Response of the Atlantic Thermohaline Circulation to Increased Atmospheric CO2 in a Coupled Model

Abstract Changes in the thermohaline circulation (THC) due to increased CO2 are important in future climate regimes. Using a coupled climate model, the Parallel Climate Model (PCM), regional responses of the THC in the North Atlantic to increased CO2 and the underlying physical processes are studied here. The Atlantic THC shows a 20-yr cycle in the control run, qualitatively agreeing with other modeling results. Compared with the control run, the simulated maximum of the Atlantic THC weakens by about 5 Sv (1 Sv ≡ 106 m3 s−1) or 14% in an ensemble of transient experiments with a 1% CO2 increase per year at the time of CO2 doubling. The weakening of the THC is accompanied by reduced poleward heat transport in the midlatitude North Atlantic. Analyses show that oceanic deep convective activity strengthens significantly in the Greenland–Iceland–Norway (GIN) Seas owing to a saltier (denser) upper ocean, but weakens in the Labrador Sea due to a fresher (lighter) upper ocean and in the south of the Denmark Strait region (SDSR) because of surface warming. The saltiness of the GIN Seas are mainly caused by an increased salty North Atlantic inflow, and reduced sea ice volume fluxes from the Arctic into this region. The warmer SDSR is induced by a reduced heat loss to the atmosphere, and a reduced sea ice flux into this region, resulting in less heat being used to melt ice. Thus, sea ice–related salinity effects appear to be more important in the GIN Seas, but sea ice–melt-related thermal effects seem to be more important in the SDSR region. On the other hand, the fresher Labrador Sea is mainly attributed to increased precipitation. These regional changes produce the overall weakening of the THC in the Labrador Sea and SDSR, and more vigorous ocean overturning in the GIN Seas. The northward heat transport south of 60°N is reduced with increased CO2, but increased north of 60°N due to the increased flow of North Atlantic water across this latitude.

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Anomalous blocking over Greenland preceded the 2013 extreme early melt of local sea ice

Annals of Glaciology ◽

10.1017/aog.2017.30 ◽

2017 ◽

Vol 59 (76pt2) ◽

pp. 181-190 ◽

Cited By ~ 5

Author(s):

Thomas J. Ballinger ◽

Edward Hanna ◽

Richard J. Hall ◽

Thomas E. Cropper ◽

Jeffrey Miller ◽

...

Keyword(s):

Sea Ice ◽

North Atlantic ◽

Regional Climate ◽

Regional Climate Change ◽

Circulation Pattern ◽

The Arctic ◽

Labrador Sea ◽

First Year ◽

Tropospheric Circulation ◽

Blocking Index

ABSTRACTThe Arctic marine environment is undergoing a transition from thick multi-year to first-year sea-ice cover with coincident lengthening of the melt season. Such changes are evident in the Baffin Bay-Davis Strait-Labrador Sea (BDL) region where melt onset has occurred ~8 days decade−1 earlier from 1979 to 2015. A series of anomalously early events has occurred since the mid-1990s, overlapping a period of increased upper-air ridging across Greenland and the northwestern North Atlantic. We investigate an extreme early melt event observed in spring 2013. (~6σ below the 1981–2010 melt climatology), with respect to preceding sub-seasonal mid-tropospheric circulation conditions as described by a daily Greenland Blocking Index (GBI). The 40-days prior to the 2013 BDL melt onset are characterized by a persistent, strong 500 hPa anticyclone over the region (GBI >+1 on >75% of days). This circulation pattern advected warm air from northeastern Canada and the northwestern Atlantic poleward onto the thin, first-year sea ice and caused melt ~50 days earlier than normal. The episodic increase in the ridging atmospheric pattern near western Greenland as in 2013, exemplified by large positive GBI values, is an important recent process impacting the atmospheric circulation over a North Atlantic cryosphere undergoing accelerated regional climate change.

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Hydrographic Preconditioning for Seasonal Sea Ice Anomalies in the Labrador Sea

Journal of Physical Oceanography ◽

10.1175/jpo-d-12-064.1 ◽

2013 ◽

Vol 43 (5) ◽

pp. 863-883 ◽

Cited By ~ 10

Author(s):

Ian Fenty ◽

Patrick Heimbach

Keyword(s):

Sea Ice ◽

Mixed Layer ◽

Critical Role ◽

The Arctic ◽

Labrador Sea ◽

Upper Ocean ◽

Quasi Equilibrium ◽

Low Salinity ◽

Thermohaline Front ◽

Subsurface Waters

Abstract This study investigates the hydrographic processes involved in setting the maximum wintertime sea ice (SI) extent in the Labrador Sea and Baffin Bay. The analysis is based on an ocean and sea ice state estimate covering the summer-to-summer 1996/97 annual cycle. The estimate is a synthesis of in situ and satellite hydrographic and ice data with a regional coupled ⅓° ocean–sea ice model. SI advective processes are first demonstrated to be required to reproduce the observed ice extent. With advection, the marginal ice zone (MIZ) location stabilizes where ice melt balances ice mass convergence, a quasi-equilibrium condition achieved via the convergence of warm subtropical-origin subsurface waters into the mixed layer seaward of the MIZ. An analysis of ocean surface buoyancy fluxes reveals a critical role of low-salinity upper ocean (100 m) anomalies for the advancement of SI seaward of the Arctic Water–Irminger Water Thermohaline Front. Anomalous low-salinity waters slow the rate of buoyancy loss–driven mixed layer deepening, shielding an advancing SI pack from the warm subsurface waters, and are conducive to a positive surface meltwater stabilization enhancement (MESEM) feedback driven by SI meltwater release. The low-salinity upper-ocean hydrographic conditions in which the MESEM efficiently operates are termed sea ice–preconditioned waters (SIPW). The SI extent seaward of the Thermohaline Front is shown to closely correspond to the distribution of SIPW. The analysis of two additional state estimates (1992/93, 2003/04) suggests that interannual hydrographic variability provides a first-order explanation for SI maximum extent anomalies in the region.

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Climate feedbacks induced by the North Atlantic freshwater forcing in a coupled model of intermediate complexity

Revista Brasileira de Meteorologia ◽

10.1590/s0102-77862010000100009 ◽

2010 ◽

Vol 25 (1) ◽

pp. 103-113 ◽

Cited By ~ 2

Author(s):

Flávio Barbosa Justino ◽

Jeferson Prietsch Machado

Keyword(s):

Heat Transport ◽

North Atlantic ◽

Thermohaline Circulation ◽

Coupled Model ◽

The North ◽

Poleward Heat Transport ◽

Freshwater Forcing ◽

Oceanic Response ◽

The North Atlantic ◽

The Tropics

Based on coupled model simulations (ECBilt-Clio), we investigate the atmospheric and oceanic response to sustained freshwater input into the North Atlantic under the glacial maximum background state. The results demonstrate that a weakening of the thermohaline circulation triggered by weaker density flux leads to rapid changes in global sea-ice volume and reduced poleward heat transport in the Northern Hemisphere (NH). In the Southern Hemisphere (SH), however, the oceanic heat transport increases substantially. This in turn leads to strong cooling over the North Atlantic whereas the SH extratropical region warms up. The suppression of the NADW also drastically changes the atmospheric circulation. The associated northward wind anomalies over the North Atlantic increase the warm air advection from the tropics and induce the transport of tropical saltier water to mid-latitudes. This negative atmospheric-oceanic feedback should play an important role to resume the NADW, after the freshwater forcing ends up.

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Simulated Arctic Ocean Freshwater Budgets in the Twentieth and Twenty-First Centuries

Journal of Climate ◽

10.1175/jcli3967.1 ◽

2006 ◽

Vol 19 (23) ◽

pp. 6221-6242 ◽

Cited By ~ 56

Author(s):

Marika M. Holland ◽

Joel Finnis ◽

Mark C. Serreze

Keyword(s):

Arctic Ocean ◽

North Atlantic ◽

Climate Model ◽

The Arctic ◽

Fram Strait ◽

Climate System Model ◽

Ice Melt ◽

East Greenland Current ◽

Freshwater Fluxes ◽

Gin Seas

Abstract The Arctic Ocean freshwater budgets in climate model integrations of the twentieth and twenty-first century are examined. An ensemble of six members of the Community Climate System Model version 3 (CCSM3) is used for the analysis, allowing the anthropogenically forced trends over the integration length to be assessed. Mechanisms driving trends in the budgets are diagnosed, and the implications of changes in the Arctic–North Atlantic exchange on the Labrador Sea and Greenland–Iceland–Norwegian (GIN) Seas properties are discussed. Over the twentieth and the twenty-first centuries, the Arctic freshens as a result of increased river runoff, net precipitation, and decreased ice growth. For many of the budget terms, the maximum 50-yr trends in the time series occur from approximately 1975 to 2025, suggesting that we are currently in the midst of large Arctic change. The total freshwater exchange between the Arctic and North Atlantic increases over the twentieth and twenty-first centuries with decreases in ice export more than compensated for by an increase in the liquid freshwater export. Changes in both the liquid and solid (ice) Fram Strait freshwater fluxes are transported southward by the East Greenland Current and partially removed from the GIN Seas. Nevertheless, reductions in GIN sea ice melt do result from the reduced Fram Strait transport and account for the largest term in the changing ocean surface freshwater fluxes in this region. This counteracts the increased ocean stability due to the warming climate and helps to maintain GIN sea deep-water formation.

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Predicting a Decadal Shift in North Atlantic Climate Variability Using the GFDL Forecast System

Journal of Climate ◽

10.1175/jcli-d-13-00476.1 ◽

2014 ◽

Vol 27 (17) ◽

pp. 6472-6496 ◽

Cited By ~ 63

Author(s):

Rym Msadek ◽

T. L. Delworth ◽

A. Rosati ◽

W. Anderson ◽

G. Vecchi ◽

...

Keyword(s):

North Atlantic ◽

Climate Model ◽

Coupled Model ◽

Climate Impacts ◽

Meridional Overturning Circulation ◽

The Arctic ◽

North Atlantic Current ◽

Forecast System ◽

Sea Ice Concentration ◽

North Atlantic Climate

Abstract Decadal prediction experiments were conducted as part of phase 5 of the Coupled Model Intercomparison Project (CMIP5) using the GFDL Climate Model, version 2.1 (CM2.1) forecast system. The abrupt warming of the North Atlantic Subpolar Gyre (SPG) that was observed in the mid-1990s is considered as a case study to evaluate forecast capabilities and better understand the reasons for the observed changes. Initializing the CM2.1 coupled system produces high skill in retrospectively predicting the mid-1990s shift, which is not captured by the uninitialized forecasts. All the hindcasts initialized in the early 1990s show a warming of the SPG; however, only the ensemble-mean hindcasts initialized in 1995 and 1996 are able to reproduce the observed abrupt warming and the associated decrease and contraction of the SPG. Examination of the physical mechanisms responsible for the successful retrospective predictions indicates that initializing the ocean is key to predicting the mid-1990s warming. The successful initialized forecasts show an increased Atlantic meridional overturning circulation and North Atlantic Current transport, which drive an increased advection of warm saline subtropical waters northward, leading to a westward shift of the subpolar front and, subsequently, a warming and spindown of the SPG. Significant seasonal climate impacts are predicted as the SPG warms, including a reduced sea ice concentration over the Arctic, an enhanced warming over the central United States during summer and fall, and a northward shift of the mean ITCZ. These climate anomalies are similar to those observed during a warm phase of the Atlantic multidecadal oscillation, which is encouraging for future predictions of North Atlantic climate.

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Investigating future changes in the volume budget of the Arctic sea ice in a coupled climate model

10.5194/tc-2017-216 ◽

2017 ◽

Cited By ~ 1

Author(s):

Ann Keen ◽

Ed Blockley

Keyword(s):

Sea Ice ◽

21St Century ◽

Climate Model ◽

Coupled Model ◽

Ice Cover ◽

Arctic Sea Ice ◽

The Arctic ◽

Ice Melt ◽

Arctic Sea ◽

Projected Changes

Abstract. We consider the volume budget of the Arctic sea ice in the CMIP5 global coupled model HadGEM2-ES, and evaluate how the budget components evolve during the 21st century under a range of different forcing scenarios. As the climate warms and the ice cover declines, the Arctic sea ice processes that change the most in HadGEM2-ES are summer melting at the top surface of the ice, and basal melting due to extra heat from the warming ocean. However, the declining ice cover affects how much impact these changes have on the ice volume budget, where the biggest contribution to Arctic ice decline is the reduction in the total amount of basal ice formation during the autumn and early winter. This highlights the importance of taking the declining ice area into account when evaluating projected changes in the sea ice budget, especially if comparing models with very different rates of decline. Changes in the volume budget during the 21st century have a distinctive seasonal cycle, with processes contributing to ice decline occurring in May/June and September to November. During July and August the total amount of sea ice melt decreases, due to the reducing ice cover. The choice of forcing scenario affects the rate of ice decline and the timing and magnitude of changes in the volume budget components, but for the HadGEM2-ES model and for the range of scenarios considered for CMIP5, the mean changes in the volume budget depend on the evolving ice area, and are independent of the speed at which the ice cover declines.

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AMOC instability during the Last Inerglacial

10.5194/egusphere-egu21-12364 ◽

2021 ◽

Author(s):

Augustin Kessler ◽

Didier Roche ◽

Eirik Galaasen ◽

Jerry Tjiputra ◽

Nathaelle Bouttes ◽

...

Keyword(s):

Sea Ice ◽

North Atlantic ◽

Thermohaline Circulation ◽

Model Simulation ◽

Arctic Sea Ice ◽

The Arctic ◽

Last Interglacial ◽

The North ◽

Arctic Sea ◽

The North Atlantic

Multiple evidences from the analysis of satellite, in-situ and proxy data show that the climate is already changing toward a warmer Earth System due to our emissions of CO2 into the atmosphere. However, the magnitude and the extent of changes remain difficult to predict. A change in the ocean thermohaline circulation and its consequences for climate, such as drought, regional sea-level and ocean carbon uptake remain under debate as this circulation has been long thought to be stable during warm Earth periods &#8211; Interglacials. However, recent high-resolution reconstructions of carbon isotopes (&#948;13C) from the deep North Atlantic challenge this idea of stability and point toward abrupt modifications in the ocean interior biogeochemistry and/or ocean thermohaline circulation during the Last Interglacial (LIG, 125ka &#8211; 115ka).&#160;Our model simulation of the LIG reproduces the observed magnitude and timescale of the reconstructed variations of &#948;13C, highlighting crucial dynamical changes in two regions of the North Atlantic deep-water formation (south of Greenland and south of Svalbard). These regions are found to drive the variations in the strength of the Atlantic Overturning Circulation (AMOC) when the Arctic sea-ice extent is perturbed.&#160;Our study suggests that the AMOC may have experienced great instability phase during some parts of the LIG. The water mass geometry reorganization from the warm onset at 125ka to the glacial inception at 115ka could also have greatly impacted the distribution of carbon in the interior Ocean. Changes in sea-ice cover either south of Svalbard or in the Southern Ocean seem to play a determining role. However, in our global warming context, our study suggests that the mechanisms responsible for the LIG AMOC instability of the LIG may not occur by the end of the century if the Arctic sea-ice retreats from the high latitudes of the North Atlantic as projected by climate models.&#160;

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Simulation of observed climate changes in 1850–2014 with climate model INM-CM5

10.5194/esd-2018-21 ◽

2018 ◽

Author(s):

Evgeny Volodin ◽

Andrey Gritsun

Keyword(s):

Sea Ice ◽

Surface Temperature ◽

Climate Changes ◽

Climate Model ◽

Coupled Model ◽

Arctic Sea Ice ◽

The Arctic ◽

Arctic Sea ◽

Arctic Sea Ice Loss ◽

Global Mean

Abstract. Climate changes observed in 1850-2014 are modeled and studied on the basis of seven historical runs with the climate model INM-CM5 under the scenario proposed for Coupled Model Intercomparison Project, Phase 6 (CMIP6). In all runs global mean surface temperature rises by 0.8 K at the end of the experiment (2014) in agreement with the observations. Periods of fast warming in 1920–1940 and 1980–2000 as well as its slowdown in 1950–1975 and 2000–2014 are correctly reproduced by the ensemble mean. The notable change here with respect to the CMIP5 results is correct reproduction of the slowdown of global warming in 2000–2014 that we attribute to more accurate description of the Solar constant in CMIP6 protocol. The model is able to reproduce correct behavior of global mean temperature in 1980–2014 despite incorrect phases of the Atlantic Multidecadal Oscillation and Pacific Decadal Oscillation indices in the majority of experiments. The Arctic sea ice loss in recent decades is reasonably close to the observations just in one model run; the model underestimates Arctic sea ice loss by the factor 2.5. Spatial pattern of model mean surface temperature trend during the last 30 years looks close the one for the ERA Interim reanalysis. Model correctly estimates the magnitude of stratospheric cooling.

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Atlantic multi-centennial variability in IPSL-CM6A-LR climate model

10.5194/egusphere-egu21-7336 ◽

2021 ◽

Author(s):

Weimin Jiang ◽

Guillaume Gastineau ◽

Francis Codron

Keyword(s):

Sea Ice ◽

Climate Model ◽

Barents Sea ◽

Deep Convection ◽

Coupled Model ◽

Lower Troposphere ◽

Arctic Sea Ice ◽

Meridional Overturning Circulation ◽

The Arctic ◽

Surface Salinity

A pronounced multi-centennial variability of the Atlantic meridional overturning circulation (AMOC) is found to be regulated by the salinity exchanges between the Atlantic and Arctic ocean in the IPSL-CM6A-LR atmosphere-ocean coupled model. The AMOC variations are preceded by salinity-driven density anomalies in the main deep convection sites in the Labrador and Greenland seas. Associated with a strong AMOC, the Arctic sea ice export through the Fram Strait reduces due to the decreased sea ice volume and anomalous northward currents. Anomalous freshwater hence accumulates at the surface in the Central Arctic. Meanwhile, the enhanced Atlantic inflow enters the Arctic through the Barents Sea and leads to a positive salinity in the Eastern Arctic subsurface. The surface freshwater anomalies last for 4 to 5 decades before they eventually reach the Lincoln Sea north of Greenland. The associated oceanic currents around Greenland reorganize, favoring the anomalous Arctic freshwater export to the North Atlantic and intensifying the stratification in deep convection sites. The AMOC then weakens, and the Central Arctic presents a positive surface salinity anomaly in turn. The oscillation switches to the opposite phase. These AMOC and sea ice fluctuations modulate climate worldwide, with a strong AMOC leading to a warming of 0.4&#176;C in the northern extratropics, reaching up to 1&#176;C in the Arctic lower troposphere during winter. In all seasons, a northward displacement of the intertropical convergence zone is also simulated.

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Centennial variability driven by salinity exchanges between the Atlantic and Arctic in a coupled climate model

10.5194/egusphere-egu2020-844 ◽

2020 ◽

Author(s):

Weimin Jiang ◽

Guillaume Gastineau ◽

Francis Codron

Keyword(s):

Sea Ice ◽

Climate Model ◽

Barents Sea ◽

Coupled Model ◽

Arctic Sea Ice ◽

Meridional Overturning Circulation ◽

Salt Transport ◽

The Arctic ◽

Surface Salinity ◽

Sea Ice Extent

The centennial to multi-centennial variability of the Atlantic Meridional Overturning Circulation (AMOC) is studied in a 1200-yr pre-industrial control simulation of the IPSL-CM6-LR atmosphere-ocean coupled model. In this run, a spectrum analysis finds a periodicity of the low-frequency variability of AMOC, with a period of about 200-year. This variability alters the Northern Hemisphere climate over the land and modulates the Arctic sea ice extent and volume. The associated density variations show large positive (negative) salinity-driven density anomalies in the Nordic Seas and subpolar gyre associated with a strong (week) AMOC state. The positive salinity anomalies in the Greenland Sea are found to be generated by anomalous southward salinity transport in the Fram Straits. The gradual AMOC increase and the associated oceanic northward heat transport melt the sea ice in the Arctic and build shallow negative salinity anomalies in the central Arctic. In parallel, the AMOC is also associated with a northward salt transport into the Eastern Arctic, by an inflow of Atlantic water from the Barents Sea to the East Siberian Ocean. The accumulated surface freshwater in the central Arctic is ultimately exported into the Atlantic mainly through the Fram Strait via intensified East Greenland Current, lowering the upper ocean density and enhancing the stratification at the regions where the cold deep limb of AMOC is formed. The positive salinity anomalies at subsurface then slowly reach the surface though diffusion, increasing the surface salinity. The oscillation then turns into the opposite phase.

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