skip to main content

Title: How Much Arctic Fresh Water Participates in the Subpolar Overturning Circulation?
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 more » future changes in Arctic freshwater outflow and precipitation, while Greenland runoff and iceberg melt are more likely to stay along the coast of Greenland. « less
; ; ; ; ; ;
Award ID(s):
2038481 1756272
Publication Date:
Journal Name:
Journal of Physical Oceanography
Page Range or eLocation-ID:
955 to 973
Sponsoring Org:
National Science Foundation
More Like this
  1. 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 amplitudemore »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.

    « less
  2. Abstract The dense outflow through Denmark Strait is the largest contributor to the lower limb of the Atlantic meridional overturning circulation, yet a description of the full velocity field across the strait remains incomplete. Here we analyze a set of 22 shipboard hydrographic–velocity sections occupied along the Látrabjarg transect at the Denmark Strait sill, obtained over the time period 1993–2018. The sections provide the first complete view of the kinematic components at the sill: the shelfbreak East Greenland Current (EGC), the combined flow of the separated EGC, and the North Icelandic Jet (NIJ), and the northward-flowing North Icelandic Irminger Current (NIIC). The total mean transport of overflow water is 3.54 ± 0.29 Sv (1 Sv ≡ 10 6 m 3 s −1 ), comparable to previous estimates. The dense overflow is partitioned in terms of water mass constituents and flow components. The mean transports of the two types of overflow water—Atlantic-origin Overflow Water and Arctic-origin Overflow Water—are comparable in Denmark Strait, while the merged NIJ–separated EGC transports 55% more water than the shelfbreak EGC. A significant degree of water mass exchange takes place between the branches as they converge in Denmark Strait. There are two dominant time-varying configurations of themore »flow that are characterized as a cyclonic state and a noncyclonic state. These appear to be wind-driven. A potential vorticity analysis indicates that the flow through Denmark Strait is subject to symmetric instability. This occurs at the top of the overflow layer, implying that the mixing/entrainment process that modifies the overflow water begins at the sill.« less
  3. A widespread theory in paleoclimatology suggests that changes in freshwater discharge to the Nordic (Greenland, Norwegian, and Icelandic) Seas from ice sheets and proglacial lakes over North America played a role in triggering episodes of abrupt climate change during deglaciation (21–8 ka) by slowing the strength of the Atlantic Meridional Overturning circulation (AMOC). Yet, proving this link has been problematic, as climate models are unable to produce centennial-to-millennial–length reductions in overturning from short-lived outburst floods, while periods of iceberg discharge during Heinrich Event 1 (ca. 16 ka) may have occurred after the climate had already begun to cool. Here, results from a series of numerical model experiments are presented to show that prior to deglaciation, sea ice could have become tens of meters thick over large parts of the Arctic Basin, forming an enormous reservoir of freshwater independent from terrestrial sources. Our model then shows that deglacial sea-level rise, changes in atmospheric circulation, and terrestrial outburst floods caused this ice to be exported through Fram Strait, where its subsequent melt freshened the Nordic Seas enough to weaken the AMOC. Given that both the volume of ice stored in the Arctic Basin and the magnitude of the simulated export events exceedmore »estimates of the volumes and fluxes of meltwater periodically discharged from proglacial Lake Agassiz, our results show that non-terrestrial freshwater sources played an important role in causing past abrupt climate change.« less
  4. We explore the mechanisms by which Arctic sea ice decline affects the Atlantic meridional overturning circulation (AMOC) in a suite of numerical experiments perturbing the Arctic sea ice radiative budget within a fully coupled climate model. The imposed perturbations act to increase the amount of heat available to melt ice, leading to a rapid Arctic sea ice retreat within 5 years after the perturbations are activated. In response, the AMOC gradually weakens over the next ~100 years. The AMOC changes can be explained by the accumulation in the Arctic and subsequent downstream propagation to the North Atlantic of buoyancy anomalies controlled by temperature and salinity. Initially, during the first decade or so, the Arctic sea ice loss results in anomalous positive heat and salinity fluxes in the subpolar North Atlantic, inducing positive temperature and salinity anomalies over the regions of oceanic deep convection. At first, these anomalies largely compensate one another, leading to a minimal change in upper ocean density and deep convection in the North Atlantic. Over the following years, however, more anomalous warm water accumulates in the Arctic and spreads to the North Atlantic. At the same time, freshwater that accumulates from seasonal sea ice melting over mostmore »of the upper Arctic Ocean also spreads southward, reaching as far as south of Iceland. These warm and fresh anomalies reduce upper ocean density and suppress oceanic deep convection. The thermal and haline contributions to these buoyancy anomalies, and therefore to the AMOC slowdown during this period, are found to have similar magnitudes. We also find that the related changes in horizontal wind-driven circulation could potentially push freshwater away from the deep convection areas and hence strengthen the AMOC, but this effect is overwhelmed by mean advection.

    « less
  5. With contributions from the US, UK, Germany, the Netherlands, Canada and China, the Overturning in the Subpolar North Atlantic Program (OSNAP) observing system was installed in the summer of 2014, which aims at measuring and understanding what drives the Atlantic Meridional Overturning Circulation (AMOC) and its variability. This coast-to-coast array of high-resolution moorings provides a continuous record of the full water column, trans-basin fluxes of heat, mass and freshwater in the subpolar North Atlantic. Data from observing system between August 2014 – June 2018 have been used to estimate those key variables for the full array as well as two sub-sections: OSNAP West, in the Labrador Sea, and OSNAP East, between Greenland and the Scottish shelf. We show notable differences in the magnitude and variability of the MOC across the full array between 2014-2016 and 2016-2018, and discuss the associated changes in the heat and freshwater transports. Differences between the fluxes across the OSNAP West and OSNAP East subsections will also be presented, along with a discussion of how this relates to the formation and transport of deep waters in the region.