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

2021 ◽  
pp. 1-56
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

AbstractThere 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 and meridional overturning streamfunctions, and sea level pressure are jointly analyzed to derive the leading mode of Atlantic sector variability. This mode accounting for 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 leads to lower surface salinity and density in the sinking region, which reduces deep convection and eventually 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.

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 The Variability of the Atlantic Meridional Overturning Circulation, the North Atlantic Oscillation, and the El Niño–Southern Oscillation in the Bergen Climate Model

2005 ◽  
Vol 18 (13) ◽  
pp. 2361-2375 ◽  
Author(s):  
Juliette Mignot ◽  
Claude Frankignoul
Keyword(s):  
North Atlantic ◽  
Climate Model ◽  
Deep Convection ◽  
Meridional Overturning Circulation ◽  
Tropical Atlantic ◽  
The North ◽  
Meridional Overturning ◽  
The North Atlantic ◽  
The North Atlantic Oscillation ◽  
The Tropical Pacific

Abstract The link between the interannual to interdecadal variability of the Atlantic meridional overturning circulation (AMOC) and the atmospheric forcing is investigated using 200 yr of a control simulation of the Bergen Climate Model, where the mean circulation cell is rather realistic, as is also the location of deep convection in the northern North Atlantic. The AMOC variability has a slightly red frequency spectrum and is primarily forced by the atmosphere. The maximum value of the AMOC is mostly sensitive to the deep convection in the Irminger Sea, which it lags by about 5 yr. The latter is mostly forced by a succession of atmospheric patterns that induce anomalous northerly winds over the area. The impact of the North Atlantic Oscillation on deep convection in the Labrador and Greenland Seas is represented realistically, but its influence on the AMOC is limited to the interannual time scale and is primarily associated with wind forcing. The tropical Pacific shows a strong variability in the model, with too strong an influence on the North Atlantic. However, its influence on the tropical Atlantic is realistic. Based on lagged correlations and the release of fictitious Lagrangian drifters, the tropical Pacific seems to influence the AMOC with a time lag of about 40 yr. The mechanism is as follows: El Niño events induce positive sea surface salinity anomalies in the tropical Atlantic that are advected northward, circulate in the subtropical gyre, and then subduct. In the ocean interior, part of the salinity anomaly is advected along the North Atlantic current, eventually reaching the Irminger and Labrador Seas after about 35 yr where they destabilize the water column and favor deep convection.

scholarly journals The Mechanisms of the Atlantic Meridional Overturning Circulation Slowdown Induced by Arctic Sea Ice Decline

2019 ◽  
Vol 32 (4) ◽  
pp. 977-996 ◽  
Author(s):  
Wei Liu ◽  
Alexey Fedorov ◽  
Florian Sévellec
Keyword(s):  
Sea Ice ◽  
North Atlantic ◽  
Deep Convection ◽  
Arctic Sea Ice ◽  
Meridional Overturning Circulation ◽  
The Arctic ◽  
The North ◽  
Arctic Sea ◽  
Meridional Overturning ◽  
The North Atlantic

We explore the mechanisms by which Arctic sea ice decline affects the Atlantic meridional overturning circulation (AMOC) in a suite of numerical experiments perturbing the Arctic sea ice radiative budget within a fully coupled climate model. The imposed perturbations act to increase the amount of heat available to melt ice, leading to a rapid Arctic sea ice retreat within 5 years after the perturbations are activated. In response, the AMOC gradually weakens over the next ~100 years. The AMOC changes can be explained by the accumulation in the Arctic and subsequent downstream propagation to the North Atlantic of buoyancy anomalies controlled by temperature and salinity. Initially, during the first decade or so, the Arctic sea ice loss results in anomalous positive heat and salinity fluxes in the subpolar North Atlantic, inducing positive temperature and salinity anomalies over the regions of oceanic deep convection. At first, these anomalies largely compensate one another, leading to a minimal change in upper ocean density and deep convection in the North Atlantic. Over the following years, however, more anomalous warm water accumulates in the Arctic and spreads to the North Atlantic. At the same time, freshwater that accumulates from seasonal sea ice melting over most of the upper Arctic Ocean also spreads southward, reaching as far as south of Iceland. These warm and fresh anomalies reduce upper ocean density and suppress oceanic deep convection. The thermal and haline contributions to these buoyancy anomalies, and therefore to the AMOC slowdown during this period, are found to have similar magnitudes. We also find that the related changes in horizontal wind-driven circulation could potentially push freshwater away from the deep convection areas and hence strengthen the AMOC, but this effect is overwhelmed by mean advection.

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 Wintertime Atmospheric Response to North Atlantic Ocean Circulation Variability in a Climate Model

2015 ◽  
Vol 28 (19) ◽  
pp. 7659-7677 ◽  
Author(s):  
Claude Frankignoul ◽  
Guillaume Gastineau ◽  
Young-Oh Kwon
Keyword(s):  
Ocean Circulation ◽  
North Atlantic ◽  
Climate Model ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Atmospheric Response ◽  
Subpolar Gyre ◽  
Climate System Model ◽  
The North ◽  
The North Atlantic

Abstract Maximum covariance analysis of a preindustrial control simulation of the NCAR Community Climate System Model, version 4 (CCSM4), shows that a barotropic signal in winter broadly resembling a negative phase of the North Atlantic Oscillation (NAO) follows an intensification of the Atlantic meridional overturning circulation (AMOC) by about 7 yr. The delay is due to the cyclonic propagation along the North Atlantic Current (NAC) and the subpolar gyre of a SST warming linked to a northward shift and intensification of the NAC, together with an increasing SST cooling linked to increasing southward advection of subpolar water along the western boundary and a southward shift of the Gulf Stream (GS). These changes result in a meridional SST dipole, which follows the AMOC intensification after 6 or 7 yr. The SST changes were initiated by the strengthening of the western subpolar gyre and by bottom torque at the crossover of the deep branches of the AMOC with the NAC on the western flank of the Mid-Atlantic Ridge and the GS near the Tail of the Grand Banks, respectively. The heat flux damping of the SST dipole shifts the region of maximum atmospheric transient eddy growth southward, leading to a negative NAO-like response. No significant atmospheric response is found to the Atlantic multidecadal oscillation (AMO), which is broadly realistic but shifted south and associated with a much weaker meridional SST gradient than the AMOC fingerprint. Nonetheless, the wintertime atmospheric response to the AMOC shows some similarity with the observed response to the AMO, suggesting that the ocean–atmosphere interactions are broadly realistic in CCSM4.

Is a Decline of AMOC Causing the Warming Hole above the North Atlantic in Observed and Modeled Warming Patterns?

2012 ◽  
Vol 25 (24) ◽  
pp. 8373-8379 ◽  
Author(s):  
Sybren Drijfhout ◽  
Geert Jan van Oldenborgh ◽  
Andrea Cimatoribus
Keyword(s):  
North Atlantic ◽  
Deep Convection ◽  
Meridional Overturning Circulation ◽  
Cmip5 Models ◽  
Climate Change Scenarios ◽  
Subpolar Gyre ◽  
Nordic Seas ◽  
The North ◽  
Warming Pattern ◽  
The North Atlantic

Abstract The pattern of global mean temperature (GMT) change is calculated by regressing local surface air temperature (SAT) to GMT for an ensemble of CMIP5 models and for observations over the last 132 years. Calculations are based on the historical period and climate change scenarios. As in the observations the warming pattern contains a warming hole over the subpolar North Atlantic. Using a bivariate regression of SAT to GMT and an index of the Atlantic meridional overturning circulation (AMOC), the warming pattern is decomposed in a radiatively forced part and an AMOC fingerprint. The North Atlantic warming hole is associated with a decline of the AMOC. The AMOC fingerprint resembles Atlantic multidecadal variability (AMV), but details of the pattern change when the AMOC decline increases, underscoring the nonlinearity in the response. The warming hole is situated south of deep convection sites, indicating that it involves an adjustment of the gyre circulation, although it should be noted that some models feature deep convection in the middle of the subpolar gyre. The warming hole is already prominent in historical runs, where the response of the AMOC to GMT is weak, which suggests that it is involved in an ocean adjustment that precedes the AMOC decline. In the more strongly forced scenario runs, the warming hole over the subpolar gyre becomes weaker, while cooling over the Nordic seas increases, consistent with previous findings that deep convection in the Labrador and Irminger Seas is more vulnerable to changes in external forcing than convection in the Nordic seas, which only reacts after a threshold is passed.

scholarly journals The Effect of Northern Hemisphere Winds on the Meridional Overturning Circulation and Stratification

2018 ◽  
Vol 48 (10) ◽  
pp. 2495-2506 ◽  
Author(s):  
Paola Cessi
Keyword(s):  
Southern Ocean ◽  
Northern Hemisphere ◽  
Wind Stress ◽  
North Atlantic ◽  
Meridional Overturning Circulation ◽  
Ekman Transport ◽  
Subpolar Gyre ◽  
The North ◽  
Meridional Overturning ◽  
The North Atlantic

AbstractThe current paradigm for the meridional overturning cell and the associated middepth stratification is that the wind stress in the subpolar region of the Southern Ocean drives a northward Ekman flow, which, together with the global diapycnal mixing across the lower boundary of the middepth waters, feeds the upper branch of the interhemispheric overturning. The resulting mass transport proceeds to the Northern Hemisphere of the North Atlantic, where it sinks, to be eventually returned to the Southern Ocean at depth. Seemingly, the wind stress in the Atlantic basin plays no role. This asymmetry occurs because the Ekman transport in the Atlantic Ocean is assumed to return geostrophically at depths much shallower than those occupied by the interhemispheric overturning. However, this vertical separation fails in the North Atlantic subpolar gyre region. Using a conceptual model and an ocean general circulation model in an idealized geometry, we show that the westerly wind stress in the northern part of the Atlantic provides two opposing effects. Mechanically, the return of the Ekman transport in the North Atlantic opposes sinking in this region, reducing the total overturning and deepening the middepth stratification; thermodynamically, the subpolar gyre advects salt poleward, promoting Northern Hemisphere sinking. Depending on which mechanism prevails, increased westerly winds in the Northern Hemisphere can reduce or augment the overturning.

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