Zonal Momentum Balance in the Antarctic Circumpolar Current
Xihan Zhang
Supervisory team: Maxim Nikurashin, Beatriz Pena Molino, Stephen Rintoul, Edward Doddridge
Maintenance of the zonal momentum balance of the Antarctic Circumpolar
Current (ACC) by barotropic dynamics
•
Depth-integrated momentum balance
of the Antarctic Circumpolar Current
(ACC) is between
wind stress
and
topographic form stress (TFS)
(e.g.,
Munk & Palm 1950)
•
Topographic form stress arises from
pressure difference across topography
•
This depth-integrated balance suggests
that momentum is ‘transmitted’ from
the surface to the bottom throughout
the water column (e.g., Johnson and
Bryden, 1989)
Stewart and Hogg 2017
Background: momentum balance between wind stress and TFS
TFS
Marshall and Radko 2003
Background: Eddies transfer momentum downwards (TEM framework)
•
wind stress matches to eddy momentum stress
in the limit of zero residual circulation
eddy momentum stress
Meridional isopycnal structure
•
TEM and isopycnal frameworks naturally put emphasis on
baroclinic
structure/eddy fluxes for the maintenance of the
ACC momentum balance
•
Howard et al (2015) shows that TFS is quickly established
through barotropic dynamics during the initial transient
phase, but then they argue that eddies are responsible for
the momentum transfer at equilibrium
•
In this study, we explore the maintenance and adjustment
of the ACC momentum balance from a Eulerian, depth-
coordinate perspective
•
We show that the ACC momentum balance is not only
initially established, but also maintained at equilibrium
and responds to changes in wind by
barotropic
dynamics
Howard et al (2015)
Background: Barotropic vs baroclinic dynamics for the ACC momentum balance
Year
TFS
Eddy momentum stress
Method: an idealized channel model with a ridge
•
MITgcm
•
Zonally re-entrant channel, depth-coordinate model
•
A Gaussian ridge in the middle of the domain
•
10-km horizontal resolution
•
Stratified (barotropic/baroclinic) & homogenous (only barotropic) simulations
Model bathymetry and mean SSH
A snapshot of surface temperature
H
L
H
L
L
H
Mechanism: Spin up from rest
y
z
•
Surface Ekman layer piles water in the north, which drives a
zonal flow through barotropic dynamics (SSH slope)
•
Zonal flow interacts with a ridge and
sets TFS
. Return flow,
balanced by TFS, closes the total volume budget (i.e.,
equilibrates SSH)
•
Momentum balances (wind stress = Coriolis at the surface
and Coriolis = TFS at the bottom) are closed without
momentum stresses (i.e.., transfer) in the interior
•
Upwelling and downwelling formed from continuity steepen
isopycnals in a stratified flow
•
Steepening isopycnals generate baroclinic pressure gradient
that opposes barotropic pressure gradient due to SSH slope
•
SSH stops tilting when baroclinic eddies appear and act to
flatten the isopycnals (i.e., baroclinic structure reaches
equilibrium and mass budget is closed)
Red
: barotropic dynamics
Blue
: baroclinic dynamics
•
Thus, SSH keeps tilting to
maintain the TFS
and
compensate for the isopycnal steepening
H
L
L
H
H
L
Mechanism: Response to changes in wind
Red
: barotropic dynamics
Blue
: baroclinic dynamics
When wind stress increases,
baroclinic
structure is
saturated, and it is the
barotropic
dynamics that
respond and adjust the system to a new equilibrium:
y
z
•
Increased wind stress drives a stronger
northward flow in the surface Ekman layer
•
SSH slopes more significantly and generates a
stronger barotropic pressure gradient
•
The stronger barotropic pressure increases the
bottom flow, resulting in stronger TFS (and
return flow) that balances the increased wind
(Ekman transport)
Results: Response to wind – saturated
baroclinicity
•
Baroclinic dynamics is saturated
•
Isopycnal slopes & Baroclinic transport have no
adjustment to wind
•
It is the barotropic transport that changes and
account for the increase in the total transport
Wind doubled
Results: Response to wind – adjustment by
barotropic dynamics
•
Depth-integrated momentum balance is
readjusted to a new balance within the
first month after doubling of the wind
•
Return flow at the bottom shows the
same adjustment as during the spin-up
stage, which is related to the
establishment of TFS
•
Stratified simulation and homogenous
simulation undergo a similar adjustment
in response to changes in wind,
suggesting that baroclinic eddies are not
essential for the wind-TFS balance
Adjustment of the return flow to Ekman
Adjustment of TFS to wind stress
Wind doubled
Summary
•
We explore the maintenance of the ACC zonal momentum balance from a Eulerian, depth-
coordinate perspective. While it is mathematically equivalent to TEM (or, isopycnal framework), it
offers a different and useful insight in the governing dynamics
•
In this framework, eddy fluxes are part of the buoyancy budget and momentum balances at the
surface and bottom are connected by flow continuity and regulated by pressure fields without
momentum stresses in the interior.
•
It is the barotropic dynamics that maintains the depth-integrated momentum balance between
wind stress and TFS, compensating also for the opposing effect of baroclinicity
•
The critical role of the barotropic dynamics in this balance explains the short time-scale of the ACC
(TFS, total transport, eddy fluxes) adjustment to wind
This study investigates the zonal momentum balance of the Antarctic Circumpolar Current (ACC) by analyzing the interplay between wind stress, topographic form stress, and eddy dynamics. The research explores the maintenance and adjustment of momentum balance in the ACC, emphasizing the roles of barotropic and baroclinic dynamics. Using an idealized channel model, the study sheds light on the complex mechanisms that regulate the zonal momentum transfer in this crucial oceanic current.
Download Presentation
Please find below an Image/Link to download the presentation.
The content on the website is provided AS IS for your information and personal use only. It may not be sold, licensed, or shared on other websites without obtaining consent from the author.If you encounter any issues during the download, it is possible that the publisher has removed the file from their server.
You are allowed to download the files provided on this website for personal or commercial use, subject to the condition that they are used lawfully. All files are the property of their respective owners.
The content on the website is provided AS IS for your information and personal use only. It may not be sold, licensed, or shared on other websites without obtaining consent from the author.
E N D
Presentation Transcript
Maintenance of the zonal momentum balance of the Antarctic Circumpolar Current (ACC) by barotropic dynamics Xihan Zhang Supervisory team: Maxim Nikurashin, Beatriz Pena Molino, Stephen Rintoul, Edward Doddridge
Background: momentum balance between wind stress and TFS Depth-integrated momentum balance of the Antarctic Circumpolar Current (ACC) is between wind stress and topographic form stress (TFS) (e.g., Munk & Palm 1950) Topographic form stress arises from pressure difference across topography TFS This depth-integrated balance suggests that momentum is transmitted from the surface to the bottom throughout the water column (e.g., Johnson and Bryden, 1989) Stewart and Hogg 2017
Background: Eddies transfer momentum downwards (TEM framework) Bringing eddy buoyancy fluxes into the zonal momentum equation, TEM framework connects the meridional overturning circulation (MOC) (i.e., isopycnal flattening) to the ACC zonal momentum via Eliassen-Palm (EP) fluxes, i.e., eddy momentum stresses. Eddies flux buoyancy meridionally (? ? ), flattening isopycnals, thereby redistributing the zonal velocity shear vertically This effect is brought into zonal momentum equation represented by eddy momentum fluxes Meridional isopycnal structure ? ??(? ? ? = eddy momentum stress ) Marshall and Radko 2003 ?? ??(?? ? ? ? ? =? ? ?? ?? ) wind stress matches to eddy momentum stress in the limit of zero residual circulation ?? ?? ?????
Background: Barotropic vs baroclinic dynamics for the ACC momentum balance TEM and isopycnal frameworks naturally put emphasis on baroclinic structure/eddy fluxes for the maintenance of the ACC momentum balance Eddy momentum stress Howard et al (2015) shows that TFS is quickly established through barotropic dynamics during the initial transient phase, but then they argue that eddies are responsible for the momentum transfer at equilibrium In this study, we explore the maintenance and adjustment of the ACC momentum balance from a Eulerian, depth- coordinate perspective TFS We show that the ACC momentum balance is not only initially established, but also maintained at equilibrium and responds to changes in wind by barotropic dynamics Year Howard et al (2015)
Method: an idealized channel model with a ridge MITgcm Zonally re-entrant channel, depth-coordinate model A Gaussian ridge in the middle of the domain 10-km horizontal resolution Stratified (barotropic/baroclinic) & homogenous (only barotropic) simulations A snapshot of surface temperature Model bathymetry and mean SSH
Mechanism: Spin up from rest Surface Ekman layer piles water in the north, which drives a zonal flow through barotropic dynamics (SSH slope) Zonal flow interacts with a ridge and sets TFS. Return flow, balanced by TFS, closes the total volume budget (i.e., equilibrates SSH) Upwelling and downwelling formed from continuity steepen isopycnals in a stratified flow Steepening isopycnals generate baroclinic pressure gradient that opposes barotropic pressure gradient due to SSH slope Thus, SSH keeps tilting to maintain the TFS and compensate for the isopycnal steepening H H L L z H L SSH stops tilting when baroclinic eddies appear and act to flatten the isopycnals (i.e., baroclinic structure reaches equilibrium and mass budget is closed) y Momentum balances (wind stress = Coriolis at the surface and Coriolis = TFS at the bottom) are closed without momentum stresses (i.e.., transfer) in the interior Red: barotropic dynamics Blue: baroclinic dynamics
Mechanism: Response to changes in wind When wind stress increases, baroclinic structure is saturated, and it is the barotropic dynamics that respond and adjust the system to a new equilibrium: Increased wind stress drives a stronger northward flow in the surface Ekman layer SSH slopes more significantly and generates a stronger barotropic pressure gradient The stronger barotropic pressure increases the bottom flow, resulting in stronger TFS (and return flow) that balances the increased wind (Ekman transport) z L H H L H L y Red: barotropic dynamics Blue: baroclinic dynamics
Baroclinic dynamics is saturated Results: Response to wind saturated baroclinicity Isopycnal slopes & Baroclinic transport have no adjustment to wind It is the barotropic transport that changes and account for the increase in the total transport Wind doubled
Results: Response to wind adjustment by barotropic dynamics Wind doubled Depth-integrated momentum balance is readjusted to a new balance within the first month after doubling of the wind Adjustment of the return flow to Ekman Return flow at the bottom shows the same adjustment as during the spin-up stage, which is related to the establishment of TFS Stratified simulation and homogenous simulation undergo a similar adjustment in response to changes in wind, suggesting that baroclinic eddies are not essential for the wind-TFS balance Adjustment of TFS to wind stress
Summary We explore the maintenance of the ACC zonal momentum balance from a Eulerian, depth- coordinate perspective. While it is mathematically equivalent to TEM (or, isopycnal framework), it offers a different and useful insight in the governing dynamics In this framework, eddy fluxes are part of the buoyancy budget and momentum balances at the surface and bottom are connected by flow continuity and regulated by pressure fields without momentum stresses in the interior. It is the barotropic dynamics that maintains the depth-integrated momentum balance between wind stress and TFS, compensating also for the opposing effect of baroclinicity The critical role of the barotropic dynamics in this balance explains the short time-scale of the ACC (TFS, total transport, eddy fluxes) adjustment to wind
