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 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.

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 Standing Eddies in the Meridional Overturning Circulation

2012 ◽  
Vol 42 (9) ◽  
pp. 1486-1508 ◽  
Author(s):  
Jan Viebahn ◽  
Carsten Eden
Keyword(s):  
Surface Layer ◽  
Southern Ocean ◽  
Ocean Model ◽  
Meridional Overturning Circulation ◽  
Transient Eddy ◽  
Meridional Transport ◽  
Overturning Circulation ◽  
Depth Contour ◽  
Meridional Overturning

Abstract The role of standing eddies for the meridional overturning circulation (MOC) is discussed. The time-mean isopycnal meridional streamfunction is decomposed into a time- and zonal-mean part, a standing-eddy part, and a transient-eddy part. It turns out that the construction of an isopycnal MOC with an exactly vanishing standing-eddy part has to be performed by zonal integration along depth-dependent horizontal isolines of time-mean density. In contrast, zonal integration along time-mean geostrophic streamlines generally only leads to an isopycnal MOC with a reduced standing-eddy part. A generalized approach of constructing meridional transport streamfunctions by two tracer fields and the generalized way to neutralize the corresponding standing-eddy part is given to summarize the discussion. Using the results of an idealized Southern Ocean model, it is demonstrated that neglecting the depth dependence of the zonal integration paths by integrating along density contours or geostrophic streamlines of a fixed depth (“contour depth”) may represent an acceptable approximation: although the standing-eddy part then exactly vanishes only at the contour depth (except for the ageostrophic surface layer using geostrophic streamlines), the overall standing-eddy part is significantly reduced for adequate contour depths. In the idealized Southern Ocean model, density contours at middepth and surface geostrophic streamlines represent the most adequate approximations. Moreover, it is found that the effect of changing the zonal integration paths from latitude circles to curvilinear paths on the zonally averaged density is of the same order as changing from Eulerian to isopycnal averaging.

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 Southern Ocean Heat Uptake, Redistribution, and Storage in a Warming Climate: The Role of Meridional Overturning Circulation

2018 ◽  
Vol 31 (12) ◽  
pp. 4727-4743 ◽  
Author(s):  
Wei Liu ◽  
Jian Lu ◽  
Shang-Ping Xie ◽  
Alexey Fedorov
Keyword(s):  
Southern Ocean ◽  
Heat Transport ◽  
Climate Models ◽  
Climate Model ◽  
Meridional Overturning Circulation ◽  
The Other ◽  
Anthropogenic Heat ◽  
Overturning Circulation ◽  
Ocean Heat Uptake ◽  
Meridional Overturning

Climate models show that most of the anthropogenic heat resulting from increased atmospheric CO2 enters the Southern Ocean near 60°S and is stored around 45°S. This heat is transported to the ocean interior by the meridional overturning circulation (MOC) with wind changes playing an important role in the process. To isolate and quantify the latter effect, we apply an overriding technique to a climate model and decompose the total ocean response to CO2 increase into two major components: one due to wind changes and the other due to direct CO2 effect. We find that the poleward-intensified zonal surface winds tend to shift and strengthen the ocean Deacon cell and hence the residual MOC, leading to anomalous divergence of ocean meridional heat transport around 60°S coupled to a surface heat flux increase. In contrast, at 45°S we see anomalous convergence of ocean heat transport and heat loss at the surface. As a result, the wind-induced ocean heat storage (OHS) peaks at 46°S at a rate of 0.07 ZJ yr−1 (° lat)−1 (1 ZJ = 1021 J), contributing 20% to the total OHS maximum. The direct CO2 effect, on the other hand, very slightly alters the residual MOC but primarily warms the ocean. It induces a small but nonnegligible change in eddy heat transport and causes OHS to peak at 42°S at a rate of 0.30 ZJ yr−1 (° lat)−1, accounting for 80% of the OHS maximum. We also find that the eddy-induced MOC weakens, primarily caused by a buoyancy flux change as a result of the direct CO2 effect, and does not compensate the intensified Deacon cell.

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