The Norwegian node for the European Multidisciplinary Seafloor and water column Observatory

2020 ◽  
Author(s):  
Thibaut Barreyre ◽  
Ilker Fer ◽  
Bénédicte Ferré
Keyword(s):  
Water Column ◽  
Ocean Circulation ◽  
Large Scale ◽  
Water Mass ◽  
Critical Role ◽  
Meridional Overturning Circulation ◽  
The Arctic ◽  
Global Ocean ◽  
Fram Strait ◽  
Nordic Seas

<p>NorEMSO is a coordinated, large-scale deep-ocean observation facility to establish the Norwegian node for the European Multidisciplinary Seafloor and water column Observatory (EMSO). The project aims to explore the under-sampled Nordic Seas to gain a better understanding of the critical role that they play in our climate system and global ocean circulation. An overarching scientific objective is to better understand the drivers for the temporal and spatial changes of water mass transformations, ocean circulation, acidification and thermo-chemical exchanges at the seafloor in the Nordic Seas, and to contribute to improvement of models and forecasting by producing and making available high quality, near real time data. NorEMSO will achieve this by combining expansion of existing and establishment of new observatory network infrastructure, as well as its coordination and integration into EMSO.</p><p>NorEMSO comprises of three main components: moored observatories, gliders, and seafloor and water column observatory at the Mohn Ridge (EMSO-Mohn).</p><p>Moored observation systems include an array of four moored observatories located at key positions in the Nordic Seas (Svinøy, Station M, South Cape, and central Fram Strait).</p><p>Gliders will be operated along five transects across both the Norwegian and the Greenland Seas to monitor circulation and water mass properties at those key locations. Transects in the Norwegian and Lofoten basins will focus on monitoring the Norwegian Atlantic Current, and a transect in Fram Strait will monitor properties and variability in the return Atlantic Water along the Polar Front in the northern Nordic Seas. In addition, transects in the Greenland and Iceland Seas will address the water mass transformation processes through wintertime open ocean convection, and the southbound transport of surface water from the Arctic Ocean and dense water that feeds the lower limb of the Atlantic Meridional Overturning Circulation in the East Greenland Current.</p><p>EMSO-Mohn will establish, at the newly discovered hydrothermal site on the Mohn Ridge, a fixed-point seabed-water-column-coupled and wireless observatory with a multidisciplinary approach – from geophysics and physical oceanography to ecology and microbiology. It is primarily directed at understanding hydrothermal fluxes and associated hydrothermal plume dynamics in the water column and how it disperses in an oceanographic front over the Mohn Ridge.</p><p>Following EMSO philosophy, NorEMSO will provide data and platforms to a large and diverse group of users, from scientists and industries to institutions and policy makers. The observations will serve climate research, ocean circulation understanding, numerical operational models, design of environmental policies, and education.</p>

scholarly journals A Diagnostic Model of the Nordic Seas and Arctic Ocean Circulation: Quantifying the Effects of a Variable Bottom Density along a Sloping Topography

2008 ◽  
Vol 38 (12) ◽  
pp. 2685-2703 ◽  
Author(s):  
Signe Aaboe ◽  
Ole Anders Nøst
Keyword(s):  
Arctic Ocean ◽  
Ocean Circulation ◽  
Large Scale ◽  
The Arctic ◽  
Diagnostic Model ◽  
Large Scale Circulation ◽  
Nordic Seas ◽  
Variable Bottom ◽  
Canadian Basin ◽  
Baroclinic Transport

Abstract A linear diagnostic model, solving for the time-mean large-scale circulation in the Nordic seas and Arctic Ocean, is presented. Solutions on depth contours that close within the Nordic seas and Arctic Ocean are found from vorticity balances integrated over the areas enclosed by the contours. Climatological data for wind stress and hydrography are used as input to the model, and the bottom geostrophic flow is assumed to follow depth contours. Comparison against velocity observations shows that the simplified dynamics in the model capture many aspects of the large-scale circulation. Special attention is given to the dynamical effects of an along-isobath varying bottom density, which leads to a transformation between barotropic and baroclinic transport. Along the continental slope, enclosing both the Nordic seas and Arctic Ocean, the along-slope barotropic transport has a maximum in the Nordic seas and a minimum in the Canadian Basin with a difference of 9 Sv (1 Sv ≡ 106 m3 s−1) between the two. This is caused by the relatively lower bottom densities in the Canadian Basin compared to the Nordic seas and suggests that most of the barotropic transport entering the Arctic Ocean through the Fram Strait is transformed to baroclinic transport. A conversion from barotropic to baroclinic flow may be highly important for the slope–basin exchange in the Nordic seas and Arctic Ocean. The model has obvious shortcomings due to its simplicity. However, the simplified physics and the agreement with observations make this model an excellent framework for understanding the large-scale circulation in the Nordic seas and Arctic Ocean.

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

2018 ◽  
Author(s):  
Svein Østerhus ◽  
Rebecca Woodgate ◽  
Héðinn Valdimarsson ◽  
Bill Turrell ◽  
Laura de Steur ◽  
...  
Keyword(s):  
Ocean Circulation ◽  
Atlantic Water ◽  
Volume Transport ◽  
Meridional Overturning Circulation ◽  
The Arctic ◽  
Global Ocean ◽  
Bering Strait ◽  
Data Set ◽  
Considerable Uncertainty ◽  
Denmark Strait

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.

Freshwater pulses in the eastern Arctic Ocean during Saalian and Early Weichselian ice-sheet collapse

2003 ◽  
Vol 60 (3) ◽  
pp. 243-251 ◽  
Author(s):  
Jochen Knies ◽  
Christoph Vogt
Keyword(s):  
Sea Ice ◽  
Arctic Ocean ◽  
Large Scale ◽  
Barents Sea ◽  
The Arctic ◽  
Fram Strait ◽  
Nordic Seas ◽  
The Arctic Ocean ◽  
Transpolar Drift ◽  
Mis 5

AbstractImproved multiparameter records from the northern Barents Sea margin show two prominent freshwater pulses into the Arctic Ocean during MIS 5 that significantly disturbed the regional oceanic regime and probably affected global climate. Both pulses are associated with major iceberg-rafted debris (IRD) events, revealing intensive iceberg/sea ice melting. The older meltwater pulse occurred near the MIS 5/6 boundary (∼131,000 yr ago); its ∼2000 year duration and high IRD input accompanied by high illite content suggest a collapse of large-scale Saalian Glaciation in the Arctic Ocean. Movement of this meltwater with the Transpolar Drift current into the Fram Strait probably promoted freshening of Nordic Seas surface water, which may have increased sea-ice formation and significantly reduced deep-water formation. A second pulse of freshwater occurred within MIS 5a (∼77,000 yr ago); its high smectite content and relatively short duration is possibly consistent with sudden discharge of Early Weichselian ice-dammed lakes in northern Siberia as suggested by terrestrial glacial geologic data. The influence of this MIS 5a meltwater pulse has been observed at a number of sites along the Transpolar Drift, through Fram Strait, and into the Nordic Seas; it may well have been a trigger for the North Atlantic cooling event C20.

The ocean response to changes of the Greenland Ice sheet in a warming climate

2020 ◽  
Author(s):  
Marianne S. Madsen ◽  
Shuting Yang ◽  
Christian Rodehacke ◽  
Guðfinna Aðalgeirsdóttir ◽  
Synne H. Svendsen ◽  
...  
Keyword(s):  
Mass Loss ◽  
Ocean Circulation ◽  
Large Scale ◽  
Climate Model ◽  
Variable Mass ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Model System ◽  
Meridional Overturning Circulation ◽  
The Arctic

<p>During recent decades, increased and highly variable mass loss from the Greenland ice sheet has been observed, implying that the ice sheet can respond to changes in ocean and atmospheric conditions on annual to decadal time scales. Changes in ice sheet topography and increased mass loss into the ocean may impact large scale atmosphere and ocean circulation. Therefore, coupling of ice sheet and climate models, to explicitly include the processes and feedbacks of ice sheet changes, is needed to improve the understanding of ice sheet-climate interactions.</p><p>Here, we present results from the coupled ice sheet-climate model system, EC-Earth-PISM. The model consists of the atmosphere, ocean and sea-ice model system EC-Earth, two-way coupled to the Parallel Ice Sheet Model, PISM. The surface mass balance (SMB) is calculated within EC-Earth, from the precipitation, evaporation and surface melt of snow and ice, to ensure conservation of mass and energy. The ice sheet model, PISM, calculates ice dynamical changes in ice discharge and basal melt as well as changes in ice extent and thickness. Idealized climate change experiments have been performed starting from pre-industrial conditions for a) constant forcing (pre-industrial control); b) abruptly quadrupling the CO<sub>2</sub> concentration; and c) gradually increasing the CO<sub>2</sub> concentration by 1% per year until 4xCO<sub>2</sub> is reached.  All three experiments are run for 350 years.</p><p>Our results show a significant impact of the interactive ice sheet component on heat and fresh water fluxes into the Arctic and North Atlantic Oceans. The interactive ice sheet causes freshening of the Arctic Ocean and affects deep water formation, resulting in a significant delay of the recovery of the Atlantic Meridional Overturning Circulation (AMOC) in the coupled 4xCO<sub>2</sub> experiments, when compared with uncoupled experiments.</p>

scholarly journals Labrador Sea freshening linked to Beaufort Gyre freshwater release

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Jiaxu Zhang ◽  
Wilbert Weijer ◽  
Michael Steele ◽  
Wei Cheng ◽  
Tarun Verma ◽  
...  
Keyword(s):  
Ocean Circulation ◽  
North Atlantic ◽  
Large Scale ◽  
The Arctic ◽  
Global Ocean ◽  
Labrador Sea ◽  
Beaufort Gyre ◽  
Short Period ◽  
Freshwater Release ◽  
The Impact

AbstractThe Beaufort Gyre (BG), the largest Arctic Ocean freshwater reservoir, has drastically increased its liquid freshwater content by 40% in the past two decades. If released within a short period, the excess freshwater could potentially impact the large-scale ocean circulation by freshening the upper subpolar North Atlantic. Here, we track BG-sourced freshwater using passive tracers in a global ocean sea-ice model and show that this freshwater exited the Arctic mostly through the Canadian Arctic Archipelago, rather than Fram Strait, during an historical release event in 1983–1995. The Labrador Sea is the most affected region in the subpolar North Atlantic, with a freshening of 0.2 psu on the western shelves and 0.4 psu in the Labrador Current. Given that the present BG freshwater content anomaly is twice the historical analog studied here, the impact of a future rapid release on Labrador Sea salinity could be significant, easily exceeding similar fluxes from Greenland meltwater.

scholarly journals Tracing water masses with <sup>129</sup>I and <sup>236</sup>U in the subpolar North Atlantic along the GEOTRACES GA01 section

2018 ◽  
Vol 15 (18) ◽  
pp. 5545-5564 ◽  
Author(s):  
Maxi Castrillejo ◽  
Núria Casacuberta ◽  
Marcus Christl ◽  
Christof Vockenhuber ◽  
Hans-Arno Synal ◽  
...  
Keyword(s):  
North Atlantic ◽  
Water Mass ◽  
Meridional Overturning Circulation ◽  
The Arctic ◽  
Global Ocean ◽  
Labrador Sea ◽  
Intermediate Water ◽  
Geochemical Tracers ◽  
Depth Profiles ◽  
Wide Range

Abstract. Pathways and timescales of water mass transport in the subpolar North Atlantic Ocean (SPNA) have been investigated by many studies due to their importance for the meridional overturning circulation and thus for the global ocean. In this sense, observational data on geochemical tracers provide complementary information to improve the current understanding of the circulation in the SPNA. To this end, we present the first simultaneous distribution of artificial 129I and 236U in 14 depth profiles and in surface waters along the GEOVIDE section covering a zonal transect through the SPNA in spring 2014. Our results show that the two tracers are distributed following the water mass structure and that their presence is largely influenced by the global fallout (GF) and liquid effluents discharged to north-western European coastal waters by the Sellafield and La Hague nuclear reprocessing plants (NRPs). As a result, 129I concentrations and 236U∕238U atom ratios and 129I∕236U atom ratios display a wide range of values: (0.2–256) ×107 at kg−1 (40–2350) ×10-12 and 0.5–200, respectively. The signal from NRPs, which is characterised by higher 129I concentrations and 129I∕236U atom ratios compared to GF, is transported by Atlantic Waters (AWs) into the SPNA, notably by the East Greenland Current (EGC)/Labrador Current (LC) at the surface and by waters overflowing the Greenland–Scotland passage at greater depths. Nevertheless, our results show that the effluents from NRPs may also directly enter the surface of the eastern SPNA through the Iceland–Scotland passage or the English Channel/Irish Sea. The use of the 236U∕238U and 129I∕236U dual tracer approach further serves to discern Polar Intermediate Water (PIW) of Canadian origin from that of Atlantic origin, which carries comparably higher tracer levels due to NRPs (particularly 129I). The cascading of these waters appears to modify the water mass composition in the bottom of the Irminger and Labrador seas, which are dominated by Denmark Strait Overflow Water (DSOW). Indeed, PIW–Atlantic, which has a high level of 129I compared to 236U, appears to contribute to the deep Irminger Sea increasing the 129I concentrations in the realm of DSOW. A similar observation can be made for 236U for PIW entering through the Canadian Archipelago into the Labrador Sea. Several depth profiles also show an increase in 129I concentrations in near bottom waters in the Iceland and the West European basins that are very likely associated with the transport of the NRP signal by the Iceland–Scotland Overflow Water (ISOW). This novel result would support current modelling studies indicating the transport of ISOW into the eastern SPNA. Finally, our tracer data from 2014 are combined with published 129I data for the deep central Labrador Sea between 1993 and 2013. The results obtained from comparing simulated and measured 129I concentrations support the previously suggested two major transport pathways for the AWs in the SPNA, i.e. a short loop through the Nordic seas into the SPNA and a longer loop, which includes recirculation of the AWs in the Arctic Ocean before it enters the western SPNA.

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

Ocean Science ◽  
2019 ◽  
Vol 15 (2) ◽  
pp. 379-399 ◽  
Author(s):  
Svein Østerhus ◽  
Rebecca Woodgate ◽  
Héðinn Valdimarsson ◽  
Bill Turrell ◽  
Laura de Steur ◽  
...  
Keyword(s):  
Ocean Circulation ◽  
Atlantic Water ◽  
Volume Transport ◽  
Meridional Overturning Circulation ◽  
The Arctic ◽  
Global Ocean ◽  
Bering Strait ◽  
Data Set ◽  
Considerable Uncertainty ◽  
Denmark Strait

Abstract. The Arctic Mediterranean (AM) is the collective name for the Arctic Ocean, the Nordic Seas, and their adjacent shelf seas. Water enters into this region through the Bering Strait (Pacific inflow) and through the passages across the Greenland–Scotland Ridge (Atlantic inflow) and is 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 and salt 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 the observational coverage has 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 and 2015. The total AM import (oceanic inflows plus freshwater) is found to be 9.1 Sv (sverdrup, 1 Sv =106 m3 s−1) with an estimated uncertainty of 0.7 Sv and has the 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 the remaining two-thirds leaving as overflow. The overflow water 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 (runoff and precipitation), but is still approximately two-thirds of 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 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 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 outflow 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.

scholarly journals The long-lived anthropogenic radionuclides I-129 and U-236 as tracers of water mass provenance, circulation timescales and mixing in the Arctic Ocean and Fram Strait

2020 ◽  
Author(s):  
Anne-Marie Wefing ◽  
Núria Casacuberta ◽  
Marcus Christl ◽  
John N. Smith ◽  
Paul A. Dodd ◽  
...  
Keyword(s):  
Arctic Ocean ◽  
Ocean Circulation ◽  
Water Mass ◽  
Atlantic Water ◽  
The Arctic ◽  
Fram Strait ◽  
Anthropogenic Radionuclides ◽  
Chemical Tracers ◽  
Pacific Waters ◽  
The Arctic Ocean

&lt;p&gt;Anthropogenic chemical tracers are powerful tools to study ocean circulation timescales, water mass provenance and mixing regimes. In the Arctic Ocean, the releases of artificial radionuclides from European nuclear reprocessing plants (RPs) act as valuable transient tracers as they label the inflowing Atlantic Waters with a distinct anthropogenic signal. In recent years, the combination of the two long-lived radionuclides &lt;sup&gt;129&lt;/sup&gt;I and &lt;sup&gt;236&lt;/sup&gt;U has emerged as a new tracer pair and several studies have shown their potential to track pathways and timescales of Atlantic Water circulation in the Arctic Ocean and Fram Strait.&lt;/p&gt;&lt;p&gt;The circulation times of Atlantic-origin waters in the Arctic Ocean that were inferred using this tracer pair (in combination with the naturally occurring &lt;sup&gt;238&lt;/sup&gt;U) agree to those obtained by means of other transient tracers. Moreover, the combination of &lt;sup&gt;129&lt;/sup&gt;I and &lt;sup&gt;236&lt;/sup&gt;U promises to be a useful marker of water mass mixing regimes both in the surface waters and the subsurface Atlantic layer. In particular, the interface between Atlantic and Pacific Waters in the polar surface layer of the Arctic Ocean can be easily identified as these two water masses are labelled by very different &lt;sup&gt;129&lt;/sup&gt;I/&lt;sup&gt;236&lt;/sup&gt;U and &lt;sup&gt;236&lt;/sup&gt;U/&lt;sup&gt;238&lt;/sup&gt;U atom ratios.&lt;/p&gt;&lt;p&gt;Here we present a compilation of &lt;sup&gt;129&lt;/sup&gt;I and &lt;sup&gt;236&lt;/sup&gt;U in a quasi-synoptic pan-arctic section including the Fram Strait and we show how this data can be used to gain information about circulation patterns. We discuss timescales and transport characteristics of Atlantic Water flow, the position and variability of the front between Atlantic and Pacific Waters and the temporal variability of Pacific Waters in the Fram Strait.&lt;/p&gt;

scholarly journals Arctic Ocean Water Mass Transformation in S–T Coordinates

2015 ◽  
Vol 45 (4) ◽  
pp. 1025-1050 ◽  
Author(s):  
Per Pemberton ◽  
Johan Nilsson ◽  
Magnus Hieronymus ◽  
H. E. Markus Meier
Keyword(s):  
Arctic Ocean ◽  
Ocean Circulation ◽  
Water Mass ◽  
Circulation Model ◽  
The Arctic ◽  
Global Ocean ◽  
Freshwater Flux ◽  
The Arctic Ocean ◽  
Water Mass Transformation ◽  
Water Mass Transformations

AbstractIn this paper, water mass transformations in the Arctic Ocean are studied using a recently developed salinity–temperature (S–T) framework. The framework allows the water mass transformations to be succinctly quantified by computing the surface and internal diffusive fluxes in S–T coordinates. This study shows how the method can be applied to a specific oceanic region, in this case the Arctic Ocean, by including the advective exchange of water masses across the boundaries of the region. Based on a simulation with a global ocean circulation model, the authors examine the importance of various parameterized mixing processes and surface fluxes for the transformation of water across isohaline and isothermal surfaces in the Arctic Ocean. The model-based results reveal a broadly realistic Arctic Ocean where the inflowing Atlantic and Pacific waters are primarily cooled and freshened before exiting back to the North Atlantic. In the model, the water mass transformation in the T direction is primarily accomplished by the surface heat flux. However, the surface freshwater flux plays a minor role in the transformation of water toward lower salinities, which is mainly driven by a downgradient mixing of salt in the interior ocean. Near the freezing line, the seasonal melt and growth of sea ice influences the transformation pattern.

scholarly journals Asynchronous warming and δ18O evolution of deep Atlantic water masses during the last deglaciation

2017 ◽  
Vol 114 (42) ◽  
pp. 11075-11080 ◽  
Author(s):  
Jiaxu Zhang ◽  
Zhengyu Liu ◽  
Esther C. Brady ◽  
Delia W. Oppo ◽  
Peter U. Clark ◽  
...  
Keyword(s):  
Ocean Circulation ◽  
North Atlantic ◽  
General Circulation ◽  
Large Scale ◽  
Water Mass ◽  
Critical Role ◽  
Circulation Model ◽  
Deep Ocean ◽  
Last Deglaciation ◽  
The Last Deglaciation

The large-scale reorganization of deep ocean circulation in the Atlantic involving changes in North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW) played a critical role in regulating hemispheric and global climate during the last deglaciation. However, changes in the relative contributions of NADW and AABW and their properties are poorly constrained by marine records, including δ18O of benthic foraminiferal calcite (δ18Oc). Here, we use an isotope-enabled ocean general circulation model with realistic geometry and forcing conditions to simulate the deglacial water mass and δ18O evolution. Model results suggest that, in response to North Atlantic freshwater forcing during the early phase of the last deglaciation, NADW nearly collapses, while AABW mildly weakens. Rather than reflecting changes in NADW or AABW properties caused by freshwater input as suggested previously, the observed phasing difference of deep δ18Oc likely reflects early warming of the deep northern North Atlantic by ∼1.4 °C, while deep Southern Ocean temperature remains largely unchanged. We propose a thermodynamic mechanism to explain the early warming in the North Atlantic, featuring a strong middepth warming and enhanced downward heat flux via vertical mixing. Our results emphasize that the way that ocean circulation affects heat, a dynamic tracer, is considerably different from how it affects passive tracers, like δ18O, and call for caution when inferring water mass changes from δ18Oc records while assuming uniform changes in deep temperatures.

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