scholarly journals What Sets the Strength of the Middepth Stratification and Overturning Circulation in Eddying Ocean Models?

2010 ◽  
Vol 40 (7) ◽  
pp. 1520-1538 ◽  
Author(s):  
Christopher L. Wolfe ◽  
Paola Cessi
Keyword(s):  
General Circulation ◽  
Large Scale ◽  
World Ocean ◽  
Circulation Model ◽  
Volume Transport ◽  
Meridional Overturning Circulation ◽  
Meridional Transport ◽  
Overturning Circulation ◽  
Meridional Overturning ◽  
Circumpolar Current

Abstract The processes maintaining stratification in the oceanic middepth (between approximately 1000 and 3000 m) are explored using an eddy-resolving general circulation model composed of a two-hemisphere, semienclosed basin with a zonal reentrant channel in the southernmost eighth of the domain. The middepth region lies below the wind-driven main thermocline but above the diffusively driven abyssal ocean. Here, it is argued that middepth stratification is determined primarily in the model’s Antarctic Circumpolar Current. Competition between mean and eddy overturning in the channel leads to steeper isotherms and thus deeper stratification throughout the basin than would exist without the channel. Isotherms that outcrop only in the channel are nearly horizontal in the semienclosed portion of the domain, whereas isotherms that also outcrop in the Northern Hemisphere deviate from horizontal and are accompanied by geostrophically balanced meridional transport. A northern source of deep water (water with temperatures in the range of those in the channel) leads to the formation of a thick middepth thermostad. Changes in wind forcing over the channel influence the stratification throughout the domain. Since the middepth stratification is controlled by adiabatic dynamics in the channel, it becomes independent of the interior diffusivity κ as κ → 0. The meridional overturning circulation (MOC), as diagnosed by the mean meridional volume transport, also shows a tendency to become independent of κ as κ → 0, whereas the MOC diagnosed by water mass transport shows a continuing dependence on κ as κ → 0. A nonlocal scaling for MOC is developed that relates the strength of the northern MOC to the depth of isotherms in the southern channel. The results of this paper compare favorably to observations of large-scale neutral density in the World Ocean.

scholarly journals Influence of the Multidecadal Atlantic Meridional Overturning Circulation Variability on European Climate

2006 ◽  
Vol 19 (23) ◽  
pp. 6062-6067 ◽  
Author(s):  
Holger Pohlmann ◽  
Frank Sienz ◽  
Mojib Latif
Keyword(s):  
General Circulation ◽  
Large Scale ◽  
Circulation Model ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Max Planck ◽  
Overturning Circulation ◽  
Multidecadal Variability ◽  
European Climate ◽  
Meridional Overturning

Abstract The influence of the natural multidecadal variability of the Atlantic meridional overturning circulation (MOC) on European climate is investigated using a simulation with the coupled atmosphere–ocean general circulation model ECHAM5/Max Planck Institute Ocean Model (MPI-OM). The results show that Atlantic MOC fluctuations, which go along with changes in the northward heat transport, in turn affect European climate. Additionally, ensemble predictability experiments with ECHAM5/MPI-OM show that the probability density functions of surface air temperatures in the North Atlantic/European region are affected by the multidecadal variability of the large-scale oceanic circulation. Thus, some useful decadal predictability may exist in the Atlantic/European sector.

scholarly journals On the Near-Inertial Resonance of the Atlantic Meridional Overturning Circulation

2013 ◽  
Vol 43 (12) ◽  
pp. 2661-2672 ◽  
Author(s):  
Florian Sévellec ◽  
Joël J.-M. Hirschi ◽  
Adam T. Blaker
Keyword(s):  
General Circulation ◽  
Numerical Models ◽  
Circulation Model ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Resonance Phenomenon ◽  
Dimensionless Number ◽  
Overturning Circulation ◽  
Wide Range ◽  
Meridional Overturning

Abstract The Atlantic meridional overturning circulation (AMOC) is a crucial component of the global climate system. It is responsible for around a quarter of the global northward heat transport and contributes to the mild European climate. Observations and numerical models suggest a wide range of AMOC variability. Recent results from an ocean general circulation model (OGCM) in a high-resolution configuration (¼°) suggest the existence of superinertial variability of the AMOC. In this study, the validity of this result in a theoretical framework is tested. At a low Rossby number and in the presence of Rayleigh friction, it is demonstrated that, unlike a typical forced damped oscillator (which shows subinertial resonance), the AMOC undergoes both super- and subinertial resonances (except at low latitudes and for high friction). A dimensionless number Sr, measuring the ratio of ageo- to geostrophic forcing (i.e., the zonal versus meridional pressure gradients), indicates which of these resonances dominates. If Sr ≪ 1, the AMOC variability is mainly driven by geostrophic forcing and shows subinertial resonance. Alternatively and consistent with the recently published ¼° OGCM experiments, if Sr ≫ 1, the AMOC variability is mainly driven by the ageostrophic forcing and shows superinertial resonance. In both regimes, a forcing of ±1 K induces an AMOC variability of ±10 Sv (1 Sv ≡ 106 m3 s−1) through these near-inertial resonance phenomena. It is also shown that, as expected from numerical simulations, the spatial structure of the near-inertial AMOC variability corresponds to equatorward-propagating waves equivalent to baroclinic Poincaré waves. The long-time average of this resonance phenomenon, raising and depressing the pycnocline, could contribute to the mixing of the ocean stratification.

A Theory of the Interhemispheric Meridional Overturning Circulation and Associated Stratification

2012 ◽  
Vol 42 (10) ◽  
pp. 1652-1667 ◽  
Author(s):  
Maxim Nikurashin ◽  
Geoffrey Vallis
Keyword(s):  
Deep Water ◽  
General Circulation ◽  
Mesoscale Eddy ◽  
Circulation Model ◽  
Meridional Overturning Circulation ◽  
Minor Role ◽  
Diapycnal Mixing ◽  
Overturning Circulation ◽  
Northern Latitudes ◽  
Meridional Overturning

Abstract A quantitative theoretical model of the meridional overturning circulation and associated deep stratification in an interhemispheric, single-basin ocean with a circumpolar channel is presented. The theory includes the effects of wind, eddies, and diapycnal mixing and predicts the deep stratification and overturning streamfunction in terms of the surface forcing and other parameters of the problem. It relies on a matching among three regions: the circumpolar channel at high southern latitudes, a region of isopycnal outcrop at high northern latitudes, and the ocean basin between. The theory describes both the middepth and abyssal cells of a circulation representing North Atlantic Deep Water and Antarctic Bottom Water. It suggests that, although the strength of the middepth overturning cell is primarily set by the wind stress in the circumpolar channel, middepth stratification results from a balance between the wind-driven upwelling in the channel and deep-water formation at high northern latitudes. Diapycnal mixing in the ocean interior can lead to warming and upwelling of deep waters. However, for parameters most representative of the present ocean mixing seems to play a minor role for the middepth cell. In contrast, the abyssal cell is intrinsically diabatic and controlled by a balance between the deep mixing-driven upwelling and the residual of the wind-driven and eddy-induced circulations in the Southern Ocean. The theory makes explicit predictions about how the stratification and overturning circulation vary with the wind strength, diapycnal diffusivity, and mesoscale eddy effects. The predictions compare well with numerical results from a coarse-resolution general circulation model.

scholarly journals An Interhemispheric Four-Box Model of the Meridional Overturning Circulation

2011 ◽  
Vol 41 (3) ◽  
pp. 516-530 ◽  
Author(s):  
Peter H. Stone ◽  
Yuriy P. Krasovskiy
Keyword(s):  
High Latitude ◽  
General Circulation ◽  
Circulation Model ◽  
Meridional Overturning Circulation ◽  
High Latitudes ◽  
Stable States ◽  
Low Latitude ◽  
Overturning Circulation ◽  
High Latitude Region ◽  
Meridional Overturning

Abstract The authors introduce a four-box interhemispheric model of the meridional overturning circulation. A single box represents high latitudes in each hemisphere, and in contrast to earlier interhemispheric box models, low latitudes are represented by two boxes—a surface box and a deep box—separated by a thermocline in which a balance is assumed between vertical advection and vertical diffusion. The behavior of the system is analyzed with two different closure assumptions for how the low-latitude upwelling depends on the density contrast between the surface and deep low-latitude boxes. The first is based on the conventional assumption that the diffusivity is a constant, and the second on the assumption that the energy input to the mixing is constant. There are three different stable equilibrium states that are closely analogous to the three found by Bryan in a single-basin interhemispheric ocean general circulation model. One is quasi-symmetric with downwelling in high latitudes of both hemispheres, and two are asymmetric solutions, with downwelling confined to high latitudes in one or the other of the two hemispheres. The quasi-symmetric solution becomes linearly unstable for strong global hydrological forcing, while the two asymmetric solutions do not. The qualitative nature of the solutions is generally similar for both the closure assumptions, in contrast to the solutions in hemispheric models. In particular, all the stable states can be destabilized by finite amplitude perturbations in the salinity or the hydrological forcing, and transitions are possible between any two states. For example, if the system is in an asymmetric state, and the moisture flux into the high-latitude region of downwelling is slowly increased, for both closure assumptions the high-latitude downwelling decreases until a critical forcing is reached where the system switches to the asymmetric state with downwelling in the opposite hemisphere. By contrast, in hemispheric models with the energy constraint, the downwelling increases and there is no loss of stability.

scholarly journals The Force Balance of the Southern Ocean Meridional Overturning Circulation

2013 ◽  
Vol 43 (6) ◽  
pp. 1193-1208 ◽  
Author(s):  
Matthew R. Mazloff ◽  
Raffaele Ferrari ◽  
Tapio Schneider
Keyword(s):  
Southern Ocean ◽  
Drake Passage ◽  
Meridional Overturning Circulation ◽  
Force Balance ◽  
Pressure Gradients ◽  
Meridional Transport ◽  
Overturning Circulation ◽  
Mean Pressure ◽  
Meridional Overturning ◽  
Circumpolar Current

Abstract The Southern Ocean (SO) limb of the meridional overturning circulation (MOC) is characterized by three vertically stacked cells, each with a transport of about 10 Sv (Sv ≡ 106 m3 s−1). The buoyancy transport in the SO is dominated by the upper and middle MOC cells, with the middle cell accounting for most of the buoyancy transport across the Antarctic Circumpolar Current. A Southern Ocean state estimate for the years 2005 and 2006 with ⅙° resolution is used to determine the forces balancing this MOC. Diagnosing the zonal momentum budget in density space allows an exact determination of the adiabatic and diapycnal components balancing the thickness-weighted (residual) meridional transport. It is found that, to lowest order, the transport consists of an eddy component, a directly wind-driven component, and a component in balance with mean pressure gradients. Nonvanishing time-mean pressure gradients arise because isopycnal layers intersect topography or the surface in a circumpolar integral, leading to a largely geostrophic MOC even in the latitude band of Drake Passage. It is the geostrophic water mass transport in the surface layer where isopycnals outcrop that accomplishes the poleward buoyancy transport.

Wind-Driven Seasonal Cycle of the Atlantic Meridional Overturning Circulation

2014 ◽  
Vol 44 (6) ◽  
pp. 1541-1562 ◽  
Author(s):  
Jian Zhao ◽  
William Johns
Keyword(s):  
Seasonal Cycle ◽  
General Circulation ◽  
Numerical Models ◽  
Circulation Model ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Ekman Transport ◽  
Western Boundary ◽  
Overturning Circulation ◽  
Meridional Overturning

Abstract The dynamical processes governing the seasonal cycle of the Atlantic meridional overturning circulation (AMOC) are studied using a variety of models, ranging from a simple forced Rossby wave model to an eddy-resolving ocean general circulation model. The AMOC variability is decomposed into Ekman and geostrophic transport components, which reveal that the seasonality of the AMOC is determined by both components in the extratropics and dominated by the Ekman transport in the tropics. The physics governing the seasonal fluctuations of the AMOC are explored in detail at three latitudes (26.5°N, 6°N, and 34.5°S). While the Ekman transport is directly related to zonal wind stress seasonality, the comparison between different numerical models shows that the geostrophic transport involves a complex oceanic adjustment to the wind forcing. The oceanic adjustment is further evaluated by separating the zonally integrated geostrophic transport into eastern and western boundary currents and interior flows. The results indicate that the seasonal AMOC cycle in the extratropics is controlled mainly by local boundary effects, where either the western or eastern boundary can be dominant at different latitudes, while in the northern tropics it is the interior flow and its lagged compensation by the western boundary current that determine the seasonal AMOC variability.

scholarly journals Meridional Overturning Circulation in a multi-basin model. Part II: Sensitivity to diffusivity and wind in warm and cool climates

2021 ◽  
Author(s):  
Jonathan A. Baker ◽  
Andrew J. Watson ◽  
Geoffrey K. Vallis
Keyword(s):  
Southern Ocean ◽  
Wind Stress ◽  
General Circulation ◽  
Circulation Model ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Buoyancy Forcing ◽  
The Pacific ◽  
Primary Determinant ◽  
Meridional Overturning

AbstractThe response of the meridional overturning circulation (MOC) to changes in Southern Ocean (SO) zonal wind forcing and Pacific basin vertical diffusivity is investigated under varying buoyancy forcings, corresponding to ‘warm’, ‘present-day’ and ‘cold’ states, in a two-basin general circulation model connected by a southern circumpolar channel. We find that the Atlantic MOC (AMOC) strengthens with increased SO wind stress or diffusivity in the model Pacific, under all buoyancy forcings. The sensitivity of the AMOC to wind stress increases as the buoyancy forcing is varied from a warm to a present-day or cold state, whereas it is most sensitive to the Pacific diffusivity in a present-day or warm state. Similarly, the AMOC is more sensitive to buoyancy forcing over the Southern Ocean under reduced wind stress or enhanced Pacific diffusivity. These results arise because of the increased importance of the Pacific pathway in the warmer climates, giving an increased linkage between the basins and so the opportunity for the diffusivity in the Pacific to affect the overturning in the Atlantic. In cooler states, such as in glacial climates, the two basins are largely decoupled and the wind strength over the SO is the primary determinant of the AMOC strength. Both wind- and diffusively-driven upwelling sustain the AMOC in the warmer (present-day) state. Changes in SO wind stress alone do not shoal the AMOC to resemble that observed at the last glacial maximum; changes in the buoyancy forcing are also needed to decouple the two basins.

scholarly journals The Onset of the Last Interglacial and Fingerprints of Transient Atlantic Meridional Overturning Circulation

2021 ◽  
Author(s):  
Paul Gierz ◽  
Gregor Knorr ◽  
Aline Govin ◽  
Emilie Capron ◽  
Nadezhda Sokolova ◽  
...  
Keyword(s):  
North Atlantic ◽  
General Circulation ◽  
Circulation Model ◽  
Atlantic Meridional Overturning Circulation ◽  
Ocean General Circulation Model ◽  
Meridional Overturning Circulation ◽  
Last Interglacial ◽  
Overturning Circulation ◽  
Climate Conditions ◽  
Meridional Overturning

Abstract Transitions from glacials to interglacials are the largest climate shifts that occurred during the Quaternary. These glacial terminations are characterized by transient changes in the Atlantic Meridional Overturning Circulation (AMOC) and associated alterations in the northward heat transport. It has been a challenge to differentiate between early last interglacial or late penultimate glacial climate conditions at 129-131 ka (thousands of years before present). Neither simulations with a stable interglacial-type nor with a freshwater perturbed AMOC state have reproduced the reconstructed sea surface temperature (SST) fingerprint in the North Atlantic. As previous approaches failed to consider the highly transient nature of the climate system at ~130 ka, the potential of transient, deglaciating AMOC responses and the corresponding impact on North Atlantic SST has yet to be examined. In this study, we employ a fully coupled Atmosphere-Ocean General Circulation Model (AOGCM) equipped with a stable-oxygen isotope module to investigate the underlying AMOC dynamics at the onset of the Last Interglacial (LIG). We demonstrate that successfully capturing both the SST patterns and the calcite δ18O signature in planktonic foraminifera from North Atlantic marine sediment cores necessitates a transiently recovering AMOC. Furthermore, this critically depends on capturing the cold, glacial ocean state prior to the onset of the interglacial.

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