PowerPoint Presentation
Mixing and Entrainment in the Orkney PassageJudy TwedtUniversity
of Washington Dept. of PhysicsNOAA, Geophysical Fluid Dynamics
LabDr. Sonya LeggDr. Marian WestleyJuly 31, 2012
This presentation is given in support of NOAAs mission to
improve scientific understanding of the changing climate system
through advances in climate modeling.NOAA symbol, into1Motivation:
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.
Using high resolution models, we are quantifying the mixing .
This mixing potentially effects the global ocean circulation and
the heat budget. OutlineOrientation to the topography and general
flow patternsThe model set up The concept and calculations of
entrainmentThe (preliminary) entrainment results*Onto higher
resolution simulationsNew questions*Please note this project is in
progress simulations are still running.Outline the Presentation5The
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.
Yellow lines mark the primary routes of Weddell Sea Deep Waters
through the Orkney and Georgia Passages
Figure: Meredith et al., 2011MethodsThe MIT general circulation
model, a 3-dimensional 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 intervalOrlanski Open Boundary Conditions were used on
all sides except the southern boundary, where inflow forcing was
specified
Discuss OB conditions- they were important for allowing flow
through the domain, and so that waves would not propagate off the
sides8 Plan View of Model Domain with Inflow Forcing
ottNThe Model Domain: Aspect 1
Inflow from Weddell SeaOutflow to Scotia SeaThe Model Domain:
Aspect 2
Inflow from Weddell SeaOutflow to Scotia SeaModel Domain: Aspect
3
Inflow from Weddell SeaOutflow to Scotia SeaProfile of Constant
Inflow Conditions
Velocity: 10 cm/sTemperature: -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 54Effects of rotation are evident in the
development of eddies. Plume roughly follows contours; significant
outflow along eastern boundary. 14Understanding 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
Dense FlowConcept of Entrainment; the formulas and calculations
(brief!) 15Quantifying Entrainment: The Streamtube ModelEntrainment
= transport out transport in + dVdt
Entrainment Rate = entrainment/plume surface
plume transport Inplume transport OutdVdtEntrainment in the
Orkney Passage
To determine the transport:Define the inflow and outflow
boundariesDetermine the boundaries of the plume by identifying
those cells that have >1% tracer concentrationIntegrate the
horizontal velocities through the boundariesTo determine the volume
derivative:Define the boundaries of the sidewallsIntegrate the
volume within the plumeCalculate the time derivativeEvolution of
Flow Over Sill
Quasi-steady state in middle of time series used for entrainment
calculations
Time average of entrainment and entrainment ratesIn comparison
to other overflows?Table Legg. et al., 2009. Data from Field
CampaignsPreliminary estimate of entrainment coefficientfrom a 1km
horizontal resolution:By running higher resolution simulations, we
hope to achieve a more accurate estimate
?These are calculated in very small regions. By looking at such
a large area with little entrainment in the Orkney trough, the
value is likely to be an underestimate. 21Model Simulations in
ProgressStretched grid from 3km to 200 meters near the Orkney
Passage and fixed resolution of 62 meters in the
horizontalVariations of the size and angle of specified inflow
Modeling turbulence has been a turbulent process!Original
simulations had problems with the inflow boundariesCurrent
simulations develop instabilities late in the run, when the plume
reaches and begins to flow out of the domainWhere were going:
higher resolution simulations that capture the dissipation of
energy more accurately22Conclusions, and New QuestionsThe Orkney
Passage does appear be a location of entrainment of Weddell Sea
Deep WatersWithin 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
possibleMany thanks to Sonya Legg and Marian Westley of GFDL for
their feedback, ideas, and supportMany thanks to you, the audience,
for participating in this presentation. I hope it sparked a
curiosity or left you with an interesting idea
ReferencesBathymetry 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.