the Fram Strait due to both increased ocean volume export and decreased salinity. Fram Strait sea ice volume export hit a record low in the 2010s and is projected to continue to decrease along with Arctic sea ice decline. We quantitatively attribute the variability of the volume, heat, and freshwater transports in the Arctic gateways to forcing within and outside the Arctic based on dedicated numerical simulations and emphasize the importance of both origins in driving the variability. 

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

   https://doi.org/10.5194/os-15-379-2019  

   Østerhus, Svein; Woodgate, Rebecca; Valdimarsson, Héðinn; Turrell, Bill; de Steur, Laura; Quadfasel, Detlef; Olsen, Steffen M.; Moritz, Martin; Lee, Craig M.; Larsen, Karin Margretha; et al (January 2019, Ocean Science)

   Abstract. The Arctic Mediterranean (AM) is the collective name forthe Arctic Ocean, the Nordic Seas, and their adjacent shelf seas. Water enters into thisregion through the Bering Strait (Pacific inflow) and through the passages across theGreenland–Scotland Ridge (Atlantic inflow) and is modified within the AM. The modifiedwaters leave the AM in several flow branches which are grouped into two differentcategories: (1) overflow of dense water through the deep passages across theGreenland–Scotland Ridge, and (2) outflow of light water – here termed surface outflow– on both sides of Greenland. These exchanges transport heat and salt into and out ofthe AM and are important for conditions in the AM. They are also part of the global oceancirculation and climate system. Attempts to quantify the transports by various methodshave been made for many years, but only recently the observational coverage has becomesufficiently complete to allow an integrated assessment of the AM exchanges based solelyon observations. In this study, we focus on the transport of water and have collecteddata on volume transport for as many AM-exchange branches as possible between 1993 and2015. The total AM import (oceanic inflows plusfreshwater) is found to be 9.1 Sv (sverdrup,1 Sv =10⁶ m³ s⁻¹) with an estimated uncertainty of 0.7 Sv and hasthe amplitude of the seasonal variation close to 1 Sv and maximum import in October.Roughly one-third of the imported water leaves the AM as surface outflow with theremaining two-thirds leaving as overflow. The overflow water is mainly produced frommodified Atlantic inflow and around 70 % of the total Atlantic inflow is convertedinto overflow, indicating a strong coupling between these two exchanges. The surfaceoutflow is fed from the Pacific inflow and freshwater (runoff and precipitation), but isstill approximately two-thirds of modified Atlantic water. For the inflowbranches and the two main overflow branches (Denmark Strait and Faroe Bank Channel),systematic monitoring of volume transport has been established since the mid-1990s, andthis enables us to estimate trends for the AM exchanges as a whole. At the 95 %confidence level, only the inflow of Pacific water through the Bering Strait showed astatistically significant trend, which was positive. Both the total AM inflow and thecombined transport of the two main overflow branches also showed trends consistent withstrengthening, but they were not statistically significant. They do suggest, however,that any significant weakening of these flows during the last two decades is unlikely andthe overall message is that the AM exchanges remained remarkably stable in the periodfrom the mid-1990s to the mid-2010s. The overflows are the densest source water for thedeep 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 tooverflow weakening or reduced overturning in the AM. Although the combined data set hasmade it possible to establish a consistent budget for the AM exchanges, the observationalcoverage for some of the branches is limited, which introduces considerable uncertainty.This lack of coverage is especially extreme for the surface outflow through the DenmarkStrait, the overflow across the Iceland–Faroe Ridge, and the inflow over the Scottishshelf. We recommend that more effort is put into observing these flows as well asmaintaining the monitoring systems established for the other exchange branches. 

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5. Observation-based estimates of heat and freshwater exchanges from the subtropical North Atlantic to the Arctic

   Li, F.; Lozier, M. S.; Holliday, N. P.; Johns, W. E.; Le Bras, I. A.; Moat, B. I.; Cunningham, S. A.; de Jong, M. F. (July 2021, Progress in Oceanography)

   Continuous measurements from the OSNAP (Overturning in the Subpolar North Atlantic Program) array yield the first estimates of trans-basin heat and salinity transports in the subpolar latitudes. For the period from August 2014 to May 2018, there is a poleward heat transport of 0.50 ± 0.05 PW and a poleward salinity transport of 12.5 ± 1.0 Sv across the OSNAP section. Based on the mass and salt budget analyses, we estimate that a surface freshwater input of 0.36 ± 0.05 Sv over the broad subpolar-Arctic region is needed to balance the ocean salinity change created by the OSNAP transports. The overturning circulation is largely responsible for setting these heat and salinity transports (and the derived surface freshwater input) derived from the OSNAP array, while the gyre (isopycnal) circulation contributes to a lesser, but still significant, extent. Despite its relatively weak overturning and heat transport, the Labrador Sea is a strong contributor to salinity and freshwater changes in the subpolar region. Combined with trans-basin transport estimates at other locations, we provide new estimates for the time-mean surface heat and freshwater divergences over a wide domain of the Arctic-North Atlantic region to the north and south of the OSNAP line. Furthermore, we estimate the total heat and freshwater exchanges across the surface area of the extratropical North Atlantic between the OSNAP and the RAPID-MOCHA (RAPID Meridional Overturning Circulation and Heat-flux Array) arrays, by combining the cross-sectional transports with vertically-integrated ocean heat and salinity content. Comparisons with the air-sea heat and freshwater fluxes from atmospheric reanalysis products show an overall consistency, yet with notable differences in the magnitudes during the observation time period. 

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