Planetary-wave induced strengthening of the AMOC forced by poleward intensified Southern Hemisphere westerly winds

2021 ◽  
pp. 1-45
Author(s):  
D. J. Webb ◽  
P. Spence ◽  
R. M. Holmes ◽  
M. H. England
Keyword(s):  
Southern Ocean ◽  
North Atlantic ◽  
Climate Models ◽  
Horizontal Resolution ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
Baroclinic Waves ◽  
The North ◽  
Shelf Slope ◽  
The North Atlantic

AbstractThe Atlantic meridional overturning circulation (AMOC) plays a key role in determining the distribution of heat and nutrients in the global ocean. Climate models suggest that Southern Ocean winds will strengthen and shift poleward in the future, which could have implications for future AMOC trends. Using a coupled global-ocean sea-ice model at 1/4°horizontal resolution, we study the response of the North Atlantic overturning to two anomalous Southern Ocean wind-forcing (τ+15%), and a poleward intensification(). In both scenarios a strengthening in the North Atlantic overturning develops within a decade, with a much stronger response in the case. In , we find that the primary link between the North Atlantic response and the Southern Ocean forcing is via the propagation of baroclinic waves. In fact, due to the rapid northward propagation of these waves, changes in the AMOC in the case appear to originate in the North Atlantic and propagate southward, whereas in the τ+15% case AMOC anomalies propagate northward from the Southern Ocean. We find the difference to be predominately caused by the sign of the baroclinic waves propagating from the forcing region into the North Atlantic; downwelling in the τ+15% case, versus upwelling in the case. In the case, upwelling waves propagating into the NADW formation regions along shelf-slope topography bringing dense water to the surface. This reduces vertical density gradients leading to deeper wintertime convective overturn of surface waters, and an intensification of the AMOC.

scholarly journals Mechanism for potential strengthening of Atlantic overturning prior to collapse

2014 ◽  
Vol 5 (1) ◽  
pp. 29-62
Author(s):  
D. Ehlert ◽  
A. Levermann
Keyword(s):  
Southern Ocean ◽  
North Atlantic ◽  
Climate Models ◽  
Meridional Overturning Circulation ◽  
Freshwater Flux ◽  
Overturning Circulation ◽  
Ocean Eddies ◽  
The North ◽  
Meridional Overturning ◽  
The North Atlantic

Abstract. The Atlantic meridional overturning circulation (AMOC) carries large amounts of heat into the North Atlantic influencing climate regionally as well as globally. Paleorecords and simulations with comprehensive climate models suggest that the positive salt-advection feedback may yield a threshold behaviour of the system. That is to say that beyond a certain amount of freshwater flux into the North Atlantic, no meridional overturning circulation can be sustained. Concepts of monitoring the AMOC and identifying its vicinity to the threshold rely on the fact that the volume flux defining the AMOC will be reduced when approaching the threshold. Here we advance conceptual models that have been used in a paradigmatic way to understand the AMOC, by introducing a density-dependent parameterization for the Southern Ocean eddies. This additional degree of freedom uncovers a mechanism by which the AMOC can increase with additional freshwater flux into the North Atlantic, before it reaches the threshold and collapses: an AMOC that is mainly wind-driven will have a constant upwelling as long as the Southern Ocean winds do not change significantly. The downward transport of tracers occurs either in the northern sinking regions or through Southern Ocean eddies. If freshwater is transported, either atmospherically or via horizontal gyres, from the low- to high-latitudes, this would reduce the eddy transport and by continuity increase the northern sinking which defines the AMOC until a threshold is reached at which the AMOC cannot be sustained. If dominant in the real ocean this mechanism would have significant consequences for monitoring the AMOC.

scholarly journals Mechanism for potential strengthening of Atlantic overturning prior to collapse

2014 ◽  
Vol 5 (2) ◽  
pp. 383-397
Author(s):  
D. Ehlert ◽  
A. Levermann
Keyword(s):  
Southern Ocean ◽  
North Atlantic ◽  
Climate Models ◽  
Meridional Overturning Circulation ◽  
Freshwater Flux ◽  
Overturning Circulation ◽  
Ocean Eddies ◽  
The North ◽  
Meridional Overturning ◽  
The North Atlantic

Abstract. The Atlantic meridional overturning circulation (AMOC) carries large amounts of heat into the North Atlantic influencing climate regionally as well as globally. Palaeo-records and simulations with comprehensive climate models suggest that the positive salt-advection feedback may yield a threshold behaviour of the system. That is to say that beyond a certain amount of freshwater flux into the North Atlantic, no meridional overturning circulation can be sustained. Concepts of monitoring the AMOC and identifying its vicinity to the threshold rely on the fact that the volume flux defining the AMOC will be reduced when approaching the threshold. Here we advance conceptual models that have been used in a paradigmatic way to understand the AMOC, by introducing a density-dependent parameterization for the Southern Ocean eddies. This additional degree of freedom uncovers a mechanism by which the AMOC can increase with additional freshwater flux into the North Atlantic, before it reaches the threshold and collapses: an AMOC that is mainly wind-driven will have a constant upwelling as long as the Southern Ocean winds do not change significantly. The downward transport of tracers occurs either in the northern sinking regions or through Southern Ocean eddies. If freshwater is transported, either atmospherically or via horizontal gyres, from the low to high latitudes, this would reduce the eddy transport and by continuity increase the northern sinking which defines the AMOC until a threshold is reached at which the AMOC cannot be sustained. If dominant in the real ocean this mechanism would have significant consequences for monitoring the AMOC.

scholarly journals North Atlantic gyre circulation in PRIMAVERA models

2021 ◽  
Author(s):  
Virna L. Meccia ◽  
Doroteaciro Iovino ◽  
Alessio Bellucci
Keyword(s):  
High Resolution ◽  
North Atlantic ◽  
Climate Models ◽  
Horizontal Resolution ◽  
Meridional Overturning Circulation ◽  
The North ◽  
Ocean Response ◽  
The North Atlantic ◽  
Gyre Circulation ◽  
The Impact

AbstractWe study the impact of horizontal resolution in setting the North Atlantic gyre circulation and representing the ocean–atmosphere interactions that modulate the low-frequency variability in the region. Simulations from five state-of-the-art climate models performed at standard and high-resolution as part of the High-Resolution Model Inter-comparison Project (HighResMIP) were analysed. In some models, the resolution is enhanced in the atmospheric and oceanic components whereas, in some other models, the resolution is increased only in the atmosphere. Enhancing the horizontal resolution from non-eddy to eddy-permitting ocean produces stronger barotropic mass transports inside the subpolar and subtropical gyres. The first mode of inter-annual variability is associated with the North Atlantic Oscillation (NAO) in all the cases. The rapid ocean response to it consists of a shift in the position of the inter-gyre zone and it is better captured by the non-eddy models. The delayed ocean response consists of an intensification of the subpolar gyre (SPG) after around 3 years of a positive phase of NAO and it is better represented by the eddy-permitting oceans. A lagged relationship between the intensity of the SPG and the Atlantic Meridional Overturning Circulation (AMOC) is stronger in the cases of the non-eddy ocean. Then, the SPG is more tightly coupled to the AMOC in low-resolution models.

scholarly journals Climatic impacts of fresh water hosing under Last Glacial Maximum conditions: a multi-model study

2013 ◽  
Vol 9 (2) ◽  
pp. 935-953 ◽  
Author(s):  
M. Kageyama ◽  
U. Merkel ◽  
B. Otto-Bliesner ◽  
M. Prange ◽  
A. Abe-Ouchi ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Fresh Water ◽  
North Atlantic ◽  
Greenland Ice Sheet ◽  
Meridional Overturning Circulation ◽  
Last Glacial ◽  
The North ◽  
Tropical North Atlantic ◽  
The North Atlantic ◽  
The Impact

Abstract. Fresh water hosing simulations, in which a fresh water flux is imposed in the North Atlantic to force fluctuations of the Atlantic Meridional Overturning Circulation, have been routinely performed, first to study the climatic signature of different states of this circulation, then, under present or future conditions, to investigate the potential impact of a partial melting of the Greenland ice sheet. The most compelling examples of climatic changes potentially related to AMOC abrupt variations, however, are found in high resolution palaeo-records from around the globe for the last glacial period. To study those more specifically, more and more fresh water hosing experiments have been performed under glacial conditions in the recent years. Here we compare an ensemble constituted by 11 such simulations run with 6 different climate models. All simulations follow a slightly different design, but are sufficiently close in their design to be compared. They all study the impact of a fresh water hosing imposed in the extra-tropical North Atlantic. Common features in the model responses to hosing are the cooling over the North Atlantic, extending along the sub-tropical gyre in the tropical North Atlantic, the southward shift of the Atlantic ITCZ and the weakening of the African and Indian monsoons. On the other hand, the expression of the bipolar see-saw, i.e., warming in the Southern Hemisphere, differs from model to model, with some restricting it to the South Atlantic and specific regions of the southern ocean while others simulate a widespread southern ocean warming. The relationships between the features common to most models, i.e., climate changes over the north and tropical Atlantic, African and Asian monsoon regions, are further quantified. These suggest a tight correlation between the temperature and precipitation changes over the extra-tropical North Atlantic, but different pathways for the teleconnections between the AMOC/North Atlantic region and the African and Indian monsoon regions.

Evolving Relative Importance of the Southern Ocean and North Atlantic in Anthropogenic Ocean Heat Uptake

2018 ◽  
Vol 31 (18) ◽  
pp. 7459-7479 ◽  
Author(s):  
Jia-Rui Shi ◽  
Shang-Ping Xie ◽  
Lynne D. Talley
Keyword(s):  
Southern Ocean ◽  
North Atlantic ◽  
Radiative Forcing ◽  
Meridional Overturning Circulation ◽  
Anthropogenic Heat ◽  
Anthropogenic Aerosols ◽  
Ocean Heat Uptake ◽  
The North ◽  
Heat Uptake ◽  
The North Atlantic

Ocean uptake of anthropogenic heat over the past 15 years has mostly occurred in the Southern Ocean, based on Argo float observations. This agrees with historical simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5), where the Southern Ocean (south of 30°S) accounts for 72% ± 28% of global heat uptake, while the contribution from the North Atlantic north of 30°N is only 6%. Aerosols preferentially cool the Northern Hemisphere, and the effect on surface heat flux over the subpolar North Atlantic opposes the greenhouse gas (GHG) effect in nearly equal magnitude. This heat uptake compensation is associated with weakening (strengthening) of the Atlantic meridional overturning circulation (AMOC) in response to GHG (aerosol) radiative forcing. Aerosols are projected to decline in the near future, reinforcing the greenhouse effect on the North Atlantic heat uptake. As a result, the Southern Ocean, which will continue to take up anthropogenic heat largely through the mean upwelling of water from depth, will be joined by increased relative contribution from the North Atlantic because of substantial AMOC slowdown in the twenty-first century. In the RCP8.5 scenario, the percentage contribution to global uptake is projected to decrease to 48% ± 8% in the Southern Ocean and increase to 26% ± 6% in the northern North Atlantic. Despite the large uncertainty in the magnitude of projected aerosol forcing, our results suggest that anthropogenic aerosols, given their geographic distributions and temporal trajectories, strongly influence the high-latitude ocean heat uptake and interhemispheric asymmetry through AMOC change.

scholarly journals Transient Overturning Compensation between Atlantic and Indo-Pacific Basins

2020 ◽  
Vol 50 (8) ◽  
pp. 2151-2172 ◽  
Author(s):  
Shantong Sun ◽  
Andrew F. Thompson ◽  
Ian Eisenman
Keyword(s):  
Southern Ocean ◽  
Time Scales ◽  
North Atlantic ◽  
Climate Models ◽  
Diffusion Processes ◽  
Formation Rate ◽  
Deep Ocean ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
Overturning Circulation

AbstractClimate models consistently project (i) a decline in the formation of North Atlantic Deep Water (NADW) and (ii) a strengthening of the Southern Hemisphere westerly winds in response to anthropogenic greenhouse gas forcing. These two processes suggest potentially conflicting tendencies of the Atlantic meridional overturning circulation (AMOC): a weakening AMOC due to changes in the North Atlantic but a strengthening AMOC due to changes in the Southern Ocean. Here we focus on the transient evolution of the global ocean overturning circulation in response to a perturbation to the NADW formation rate. We propose that the adjustment of the Indo-Pacific overturning circulation is a critical component in mediating AMOC changes. Using a hierarchy of ocean and climate models, we show that the Indo-Pacific overturning circulation provides the first response to AMOC changes through wave processes, whereas the Southern Ocean overturning circulation responds on longer (centennial to millennial) time scales that are determined by eddy diffusion processes. Changes in the Indo-Pacific overturning circulation compensate AMOC changes, which allows the Southern Ocean overturning circulation to evolve independently of the AMOC, at least over time scales up to many decades. In a warming climate, the Indo-Pacific develops an overturning circulation anomaly associated with the weakening AMOC that is characterized by a northward transport close to the surface and a southward transport in the deep ocean, which could effectively redistribute heat between the basins. Our results highlight the importance of interbasin exchange in the response of the global ocean overturning circulation to a changing climate.

scholarly journals The mean state and variability of the North Atlantic circulation: a perspective from ocean reanalyses

2020 ◽  
Author(s):  
Laura Jackson ◽  
Clotilde Dubois ◽  
Gael Forget ◽  
Keith Haines ◽  
Matt Harrison ◽  
...  
Keyword(s):  
North Atlantic ◽  
Decadal Variability ◽  
Heat Content ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
The North ◽  
Ocean Reanalyses ◽  
The Mean ◽  
The North Atlantic ◽  
Mean State

<p>The observational network around the North Atlantic has improved significantly over the last few decades with the advent of Argo and satellite observations, and the more recent efforts to monitor the Atlantic Meridional Overturning Circulation (AMOC) using arrays such as RAPID and OSNAP. These have shown decadal timescale changes across the North Atlantic including in heat content, heat transport and the circulation. </p><p>However there are still significant gaps in the observational coverage, and significant uncertainties around some observational products. Ocean reanalyses integrate the observations with a dynamically consistent ocean model and are potentially tools that can be used to understand the observed changes. However the suitability of the reanalyses for the task must also be assessed.<br>We use an ensemble of global ocean reanalyses in comparison with observations in order to examine the mean state and interannual-decadal variability of the North Atlantic ocean since 1993. We assess how well the reanalyses are able to capture different processes and whether any understanding can be inferred. In particular we look at ocean heat content, transports, the AMOC and gyre strengths, water masses and convection. </p><p> </p>

scholarly journals Dependence of regional ocean heat uptake on anthropogenic warming scenarios

2020 ◽  
Vol 6 (45) ◽  
pp. eabc0303
Author(s):  
Xiaofan Ma ◽  
Wei Liu ◽  
Robert J. Allen ◽  
Gang Huang ◽  
Xichen Li
Keyword(s):  
Southern Ocean ◽  
North Atlantic ◽  
Meridional Overturning Circulation ◽  
Anthropogenic Aerosols ◽  
Ocean Heat Uptake ◽  
The North ◽  
Heat Uptake ◽  
The North Atlantic ◽  
Aerosol Effects ◽  
Business As Usual

The North Atlantic and Southern Ocean exhibit enhanced ocean heat uptake (OHU) during recent decades while their future OHU changes are subject to great uncertainty. Here, we show that regional OHU patterns in these two basins are highly dependent on the trajectories of aerosols and greenhouse gases (GHGs) in future scenarios. During the 21st century, North Atlantic and Southern Ocean OHU exhibit similarly positive trends under a business-as-usual scenario but respectively positive and negative trends under a mitigation scenario. The opposite centurial OHU trends in the Southern Ocean can be attributed partially to distinct GHG trajectories under the two scenarios while the common positive centurial OHU trends in the North Atlantic are mainly due to aerosol effects. Under both scenarios, projected decline of anthropogenic aerosols potentially induces a weakening of the Atlantic Meridional Overturning Circulation and a divergence of meridional oceanic heat transport, which leads to enhanced OHU in the subpolar North Atlantic.

North Atlantic Ocean Circulation Response to Stochastic Mesoscale Weather Systems

2021 ◽  
Author(s):  
Shenjie Zhou ◽  
Xiaoming Zhai ◽  
Ian Renfrew
Keyword(s):  
Ocean Circulation ◽  
North Atlantic ◽  
Climate Models ◽  
Meridional Overturning Circulation ◽  
Ocean Dynamics ◽  
Ensemble Prediction ◽  
Subpolar Gyre ◽  
Atmospheric Forcing ◽  
The North ◽  
The North Atlantic

<p>The ocean is forced by the atmosphere on a range of spatial and temporal scales. In ocean and climate models the resolution of the atmospheric forcing sets a limit on the scales that are represented. For typical climate models this means mesoscale (< 400 km) atmospheric forcing is absent. Previous studies have demonstrated that mesoscale forcing significantly affects key ocean circulation systems such as the North Atlantic Subpolar gyre and the Atlantic Meridional Overturning Circulation (AMOC). However, the approach of these studies has either been ad hoc or limited in resolution. Here we present ocean model simulations with and without realistic mesoscale atmospheric forcing that represents scales down to 10 km. We use a novel stochastic parameterization – based on a cellular automaton algorithm that is common in weather forecasting ensemble prediction systems<sup> </sup>– to represent spatially coherent weather systems over a range of scales, including down to the smallest resolvable by the ocean grid. The parameterization is calibrated spatially and temporally using marine wind observations. The addition of mesoscale atmospheric forcing leads to coherent patterns of change in the sea surface temperature and mixed-layer depth. It also leads to non-negligible changes in the volume transport in the North Atlantic subtropical gyre (STG) and subpolar gyre (SPG) and in the AMOC. A non-systematic basin-scale circulation response to the mesoscale wind perturbation emerges – an in-phase oscillation in northward heat transport across the gyre boundary, partly driven by the constantly enhanced STG, correspoding to an oscillatory behaviour in SPG and AMOC indices with a typical time scale of 5-year, revealing the importance of ocean dynamics in generating non-local ocean response to the stochastic mesoscale atmospheric forcing. Atmospheric convection-permitting regional climate simulations predict changes in the intensity and frequency of mesoscale weather systems this century, so representing these systems in coupled climate models could bring higher fidelity in future climate projections.</p>

scholarly journals Assimilation of Radiocarbon and Chlorofluorocarbon Data to Constrain Deep and Bottom Water Transports in the World Ocean

2007 ◽  
Vol 37 (2) ◽  
pp. 259-276 ◽  
Author(s):  
Reiner Schlitzer
Keyword(s):  
Southern Ocean ◽  
North Atlantic ◽  
Bottom Water ◽  
World Ocean ◽  
Weddell Sea ◽  
Global Ocean ◽  
Western Boundary ◽  
Boundary Current ◽  
The North ◽  
The North Atlantic

Abstract A coarse-resolution global model with time-invariant circulation is fitted to hydrographic and tracer data by means of the adjoint method. Radiocarbon and chlorofluorocarbon (CFC-11 and CFC-12) data are included to constrain deep and bottom water transport rates and spreading pathways as well as the strength of the global overturning circulation. It is shown that realistic global ocean distributions of hydrographic parameters and tracers can be obtained simultaneously. The model correctly reproduces the deep ocean radiocarbon field and the concentrations gradients between different basins. The spreading of CFC plumes in the deep and bottom waters is simulated in a realistic way, and the spatial extent as well as the temporal evolution of these plumes agrees well with observations. Radiocarbon and CFC observations place upper bounds on the northward transports of Antarctic Bottom Water (AABW) into the Pacific, Atlantic, and Indian Oceans. Long-term mean AABW transports larger than 5 Sv (Sv ≡ 106 m3 s−1) through the Vema and Hunter Channels in the South Atlantic and net AABW transports across 30°S into the Indian Ocean larger than 10 Sv are found to be incompatible with CFC data. The rates of equatorward deep and bottom water transports from the North Atlantic and Southern Ocean are of similar magnitude (15.7 Sv at 50°N and 17.9 Sv at 50°S). Deep and bottom water formation in the Southern Ocean occurs at multiple sites around the Antarctic continent and is not confined to the Weddell Sea. A CFC forecast based on the assumption of unchanged abyssal transports shows that by 2030 the entire deep west Atlantic exhibits CFC-11 concentrations larger than 0.1 pmol kg−1, while most of the deep Indian and Pacific Oceans remain CFC free. By 2020 the predicted CFC concentrations in the deep western boundary current (DWBC) in the North Atlantic exceed surface water concentrations and the vertical CFC gradients start to reverse.

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