scholarly journals The Ocean General Circulation near 1000-m Depth

2014 ◽  
Vol 44 (1) ◽  
pp. 384-409 ◽  
Author(s):  
Michel Ollitrault ◽  
Alain Colin de Verdière
Keyword(s):  
Ocean Circulation ◽  
General Circulation ◽  
Neumann Boundary ◽  
Reference Level ◽  
Western Boundary ◽  
Boundary Currents ◽  
Zonal Jets ◽  
The Mean ◽  
Circumpolar Current ◽  
The Antarctic

Abstract The mean ocean circulation near 1000-m depth is estimated with 100-km resolution from the Argo float displacements collected before 1 January 2010. After a thorough validation, the 400 000 or so displacements found in the 950–1150 dbar layer and with parking times between 4 and 17 days allow the currents to be mapped at intermediate depths with unprecedented details. The Antarctic Circumpolar Current (ACC) is the most prominent feature, but western boundary currents (and their recirculations) and alternating zonal jets in the tropical Atlantic and Pacific are also well defined. Eddy kinetic energy (EKE) gives the mesoscale variability (on the order of 10 cm2 s−2 in the interior), which is compared to the surface geostrophic altimetric EKE showing e-folding depths greater than 700 m in the ACC and northern subpolar regions. Assuming planetary geostrophy, the geopotential height of the 1000-dbar isobar is estimated to obtain an absolute and deep reference level worldwide. This is done by solving numerically the Poisson equation that results from taking the divergence of the geostrophic equations on the sphere, assuming Neumann boundary conditions.

scholarly journals Ocean and Atmosphere Storm Tracks: The Role of Eddy Vorticity Forcing

2007 ◽  
Vol 37 (9) ◽  
pp. 2267-2289 ◽  
Author(s):  
Richard G. Williams ◽  
Chris Wilson ◽  
Chris W. Hughes
Keyword(s):  
Storm Track ◽  
Heat Fluxes ◽  
Storm Tracks ◽  
Mean Flow ◽  
Ocean Eddies ◽  
Boundary Currents ◽  
The Mean ◽  
Circumpolar Current ◽  
The Antarctic

Abstract Signatures of eddy variability and vorticity forcing are diagnosed in the atmosphere and ocean from weather center reanalysis and altimetric data broadly covering the same period, 1992–2002. In the atmosphere, there are localized regions of eddy variability referred to as storm tracks. At the entrance of the storm track the eddies grow, providing a downgradient heat flux and accelerating the mean flow eastward. At the exit and downstream of the storm track, the eddies decay and instead provide a westward acceleration. In the ocean, there are similar regions of enhanced eddy variability along the extension of midlatitude boundary currents and the Antarctic Circumpolar Current. Within these regions of high eddy kinetic energy, there are more localized signals of high Eady growth rate and downgradient eddy heat fluxes. As in the atmosphere, there are localized regions in the Southern Ocean where ocean eddies provide statistically significant vorticity forcing, which acts to accelerate the mean flow eastward, provide torques to shift the jet, or decelerate the mean flow. These regions of significant eddy vorticity forcing are often associated with gaps in the topography, suggesting that the ocean jets are being locally steered by topography. The eddy forcing may also act to assist in the separation of boundary currents, although the diagnostics of this study suggest that this contribution is relatively small when compared with the advection of planetary vorticity by the time-mean flow.

scholarly journals Radiating Instability of a Meridional Boundary Current

2008 ◽  
Vol 38 (10) ◽  
pp. 2294-2307 ◽  
Author(s):  
Hristina G. Hristova ◽  
Joseph Pedlosky ◽  
Michael A. Spall
Keyword(s):  
Western Boundary ◽  
Far Field ◽  
Boundary Current ◽  
Important Conclusion ◽  
Western Boundary Currents ◽  
Horizontal Structure ◽  
Boundary Currents ◽  
Zonal Jets ◽  
Eastern Boundary ◽  
Eastern Boundary Current

Abstract A linear stability analysis of a meridional boundary current on the beta plane is presented. The boundary current is idealized as a constant-speed meridional jet adjacent to a semi-infinite motionless far field. The far-field region can be situated either on the eastern or the western side of the jet, representing a western or an eastern boundary current, respectively. It is found that when unstable, the meridional boundary current generates temporally growing propagating waves that transport energy away from the locally unstable region toward the neutral far field. This is the so-called radiating instability and is found in both barotropic and two-layer baroclinic configurations. A second but important conclusion concerns the differences in the stability properties of eastern and western boundary currents. An eastern boundary current supports a greater number of radiating modes over a wider range of meridional wavenumbers. It generates waves with amplitude envelopes that decay slowly with distance from the current. The radiating waves tend to have an asymmetrical horizontal structure—they are much longer in the zonal direction than in the meridional, a consequence of which is that unstable eastern boundary currents, unlike western boundary currents, have the potential to act as a source of zonal jets for the interior of the ocean.

scholarly journals Interannual response of global ocean hindcasts to a satellite-based correction of precipitation fluxes

2012 ◽  
Vol 9 (2) ◽  
pp. 611-648 ◽  
Author(s):  
A. Storto ◽  
I. Russo ◽  
S. Masina
Keyword(s):  
Ocean Circulation ◽  
Surface Current ◽  
Global Ocean ◽  
Surface Salinity ◽  
Near Surface ◽  
Convergence Zones ◽  
Circumpolar Current ◽  
The Tropics ◽  
Spatially Varying ◽  
The Antarctic

Abstract. We present a methodology to correct precipitation fluxes from the ECMWF atmospheric reanalysis (ERA-Interim) for oceanographic applications. The correction is performed by means of a spatially varying monthly climatological coefficient, computed within the period 1989–2008 by comparison between ERA-Interim and a satellite-based passive microwave precipitation product. ERA-Interim exhibits a systematic over-estimation of precipitation within the inter-tropical convergence zones (up to 3 mm d−1) and under-estimation at mid- and high- latitudes (up to −4 mm d−1). The correction has been validated within eddy-permitting resolution global ocean hindcasts (1989–2009), demonstrating the ability of our strategy in attenuating the 20-yr mean global EMP negative imbalance by 16%, reducing the near-surface salinity fresh bias in the Tropics up to 1 psu and improving the representation of the sea level interannual variability, with an SSH error decrease of 8%. The ocean circulation is also proved to benefit from the correction, especially in correspondence of the Antarctic Circumpolar Current, where the error in the near-surface current speed decreases by a 9%. Finally, we show that the correction leads to volume and freshwater transports that better agree with independent estimates.

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.

Global Ocean Meridional Overturning

2007 ◽  
Vol 37 (10) ◽  
pp. 2550-2562 ◽  
Author(s):  
Rick Lumpkin ◽  
Kevin Speer
Keyword(s):  
Ocean Circulation ◽  
Global Scale ◽  
The Arctic ◽  
Global Ocean ◽  
Mixing Processes ◽  
Boundary Currents ◽  
Antarctic Continent ◽  
Meridional Overturning ◽  
Global Ocean Circulation ◽  
The Antarctic

Abstract A decade-mean global ocean circulation is estimated using inverse techniques, incorporating air–sea fluxes of heat and freshwater, recent hydrographic sections, and direct current measurements. This information is used to determine mass, heat, freshwater, and other chemical transports, and to constrain boundary currents and dense overflows. The 18 boxes defined by these sections are divided into 45 isopycnal (neutral density) layers. Diapycnal transfers within the boxes are allowed, representing advective fluxes and mixing processes. Air–sea fluxes at the surface produce transfers between outcropping layers. The model obtains a global overturning circulation consistent with the various observations, revealing two global-scale meridional circulation cells: an upper cell, with sinking in the Arctic and subarctic regions and upwelling in the Southern Ocean, and a lower cell, with sinking around the Antarctic continent and abyssal upwelling mainly below the crests of the major bathymetric ridges.

scholarly journals Time-Mean Flow as the Prevailing Contribution to the Poleward Heat Flux across the Southern Flank of the Antarctic Circumpolar Current: A Case Study in the Fawn Trough, Kerguelen Plateau

2013 ◽  
Vol 43 (3) ◽  
pp. 583-601 ◽  
Author(s):  
H. Sekma ◽  
Y.-H. Park ◽  
F. Vivier
Keyword(s):  
Heat Flux ◽  
Southern Ocean ◽  
Antarctic Circumpolar Current ◽  
Mean Flow ◽  
Kerguelen Plateau ◽  
The Mean ◽  
The Cross ◽  
Circumpolar Current ◽  
The Antarctic ◽  
Southern Flank

Abstract The major mechanisms of the oceanic poleward heat flux in the Southern Ocean are still in debate. The long-standing belief stipulates that the poleward heat flux across the Antarctic Circumpolar Current (ACC) is mainly due to mesoscale transient eddies and the cross-stream heat flux by time-mean flow is insignificant. This belief has recently been challenged by several numerical modeling studies, which stress the importance of mean flow for the meridional heat flux in the Southern Ocean. Here, this study analyzes moored current meter data obtained recently in the Fawn Trough, Kerguelen Plateau, to estimate the cross-stream heat flux caused by the time-mean flow and transient eddies. It is shown that the poleward eddy heat flux in this southern part of the ACC is negligible, while that from the mean flow is overwhelming by two orders of magnitude. This is due to the unusual anticlockwise turning of currents with decreasing depth, which is associated with significant bottom upwelling engendered by strong bottom currents flowing over the sloping topography of the trough. The circumpolar implications of these local observations are discussed in terms of the depth-integrated linear vorticity budget, which suggests that the six topographic features along the southern flank of the ACC equivalent to the Fawn Trough case would yield sufficient poleward heat flux to balance the oceanic heat loss in the subpolar region. As eddy activity on the southern flank of the ACC is too weak to transport sufficient heat poleward, the nonequivalent barotropic structure of the mean flow in several topographically constricted passages should accomplish the required task.

scholarly journals Energy of the jets of the Antarctic circumpolar current and of their eddies in the surface layer of the Southern Ocean

2019 ◽  
Vol 59 (3) ◽  
pp. 325-334
Author(s):  
M. N. Koshlyakov ◽  
D. S. Savchenko ◽  
R. Yu. Tarakanov
Keyword(s):  
Surface Layer ◽  
Kinetic Energy ◽  
Antarctic Circumpolar Current ◽  
The Internet ◽  
Satellite Altimeter ◽  
Synoptic Eddies ◽  
Ocean Layer ◽  
The Mean ◽  
Circumpolar Current ◽  
The Antarctic

Kinetic energy of six jets of the Antarctic Circumpolar Current (ACC) and of the cyclonic and anticyclonic synoptic eddies generated by these jets is studied in application to the surface layer of Antarctic Circle. The study is based on the data of satellite altimeter observations during 1993–2015 available in the Internet (http://aviso.altimetry.fr). Main results of the study: a) five times excess of the mean energy of jets proper over the mean summary (cyclones plus anticyclones) energy of eddies; b) prevalence of the energy of middle jet of Subantarctic Current over energy of the rest ACC jets in the whole of Antarctic Circle; c) two times excess of mean energy of cyclonic eddies over energy of anticyclones in the upper ocean layer.

The Lagrangian divergence of heat and salt: A new method to determine water mass transformations

2021 ◽  
Author(s):  
Sara Berglund ◽  
Kristofer Döös
Keyword(s):  
Ocean Circulation ◽  
Water Mass ◽  
Mixed Layer Depth ◽  
Conveyor Belt ◽  
New Method ◽  
The North ◽  
Internal Mixing ◽  
Circumpolar Current ◽  
The Tropics ◽  
The Antarctic

<p>Water mass transformation is an important part of the Ocean circulation. Lagrangian trajectories can be used to connect pathways with water mass properties such as temperature and salinity. Here, we will introduce the Lagrangian divergence of heat and salt that can be computed using Lagrangian trajectories. This is a new method that can be used to determine where water masses are changing temperature or salinity geographically.<br>Further, the following two examples on how to use the Lagrangian divergence will be given:</p><p>(1) In the Atlantic Ocean water flows northward and transform from warm and saline to cold and fresh. The Lagrangian divergence has been used to show that this cooling and freshening is confined to the North Atlantic Subtropical Gyre.</p><p>(2) Waters in the upper limb of the Southern Hemisphere Conveyor Belt circulation converts from cold and fresh to warm and saline as it travels from the Southern Ocean to the tropics. The Lagrangian divergence shows that this warming and salinification are confined to the Antarctic Circumpolar Current, the southern subtropical gyres, and the equator. In this study, the Lagrangian divergence are separated by the mixed layer depth, which distinguishes if a change in heat and salt is driven by internal mixing or air--sea interactions.</p>

Barotropic and baroclinic inverse kinetic energy cascade in the Antarctic Circumpolar Current

2021 ◽  
Author(s):  
Hongjie Li ◽  
Yongsheng Xu
Keyword(s):  
Kinetic Energy ◽  
General Circulation ◽  
Antarctic Circumpolar Current ◽  
General Circulation Models ◽  
Energy Cascade ◽  
Turbulence Theory ◽  
Inverse Cascade ◽  
Spectral Flux ◽  
Circumpolar Current ◽  
The Antarctic

AbstractStratified geostrophic turbulence theory predicts an inverse energy cascade for the barotropic (BT) mode. Satellite altimetry has revealed a net inverse cascade in the baroclinic (BC) mode. Here the spatial variabilities of BT and BC kinetic energy fluxes in the Antarctic Circumpolar Current (ACC) were investigated using ECCO2 data, which synthesizes satellite data and in situ measurements with an eddy-permitting general circulation models containing realistic bathymetry and wind forcing. The BT and BC inverse kinetic energy cascades both reveal complex spatial variations that could not be explained fully by classical arguments. For example, the BC injection scales match better with most unstable scales than with the first-mode deformation scales, but the opposite is true for the BT mode. In addition, the BT and BC arrest scales do not follow the Rhines scale well in term of spatial variation, but show better consistency with their own energy-containing scales. The reverse cascade of the BT and BC modes was found related to their EKE, and better correlation was found between the BT inverse cascade and barotropization. Speculations of the findings were proposed. however, further observations and modeling experiments are needed to test these interpretations. Spectral flux anisotropy exhibits a feature associated with oceanic jets that is consistent with classical expectations. Specifically, the spectral flux along the along-stream direction remains negative at scales up to that of the studied domain (~2000km), while that in the perpendicular direction becomes positive close to the scale of the width of a typical jet.

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