A climatology of the North Atlantic subpolar gyre boundary

2021 ◽  
Author(s):  
Sam Jones ◽  
Stuart Cunningham ◽  
Neil Fraser ◽  
Mark Inall
Keyword(s):  
North Atlantic ◽  
Satellite Altimetry ◽  
Large Scale ◽  
Current Strength ◽  
Meridional Overturning Circulation ◽  
Density Gradients ◽  
Subpolar Gyre ◽  
Boundary Current ◽  
Boundary Flow ◽  
The Impact

<p>Circulation at the boundary of the subpolar North Atlantic influences both the horizontal (gyre) and vertical (overturning) components of the flow structure. While boundary current transport projects directly onto subpolar gyre strength, recent modelling studies have highlighted that buoyancy fluxes between the basin interior and the boundary, followed by rapid buoyancy export by boundary currents, are crucial steps in projecting air-sea interaction onto the strength of the Atlantic Meridional Overturning Circulation (AMOC). This work seeks observational insights into these key boundary processes.<br>To achieve this, we have constructed a robust boundary climatology from quality controlled CTD and Argo hydrography since the turn of the millennium. Following the 1000 m isobath north of 47 °N and aggregating data into 100 km bins, we build a picture of the typical large-scale temperature and salinity structure for each month.<br>This product will allow us to identify where and when important interior-boundary buoyancy fluxes take place over a seasonal cycle. A first step is to evaluate geostrophic flow into the boundary, and hence describe the vertical structure of advective buoyancy exchange. By appealing to satellite altimetry and Argo trajectories, we can also estimate turbulent eddy fluxes both at the surface and 1000 m depth. Models indicate these parameters are key in dictating the pathways for the AMOC lower limb, and we will place our observational findings in the context of these results.<br>Boundary current strength is another key parameter dictating the export of dense water from the subpolar gyre. We will appeal to satellite altimetry to build corresponding climatologies for barotropic boundary flow. Furthermore, along-slope density gradients give rise to a baroclinic boundary current forcing term, which we aim to investigate here. Water density generally increases as we follow the gyre counter-clockwise, with the notable exception of the West Greenland Current section, and our product allows us to partition the spatially-varying contribution of temperature and salinity towards these density gradients. For example, we can evaluate the impact of cooling along the eastern boundary, or surface freshening around southern Greenland, on the dynamics of boundary flow. Ultimately, we would like to understand the evolution of the dynamical balance experienced by a hypothetical fluid parcel traversing the entire subpolar gyre.</p>

scholarly journals Size Matters: Another Reason Why the Atlantic Is Saltier than the Pacific

2017 ◽  
Vol 47 (11) ◽  
pp. 2843-2859 ◽  
Author(s):  
C. S. Jones ◽  
Paola Cessi
Keyword(s):  
North Atlantic ◽  
Deep Water ◽  
Western Boundary Current ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Subpolar Gyre ◽  
Boundary Current ◽  
Surface Salinity ◽  
The North ◽  
The North Atlantic

AbstractThe surface salinity in the North Atlantic controls the position of the sinking branch of the meridional overturning circulation (MOC); the North Atlantic has higher salinity, so deep-water formation occurs there rather than in the North Pacific. Here, it is shown that in a 3D primitive equation model of two basins of different widths connected by a reentrant channel, there is a preference for sinking in the narrow basin even under zonally uniform surface forcing. This preference is linked to the details of the velocity and salinity fields in the “sinking” basin. The southward western boundary current associated with the wind-driven subpolar gyre has higher velocity in the wide basin than in the narrow basin. It overwhelms the northward western boundary current associated with the MOC for wide-basin sinking, so freshwater is brought from the far north of the domain southward and forms a pool on the western boundary in the wide basin. The fresh pool suppresses local convection and spreads eastward, leading to low salinities in the north of the wide basin for wide-basin sinking. This pool of freshwater is much less prominent in the narrow basin for narrow-basin sinking, where the northward MOC western boundary current overcomes the southward western boundary current associated with the wind-driven subpolar gyre, bringing salty water from lower latitudes northward and enabling deep-water mass formation.

scholarly journals The Atlantic Meridional Overturning Circulation and the Cabbeling Effect

2020 ◽  
Vol 50 (9) ◽  
pp. 2561-2572
Author(s):  
Fabian Schloesser
Keyword(s):  
North Atlantic ◽  
Hydrological Cycle ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Density Gradients ◽  
Overturning Circulation ◽  
Mean Temperature ◽  
Freshwater Forcing ◽  
Meridional Overturning ◽  
The Impact

AbstractNorth Atlantic meridional density gradients have been identified as a main driver of the Atlantic meridional overturning circulation (AMOC). Due to the cabbeling effect, these density gradients are increasingly dominated by temperature gradients in a warming ocean, and a direct link exists between North Atlantic mean temperature and AMOC strength. This paper quantifies the impact of this mechanism in the Stommel and Gnanadesikan models. Owing to different feedback mechanisms being included, a 1°C warming of North Atlantic mean ocean temperature strengthens the AMOC by 3% in the Gnanadesikan model and 8% in the Stommel model. In the Gnanadesikan model that increase is equivalent to a 4% strengthening of Southern Hemisphere winds and can compensate for a 14% increase in the hydrological cycle. Furthermore, mean temperature strongly controls a freshwater forcing threshold for the strong AMOC state, suggesting that the cabbeling effect needs to be considered to explain past and future AMOC variability.

Interaction of Atlantic Meridional Overturning Circulation and Sub-Polar Gyre on decadal timescale

2021 ◽  
Author(s):  
Stephen Ogungbenro ◽  
Leonard Borchert ◽  
Sebastian Brune ◽  
Vimal Koul ◽  
Levke Caesar ◽  
...  
Keyword(s):  
North Atlantic ◽  
Large Scale ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Max Planck ◽  
Overturning Circulation ◽  
The North ◽  
Meridional Overturning ◽  
The North Atlantic ◽  
The Impact

<p>North Atlantic climate variability is dominated by two important subsystems, the Atlantic Meridional Overturning Circulation (AMOC) and the Sub-Polar Gyre (SPG). While the AMOC is responsible for the transport of mass and heat into higher latitudes, SPG has been linked with large-scale changes in the subpolar marine environment. The changes in strength, intensity and positions of the constituent currents of the SPG impose variabilities in the distribution of heat and salt in the North Atlantic Ocean. Consequently, the predictability on decadal scales of the two subsystems is of huge importance for the understanding of variability in the North Atlantic.</p><p>Our contribution investigates the decadal and multi-decadal predictability of these subsystems within the Max Planck Institute for Meteorology Earth System Model (MPI-ESM). We analyse the model’s capability to predict these subsystems as well as the dependence of the two subsystems on each other. These investigations open new opportunities for a better understanding of the impact of the North Atlantic onto important marine ecosystems and its changes in the upcoming decade.</p>

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.

Subpolar Gyre – AMOC – Atmosphere Interactions on Multidecadal Timescales in a Version of the Kiel Climate Model

2021 ◽  
Author(s):  
Jing Sun ◽  
Mojib Latif ◽  
Wonsun Park
Keyword(s):  
North Atlantic ◽  
Climate Model ◽  
Deep Convection ◽  
Meridional Overturning Circulation ◽  
Subpolar Gyre ◽  
Surface Salinity ◽  
The North ◽  
Ocean Coupling ◽  
Meridional Overturning ◽  
The North Atlantic

<p>There is a controversy about the nature of multidecadal climate variability in the North Atlantic (NA) region, concerning the roles of ocean circulation and atmosphere-ocean coupling. Here we describe NA multidecadal variability from a version of the Kiel Climate Model, in which both subpolar gyre (SPG)-Atlantic Meridional Overturning Circulation (AMOC) and atmosphere-ocean coupling are essential. The oceanic barotropic streamfuntions, meridional overturning streamfunctions, and sea level pressure are jointly analyzed to derive the leading mode of Atlantic variability. This mode accounting for about 23.7 % of the total combined variance is oscillatory with an irregular periodicity of 25-50 years and an e-folding time of about a decade. SPG and AMOC mutually influence each other and together provide the delayed negative feedback necessary for maintaining the oscillation. An anomalously strong SPG, for example, drives higher surface salinity and density in the NA’s sinking region. In response, oceanic deep convection and AMOC intensify, which, with a time delay of about a decade, reduces SPG strength by enhancing upper-ocean heat content. The weaker gyre circulation leads to lower surface salinity and density in the sinking region, which eventually reduces deep convection and AMOC strength. There is a positive ocean-atmosphere feedback between the sea surface temperature and low-level atmospheric circulation over the Southern Greenland area, with related wind stress changes reinforcing SPG changes, thereby maintaining the (damped) multidecadal oscillation against dissipation. Stochastic surface heat-flux forcing associated with the North Atlantic Oscillation drives the eigenmode.</p>

scholarly journals Climate impacts on multidecadal <i>p</i>CO<sub>2</sub> variability in the North Atlantic: 1948–2009

2015 ◽  
Vol 12 (17) ◽  
pp. 15223-15244
Author(s):  
M. L. Breeden ◽  
G. A. McKinley
Keyword(s):  
North Atlantic ◽  
Large Scale ◽  
Dissolved Inorganic Carbon ◽  
Atlantic Multidecadal Oscillation ◽  
Climate Impacts ◽  
Co2 Uptake ◽  
Subpolar Gyre ◽  
The North ◽  
The North Atlantic

Abstract. The North Atlantic is the most intense region of ocean CO2 uptake. Here, we investigate multidecadal timescale variability of the partial pressure CO2 (pCO2) that is due to the natural carbon cycle using a regional model forced with realistic climate and pre-industrial atmospheric pCO2 for 1948–2009. Large-scale patterns of natural pCO2 variability are primarily associated with basin-averaged sea surface temperature (SST) that, in turn, is composed of two parts: the Atlantic Multidecadal Oscillation (AMO) and a long-term positive SST trend. The North Atlantic Oscillation (NAO) drives a secondary mode of variability. For the primary mode, positive AMO and the SST trend modify pCO2 with different mechanisms and spatial patterns. Warming with the positive AMO increases subpolar gyre pCO2, but there is also a significant reduction of dissolved inorganic carbon (DIC) due primarily to reduced vertical mixing. The net impact of positive AMO is to reduce pCO2 in the subpolar gyre. Through direct impacts on SST, the net impacts of positive AMO is to increase pCO2 in the subtropical gyre. From 1980 to present, long-term SST warming has amplified AMO impacts on pCO2.

scholarly journals Response of North Atlantic Ocean Circulation to Atmospheric Weather Regimes

2014 ◽  
Vol 44 (1) ◽  
pp. 179-201 ◽  
Author(s):  
Nicolas Barrier ◽  
Christophe Cassou ◽  
Julie Deshayes ◽  
Anne-Marie Treguier
Keyword(s):  
Atlantic Ocean ◽  
Wind Stress ◽  
Ocean Circulation ◽  
North Atlantic ◽  
North Atlantic Ocean ◽  
Meridional Overturning Circulation ◽  
Subpolar Gyre ◽  
Wind Stress Curl ◽  
Overturning Circulation ◽  
Weather Regimes

Abstract A new framework is proposed for investigating the atmospheric forcing of North Atlantic Ocean circulation. Instead of using classical modes of variability, such as the North Atlantic Oscillation (NAO) or the east Atlantic pattern, the weather regimes paradigm was used. Using this framework helped avoid problems associated with the assumptions of orthogonality and symmetry that are particular to modal analysis and known to be unsuitable for the NAO. Using ocean-only historical and sensitivity experiments, the impacts of the four winter weather regimes on horizontal and overturning circulations were investigated. The results suggest that the Atlantic Ridge (AR), negative NAO (NAO−), and positive NAO (NAO+) regimes induce a fast (monthly-to-interannual time scales) adjustment of the gyres via topographic Sverdrup dynamics and of the meridional overturning circulation via anomalous Ekman transport. The wind anomalies associated with the Scandinavian blocking regime (SBL) are ineffective in driving a fast wind-driven oceanic adjustment. The response of both gyre and overturning circulations to persistent regime conditions was also estimated. AR causes a strong, wind-driven reduction in the strengths of the subtropical and subpolar gyres, while NAO+ causes a strengthening of the subtropical gyre via wind stress curl anomalies and of the subpolar gyre via heat flux anomalies. NAO− induces a southward shift of the gyres through the southward displacement of the wind stress curl. The SBL is found to impact the subpolar gyre only via anomalous heat fluxes. The overturning circulation is shown to spin up following persistent SBL and NAO+ and to spin down following persistent AR and NAO− conditions. These responses are driven by changes in deep water formation in the Labrador Sea.

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.

scholarly journals Impact of Agulhas Leakage on the Atlantic Overturning Circulation in the CCSM4

2014 ◽  
Vol 27 (1) ◽  
pp. 101-110 ◽  
Author(s):  
Wilbert Weijer ◽  
Erik van Sebille
Keyword(s):  
North Atlantic ◽  
South Atlantic ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Climate System Model ◽  
The North ◽  
Salinity Variability ◽  
Meridional Overturning ◽  
The North Atlantic ◽  
The Impact

Abstract The impact of Agulhas leakage variability on the strength of the Atlantic meridional overturning circulation (AMOC) in the Community Climate System Model, version 4 (CCSM4) is investigated. In this model an advective connection exists that transports salinity anomalies from the Agulhas region into the North Atlantic on decadal (30–40 yr) time scales. However, there is no identifiable impact of Agulhas leakage on the strength of the AMOC, suggesting that the salinity variations are too weak to significantly modify the stratification in the North Atlantic. It is argued that this study is inconclusive with respect to an impact of Agulhas leakage on the AMOC. Salinity biases leave the South Atlantic and Indian Oceans too homogeneous, in particular erasing the observed salinity front in the Agulhas retroflection region. Consequently, salinity variability in the southeastern South Atlantic is found to be much weaker than observed.

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