scholarly journals Distinct controls on the strength of the abyssal overturning circulation: channel versus basin dynamics

2021 ◽  
Author(s):  
Chiung-Yin Chang ◽  
Malte F. Jansen
Keyword(s):  
Southern Ocean ◽  
General Circulation ◽  
Bottom Topography ◽  
Strong Dependence ◽  
Circulation Model ◽  
Overturning Circulation ◽  
Basin Scale ◽  
Diapycnal Diffusivity ◽  
Surface Buoyancy ◽  
Buoyancy Loss

AbstractAlthough the reconfiguration of the abyssal overturning circulation has been argued to be a salient feature of Earth’s past climate changes, our understanding of the physical mechanisms controlling its strength remains limited. In particular, existing scaling theories disagree on the relative importance of the dynamics in the Southern Ocean versus the dynamics in the basins to the north. In this study, we systematically investigate these theories and compare them with a set of numerical simulations generated from an ocean general circulation model with idealized geometry, designed to capture only the basic ingredients considered by the theories. It is shown that the disagreement between existing theories can be partially explained by the fact that the overturning strengths measured in the channel and in the basin scale distinctly with the external parameters, including surface buoyancy loss, diapycnal diffusivity, wind stress, and eddy diffusivity. The overturning in the re-entrant channel, which represents the Southern Ocean, is found to be sensitive to all these parameters, in addition to a strong dependence on bottom topography. By contrast, the basin overturning varies with the integrated surface buoyancy loss rate and diapycnal diffusivity but is mostly unaffected by winds and channel topography. The simulated parameter dependence of the basin overturning can be described by a scaling theory that is based only on basin dynamics.

scholarly journals Eddy Saturation of Equilibrated Circumpolar Currents

2013 ◽  
Vol 43 (3) ◽  
pp. 507-532 ◽  
Author(s):  
David R. Munday ◽  
Helen L. Johnson ◽  
David P. Marshall
Keyword(s):  
Southern Ocean ◽  
Wind Stress ◽  
General Circulation ◽  
Circulation Model ◽  
Ocean General Circulation Model ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Meridional Overturning ◽  
Diapycnal Diffusivity ◽  
Ocean Wind

Abstract This study uses a sector configuration of an ocean general circulation model to examine the sensitivity of circumpolar transport and meridional overturning to changes in Southern Ocean wind stress and global diapycnal mixing. At eddy-permitting, and finer, resolution, the sensitivity of circumpolar transport to forcing magnitude is drastically reduced. At sufficiently high resolution, there is little or no sensitivity of circumpolar transport to wind stress, even in the limit of no wind. In contrast, the meridional overturning circulation continues to vary with Southern Ocean wind stress, but with reduced sensitivity in the limit of high wind stress. Both the circumpolar transport and meridional overturning continue to vary with diapycnal diffusivity at all model resolutions. The circumpolar transport becomes less sensitive to changes in diapycnal diffusivity at higher resolution, although sensitivity always remains. In contrast, the overturning circulation is more sensitive to change in diapycnal diffusivity when the resolution is high enough to permit mesoscale eddies.

Overturning Circulation Pathways in a Two-Basin Ocean Model

2020 ◽  
Vol 50 (8) ◽  
pp. 2105-2122
Author(s):  
Louis-Philippe Nadeau ◽  
Malte F. Jansen
Keyword(s):  
Southern Ocean ◽  
General Circulation ◽  
Circulation Model ◽  
Deep Ocean ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
Direct Result ◽  
Diapycnal Mixing ◽  
Overturning Circulation ◽  
Deep Ocean Water

AbstractA toy model for the deep ocean overturning circulation in multiple basins is presented and applied to study the role of buoyancy forcing and basin geometry in the ocean’s global overturning. The model reproduces the results from idealized general circulation model simulations and provides theoretical insights into the mechanisms that govern the structure of the overturning circulation. The results highlight the importance of the diabatic component of the meridional overturning circulation (MOC) for the depth of North Atlantic Deep Water (NADW) and for the interbasin exchange of deep ocean water masses. This diabatic component, which extends the upper cell in the Atlantic below the depth of adiabatic upwelling in the Southern Ocean, is shown to be sensitive to the global area-integrated diapycnal mixing rate and the density contrast between NADW and Antarctic Bottom Water (AABW). The model also shows that the zonally averaged global overturning circulation is to zeroth-order independent of whether the ocean consists of one or multiple connected basins, but depends on the total length of the southern reentrant channel region (representing the Southern Ocean) and the global ocean area integrated diapycnal mixing. Common biases in single-basin simulations can thus be understood as a direct result of the reduced domain size.

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 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 Control of Lower-Limb Overturning Circulation in the Southern Ocean by Diapycnal Mixing and Mesoscale Eddy Transfer

2008 ◽  
Vol 38 (12) ◽  
pp. 2832-2845 ◽  
Author(s):  
Taka Ito ◽  
John Marshall
Keyword(s):  
Southern Ocean ◽  
Lower Limb ◽  
General Circulation ◽  
Mesoscale Eddy ◽  
Circulation Model ◽  
Ocean General Circulation Model ◽  
Meridional Overturning Circulation ◽  
Diapycnal Mixing ◽  
Overturning Circulation ◽  
Diabatic Forcing

Abstract A simple model is developed of the lower limb of the meridional overturning circulation in the Southern Ocean based on residual-mean theory. It is hypothesized that the strength of the lower-limb overturning (Ψ) is strongly controlled by the magnitude of abyssal diapycnal mixing (κ) and that of mesoscale eddy transfer (K). In particular, it is argued that Ψ ∝ κK. The scaling and associated theory find support in a suite of sensitivity experiments with an idealized ocean general circulation model. This study shows that intense diapycnal mixing is required to close the buoyancy budget of the lower-limb overturning circulation, in contrast to the upper limb, where air–sea buoyancy fluxes can provide the required diabatic forcing.

The Adiabatic Pole-to-Pole Overturning Circulation

2011 ◽  
Vol 41 (9) ◽  
pp. 1795-1810 ◽  
Author(s):  
Christopher L. Wolfe ◽  
Paola Cessi
Keyword(s):  
Simple Model ◽  
Northern Hemisphere ◽  
General Circulation ◽  
Circulation Model ◽  
Intermediate Water ◽  
Overturning Circulation ◽  
Pole Cell ◽  
Circumpolar Current ◽  
Surface Buoyancy ◽  
Averaged Model

Abstract The adiabatic pole-to-pole cell of the residual overturning circulation (ROC) is studied in a two-hemisphere, semienclosed basin, with a zonally reentrant channel occupying the southernmost eighth of the domain. Three different models of increasing complexity are used: a simple, analytically tractable zonally averaged model; a coarse-resolution numerical model with parameterized eddies; and an eddy-resolving general circulation model. Two elements are found to be necessary for the existence of an adiabatic pole-to-pole cell: 1) a thermally indirect, wind-driven overturning circulation in the zonally reentrant channel, analogous to the Deacon cell in the Antarctic Circumpolar Current (ACC) region, and 2) a set of outcropping isopycnals shared between the channel and the semienclosed region of the Northern Hemisphere. These points are supported by several computations varying the domain geometry, the surface buoyancy distribution, and the wind forcing. All three models give results that are qualitatively very similar, indicating that the two requirements above are general and robust. The zonally averaged model parameterizes the streamfunction associated with adiabatic buoyancy fluxes as downgradient diffusion of buoyancy thickness, with a diffusivity in the semienclosed region of the Northern Hemisphere much larger than that in the ACC region. In the simple model, the disparity in diffusivities is necessary to obtain a substantial pole-to-pole ROC. The simple model also illustrates how the geometry of the isopycnals is shaped by the interhemispheric ROC, leading to three major thermostads, which the authors identify with the major water masses of the Atlantic: that is, North Atlantic Deep Water, Antarctic Intermediate Water, and Antarctic Bottom Water.

scholarly journals Meridional Overturning Circulation in a Multibasin Model. Part I: Dependence on Southern Ocean Buoyancy Forcing

2020 ◽  
Vol 50 (5) ◽  
pp. 1159-1178 ◽  
Author(s):  
Jonathan A. Baker ◽  
Andrew J. Watson ◽  
Geoffrey K. Vallis
Keyword(s):  
Southern Ocean ◽  
Deep Water ◽  
General Circulation ◽  
Circulation Model ◽  
Meridional Overturning Circulation ◽  
Pacific Basin ◽  
Overturning Circulation ◽  
Buoyancy Forcing ◽  
The Pacific ◽  
Meridional Overturning

AbstractThe variation in the strength and structure of the overturning circulation under varying Southern Ocean buoyancy forcing, corresponding to present day, a cooler (glacial) state, and a possible future warmer state is analyzed in an idealized two-basin general circulation model connected by a southern circumpolar channel. A connection between the North Atlantic Deep Water (NADW) cell in the Atlantic basin and the Pacific Deep Water (PDW) cell in the Pacific basin occurs with a direct flow of NADW into the channel’s lower cell, while PDW upwelled in the Pacific basin can flow directly into the upper wind-driven cell in the channel. The intersection of these cells along with direct zonal flows between the basins completes the interbasin circulation. The present-day Atlantic meridional overturning circulation (AMOC) in the model is upwelled both by wind-driven upwelling in the Southern Ocean and by diffusion in the Pacific and Atlantic. In a cooler climate with enhanced sea ice, the NADW cell shoals, which can then no longer flow directly into the channel’s lower cell, reducing the Pacific pathway of NADW. This leads to a substantial weakening of the AMOC, suggesting buoyancy forcing changes can play a substantial role in the transition of the AMOC to a glacial state. In contrast, in a warmer equilibrium climate state with reduced AABW formation, the NADW cell strengthens and deepens. NADW is increasingly directed along the Pacific pathway, while the direct upwelling in the channel’s wind-driven upper cell plays a smaller role.

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 Temperature Projections over the Indus River Basin of Pakistan Using Statistical Downscaling

Atmosphere ◽  
2021 ◽  
Vol 12 (2) ◽  
pp. 195
Author(s):  
Muhammad Saleem Pomee ◽  
Elke Hertig
Keyword(s):  
21St Century ◽  
Statistical Downscaling ◽  
General Circulation ◽  
Winter Season ◽  
Circulation Model ◽  
Indus Basin ◽  
Indus River ◽  
Temperature Changes ◽  
Basin Scale ◽  
Maximum Heating

We assessed maximum (Tmax) and minimum (Tmin) temperatures over Pakistan’s Indus basin during the 21st century using statistical downscaling. A particular focus was given to spatiotemporal heterogeneity, reference and General Circulation Model (GCM) uncertainties, and statistical skills of regression models using an observational profile that could significantly be improved by recent high-altitude observatories. First, we characterized the basin into homogeneous climate regions using K-means clustering. Predictors from ERA-Interim reanalysis were then used to model observed temperatures skillfully and quantify reference and GCM uncertainties. Thermodynamical (dynamical) variables mainly governed reference (GCM) uncertainties. The GCM predictors under RCP4.5 and RCP8.5 scenarios were used as “new” predictors in statistical models to project ensemble temperature changes. Our analysis projected non-uniform warming but could not validate elevation-dependent warming (EDW) at the basin scale. We obtained more significant warming during the westerly-dominated seasons, with maximum heating during the winter season through Tmin changes. The most striking feature is a low-warming monsoon (with the possibility of no change to slight cooling) over the Upper Indus Basin (UIB). Therefore, the likelihood of continuing the anomalous UIB behavior during the primary melt season may not entirely be ruled out at the end of the 21st century under RCP8.5.

scholarly journals An Eddy-Permitting Southern Ocean State Estimate

2010 ◽  
Vol 40 (5) ◽  
pp. 880-899 ◽  
Author(s):  
Matthew R. Mazloff ◽  
Patrick Heimbach ◽  
Carl Wunsch
Keyword(s):  
Southern Ocean ◽  
General Circulation Model ◽  
General Circulation ◽  
Microwave Radiometer ◽  
World Ocean ◽  
Circulation Model ◽  
Great Majority ◽  
Drake Passage ◽  
Volume Transport ◽  
Intermediate Cell

Abstract An eddy-permitting general circulation model of the Southern Ocean is fit by constrained least squares to a large observational dataset during 2005–06. Data used include Argo float profiles, CTD synoptic sections, Southern Elephant Seals as Oceanographic Samplers (SEaOS) instrument-mounted seal profiles, XBTs, altimetric observations [Envisat, Geosat, Jason-1, and Ocean Topography Experiment (TOPEX)/Poseidon], and infrared and microwave radiometer observed sea surface temperature. An adjoint model is used to determine descent directions in minimizing a misfit function, each of whose elements has been weighted by an estimate of the observational plus model error. The model is brought into near agreement with the data by adjusting its control vector, here consisting of initial and meteorological boundary conditions. Although total consistency has not yet been achieved, the existing solution is in good agreement with the great majority of the 2005 and 2006 Southern Ocean observations and better represents these data than does the World Ocean Atlas 2001 (WOA01) climatological product. The estimate captures the oceanic temporal variability and in this respect represents a major improvement upon earlier static inverse estimates. During the estimation period, the Drake Passage volume transport is 153 ± 5 Sv (1 Sv ≡ 106 m3 s−1). The Ross and Weddell polar gyre transports are 20 ± 5 Sv and 40 ± 8 Sv, respectively. Across 32°S there is a surface meridional overturning cell of 12 ± 12 Sv, an intermediate cell of 17 ± 12 Sv, and an abyssal cell of 13 ± 6 Sv. The northward heat and freshwater anomaly transports across 30°S are −0.3 PW and 0.7 Sv, with estimated uncertainties of 0.5 PW and 0.2 Sv. The net rate of wind work is 2.1 ± 1.1 TW. Southern Ocean theories involving short temporal- and spatial-scale dynamics may now be tested with a dynamically and thermodynamically realistic general circulation model solution that is known to be compatible with the modern observational datasets.

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