scholarly journals Planetary–Geometric Constraints on Isopycnal Slope in the Southern Ocean

2015 ◽  
Vol 45 (12) ◽  
pp. 2991-3004 ◽  
Author(s):  
Daniel C. Jones ◽  
Takamitsu Ito ◽  
Thomas Birner ◽  
Andreas Klocker ◽  
David Munday
Keyword(s):  
Southern Ocean ◽  
Large Scale ◽  
Baroclinic Instability ◽  
Surface Wind ◽  
Relative Vorticity ◽  
Geometric Constraints ◽  
Coriolis Parameter ◽  
Sector Model ◽  
Surface Wind Stress ◽  
Flux Parameterization

AbstractOn planetary scales, surface wind stress and differential buoyancy forcing act together to produce isopycnal surfaces that are relatively flat in the tropics/subtropics and steep near the poles, where they tend to outcrop. Tilted isopycnals in a rapidly rotating fluid are subject to baroclinic instability. The turbulent, mesoscale eddies generated by this instability have a tendency to homogenize potential vorticity (PV) along density surfaces. In the Southern Ocean (SO), the tilt of isopycnals is largely maintained by competition between the steepening effect of surface forcing and the flattening effect of turbulent, spatially inhomogeneous eddy fluxes of PV. Here quasigeostrophic theory is used to investigate the influence of a planetary–geometric constraint on the equilibrium slope of tilted density/buoyancy surfaces in the SO. If the meridional gradients of relative vorticity and PV are small relative to β, then quasigeostrophic theory predicts ds/dz = β/f0 = cot(ϕ0)/a, or equivalently r ≡ |∂zs/(β/f0)| = 1, where f is the Coriolis parameter, β is the meridional gradient of f, s is the isopycnal slope, ϕ0 is a reference latitude, a is the planetary radius, and r is the depth-averaged criticality parameter. It is found that the strict r = 1 condition holds over specific averaging volumes in a large-scale climatology. A weaker r = O(1) condition for depth-averaged quantities is generally satisfied away from large bathymetric features. The r = O(1) constraint is employed to derive a depth scale to characterize large-scale interior stratification, and an idealized sector model is used to test the sensitivity of this relationship to surface wind forcing. Finally, the possible implications for eddy flux parameterization and for the sensitivity of SO circulation/stratification to changes in forcing are discussed.

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

2020 ◽  
Vol 33 (4) ◽  
pp. 1209-1226 ◽  
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.

Spatially amplifying modes of the Charney baroclinic-instability problem

1986 ◽  
Vol 170 ◽  
pp. 293-317 ◽  
Author(s):  
R. T. Pierrehumbert
Keyword(s):  
Broad Class ◽  
Baroclinic Instability ◽  
Surface Wind ◽  
Stationary Wave ◽  
Vertical Shear ◽  
Coriolis Parameter ◽  
Spatial Instability ◽  
Wide Range ◽  
Amplification Rate ◽  
Zero Frequency

We determine the circumstances under which baroclinic instability in the Charney model subjected to localized time-periodic forcing manifests itself as a wavetrain that oscillates at the source frequency and amplifies in space with distance from the source; analytical and numerical results describing the salient characteristics of such waves are presented. The spatially amplifying disturbance is a hitherto unsuspected part of the response to a pulsating source, and coexists with the more familiar neutral Rossby wavetrains; it is likely to play a role in a wide range of atmospheric and oceanic phenomena.The central results rely on a careful application of a causality criterion due to Briggs. These results illustrate a practical means of attacking spatial instability problems, which can be applied to a broad class of systems besides the one at hand. We have found that the Charney problem with positive vertical shear is not absolutely unstable, so long as the wind at the ground is non-negative. This implies that spatial instability and forced stationary-wave problems are well posed in an open domain under typical atmospheric circumstances.The amplifying waves appear on the downstream side of the source, have eastward (downstream) phase propagation and have wavelengths that increase monotonically with decreasing frequency, becoming infinite at zero frequency. When the surface wind is not too large, the spatial amplification rate has a single maximum near the frequency ωm= (f/N)Uz, wherefis the Coriolis parameter,Nis the stability frequency andUzis the vertical shear; the rate approaches zero at zero frequency and asymptotes algebraically to zero at large frequency for any positive surface wind. Distinct Charney and Green modes do not appear until the surface wind is made very large. The amplification rate at ωmbecomes infinite as surface wind approaches zero, suggesting a mechanism for the concentration of eddy activity.We also discuss the relationship of these results to the structure of low- and high-frequency atmospheric variability.

scholarly journals Representation of Southern Ocean Properties across Coupled Model Intercomparison Project Generations: CMIP3 to CMIP6

2020 ◽  
Vol 33 (15) ◽  
pp. 6555-6581 ◽  
Author(s):  
R. L. Beadling ◽  
J. L. Russell ◽  
R. J. Stouffer ◽  
M. Mazloff ◽  
L. D. Talley ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Ocean Circulation ◽  
Large Scale ◽  
Dominant Role ◽  
Surface Wind ◽  
Coupled Model ◽  
Anthropogenic Heat ◽  
Model Intercomparison ◽  
Observational Uncertainty ◽  
Intercomparison Project

AbstractThe air–sea exchange of heat and carbon in the Southern Ocean (SO) plays an important role in mediating the climate state. The dominant role the SO plays in storing anthropogenic heat and carbon is a direct consequence of the unique and complex ocean circulation that exists there. Previous generations of climate models have struggled to accurately represent key SO properties and processes that influence the large-scale ocean circulation. This has resulted in low confidence ascribed to twenty-first-century projections of the state of the SO from previous generations of models. This analysis provides a detailed assessment of the ability of models contributed to the sixth phase of the Coupled Model Intercomparison Project (CMIP6) to represent important observationally based SO properties. Additionally, a comprehensive overview of CMIP6 performance relative to CMIP3 and CMIP5 is presented. CMIP6 models show improved performance in the surface wind stress forcing, simulating stronger and less equatorward-biased wind fields, translating into an improved representation of the Ekman upwelling over the Drake Passage latitudes. An increased number of models simulate an Antarctic Circumpolar Current (ACC) transport within observational uncertainty relative to previous generations; however, several models exhibit extremely weak transports. Generally, the upper SO remains biased warm and fresh relative to observations, and Antarctic sea ice extent remains poorly represented. While generational improvement is found in many metrics, persistent systematic biases are highlighted that should be a priority during model development. These biases need to be considered when interpreting projected trends or biogeochemical properties in this region.

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.

scholarly journals Continental Shelf Baroclinic Instability. Part II: Oscillating Wind Forcing

2016 ◽  
Vol 46 (2) ◽  
pp. 569-582 ◽  
Author(s):  
K. H. Brink ◽  
H. Seo
Keyword(s):  
Flow Field ◽  
Continental Shelf ◽  
Large Scale ◽  
Baroclinic Instability ◽  
Wind Forcing ◽  
Develop Model ◽  
Bottom Slope ◽  
Coriolis Parameter ◽  
Wide Range ◽  
Eddy Field

AbstractContinental shelf baroclinic instability energized by fluctuating alongshore winds is treated using idealized primitive equation numerical model experiments. A spatially uniform alongshore wind, sinusoidal in time, alternately drives upwelling and downwelling and so creates highly variable, but slowly increasing, available potential energy. For all of the 30 model runs, conducted with a wide range of parameters (varying Coriolis parameter, initial stratification, bottom friction, forcing period, wind strength, and bottom slope), a baroclinic instability and subsequent eddy field develop. Model results and scalings show that the eddy kinetic energy increases with wind amplitude, forcing period, stratification, and bottom slope. The dominant alongshore length scale of the eddy field is essentially an internal Rossby radius of deformation. The resulting depth-averaged alongshore flow field is dominated by the large-scale, periodic wind forcing, while the cross-shelf flow field is dominated by the eddy variability. The result is that correlation length scales for alongshore flow are far greater than those for cross-shelf velocity. This scale discrepancy is qualitatively consistent with midshelf observations by Kundu and Allen, among others.

scholarly journals Decadal Changes of Wind Stress over the Southern Ocean Associated with Antarctic Ozone Depletion

2007 ◽  
Vol 20 (14) ◽  
pp. 3395-3410 ◽  
Author(s):  
Xiao-Yi Yang ◽  
Rui Xin Huang ◽  
Dong Xiao Wang
Keyword(s):  
Southern Ocean ◽  
Wind Stress ◽  
Zonal Wind ◽  
Ozone Depletion ◽  
Surface Wind ◽  
Southern Annular Mode ◽  
Mean Flow ◽  
Annular Mode ◽  
Surface Wind Stress ◽  
Antarctic Ozone

Abstract Using 40-yr ECMWF Re-Analysis (ERA-40) data and in situ observations, the positive trend of Southern Ocean surface wind stress during two recent decades is detected, and its close linkage with spring Antarctic ozone depletion is established. The spring Antarctic ozone depletion affects the Southern Hemisphere lower-stratospheric circulation in late spring/early summer. The positive feedback involves the strengthening and cooling of the polar vortex, the enhancement of meridional temperature gradients and the meridional and vertical potential vorticity gradients, the acceleration of the circumpolar westerlies, and the reduction of the upward wave flux. This feedback loop, together with the ozone-related photochemical interaction, leads to the upward tendency of lower-stratospheric zonal wind in austral summer. In addition, the stratosphere–troposphere coupling, facilitated by ozone-related dynamics and the Southern Annular Mode, cooperates to relay the zonal wind anomalies to the upper troposphere. The wave–mean flow interaction and the meridional circulation work together in the form of the Southern Annular Mode, which transfers anomalous wind signals downward to the surface, triggering a striking strengthening of surface wind stress over the Southern Ocean.

On the theory of the wind-driven ocean circulation

1962 ◽  
Vol 12 (1) ◽  
pp. 49-80 ◽  
Author(s):  
G. F. Carrier ◽  
A. R. Robinson
Keyword(s):  
Wind Stress ◽  
Ocean Circulation ◽  
Surface Wind ◽  
Inviscid Flow ◽  
Relative Vorticity ◽  
Wind Stress Curl ◽  
Meridional Transport ◽  
Surface Distribution ◽  
Surface Wind Stress ◽  
Mean Circulation

A surface distribution of stress is imposed on an ocean enclosed by two continental boundaries; the resulting transport circulation is studied between two latitudes of zero surface wind-stress curl, within which the curl reaches a single maximum. Under the assumption that turbulent transfer of relative vorticity has a minimum effect on the mean circulation, inviscid flow patterns are deduced in the limit of small transport Rossby number. Inertial currents, or naturally scaled regions of high relative vorticity, occur on both the eastern and the western continental coasts. Limits on the relative transports of the currents are obtained and found to depend on the direction of variation of the wind-stress curl with latitude, relative to that of the Coriolis accelerations. The most striking feature of the inviscid flow is a narrow inertial current the axis of which lies along the latitude of maximum wind-stress curl. All eastward flow occurs in this midlatitude jet.A feature of the flow which cannot remain essentially free of turbulent processes is the integrated vorticity relationship, since the imposed wind-stress distribution acts as a net source of vorticity for the ocean. Heuristic arguments are used together with this integral constraint to deduce the presence and strength of the turbulent diffusion which must occur in the region of the mid-latitude jet. It is further inferred that the turbulent meanders of the jet must effect a net meridional transport of relative vorticity.

Estimates and Implications of Surface Eddy Diffusivity in the Southern Ocean Derived from Tracer Transport

2006 ◽  
Vol 36 (9) ◽  
pp. 1806-1821 ◽  
Author(s):  
John Marshall ◽  
Emily Shuckburgh ◽  
Helen Jones ◽  
Chris Hill
Keyword(s):  
Southern Ocean ◽  
Antarctic Circumpolar Current ◽  
Surface Wind ◽  
Effective Diffusivity ◽  
Eddy Diffusivity ◽  
Tracer Transport ◽  
Near Surface ◽  
Surface Wind Stress ◽  
Circumpolar Current ◽  
The Antarctic

Abstract Near-surface “effective diffusivities” associated with geostrophic eddies in the Southern Ocean are estimated by numerically monitoring the lengthening of idealized tracer contours as they are strained by surface geostrophic flow observed by satellite altimetry. The resulting surface diffusivities show considerable spatial variability and are large (2000 m2 s−1) on the equatorward flank of the Antarctic Circumpolar Current and are small (500 m2 s−1) at the jet axis. Regions of high and low effective diffusivity are shown to be collocated with regions of, respectively, weak and strong isentropic potential vorticity gradients. The maps of diffusivity are used, along with climatological estimates of surface wind stress and air–sea buoyancy flux, to estimate surface meridional residual flows and the relative importance of Eulerian and eddy-induced circulation in the streamwise-averaged dynamics of the Antarctic Circumpolar Current.

Observations of SST-Induced Perturbations of the Wind Stress Field over the Southern Ocean on Seasonal Timescales

2003 ◽  
Vol 16 (14) ◽  
pp. 2340-2354 ◽  
Author(s):  
Larry W. O'Neill ◽  
Dudley B. Chelton ◽  
Steven K. Esbensen
Keyword(s):  
Southern Ocean ◽  
Wind Stress ◽  
Surface Wind ◽  
Wind Stress Curl ◽  
Latitude Range ◽  
Surface Wind Stress ◽  
Secondary Circulations ◽  
Vertical Turbulent Mixing ◽  
Wind Stress Field ◽  
Sst Fronts

Abstract The surface wind stress response to sea surface temperature (SST) over the latitude range 30°–60°S in the Southern Ocean is described from the National Aeronautics and Space Administration's QuikSCAT scatterometer observations of wind stress and Reynolds analyses of SST during the 2-yr period August 1999 to July 2001. While ocean–atmosphere coupling at midlatitudes has previously been documented from several case studies, this is the first study to quantify this relation over the entire Southern Ocean. The spatial structures of the surface wind perturbations with wavelengths shorter than 10° latitude by 30° longitude are closely related to persistent spatial variations of the SST field on the same scales. The wind stress curl and divergence are shown to be linearly related, respectively, to the crosswind and downwind components of the SST gradient. The curl response has a magnitude only about half that of the divergence response. This observed coupling is consistent with the hypothesis that SST modification of marine atmospheric boundary layer (MABL) stability affects vertical turbulent mixing of momentum, inducing perturbations in the surface winds. The nonequivalence between the responses of the curl and divergence to the crosswind and downwind SST gradients suggests that secondary circulations in the MABL may also play an important role by producing significant perturbations in the surface wind field near SST fronts that are distinct from the vertical turbulent transfer of momentum. The importance of the wind stress curl in driving Ekman vertical velocity in the open ocean implies that the coupling between winds and SST may have important feedback effects on upper ocean processes near SST fronts.

scholarly journals Southern Ocean surface pressure and winds during the 20th century from proxy-data assimilation

2021 ◽  
Author(s):  
Gemma O'Connor ◽  
Eric Steig ◽  
Gregory Hakim
Keyword(s):  
Data Assimilation ◽  
Southern Ocean ◽  
20Th Century ◽  
Ocean Circulation ◽  
Large Scale ◽  
Surface Wind ◽  
Climate Data ◽  
Sea Ice Extent ◽  
Amundsen Sea ◽  
Global Database

Abstract Winds and pressure over the Southern Ocean are critical to many aspects of the climate system, including ocean circulation and carbon uptake, sea ice extent, and the mass balance of Antarctica. However, reliable climate data around Antarctica begin only in 1979. Here, we reconstruct sea level pressure and zonal surface wind anomalies over the Southern Ocean through the 20th century, using data assimilation with a global database of paleoclimate proxy records. There is very good agreement between the reconstructions and satellite-based reanalysis products both at the large scale and in the smaller Amundsen Sea, a key region of West Antarctica where rapid glacier retreat has occurred in recent decades. The reconstructions show insignificant trends in the zonal-average circumpolar westerlies, but a significant strengthening in mid-latitude Pacific westerlies, associated with a deepening of the Amundsen Sea Low, beginning well before the satellite era. The mean zonal-wind trend along the continental shelf break in the Amundsen Sea is easterly through the 20th century, contrasting with previous results that have suggested that glacier change in this region can be attributed to strengthening westerlies. Our reconstructions underscore the value of using paleoclimate data assimilation methods in assessing historical changes in Southern Hemisphere climate and suggest that zonally asymmetric features of atmospheric circulation may be key for understanding high-latitude climate and associated ice-sheet changes.

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