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.