Roles of meridional overturning in subpolar Southern Ocean SST trends: Insights from ensemble simulations

Surface Cooling ◽

Recent Climate ◽

Meridional Overturning ◽

Cooling Trend

Abstract One of the most puzzling observed features of recent climate has been a multidecadal surface cooling trend over the subpolar Southern Ocean (SO). In this study we use large ensembles of simulations with multiple climate models to study the role of the SO meridional overturning circulation (MOC) in these sea surface temperature (SST) trends. We find that multiple competing processes play prominent roles, consistent with multiple mechanisms proposed in the literature for the observed cooling. Early in the simulations (20th century and early 21st century) internal variability of the MOC can have a large impact, in part due to substantial simulated multidecadal variability of the MOC. Ensemble members with initially strong convection (and related surface warming due to convective mixing of subsurface warmth to the surface) tend to subsequently cool at the surface as convection associated with internal variability weakens. A second process occurs in the late 20th and 21st centuries, as weakening of oceanic convection associated with global warming and high latitude freshening can contribute to the surface cooling trend by suppressing convection and associated vertical mixing of subsurface heat. As the simulations progress, the multidecadal SO variability is suppressed due to forced changes in the mean state and increased oceanic stratification. As a third process, the shallower mixed layers can then rapidly warm due to increasing forcing from greenhouse gas warming. Also, during this period the ensemble spread of SO SST trend partly arises from the spread of the wind-driven Deacon cell strength. Thus, different processes could conceivably have led to the observed cooling trend, consistent with the range of possibilities presented in the literature. To better understand the causes of the observed trend it is important to better understand the characteristics of internal low-frequency variability in the SO and the response of that variability to global warming.

Increased risk of near term global warming due to a recent AMOC weakening

Nature Communications ◽

10.1038/s41467-021-26370-0 ◽

2021 ◽

Vol 12 (1) ◽

Author(s):

Rémy Bonnet ◽

Didier Swingedouw ◽

Guillaume Gastineau ◽

Olivier Boucher ◽

Julie Deshayes ◽

...

Keyword(s):

Global Warming ◽

Low Frequency ◽

Internal Variability ◽

The Past ◽

Increased Risk ◽

Meridional Overturning ◽

Near Term ◽

New Generation

AbstractSome of the new generation CMIP6 models are characterised by a strong temperature increase in response to increasing greenhouse gases concentration1. At first glance, these models seem less consistent with the temperature warming observed over the last decades. Here, we investigate this issue through the prism of low-frequency internal variability by comparing with observations an ensemble of 32 historical simulations performed with the IPSL-CM6A-LR model, characterized by a rather large climate sensitivity. We show that members with the smallest rates of global warming over the past 6-7 decades are also those with a large internally-driven weakening of the Atlantic Meridional Overturning Circulation (AMOC). This subset of members also matches several AMOC observational fingerprints, which are in line with such a weakening. This suggests that internal variability from the Atlantic Ocean may have dampened the magnitude of global warming over the historical era. Taking into account this AMOC weakening over the past decades means that it will be harder to avoid crossing the 2 °C warming threshold.

Southern Ocean Heat Uptake, Redistribution, and Storage in a Warming Climate: The Role of Meridional Overturning Circulation

Journal of Climate ◽

10.1175/jcli-d-17-0761.1 ◽

2018 ◽

Vol 31 (12) ◽

pp. 4727-4743 ◽

Cited By ~ 24

Author(s):

Wei Liu ◽

Jian Lu ◽

Shang-Ping Xie ◽

Alexey Fedorov

Keyword(s):

Southern Ocean ◽

Heat Transport ◽

Climate Models ◽

Climate Model ◽

The Other ◽

Anthropogenic Heat ◽

Overturning Circulation ◽

Ocean Heat Uptake ◽

Climate models show that most of the anthropogenic heat resulting from increased atmospheric CO2 enters the Southern Ocean near 60°S and is stored around 45°S. This heat is transported to the ocean interior by the meridional overturning circulation (MOC) with wind changes playing an important role in the process. To isolate and quantify the latter effect, we apply an overriding technique to a climate model and decompose the total ocean response to CO2 increase into two major components: one due to wind changes and the other due to direct CO2 effect. We find that the poleward-intensified zonal surface winds tend to shift and strengthen the ocean Deacon cell and hence the residual MOC, leading to anomalous divergence of ocean meridional heat transport around 60°S coupled to a surface heat flux increase. In contrast, at 45°S we see anomalous convergence of ocean heat transport and heat loss at the surface. As a result, the wind-induced ocean heat storage (OHS) peaks at 46°S at a rate of 0.07 ZJ yr−1 (° lat)−1 (1 ZJ = 1021 J), contributing 20% to the total OHS maximum. The direct CO2 effect, on the other hand, very slightly alters the residual MOC but primarily warms the ocean. It induces a small but nonnegligible change in eddy heat transport and causes OHS to peak at 42°S at a rate of 0.30 ZJ yr−1 (° lat)−1, accounting for 80% of the OHS maximum. We also find that the eddy-induced MOC weakens, primarily caused by a buoyancy flux change as a result of the direct CO2 effect, and does not compensate the intensified Deacon cell.

OSNAP and water mass transport in CMIP6 models

10.5194/egusphere-egu21-2725 ◽

2021 ◽

Author(s):

Laura Jackson

Keyword(s):

North Atlantic ◽

Climate Models ◽

Climate Model ◽

Water Mass ◽

Deep Convection ◽

Low Frequency ◽

The West ◽

Low Frequency Variability ◽

The Atlantic Meridional Overturning Circulation (AMOC) influences our climate by transporting heat northwards in the Atlantic ocean. The subpolar North Atlantic plays an important role in this circulation, with transformation of water to higher densities, deep convection and formation of deep water. Recent OSNAP observations have shown that the overturning is stronger to the east of Greenland than the west.Here we analyse a CMIP6 climate model at two resolutions (HadGEM3 GC3.1 LL and MM) and show both compare well with the OSNAP observations. We explore the source of low frequency variability of the AMOC and how it is related to the surface water mass transformation in different regions. We also investigate time-mean and low frequency water mass transformations in other CMIP6 climate models.

Mechanism for potential strengthening of Atlantic overturning prior to collapse

Earth System Dynamics Discussions ◽

10.5194/esdd-5-29-2014 ◽

2014 ◽

Vol 5 (1) ◽

pp. 29-62

Author(s):

D. Ehlert ◽

A. Levermann

Keyword(s):

Southern Ocean ◽

North Atlantic ◽

Climate Models ◽

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.

Mechanism for potential strengthening of Atlantic overturning prior to collapse

Earth System Dynamics ◽

10.5194/esd-5-383-2014 ◽

2014 ◽

Vol 5 (2) ◽

pp. 383-397

Author(s):

D. Ehlert ◽

A. Levermann

Keyword(s):

Southern Ocean ◽

North Atlantic ◽

Climate Models ◽

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.

Glacial mode shift of the Atlantic meridional overturning circulation by warming over the Southern Ocean

Communications Earth & Environment ◽

10.1038/s43247-021-00226-3 ◽

2021 ◽

Vol 2 (1) ◽

Author(s):

Akira Oka ◽

Ayako Abe-Ouchi ◽

Sam Sherriff-Tadano ◽

Yusuke Yokoyama ◽

Kenji Kawamura ◽

...

Keyword(s):

Southern Ocean ◽

Southern Hemisphere ◽

General Circulation ◽

Sea Surface ◽

Surface Cooling ◽

Thermal Threshold ◽

Overturning Circulation ◽

AbstractAbrupt climate warming events, known as Dansgaard-Oeschger events, occurred frequently during glacial periods, and are thought to be linked to changes in the Atlantic meridional overturning circulation. However, the mechanism responsible is not fully understood. Here, we present numerical simulations with a sea-ice coupled ocean general circulation model that systematically investigate the thermal threshold where deep water formation, and hence the overturning circulation, shift abruptly when the sea surface cools or warms sufficiently. Specifically, in our simulations where the magnitude of the sea surface cooling is changed separately or simultaneously in the Northern and Southern Hemispheres, a prominent threshold is identified when the Southern Hemisphere is slightly warmer than during glacial maxima. Abrupt mode changes of the Atlantic Meridional Overturning Circulation, like those during Dansgaard-Oeschger events, occur past a threshold in a transient simulation where the Southern Hemisphere is gradually warmed. We propose that the Southern Ocean plays a role in controlling the thermal threshold of the Atlantic Meridional Overturning Circulation in a glacial climate and that Southern Ocean warming may have triggered Dansgaard-Oeschger events which occurred with long interval.

Surface Constraints on the Depth of the Atlantic Meridional Overturning Circulation: Southern Ocean versus North Atlantic

Journal of Climate ◽

10.1175/jcli-d-19-0546.1 ◽

2020 ◽

Vol 33 (8) ◽

pp. 3125-3149 ◽

Cited By ~ 2

Author(s):

Shantong Sun ◽

Ian Eisenman ◽

Laure Zanna ◽

Andrew L. Stewart

Keyword(s):

Southern Ocean ◽

North Atlantic ◽

Surface Density ◽

Climate Models ◽

Geometric Model ◽

Overturning Circulation ◽

Meridional Overturning ◽

Surface Buoyancy

AbstractPaleoclimate proxy evidence suggests that the Atlantic meridional overturning circulation (AMOC) was about 1000 m shallower at the Last Glacial Maximum (LGM) compared to the present. Yet it remains unresolved what caused this glacial shoaling of the AMOC, and many climate models instead simulate a deeper AMOC under LGM forcing. While some studies suggest that Southern Ocean surface buoyancy forcing controls the AMOC depth, others have suggested alternatively that North Atlantic surface forcing or interior diabatic mixing plays the dominant role. To investigate the key processes that set the AMOC depth, here we carry out a number of MITgcm ocean-only simulations with surface forcing fields specified from the simulation results of three coupled climate models that span much of the range of glacial AMOC depth changes in phase 3 of the Paleoclimate Model Intercomparison Project (PMIP3). We find that the MITgcm simulations successfully reproduce the changes in AMOC depth between glacial and modern conditions simulated in these three PMIP3 models. By varying the restoring time scale in the surface forcing, we show that the AMOC depth is more strongly constrained by the surface density field than the surface buoyancy flux field. Based on these results, we propose a mechanism by which the surface density fields in the high latitudes of both hemispheres are connected to the AMOC depth. We illustrate the mechanism using MITgcm simulations with idealized surface forcing perturbations as well as an idealized conceptual geometric model. These results suggest that the AMOC depth is largely determined by the surface density fields in both the North Atlantic and the Southern Ocean.

Influence of the Southern Annular Mode on Projected Weakening of the Atlantic Meridional Overturning Circulation

Journal of Climate ◽

10.1175/jcli-d-12-00663.1 ◽

2013 ◽

Vol 26 (20) ◽

pp. 8017-8036 ◽

Cited By ~ 3

Author(s):

Peter T. Spooner ◽

Helen L. Johnson ◽

Tim J. Woollings

Keyword(s):

Climate Change ◽

Southern Ocean ◽

Greenhouse Gas ◽

Climate Models ◽

Southern Annular Mode ◽

Overturning Circulation ◽

Annular Mode ◽

Abstract Coupled climate models predict density-driven weakening of the Atlantic meridional overturning circulation (AMOC) under greenhouse gas forcing, with considerable spread in the response between models. There is also a large spread in the predicted increase of the southern annular mode (SAM) index across these models. Regression analysis across model space using 11 non-eddy-resolving models suggests that up to 35% of the intermodel spread in the AMOC response may be associated with uncertainty in the magnitude of the increase in the SAM. Models with a large, positive SAM index response generally display a smaller weakening of the AMOC under greenhouse gas forcing. The initial AMOC strength is also a major cause of intermodel spread in its response to climate change. The increase in the SAM acts to reduce the weakening of the AMOC over the next century by around ⅓, through increases in wind stress over the Southern Ocean, northward Ekman transport, and upwelling around Antarctica. The SAM response is also related to an increase in the northward salt flux across 30°S and to salinity anomalies in the high-latitude North Atlantic. These provide a positive feedback by further reinforcement of the AMOC. The results suggest that, compared with the real ocean where eddies oppose wind-driven changes in Southern Ocean circulation, climate models underestimate the effects of anthropogenic climate change on the AMOC.

Atlantic Salinity Pileup as a Remote Fingerprint of Weakening Atlantic Overturning Circulation under Anthropogenic Warming

10.5194/egusphere-egu2020-3790 ◽

2020 ◽

Author(s):

Chenyu Zhu ◽

Zhengyu Liu

Keyword(s):

Global Warming ◽

North Atlantic ◽

Climate Models ◽

South Atlantic ◽

The South ◽

Overturning Circulation ◽

The North ◽

Direct Measurements ◽

Climate models show a weakening Atlantic meridional overturning circulation (AMOC) under global warming. Limited by short direct measurements, this AMOC slowdown has been inferred, with some uncertainties, indirectly from some AMOC fingerprints locally over the subpolar North Atlantic region. Here we present observational and modeling evidences of the first remote fingerprint of AMOC slowdown outside the North Atlantic. Under global warming, the weakening AMOC reduces the salinity divergence and then leads to a remote fingerprint of &#8220;salinity pileup&#8221; in the South Atlantic. Our study supports the AMOC slowdown under anthropogenic warming and, furthermore, shows that this weakening has occurred all the way into the South Atlantic.

Low frequency variability and sensitivity of the Atlantic meridional overturning circulation in selected IPCC climate models

Russian Journal of Numerical Analysis and Mathematical Modelling ◽

10.1515/rnam-2018-0029 ◽

2018 ◽

Vol 33 (6) ◽

pp. 341-350 ◽

Cited By ~ 1

Author(s):

Andrey Gritsun

Keyword(s):

Climate Models ◽

Thermohaline Circulation ◽

Low Frequency ◽

Data Series ◽

Overturning Circulation ◽

Low Frequency Variability ◽

Meridional Overturning ◽

Analysis Of Stability

Abstract The structure of main modes of the decadal and multidecadal variability of the Atlantic meridional overturning circulation (AMOC) is analyzed for the climate models INM-CM5 (INM RAS), CCSM4 (NCAR), MPI-LR and MPI-MR (MPI). It is shown that oscillations with characteristic periods of 25–35 and 50–70 years are recognized in the models, and the corresponding spatial structures are variations of the intensity and position of the AMOC. Relations connecting changes of thermohaline circulation with external impacts on the system are constructed for the above models. The analysis of stability of calculations relative to the length of data series used for calculations is performed. The optimal impacts leading to the greatest response of the AMOC and influence functions causing its response along the leading modes of low-frequency variability are constructed. The optimal impacts have the form of density anomalies localized in the North Atlantic, and the structures of the corresponding responses are dominated by leading low-frequency modes of variability. The structure of optimal impacts varies greatly from model to model.