scholarly journals On the Relationship between Southern Ocean Overturning and ACC Transport

2013 ◽  
Vol 43 (1) ◽  
pp. 140-148 ◽  
Author(s):  
Adele K. Morrison ◽  
Andrew McC. Hogg
Keyword(s):  
Southern Ocean ◽  
Wind Stress ◽  
Large Scale ◽  
Meridional Overturning Circulation ◽  
Meridional Transport ◽  
Stress Changes ◽  
Transport Response ◽  
Meridional Overturning ◽  
Circumpolar Current ◽  
The Impact

Abstract The eddy field in the Southern Ocean offsets the impact of strengthening winds on the meridional overturning circulation and Antarctic Circumpolar Current (ACC) transport. There is widespread belief that the sensitivities of the overturning and ACC transport are dynamically linked, with limitation of the ACC transport response implying limitation of the overturning response. Here, an idealized numerical model is employed to investigate the response of the large-scale circulation in the Southern Ocean to wind stress perturbations at eddy-permitting to eddy-resolving scales. Significant differences are observed between the sensitivities and the resolution dependence of the overturning and ACC transport, indicating that they are controlled by distinct dynamical mechanisms. The modeled overturning is significantly more sensitive to change than the ACC transport, with the possible implication that the Southern Ocean overturning may increase in response to future wind stress changes without measurable changes in the ACC transport. It is hypothesized that the dynamical distinction between the zonal and meridional transport sensitivities is derived from the depth dependence of the extent of cancellation between the Ekman and eddy-induced transports.

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.

scholarly journals Diagnosing the Southern Ocean Overturning from Tracer Fields

2009 ◽  
Vol 39 (11) ◽  
pp. 2926-2940 ◽  
Author(s):  
Jan D. Zika ◽  
Bernadette M. Sloyan ◽  
Trevor J. McDougall
Keyword(s):  
Southern Ocean ◽  
Vertical Mixing ◽  
Meridional Overturning Circulation ◽  
Meridional Transport ◽  
East Indian ◽  
Density Distributions ◽  
Mixing Coefficient ◽  
Mixing Rates ◽  
Meridional Overturning ◽  
Circumpolar Current

Abstract The strength and structure of the Southern Hemisphere meridional overturning circulation (SMOC) is related to the along-isopycnal and vertical mixing coefficients by analyzing tracer and density fields from a hydrographic climatology. The meridional transport of Upper Circumpolar Deep Water (UCDW) across the Antarctic Circumpolar Current (ACC) is expressed in terms of the along-isopycnal (K) and diapycnal (D) tracer diffusivities and in terms of the along-isopycnal potential vorticity mixing coefficient (KPV). Uniform along-isopycnal (<600 m2 s−1) and low vertical mixing (10−5 m2 s−1) can maintain a southward transport of less than 60 Sv (Sv = 106 m2 s−1) of UCDW across the ACC, which is distributed largely across the South Pacific and east Indian Ocean basins. For vertical mixing rates of O(10−4 m2 s−1) or greater, the inferred transport is significantly enhanced. The transports inferred from both tracer and density distributions suggest a ratio K to D of O(2 × 106) particularly on deeper layers of UCDW. Given the range of observed southward transports of UCDW, it is found that K = 300 ± 150 m2 s−1 and D = 10−4 ± 0.5 × 10−4 m2 s−1 in the Southern Ocean interior. A view of the SMOC is revealed where dense waters are converted to lighter waters not only at the ocean surface, but also on depths below that of the mixed layer with vertical mixing playing an important role.

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 Semi-Adiabatic Model of the Deep Stratification and Meridional Overturning

2011 ◽  
Vol 41 (4) ◽  
pp. 757-780 ◽  
Author(s):  
Timour Radko ◽  
Igor Kamenkovich
Keyword(s):  
Large Scale ◽  
Formation Rate ◽  
Three Dimensional ◽  
Meridional Overturning Circulation ◽  
Small Scale ◽  
Western Boundary ◽  
Meridional Transport ◽  
The North ◽  
Meridional Overturning ◽  
Circumpolar Current

An analytical model of the Atlantic deep stratification and meridional overturning circulation is presented that illustrates the dynamic coupling between the Southern Ocean and the midlatitude gyres. The model, expressed here in terms of the two-and-a-half-layer framework, predicts the stratification and meridional transport as a function of the mechanical and thermodynamic forcing at the sea surface. The approach is based on the classical elements of large-scale circulation theory—ideal thermocline, inertial western boundary currents, and eddy-controlled Antarctic Circumpolar Current (ACC) models—which are combined to produce a consistent three-dimensional view of the global overturning. The analytical tractability is achieved by assuming and subsequently verifying that the pattern of circulation in the model is largely controlled by adiabatic processes: the time-mean and eddy-induced isopycnal advection of buoyancy. The mean stratification of the lower thermocline is determined by the surface forcing in the ACC and, to a lesser extent, by the North Atlantic Deep Water formation rate. Although the vertical small-scale mixing and the diapycnal eddy-flux components can substantially influence the magnitude of overturning, their effect on the net stratification of the midlatitude ocean is surprisingly limited. The analysis in this paper suggests the interpretation of the ACC as an active lateral boundary layer that does not passively adjust to the prescribed large-scale solution but instead forcefully controls the interior pattern.

scholarly journals Using Eulerian and Lagrangian Approaches to Investigate Wind-Driven Changes in the Southern Ocean Abyssal Circulation

2013 ◽  
Vol 44 (2) ◽  
pp. 662-675 ◽  
Author(s):  
Paul Spence ◽  
Erik van Sebille ◽  
Oleg A. Saenko ◽  
Matthew H. England
Keyword(s):  
Southern Ocean ◽  
Wind Stress ◽  
Climate Model ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
Lagrangian Particle ◽  
Stress Changes ◽  
Abyssal Circulation ◽  
Flow Pathways ◽  
Ocean Wind

Abstract This study uses a global ocean eddy-permitting climate model to explore the export of abyssal water from the Southern Ocean and its sensitivity to projected twenty-first-century poleward-intensifying Southern Ocean wind stress. The abyssal flow pathways and transport are investigated using a combination of Lagrangian and Eulerian techniques. In an Eulerian format, the equator- and poleward flows within similar abyssal density classes are increased by the wind stress changes, making it difficult to explicitly diagnose changes in the abyssal export in a meridional overturning circulation framework. Lagrangian particle analyses are used to identify the major export pathways of Southern Ocean abyssal waters and reveal an increase in the number of particles exported to the subtropics from source regions around Antarctica in response to the wind forcing. Both the Lagrangian particle and Eulerian analyses identify transients as playing a key role in the abyssal export of water from the Southern Ocean. Wind-driven modifications to the potential energy component of the vorticity balance in the abyss are also found to impact the Southern Ocean barotropic circulation.

Reduced Stability of the Atlantic Meridional Overturning Circulation due to Wind Stress Feedback during Glacial Times

2008 ◽  
Vol 21 (23) ◽  
pp. 6260-6282 ◽  
Author(s):  
Olivier Arzel ◽  
Matthew H. England ◽  
Willem P. Sijp
Keyword(s):  
Sea Ice ◽  
Wind Stress ◽  
Surface Wind ◽  
Meridional Overturning Circulation ◽  
Nonlinear Dependence ◽  
Upper Ocean ◽  
Overturning Circulation ◽  
Climate Conditions ◽  
Meridional Overturning ◽  
The Impact

Abstract A previous study by Mikolajewicz suggested that the wind stress feedback stabilizes the Atlantic thermohaline circulation. This result was obtained under modern climate conditions, for which the presence of the massive continental ice sheets characteristic of glacial times is missing. Here a coupled ocean–atmosphere–sea ice model of intermediate complexity, set up in an idealized spherical sector geometry of the Atlantic basin, is used to show that, under glacial climate conditions, wind stress feedback actually reduces the stability of the meridional overturning circulation (MOC). The analysis reveals that the influence of the wind stress feedback on the glacial MOC response to an external source of freshwater applied at high northern latitudes is controlled by the following two distinct processes: 1) the interactions between the wind field and the sea ice export in the Northern Hemisphere (NH), and 2) the northward Ekman transport in the tropics and upward Ekman pumping in the core of the NH subpolar gyre. The former dominates the response of the coupled system; it delays the recovery of the MOC, and in some cases even stabilizes collapsed MOC states achieved during the hosing period. The latter plays a minor role and mitigates the impact of the former process by reducing the upper-ocean freshening in deep-water formation regions. Hence, the wind stress feedback delays the recovery of the glacial MOC, which is the opposite of what occurs under modern climate conditions. Close to the critical transition threshold beyond which the circulation collapses, the glacial MOC appears to be very sensitive to changes in surface wind stress forcing and exhibits, in the aftermath of the freshwater pulse, a nonlinear dependence upon the wind stress feedback magnitude: a complete and irreversible MOC shutdown occurs only for intermediate wind stress feedback magnitudes. This behavior results from the competitive effects of processes 1 and 2 on the midlatitude upper-ocean salinity during the shutdown phase of the MOC. The mechanisms presented here may be relevant to the large meltwater pulses that punctuated the last glacial period.

scholarly journals Improving Estimates of the Antarctic Circumpolar Current Streamlines in Drake Passage

2008 ◽  
Vol 38 (5) ◽  
pp. 1000-1010 ◽  
Author(s):  
Yueng-Djern Lenn ◽  
Teresa K. Chereskin ◽  
Janet Sprintall
Keyword(s):  
Southern Ocean ◽  
Antarctic Circumpolar Current ◽  
Acoustic Doppler Current Profiler ◽  
Drake Passage ◽  
Meridional Overturning Circulation ◽  
Dynamic Height ◽  
Current Profiler ◽  
Meridional Overturning ◽  
The Mean ◽  
Circumpolar Current

Abstract Accurately resolving the mean Antarctic Circumpolar Current (ACC) is essential for determining Southern Ocean eddy fluxes that are important to the global meridional overturning circulation. Previous estimates of the mean ACC have been limited by the paucity of Southern Ocean observations. A new estimate of the mean surface ACC in Drake Passage is presented that combines sea surface height anomalies measured by satellite altimetry with a recent dataset of repeat high-resolution acoustic Doppler current profiler observations. A mean streamfunction (surface height field), objectively mapped from the mean currents, is used to validate two recent dynamic height climatologies. The new streamfunction has narrower and stronger ACC fronts separated by quiescent zones of much weaker flow, thereby improving on the resolution of ACC fronts observed in the other climatologies. Distinct streamlines can be associated with particular ACC fronts and tracked in time-dependent maps of dynamic height. This analysis shows that varying degrees of topographic control are evident in the preferred paths of the ACC fronts through Drake Passage.

Southern Ocean Circulation and Eddy Compensation in CMIP5 Models

2013 ◽  
Vol 26 (18) ◽  
pp. 7198-7220 ◽  
Author(s):  
Stephanie M. Downes ◽  
Andrew McC. Hogg
Keyword(s):  
Southern Ocean ◽  
Wind Stress ◽  
Surface Wind ◽  
Surface Heat ◽  
Meridional Overturning Circulation ◽  
Cmip5 Models ◽  
Strong Increase ◽  
Coupled Models ◽  
Overturning Circulation ◽  
Meridional Overturning

Abstract Thirteen state-of-the-art climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) are used to evaluate the response of the Antarctic Circumpolar Current (ACC) transport and Southern Ocean meridional overturning circulation to surface wind stress and buoyancy changes. Understanding how these flows—fundamental players in the global distribution of heat, gases, and nutrients—respond to climate change is currently a widely debated issue among oceanographers. Here, the authors analyze the circulation responses of these coarse-resolution coupled models to surface fluxes. Under a future CMIP5 climate pathway where the equivalent atmospheric CO2 reaches 1370 ppm by 2100, the models robustly project reduced Southern Ocean density in the upper 2000 m accompanied by strengthened stratification. Despite an overall increase in overlying wind stress (~20%), the projected ACC transports lie within ±15% of their historical state, and no significant relationship with changes in the magnitude or position of the wind stress is identified. The models indicate that a weakening of ACC transport at the end of the twenty-first century is correlated with a strong increase in the surface heat and freshwater fluxes in the ACC region. In contrast, the surface heat gain across the ACC region and the wind-driven surface transports are significantly correlated with an increased upper and decreased lower Eulerian-mean meridional overturning circulation. The change in the eddy-induced overturning in both the depth and density spaces is quantified, and it is found that the CMIP5 models project partial eddy compensation of the upper and lower overturning cells.

The impact of topography and eddy parameterization on the simulated Southern Ocean circulation response to changes in surface wind stress

2020 ◽  
Author(s):  
Hailu Kong ◽  
Malte F. Jansen
Keyword(s):  
Southern Ocean ◽  
Wind Stress ◽  
Ocean Circulation ◽  
Surface Wind ◽  
Flat Bottom ◽  
Coarse Resolution ◽  
Surface Wind Stress ◽  
Stress Changes ◽  
The Impact ◽  
Meso Scale

AbstractIt remains uncertain how the Southern Ocean circulation responds to changes in surface wind stress, and whether coarse resolution simulations, where meso-scale eddy fluxes are parameterized, can adequately capture the response. We address this problem using two idealized model setups mimicking the Southern Ocean: a flat bottom channel, and a channel with moderately complex topography. Under each topographic configuration and varying wind stress, we compare several coarse resolution simulations, configured with different eddy parameterizations, against an eddy-resolving simulation. We find that: (1) without topography, sensitivity of the Antarctic Circumpolar Current (ACC) to wind stress is overestimated by coarse resolution simulations, due to an underestimate of the sensitivity of the eddy diffusivity; (2) in the presence of topography, stationary eddies dominate over transient eddies in counteracting the direct response of the ACC and overturning circulation to wind stress changes; (3) coarse resolution simulations with parameterized eddies capture this counteracting effect reasonably well, largely due to their ability to resolve stationary eddies. Our results highlight the importance of topography in modulating the response of the Southern Ocean circulation to changes in surface wind stress. The interaction between meso-scale eddies and stationary meanders induced by topography requires more attention in future development and testing of eddy parameterizations.

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