Mixing and Entrainment in the
Orkney Passage
Judy Twedt
University of Washington Dept. of Physics
NOAA, Geophysical Fluid Dynamics Lab
Dr. Sonya Legg
Dr. Marian Westley
July 31, 2012
This presentation is given in support of NOAA’s mission to improve scientific understanding of
the changing climate system through advances in climate modeling.
Motivation: Deep, cold waters exiting the Weddell Sea enter the
global ocean abyss through the Orkney Passage. As they navigate this
tortuous topography, they experience mixing and turbulence.
Motivation: Deep Waters Exiting the Weddell Sea enter the global
ocean abyss through the Orkney Passage. As they navigate this
tortuous topography, they experience mixing and turbulence.
This mixing potentially effects the
global ocean circulation and the
heat budget.
Motivation: Deep Waters Exiting the Weddell Sea enter the global
ocean abyss through the Orkney Passage. As they navigate this
tortuous topography, they experience mixing and turbulence.
This mixing potentially effects the
global ocean circulation and the
heat budget.
Using high resolution models, we are quantifying the mixing .
Outline
• Orientation to the topography and general
flow patterns
• The model set up
• The concept and calculations of entrainment
• The (preliminary) entrainment results*
• Onto higher resolution simulations
• New questions…
*Please note this project is in progress – simulations are still running.
The Orkney Passage is a deep cleft in the South Scotia
Ridge and controls cold, dense waters flowing out from
the Weddell Shelf.
Approximately 1/3 of Antarctic deep waters entering the global ocean
pass through the Orkney Passage (Naveira Garabato et al., 2002)
We know less about Antarctic Overflows than others because the
environment presents many challenges for field work. However, recent
observations have observed warming trends in the abyssal waters flowing out
from the Weddell Sea.
Figure: Meredith et al., 2011
Yellow lines mark the primary routes of Weddell Sea Deep Waters
through the Orkney and Georgia Passages
Methods
• The MIT general circulation model, a 3dimensional z-level finite volume model was used
in hydrostatic mode
• The horizontal resolution is 1 km; the vertical
resolution is 62 meters
• The model simulated ~ 54 days with a 50 second
time interval
• Orlanski Open Boundary Conditions were used
on all sides except the southern boundary, where
inflow forcing was specified
Plan View of Model Domain with
Inflow Forcing
N
The Model Domain: Aspect 1
Outflow to Scotia Sea
Inflow from Weddell Sea
The Model Domain: Aspect 2
Outflow to Scotia Sea
Inflow from Weddell Sea
Model Domain: Aspect 3
Inflow from Weddell Sea
Outflow to Scotia Sea
Profile of Constant Inflow Conditions
Velocity: 10 cm/s
Temperature: -0.7 C
Salinity: 33.65ppm
Chosen to match observed values.
The inflow is also marked with a tracer equal to one.
The inflow is colder and denser than the surrounding waters.
Time Evolution of Plume Thickness
From day 0 to day 54
Understanding Entrainment
• Concept: As dense water flows downslope
through a confined region, it accelerates under
gravity and may develop a strong shear at the
interface with the lighter waters and with the
topography.
• This may cause turbulent mixing and the lighter,
overlaying water becomes entrained with the
dense flowing bottom layer.
• This increases the volume of the plume, and
changes its properties
Quantifying Entrainment:
The Streamtube Model
Entrainment =
transport out – transport in + dVdt
dVdt
plume transport In
plume transport Out
Entrainment Rate = entrainment/plume surface
Entrainment in the Orkney Passage
To determine the transport:
• Define the inflow and
outflow boundaries
• Determine the boundaries of
the plume by identifying
those cells that have >1%
tracer concentration
• Integrate the horizontal
velocities through the
boundaries
To determine the volume derivative:
• Define the boundaries of the sidewalls
• Integrate the volume within the plume
• Calculate the time derivative
Evolution of Flow Over Sill
Quasi-steady state in middle of time series used for
entrainment calculations
Time average of entrainment and entrainment rates
In comparison to other overflows…
Preliminary estimate of
entrainment coefficient
from a 1km horizontal
resolution:
?
?
Table Legg. et al., 2009. Data from Field Campaigns
𝟑. 𝟔𝒙𝟏𝟎−𝟑
By running higher
resolution simulations,
we hope to achieve a
more accurate estimate
Model Simulations in Progress
• Stretched grid from 3km to 200 meters near
the Orkney Passage and fixed resolution of 62
meters in the horizontal
• Variations of the size and angle of specified
inflow
Modeling turbulence has been a
turbulent process!
• Original simulations had problems with the inflow boundaries
• Current simulations develop instabilities late in the run, when the
plume reaches and begins to flow out of the domain
Conclusions, and New Questions
• The Orkney Passage does appear be a location of
entrainment of Weddell Sea Deep Waters
• Within the models, how sensitive is the
entrainment to horizontal and vertical
resolutions?
• Do wind-driven changes in circulation within the
Weddell Sea affect mixing in the Orkney Passage?
• How does this compare with current field
estimates?
Gratitude!
• Many thanks to the friendly and supportive staff
of the NOAA Hollings Scholarship Program, for
making this experience possible
• Many thanks to Sonya Legg and Marian Westley
of GFDL for their feedback, ideas, and support
• Many thanks to you, the audience, for
participating in this presentation. I hope it
sparked a curiosity or left you with an interesting
idea
References
• Bathymetry map accessed at 7/29/12
http://topex.ucsd.edu/marine_topo/jpg_images/topo16.jpg
• Gordon, A. et. al., 2001. Export of Weddell Sea Deep and Bottom
Water, J. of Geophysical Research Vol. 106, 9005-9017.
• Meredith, M. et. Al., 2011. Synchronous Intensification and
Warming of Antarctic Bottom Water Outflow from the Weddell
Gyre. Geophysical Research Letters, Vol 30., L03603
• Riemenschneider, Ulrike and Sonya Legg,2007. Regional
Simulations of the Faroe Bank Channel Overflow in a Level Model.
Ocean Modelling 17, 92-122.