What Causes the AMOC to Weaken in CMIP5?

2020 ◽  
Vol 33 (4) ◽  
pp. 1535-1545 ◽  
Author(s):  
Samuel J. Levang ◽  
Raymond W. Schmitt
Keyword(s):  
Meridional Overturning Circulation ◽  
Geostrophic Balance ◽  
Warming Scenario ◽  
Salinity Changes ◽  
The North ◽  
Intermediate Layers ◽  
Freshwater Forcing ◽  
Meridional Overturning ◽  
Surface Buoyancy

AbstractIn a transient warming scenario, the North Atlantic is influenced by a complex pattern of surface buoyancy flux changes that ultimately weaken the Atlantic meridional overturning circulation (AMOC). Here we study the AMOC response in the CMIP5 experiment, using the near-geostrophic balance of the AMOC on interannual time scales to identify the role of temperature and salinity changes in altering the circulation. The thermal wind relationship is used to quantify changes in the zonal density gradients that control the strength of the flow. At 40°N, where the overturning cell is at its strongest, weakening of the AMOC is largely driven by warming between 1000- and 2000-m depth along the western margin. Despite significant subpolar surface freshening, salinity changes are small in the deep branch of the circulation. This is likely due to the influence of anomalously salty water in the subpolar intermediate layers, which is carried northward from the subtropics in the upper limb of the AMOC. In the upper 1000 m at 40°N, salty anomalies due to increased evaporation largely cancel the buoyancy increase due to warming. Therefore, in CMIP5, temperature dynamics are responsible for AMOC weakening, while freshwater forcing instead acts to strengthen the circulation in the net. These results indicate that past modeling studies of AMOC weakening, which rely on freshwater hosing in the subpolar gyre, may not be directly applicable to a more complex warming scenario.

The Role of Eddies in the Zonal and Meridional Overturning Circulations of Buoyancy-Forced Basins

2021 ◽  
Vol 51 (2) ◽  
pp. 575-590
Author(s):  
Suyash Bire ◽  
Christopher L.P. Wolfe
Keyword(s):  
Meridional Overturning Circulation ◽  
Swash Zone ◽  
Boundary Current ◽  
Overturning Circulation ◽  
Southern Boundary ◽  
Eastern Boundary ◽  
Meridional Overturning ◽  
Surface Buoyancy ◽  
Upwelling And Downwelling

AbstractThe zonal and meridional overturning circulations of buoyancy-forced basins are studied in an eddy-resolving model. The zonal overturning circulation (ZOC) is driven by the meridional gradient of buoyancy at the surface and stratification at the southern boundary. The ZOC, in turn, produces zonal buoyancy gradients through upwelling and downwelling at the western and eastern boundaries, respectively. The meridional overturning circulation (MOC) is driven by these zonal gradients rather than being directly driven by meridional gradients. Eddies lead to a broadening of the upwelling and downwelling limbs of the ZOC, as well as a decoupling of the locations of vertical and diapycnal transport. This broadening is more prominent on the eastern boundary, where westward-moving eddies transport warm water away from a poleward-flowing eastern boundary current. Most of the diapycnal downwelling occurs in the “swash zone”—the region where the isopycnals intermittently come in contact with the surface and lose buoyancy to the atmosphere. A scaling for the overturning circulations, which depends on the background stratification and the surface buoyancy gradient, is derived and found to be an excellent fit to the numerical experiments.

scholarly journals Impact of North Atlantic Freshwater Forcing on the Pacific Meridional Overturning Circulation under Glacial and Interglacial Conditions

2019 ◽  
Vol 32 (15) ◽  
pp. 4641-4659
Author(s):  
Hyo-Jeong Kim ◽  
Soon-Il An
Keyword(s):  
North Atlantic ◽  
North Pacific ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
Overturning Circulation ◽  
The North ◽  
Freshwater Forcing ◽  
The Pacific ◽  
Meridional Overturning ◽  
The North Pacific

Abstract The Pacific meridional overturning circulation (PMOC) is not well known compared to the Atlantic meridional overturning circulation (AMOC), due to its absence today. However, considering PMOC development under different climate conditions shown by proxy and modeling studies, a better understanding of PMOC is appropriate to properly assess the past and future climate change associated with global ocean circulation. Here, the PMOC response to freshwater forcing in the North Atlantic (NA) is investigated using an Earth system model of intermediate complexity under glacial (i.e., Last Glacial Maximum) and interglacial [i.e., preindustrial with/without inflow through Bering Strait (BS)] conditions. The water hosing over NA led to the shutdown of the AMOC, which accompanied an active PMOC except for the preindustrial condition with the opening BS, indicating that the emergence of the PMOC is constrained by the freshwater inflow through the BS, which hinders its destabilization through enhancing ocean stratification. However, the closure of the BS itself could not explain how the sinking motion is maintained in the North Pacific. Here we found that various atmospheric and oceanic processes are involved to sustain the active PMOC. First, an atmospheric teleconnection associated with the collapsed AMOC encouraged the evaporation in the sinking region, causing buoyancy loss at the surface of the North Pacific. Second, the strengthened subpolar gyre transported saltier water northward, enhancing dense water formation. Finally, the vigorous upwelling in the Southern Ocean enabled a consistent mass supply to the sinking region, with the aid of enhanced westerlies.

scholarly journals Hosed vs. unhosed: interruptions of the Atlantic Meridional Overturning Circulation in a global coupled model, with and without freshwater forcing

2016 ◽  
Vol 12 (8) ◽  
pp. 1663-1679 ◽  
Author(s):  
Nicolas Brown ◽  
Eric D. Galbraith
Keyword(s):  
North Atlantic ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Abrupt Climate Change ◽  
Freshwater Input ◽  
The North ◽  
Freshwater Forcing ◽  
Meridional Overturning ◽  
The North Atlantic ◽  
Background Climate

Abstract. It is well known that glacial periods were punctuated by abrupt climate changes, with large impacts on air temperature, precipitation, and ocean circulation across the globe. However, the long-held idea that freshwater forcing, caused by massive iceberg discharges, was the driving force behind these changes has been questioned in recent years. This throws into doubt the abundant literature on modelling abrupt climate change through “hosing” experiments, whereby the Atlantic Meridional Overturning Circulation (AMOC) is interrupted by an injection of freshwater to the North Atlantic: if some, or all, abrupt climate change was not driven by freshwater input, could its character have been very different than the typical hosed experiments? Here, we describe spontaneous, unhosed oscillations in AMOC strength that occur in a global coupled ocean–atmosphere model when integrated under a particular background climate state. We compare these unhosed oscillations to hosed oscillations under a range of background climate states in order to examine how the global imprint of AMOC variations depends on whether or not they result from external freshwater input. Our comparison includes surface air temperature, precipitation, dissolved oxygen concentrations in the intermediate-depth ocean, and marine export production. The results show that the background climate state has a significant impact on the character of the freshwater-forced AMOC interruptions in this model, with particularly marked variations in tropical precipitation and in the North Pacific circulation. Despite these differences, the first-order patterns of response to AMOC interruptions are quite consistent among all simulations, implying that the ocean–sea ice–atmosphere dynamics associated with an AMOC weakening dominate the global response, regardless of whether or not freshwater input is the cause. Nonetheless, freshwater addition leads to a more complete shutdown of the AMOC than occurs in the unhosed oscillations, with amplified global impacts, evocative of Heinrich stadials. In addition, freshwater inputs can directly impact the strength of other polar haloclines, particularly that of the Southern Ocean, to which freshwater can be transported relatively quickly after injection in the North Atlantic.

scholarly journals Hosed vs. unhosed: global response to interruptions of the Atlantic Meridional Overturning, with and without freshwater forcing

2015 ◽  
Vol 11 (5) ◽  
pp. 4669-4700 ◽  
Author(s):  
N. Brown ◽  
E. D. Galbraith
Keyword(s):  
Climate Change ◽  
North Atlantic ◽  
Meridional Overturning Circulation ◽  
Abrupt Climate Change ◽  
Freshwater Input ◽  
Global Response ◽  
The North ◽  
Freshwater Forcing ◽  
Meridional Overturning ◽  
The North Atlantic

Abstract. It is well known that glacial periods were punctuated by abrupt climate changes, with large impacts on air temperature, precipitation, and ocean circulation across the globe. However, the long-held idea that freshwater forcing, caused by massive iceberg discharges, was the driving force behind these changes has been questioned in recent years. This throws into doubt the abundant literature on modelling abrupt climate change through "hosing" experiments, whereby the Atlantic Meridional Overturning Circulation (AMOC) is interrupted by an injection of freshwater to the North Atlantic: if some, or all, abrupt climate change was not driven by freshwater input, could its character have been very different than the typical hosed experiments? Here, we take advantage of a global coupled ocean–atmosphere model that exhibits spontaneous, unhosed oscillations in AMOC strength, in order to examine how the global imprint of AMOC variations depends on whether or not it is the result of external freshwater input. The results imply that, to first order, the ocean–ice–atmosphere dynamics associated with an AMOC weakening dominate the global response, regardless of whether or not freshwater input is the cause. The exception lies in the impact freshwater inputs can have on the strength of other polar haloclines, particularly the Southern Ocean, to which freshwater can be transported relatively quickly after injection in the North Atlantic.

Mechanisms linking the Labrador Sea with subtropical Atlantic overturning

2021 ◽  
Author(s):  
Yavor Kostov ◽  
Marie-José Messias ◽  
Helen Johnson ◽  
Herlé Mercier ◽  
David Marshall
Keyword(s):  
North Atlantic ◽  
Causal Chain ◽  
Meridional Overturning Circulation ◽  
Sea Surface ◽  
Labrador Sea ◽  
State Estimate ◽  
The North ◽  
Denmark Strait ◽  
Meridional Overturning ◽  
Surface Buoyancy

<p>We analyze the causal chain linking sea surface buoyancy anomalies in the Labrador Sea and variability in the subtropical Atlantic meridional overturning circulation (AMOC) in the ECCO ocean state estimate on inter-annual timescales. Our study highlights the importance of Lower North Atlantic Deep Water (LNADW) for the north-south connectivity in the Atlantic Ocean. We identify important mechanisms that allow the Labrador Sea to impact the southward transport of LNADW. We show that NAC plays an essential role in the export of buoyancy anomalies from the Labrador Sea – and it furthermore exerts a positive feedback that amplifies these upper ocean anomalies in the eastern subpolar gyre – before they reach the denser water masses along the lower limb of the AMOC. Our results also highlight the contribution of the western Labrador Sea for the surface uptake of tracers that penetrate the LNADW near Denmark Strait, which has implications for the redistribution of ocean heat anomalies.</p>

scholarly journals Reconciling the north–south density difference scaling for the Meridional Overturning Circulation strength with geostrophy

2013 ◽  
Vol 10 (6) ◽  
pp. 2461-2479 ◽  
Author(s):  
A. A. Cimatoribus ◽  
S. Drijfhout ◽  
H. A. Dijkstra
Keyword(s):  
Large Scale ◽  
Climate Models ◽  
Ocean Basin ◽  
Meridional Overturning Circulation ◽  
Density Difference ◽  
Overturning Circulation ◽  
Common Assumption ◽  
Geostrophic Balance ◽  
The North ◽  
Meridional Overturning

Abstract. Since the formulation of the Stommel two-box model for the meridional overturning circulation (MOC), various theoretical and conceptual models for the MOC emerged based on scaling the MOC strength with the north south density difference. At the same time the MOC should obey geostrophic balance with an east-west density difference. Scaling with the north south density gradient seems to violate the common assumption of geostrophic balance for the large-scale circulation, which implies that the pressure gradient is orthogonal to the flow. In this brief report, we report on the results of a series of numerical simulations in an idealized ocean basin (with a zonally periodic channel at its southern end). The simulations performed with different surface forcing conditions indicate that the meridional and zonal density gradients, important for the MOC strength, are in fact related to each other through the stratification located at the northern end of the periodic channel. The results suggest that the water properties at the northern end of the periodic channel play a crucial role in setting the MOC strength, possibly explaining the sensitivity of climate models to the conditions in this area.

scholarly journals The Impact of the North Atlantic Oscillation on Climate through Its Influence on the Atlantic Meridional Overturning Circulation

2016 ◽  
Vol 29 (3) ◽  
pp. 941-962 ◽  
Author(s):  
Thomas L. Delworth ◽  
Fanrong Zeng
Keyword(s):  
North Atlantic ◽  
Time Scale ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
The North ◽  
Hemispheric Temperature ◽  
Meridional Overturning ◽  
The North Atlantic Oscillation ◽  
The Impact ◽  
Model Ocean

Abstract The impact of the North Atlantic Oscillation (NAO) on the Atlantic meridional overturning circulation (AMOC) and large-scale climate is assessed using simulations with three different climate models. Perturbation experiments are conducted in which a pattern of anomalous heat flux corresponding to the NAO is added to the model ocean. Differences between the perturbation experiments and a control illustrate how the model ocean and climate system respond to the NAO. A positive phase of the NAO strengthens the AMOC by extracting heat from the subpolar gyre, thereby increasing deep-water formation, horizontal density gradients, and the AMOC. The flux forcings have the spatial structure of the observed NAO, but the amplitude of the forcing varies in time with distinct periods varying from 2 to 100 yr. The response of the AMOC to NAO variations is small at short time scales but increases up to the dominant time scale of internal AMOC variability (20–30 yr for the models used). The amplitude of the AMOC response, as well as associated oceanic heat transport, is approximately constant as the time scale of the forcing is increased further. In contrast, the response of other properties, such as hemispheric temperature or Arctic sea ice, continues to increase as the time scale of the forcing becomes progressively longer. The larger response is associated with the time integral of the anomalous oceanic heat transport at longer time scales, combined with an increased impact of radiative feedback processes. It is shown that NAO fluctuations, similar in amplitude to those observed over the last century, can modulate hemispheric temperature by several tenths of a degree.

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