Eddy-Mean flow oscillations in the Southern Ocean

Characteristic Time Scale ◽

Intrinsic Variability ◽

Mean Flow ◽

Flow Interaction ◽

Stress Changes ◽

Non Linear Oscillator

Geostrophic eddies have a leading order effect on the dynamics of the Southern Ocean (SO), and numerous studies have shown that they are also key to the response of both the zonal transport and the meridional overturning circulation to wind stress changes. The role played by eddies in setting the intrinsic variability of the SO, however, is less well-understood. Here, inspired by recent work on the atmospheric jet, we investigate whether the eddy-mean flow interaction in the Antarctic Circumpolar Current can be described by a prey-predator nonlinear model.&#160;To this end, we analyse data from a high-resolution eddy-resolving configuration of the MIT general circulation model: an idealised &#8220;channel&#8221; model with mechanical and thermodynamical forcing at the surface, and plausible zonal and meridional circulations.&#160;Here, we show that a mechanism of eddy-mean flow interaction driving the intrinsic variability of the SO-like model is well described by a stochastic non-linear oscillator with damping. This model is a generalisation of the Ambaum-Novak oscillator, which has been successfully employed to describe the atmospheric storm track variability.&#160;We find that, on length scales similar to that of individual zonal jets, the eddy-mean flow interaction is characterised by a high-frequency oscillatory mode, and that the characteristic time scale of the oscillation is comparable with classical estimates of the baroclinic life-cycle. A Gaussian smoothing of the phase space diagram also reveals the damped oscillatory character of the oscillation: this is in contrast with the atmospheric case, where damping is negligible and orbits are confined to energy surfaces.&#160;This result may help inform the interpretation of the SO intrinsic and forced variability (such as, for example, the response to wind stress changes), and pave the way to further studies featuring more realistic model configurations.

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

10.1175/jpo-d-13-0108.1 ◽

2013 ◽

Vol 44 (2) ◽

pp. 662-675 ◽

Cited By ~ 8

Author(s):

Paul Spence ◽

Erik van Sebille ◽

Oleg A. Saenko ◽

Matthew H. England

Keyword(s):

Southern Ocean ◽

Wind Stress ◽

Climate Model ◽

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.

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

10.1175/jpo-d-20-0121.1 ◽

2021 ◽

Author(s):

Jonathan A. Baker ◽

Andrew J. Watson ◽

Geoffrey K. Vallis

Keyword(s):

Southern Ocean ◽

Wind Stress ◽

General Circulation ◽

Circulation Model ◽

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.

Eddy Saturation of Equilibrated Circumpolar Currents

10.1175/jpo-d-12-095.1 ◽

2013 ◽

Vol 43 (3) ◽

pp. 507-532 ◽

Cited By ~ 119

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 ◽

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.

On the Relationship between Southern Ocean Overturning and ACC Transport

10.1175/jpo-d-12-057.1 ◽

2013 ◽

Vol 43 (1) ◽

pp. 140-148 ◽

Cited By ~ 88

Author(s):

Adele K. Morrison ◽

Andrew McC. Hogg

Keyword(s):

Southern Ocean ◽

Wind Stress ◽

Large Scale ◽

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.

Overturning response to a surface wind stress doubling in an eddying and a non-eddying ocean

10.1175/jpo-d-20-0176.1 ◽

2021 ◽

Author(s):

Veit Lüschow ◽

Jochem Marotzke ◽

Jin-Song von Storch

Keyword(s):

Wind Stress ◽

Surface Wind ◽

Relative Vorticity ◽

Ocean Model ◽

Western Boundary ◽

Boundary Current ◽

Mean Flow ◽

Surface Wind Stress ◽

Stress Changes

AbstractIn this paper, the overturning responses to wind stress changes of an eddying and a non-eddying ocean are compared. Differences are found in the deep overturning cell in the low-latitude North Atlantic with substantial implications for the deep western boundary current (DWBC). In an ocean-only twin experiment with one eddying and one non-eddying configuration of the MPI ocean model, two different forcings are being applied: the standard NCEP forcing and the NCEP forcing with 2x surface wind stress. The response to the wind stress doubling in the Atlantic meridional overturning circulation is similar in the eddying and the non-eddying configuration, showing an increase by about 4 Sv (~25%, 1 Sv = 106 m3s−1). In contrast, the DWBC responds with a speedup in the non-eddying and a slowdown in the eddying configuration. This paper demonstrates that the DWBC slowdown in the eddying configuration is largely balanced by eddy vorticity fluxes. Because those fluxes are not resolved and also not captured by an eddy parameterization in the non-eddying configuration, such a DWBC slowdown is likely not to occur in non-eddying ocean models which therefore might not capture the whole range of overturning responses. Furthermore, evidence is provided that the balancing effect of the eddies is not a passive reaction to a remotely triggered DWBC slowdown. Instead, deep eddies which are sourced from the upper ocean provide an excess input of relative vorticity which then actively forces the DWBC mean flow to slow down.

Mean, Variability, and Trend of Southern Ocean Wind Stress: Role of Wind Fluctuations

Journal of Climate ◽

10.1175/jcli-d-17-0481.1 ◽

2018 ◽

Vol 31 (9) ◽

pp. 3557-3573 ◽

Cited By ~ 13

Author(s):

Xia Lin ◽

Xiaoming Zhai ◽

Zhaomin Wang ◽

David R. Munday

Keyword(s):

Southern Ocean ◽

Wind Stress ◽

Southern Annular Mode ◽

Westerly Wind ◽

Wind Variability ◽

Stress Studies ◽

Stress Changes ◽

The Mean

The Southern Ocean (SO) surface westerly wind stress plays a fundamental role in driving the Antarctic Circumpolar Current and the global meridional overturning circulation. Here, the authors investigate the contributions of atmospheric wind fluctuations to the mean, variability, and trend of SO wind stress over the last four decades using NCEP reanalysis and ERA-Interim products. Including wind variability at synoptic frequencies (2–8 days) and higher in the stress calculation is found to increase the strength of the mean SO wind stress by almost 40% in both reanalysis products. The southern annular mode index is found to be a good indicator for the strength of the mean wind and mean wind stress, but not as good an indicator for wind fluctuations, at least for the chosen study period. Large discrepancies between reanalysis products emerge regarding the contributions of wind fluctuations to the strengthening trend of SO wind stress. Between one-third and one-half of the stress trend in NCEP can be explained by the increase in the intensity of wind fluctuations, while the stress trend in ERA-Interim is due entirely to the increasing strength of the mean westerly wind. This trend discrepancy may have important climatic implications since the sensitivity of SO circulation to wind stress changes depends strongly on how these stress changes are brought about. Given the important role of wind fluctuations in shaping the SO wind stress, studies of the SO response to wind stress changes need to account for changes of wind fluctuations in the past and future.

Southern Ocean Wind Stress in CMIP5 Models: Role of Wind Fluctuations

Journal of Climate ◽

10.1175/jcli-d-19-0466.1 ◽

2020 ◽

Vol 33 (4) ◽

pp. 1209-1226 ◽

Cited By ~ 2

Author(s):

Xia Lin ◽

Xiaoming Zhai ◽

Zhaomin Wang ◽

David R. Munday

Keyword(s):

Southern Ocean ◽

Time Scales ◽

Wind Stress ◽

Surface Wind ◽

Coupled Model ◽

Cmip5 Models ◽

Coupled Models ◽

Surface Wind Stress ◽

Stress Changes ◽

The Mean

AbstractThe Southern Ocean (SO) surface wind stress is a major atmospheric forcing for driving the Antarctic Circumpolar Current and the global overturning circulation. Here the effects of wind fluctuations at different time scales on SO wind stress in 18 models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) are investigated. It is found that including wind fluctuations, especially on time scales associated with synoptic storms, in the stress calculation strongly enhances the mean strength, modulates the seasonal cycle, and significantly amplifies the trends of SO wind stress. In 11 out of the 18 CMIP5 models, the SO wind stress has strengthened significantly over the period of 1960–2005. Among them, the strengthening trend of SO wind stress in one CMIP5 model is due to the increase in the intensity of wind fluctuations, while in all the other 10 models the strengthening trend is due to the increasing strength of the mean westerly wind. These discrepancies in SO wind stress trend in CMIP5 models may explain some of the diverging behaviors in the model-simulated SO circulation. Our results suggest that to reduce the uncertainty in SO responses to wind stress changes in the coupled models, both the mean wind and wind fluctuations need to be better simulated.

A Geometric Interpretation of Southern Ocean Eddy Form Stress

10.1175/jpo-d-18-0220.1 ◽

2019 ◽

Vol 49 (10) ◽

pp. 2553-2570 ◽

Cited By ~ 2

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.

An Adjoint Analysis of the Meridional Overturning Circulation in a Hybrid Coupled Model

Journal of Climate ◽

10.1175/jcli3821.1 ◽

2006 ◽

Vol 19 (15) ◽

pp. 3751-3767 ◽

Cited By ~ 19

Author(s):

Véronique Bugnion ◽

Chris Hill ◽

Peter H. Stone

Keyword(s):

Boundary Conditions ◽

Wind Stress ◽

Ocean Circulation ◽

General Circulation ◽

Global Ocean ◽

Model Parameters ◽

Overturning Circulation ◽

Near Surface ◽

Meridional Overturning

Abstract Multicentury sensitivities in a realistic geometry global ocean general circulation model are analyzed using an adjoint technique. This paper takes advantage of the adjoint model’s ability to generate maps of the sensitivity of a diagnostic (i.e., the meridional overturning’s strength) to all model parameters. This property of adjoints is used to review several theories, which have been elaborated to explain the strength of the North Atlantic’s meridional overturning. This paper demonstrates the profound impact of boundary conditions in permitting or suppressing mechanisms within a realistic model of the contemporary ocean circulation. For example, the so-called Drake Passage Effect in which wind stress in the Southern Ocean acts as the main driver of the overturning’s strength, is shown to be an artifact of boundary conditions that restore the ocean’s surface temperature and salinity toward prescribed climatologies. Advective transports from the Indian and Pacific basins play an important role in setting the strength of the overturning circulation under “mixed” boundary conditions, in which a flux of freshwater is specified at the ocean’s surface. The most “realistic” regime couples an atmospheric energy and moisture balance model to the ocean. In this configuration, inspection of the global maps of sensitivity to wind stress and diapycnal mixing suggests a significant role for near-surface Ekman processes in the Tropics. Buoyancy also plays an important role in setting the overturning’s strength, through direct thermal forcing near the sites of convection, or through the advection of salinity anomalies in the Atlantic basin.

Overturning Circulation Pathways in a Two-Basin Ocean Model

10.1175/jpo-d-20-0034.1 ◽

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 ◽

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.