What Sets the Surface Eddy Mass Flux in the Southern Ocean?

2005 ◽  
Vol 35 (11) ◽  
pp. 2152-2166 ◽  
Author(s):  
S. S. Drijfhout
Keyword(s):  
Southern Ocean ◽  
Ocean Circulation ◽  
Mass Flux ◽  
General Circulation ◽  
Circulation Model ◽  
Ekman Transport ◽  
Thermocline Depth ◽  
Mean Flow ◽  
Rotational Components ◽  
Eddy Fluxes

Abstract The Ocean Circulation and Climate Advanced Modelling (OCCAM) global, eddy-permitting ocean general circulation model has been used to investigate the surface eddy mass flux in the Southern Ocean. The isopycnal eddy mass flux in the surface layer is almost uniformly poleward and scales well with the local Ekman transport. This seems at odds with other models and observations suggesting topographic localization of the eddy fluxes with locally, large rotational components. Integrated over the thermocline depth the eddy fluxes do show such topographic localization. The surface eddy mass flux is mainly a consequence of the intermittent deepening of the mixed layer with the seasonal cycle, which redistributes the Ekman transport over the stack of layers that eventually become ventilated. Baroclinic instability gives rise to much smaller eddy-induced transports. Independent of the framework in which the residual mean flow is analyzed (isopycnal or geometric), the eddy-induced transport that opposes the wind-driven Ekman flow only partially compensates the Deacon cell. The associated overturning cell is about 5 Sv (where 1 Sv ≡ 106 m3 s−1), responsible for a cancellation of the Deacon cell of 30%. In geometric coordinates, a strong signature (14 Sv) of the Deacon cell remains for the residual mean flow. Only after transformation to density coordinates is a further reduction with 10 Sv obtained. Zonal tilting of isopycnals makes along-isopycnal recirculations appear as vertical overturning cells in geometric coordinates. These cells disappear in the isopycnal framework without any eddy-induced transport being involved.

On the impact of sea ice in a global ocean circulation model

1997 ◽  
Vol 25 ◽  
pp. 111-115 ◽  
Author(s):  
Achim Stössel
Keyword(s):  
Southern Ocean ◽  
Sea Ice ◽  
Ocean Circulation ◽  
General Circulation ◽  
Thermohaline Circulation ◽  
Circulation Model ◽  
Deep Ocean ◽  
Global Ocean ◽  
Convective Activity ◽  
The Impact

This paper investigates the long-term impact of sea ice on global climate using a global sea-ice–ocean general circulation model (OGCM). The sea-ice component involves state-of-the-art dynamics; the ocean component consists of a 3.5° × 3.5° × 11 layer primitive-equation model. Depending on the physical description of sea ice, significant changes are detected in the convective activity, in the hydrographic properties and in the thermohaline circulation of the ocean model. Most of these changes originate in the Southern Ocean, emphasizing the crucial role of sea ice in this marginally stably stratified region of the world's oceans. Specifically, if the effect of brine release is neglected, the deep layers of the Southern Ocean warm up considerably; this is associated with a weakening of the Southern Hemisphere overturning cell. The removal of the commonly used “salinity enhancement” leads to a similar effect. The deep-ocean salinity is almost unaffected in both experiments. Introducing explicit new-ice thickness growth in partially ice-covered gridcells leads to a substantial increase in convective activity, especially in the Southern Ocean, with a concomitant significant cooling and salinification of the deep ocean. Possible mechanisms for the resulting interactions between sea-ice processes and deep-ocean characteristics are suggested.

scholarly journals A Geometric Interpretation of Southern Ocean Eddy Form Stress

2019 ◽  
Vol 49 (10) ◽  
pp. 2553-2570 ◽  
Author(s):  
Mads B. Poulsen ◽  
Markus Jochum ◽  
James R. Maddison ◽  
David P. Marshall ◽  
Roman Nuterman
Keyword(s):  
Southern Ocean ◽  
General Circulation ◽  
Vertical Structure ◽  
Circulation Model ◽  
Ocean General Circulation Model ◽  
Geometric Interpretation ◽  
Mean Flow ◽  
Geometric Framework ◽  
The Mean ◽  
Circumpolar Current

AbstractAn interpretation of eddy form stress via the geometry described by the Eliassen–Palm flux tensor is explored. Complimentary to previous works on eddy Reynolds stress geometry, this study shows that eddy form stress is fully described by a vertical ellipse, whose size, shape, and orientation with respect to the mean flow shear determine the strength and direction of vertical momentum transfers. Following a recent proposal, this geometric framework is here used to form a Gent–McWilliams eddy transfer coefficient that depends on eddy energy and a nondimensional geometric parameter α, bounded in magnitude by unity. The parameter α expresses the efficiency by which eddies exchange energy with baroclinic mean flow via along-gradient eddy buoyancy flux—a flux equivalent to eddy form stress along mean buoyancy contours. An eddy-resolving ocean general circulation model is used to estimate the spatial structure of α in the Southern Ocean and assess its potential to form a basis for parameterization. The eddy efficiency α averages to a low but positive value of 0.043 within the Antarctic Circumpolar Current, consistent with an inefficient eddy field extracting energy from the mean flow. It is found that the low eddy efficiency is mainly the result of that eddy buoyancy fluxes are weakly anisotropic on average. The eddy efficiency is subject to pronounced vertical structure and is maximum at ~3-km depth, where eddy buoyancy fluxes tend to be directed most downgradient. Since α partly sets the eddy form stress in the Southern Ocean, a parameterization for α must reproduce its vertical structure to provide a faithful representation of vertical stress divergence and eddy forcing.

scholarly journals The Emergence of Shallow Easterly Jets within QBO Westerlies

2018 ◽  
Vol 75 (1) ◽  
pp. 21-40 ◽  
Author(s):  
Peter Hitchcock ◽  
Peter H. Haynes ◽  
William J. Randel ◽  
Thomas Birner
Keyword(s):  
Potential Vorticity ◽  
General Circulation ◽  
Circulation Model ◽  
Shear Zones ◽  
Wave Activity ◽  
Mean Flow ◽  
Quasi Biennial Oscillation ◽  
Westerly Jet ◽  
Eddy Fluxes ◽  
Biennial Oscillation

A configuration of an idealized general circulation model has been obtained in which a deep, stratospheric, equatorial, westerly jet is established that is spontaneously and quasi-periodically disrupted by shallow easterly jets. Similar to the disruption of the quasi-biennial oscillation (QBO) observed in early 2016, meridional fluxes of wave activity are found to play a central role. The possible relevance of two feedback mechanisms to these disruptions is considered. The first involves the secondary circulation produced in the shear zones on the upper and lower flanks of the easterly jet. This is found to play a role in maintaining the aspect ratio of the emerging easterly jet. The second involves the organization of the eddy fluxes by the mean flow: the presence of a weak easterly anomaly within a tall, tropical, westerly jet is demonstrated to produce enhanced and highly focused wave activity fluxes that reinforce and strengthen the easterly anomalies. The eddies appear to be organized by the formation of strong potential vorticity gradients on the subtropical flanks of the easterly anomaly. Similar wave activity and potential vorticity structures are found in the ERA-Interim for the observed QBO disruption, indicating this second feedback was active then.

scholarly journals Prognostic Residual Mean Flow in an Ocean General Circulation Model and its Relation to Prognostic Eulerian Mean Flow

2015 ◽  
Vol 45 (9) ◽  
pp. 2247-2260 ◽  
Author(s):  
Juan A. Saenz ◽  
Qingshan Chen ◽  
Todd Ringler
Keyword(s):  
General Circulation Model ◽  
Ocean Circulation ◽  
General Circulation ◽  
Mesoscale Eddy ◽  
Circulation Model ◽  
Ocean General Circulation Model ◽  
Mean Flow ◽  
Starting Point ◽  
Horizontal Momentum ◽  
Ocean General Circulation

AbstractRecent work has shown that taking the thickness-weighted average (TWA) of the Boussinesq equations in buoyancy coordinates results in exact equations governing the prognostic residual mean flow where eddy–mean flow interactions appear in the horizontal momentum equations as the divergence of the Eliassen–Palm flux tensor (EPFT). It has been proposed that, given the mathematical tractability of the TWA equations, the physical interpretation of the EPFT, and its relation to potential vorticity fluxes, the TWA is an appropriate framework for modeling ocean circulation with parameterized eddies. The authors test the feasibility of this proposition and investigate the connections between the TWA framework and the conventional framework used in models, where Eulerian mean flow prognostic variables are solved for. Using the TWA framework as a starting point, this study explores the well-known connections between vertical transfer of horizontal momentum by eddy form drag and eddy overturning by the bolus velocity, used by Greatbatch and Lamb and Gent and McWilliams to parameterize eddies. After implementing the TWA framework in an ocean general circulation model, the analysis is verified by comparing the flows in an idealized Southern Ocean configuration simulated using the TWA and conventional frameworks with the same mesoscale eddy parameterization.

scholarly journals Passive tracers in a general circulation model of the Southern Ocean

1999 ◽  
Vol 17 (7) ◽  
pp. 971-982 ◽  
Author(s):  
I. G. Stevens ◽  
D. P. Stevens
Keyword(s):  
Southern Ocean ◽  
Ocean Circulation ◽  
General Circulation ◽  
Circulation Model ◽  
Conveyor Belt ◽  
Water Masses ◽  
Deep Circulation ◽  
The United Kingdom ◽  
Passive Tracers ◽  
Fine Resolution

Abstract. Passive tracers are used in an off-line version of the United Kingdom Fine Resolution Antarctic Model (FRAM) to highlight features of the circulation and provide information on the inter-ocean exchange of water masses. The use of passive tracers allows a picture to be built up of the deep circulation which is not readily apparent from examination of the velocity or density fields. Comparison of observations with FRAM results gives good agreement for many features of the Southern Ocean circulation. Tracer distributions are consistent with the concept of a global "conveyor belt" with a return path via the Agulhas retroflection region for the replenishment of North Atlantic Deep Water.Key words. Oceanography: general (numerical modeling; water masses) · Oceanography: physical (general circulation)

scholarly journals Southern Ocean controls of the vertical marine <i>δ</i><sup>13</sup>C gradient – a modelling study

2018 ◽  
Vol 15 (23) ◽  
pp. 7205-7223 ◽  
Author(s):  
Anne L. Morée ◽  
Jörg Schwinger ◽  
Christoph Heinze
Keyword(s):  
Gas Exchange ◽  
Southern Ocean ◽  
Ocean Circulation ◽  
General Circulation ◽  
Dissolved Inorganic Carbon ◽  
Circulation Model ◽  
Ocean Basin ◽  
Global Ocean ◽  
Basin Scale ◽  
Poc Export

Abstract. δ13C, the standardised 13C ∕ 12C ratio expressed in per mille, is a widely used ocean tracer to study changes in ocean circulation, water mass ventilation, atmospheric pCO2, and the biological carbon pump on timescales ranging from decades to tens of millions of years. δ13C data derived from ocean sediment core analysis provide information on δ13C of dissolved inorganic carbon and the vertical δ13C gradient (i.e. Δδ13C) in past oceans. In order to correctly interpret δ13C and Δδ13C variations, a good understanding is needed of the influence from ocean circulation, air–sea gas exchange and biological productivity on these variations. The Southern Ocean is a key region for these processes, and we show here that Δδ13C in all ocean basins is sensitive to changes in the biogeochemical state of the Southern Ocean. We conduct a set of idealised sensitivity experiments with the ocean biogeochemistry general circulation model HAMOCC2s to explore the effect of biogeochemical state changes of the Southern and Global Ocean on atmospheric δ13C, pCO2, and marine δ13C and Δδ13C. The experiments cover changes in air–sea gas exchange rates, particulate organic carbon sinking rates, sea ice cover, and nutrient uptake efficiency in an unchanged ocean circulation field. Our experiments show that global mean Δδ13C varies by up to about ±0.35 ‰ around the pre-industrial model reference (1.2 ‰) in response to biogeochemical change. The amplitude of this sensitivity can be larger at smaller scales, as seen from a maximum sensitivity of about −0.6 ‰ on ocean basin scale. The ocean's oldest water (North Pacific) responds most to biological changes, the young deep water (North Atlantic) responds strongly to air–sea gas exchange changes, and the vertically well-mixed water (SO) has a low or even reversed Δδ13C sensitivity compared to the other basins. This local Δδ13C sensitivity depends on the local thermodynamic disequilibrium and the Δδ13C sensitivity to local POC export production changes. The direction of both glacial (intensification of Δδ13C) and interglacial (weakening of Δδ13C) Δδ13C change matches the direction of the sensitivity of biogeochemical processes associated with these periods. This supports the idea that biogeochemistry likely explains part of the reconstructed variations in Δδ13C, in addition to changes in ocean circulation.

scholarly journals Southern Ocean controls of the vertical marine δ<sup>13</sup>C gradient – a modelling study

2018 ◽  
Author(s):  
Anne L. Morée ◽  
Jörg Schwinger ◽  
Christoph Heinze
Keyword(s):  
Gas Exchange ◽  
Southern Ocean ◽  
Sediment Core ◽  
Ocean Circulation ◽  
General Circulation ◽  
Dissolved Inorganic Carbon ◽  
Circulation Model ◽  
Geochemical Environment ◽  
Ocean Sediment ◽  
Carbon Pump

Abstract. The standardized 13C isotope, δ13C, is a widely used ocean tracer to study changes in ocean circulation, water mass ventilation, atmospheric pCO2 and the biological carbon pump on timescales ranging from decades to 10s of millions of years. δ13C data derived from ocean sediment core analysis provide information on δ13C of dissolved inorganic carbon and the vertical δ13C gradient (i.e., Δδ13C) in past oceans. In order to correctly interpret δ13C and Δδ13C variations, a good understanding is needed of the influence from ocean circulation, air-sea gas exchange and biological productivity on these variations. The Southern Ocean is a key region for these processes, and we show here that global mean Δδ13C is sensitive to changes in the biogeochemical state of the Southern Ocean. We conduct four idealised sensitivity experiments with the ocean biogeochemistry general circulation model HAMOCC2s to explore the effect of biogeochemical state changes of the (Southern) Oceans on atmospheric δ13C, pCO2, and marine δ13C and Δδ13C. The experiments cover changes in air-sea gas exchange rates, particulate organic carbon sinking rates, sea ice cover, and nutrient uptake efficiency – in an unchanged ocean circulation field. We conclude that the maximum variation of mean marine Δδ13C in response to (bio)geochemical change is ~ 0.5 ‰, which is about half of the reconstructed variation in Δδ13C over glacial-interglacial timescales. Locally, Δδ13C variations can surpass or even mirror the mean effects on Δδ13C due to the spatial variation in the sensitivity of δ13C to biogeochemical change. The (bio)geochemical environment of a sediment core thus needs to be well constrained in order to be able to interpret reconstructed Δδ13C variations in such a core. The sensitivity of Δδ13C varies spatially depending on the contribution of air-sea gas exchange versus biological export productivity to the local δ13C signature. Interestingly, the direction of both glacial (intensification of Δδ13C) and interglacial (weakening of Δδ13C) Δδ13C change matches biogeochemical processes associated with these periods. This supports the idea that biogeochemistry likely explains part of the reconstructed variations in Δδ13C, and not only ocean circulation.

scholarly journals On the impact of sea ice in a global ocean circulation model

1997 ◽  
Vol 25 ◽  
pp. 111-115 ◽  
Author(s):  
Achim Stössel
Keyword(s):  
Southern Ocean ◽  
Sea Ice ◽  
Ocean Circulation ◽  
General Circulation ◽  
Thermohaline Circulation ◽  
Circulation Model ◽  
Deep Ocean ◽  
Global Ocean ◽  
Convective Activity ◽  
The Impact

This paper investigates the long-term impact of sea ice on global climate using a global sea-ice–ocean general circulation model (OGCM). The sea-ice component involves state-of-the-art dynamics; the ocean component consists of a 3.5° × 3.5° × 11 layer primitive-equation model. Depending on the physical description of sea ice, significant changes are detected in the convective activity, in the hydrographic properties and in the thermohaline circulation of the ocean model. Most of these changes originate in the Southern Ocean, emphasizing the crucial role of sea ice in this marginally stably stratified region of the world's oceans. Specifically, if the effect of brine release is neglected, the deep layers of the Southern Ocean warm up considerably; this is associated with a weakening of the Southern Hemisphere overturning cell. The removal of the commonly used “salinity enhancement” leads to a similar effect. The deep-ocean salinity is almost unaffected in both experiments. Introducing explicit new-ice thickness growth in partially ice-covered gridcells leads to a substantial increase in convective activity, especially in the Southern Ocean, with a concomitant significant cooling and salinification of the deep ocean. Possible mechanisms for the resulting interactions between sea-ice processes and deep-ocean characteristics are suggested.

scholarly journals The Global Zonally Integrated Ocean Circulation, 1992–2006: Seasonal and Decadal Variability

2009 ◽  
Vol 39 (2) ◽  
pp. 351-368 ◽  
Author(s):  
Carl Wunsch ◽  
Patrick Heimbach
Keyword(s):  
Southern Ocean ◽  
Time Scales ◽  
Ocean Circulation ◽  
General Circulation ◽  
Decadal Variability ◽  
Circulation Model ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
Oceanic Circulation ◽  
Meridional Transport

Abstract The zonally integrated meridional and vertical velocities as well as the enthalpy transports and fluxes in a least squares adjusted general circulation model are used to estimate the top-to-bottom oceanic meridional overturning circulation (MOC) and its variability from 1992 to 2006. A variety of simple theories all produce time scales suggesting that the mid- and high-latitude oceans should respond to atmospheric driving only over several decades. In practice, little change is seen in the MOC and associated heat transport except very close to the sea surface, at depth near the equator, and in parts of the Southern Ocean. Variability in meridional transports in both volume and enthalpy is dominated by the annual cycle and secondarily by the semiannual cycle, particularly in the Southern Ocean. On time scales longer than a year, the solution exhibits small trends with complicated global spatial patterns. Apart from a net uptake of heat from the atmosphere (forced by the NCEP–NCAR reanalysis, which produces net ocean heating), the origins of the meridional transport trends are not distinguishable and are likely a combination of model disequilibrium, shifts in the observing system, other trends (real or artificial) in the meteorological fields, and/or true oceanic secularities. None of the results, however, supports an inference of oceanic circulation shifts taking the system out of the range in which changes are more than small perturbations. That the oceanic observations do not conflict with an apparent excess heat uptake from the atmosphere implies a continued undersampling of the global ocean, even in the upper layers.

Interannual to Decadal Changes in the ECCO Global Synthesis

2007 ◽  
Vol 37 (2) ◽  
pp. 313-337 ◽  
Author(s):  
A. Köhl ◽  
D. Stammer ◽  
B. Cornuelle
Keyword(s):  
Southern Ocean ◽  
Ocean Circulation ◽  
North Atlantic ◽  
General Circulation ◽  
World Ocean ◽  
Circulation Model ◽  
Sea Surface Height ◽  
Sea Surface ◽  
Global Ocean ◽  
The World

Abstract An estimate of the time-varying global ocean circulation for the period 1992–2002 was obtained by combining most of the World Ocean Circulation Experiment (WOCE) ocean datasets with a general circulation model on a 1° horizontal grid. The estimate exactly satisfies the model equations without artificial sources or sinks of momentum, heat, and freshwater. To bring the model into agreement with observations, its initial temperature and salinity conditions were permitted to change, as were the time-dependent surface fluxes of momentum, heat, and freshwater. The estimation of these “control variables” is largely consistent with accepted uncertainties in the hydrographic climatology and meteorological analyses. The estimated time-mean horizontal transports of volume, heat, and freshwater, which were largely underestimated in the previous 2° optimization performed by Stammer et al., have converged with time-independent estimates from box inversions over most parts of the World Ocean. Trends in the model’s heat content are 7% larger than those reported by Levitus and correspond to a global net heat uptake of about 1.1 W m−2 over the model domain. The associated model trend in sea surface height over the estimation period resembles the observations from Ocean Topography Experiment (TOPEX)/Poseidon over most of the global ocean. Sea surface height changes in the model are primarily steric but show contributions from mass redistributions from the subpolar North Atlantic Ocean and the Southern Ocean to the subtropical Pacific Ocean gyres. Steric contributions are primarily temperature based but are partly compensated by salt variation. However, the North Atlantic and the Southern Ocean reveal a clear contribution of salt to large-scale sea level variations.

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