Mixing on the Western Antarctic Mixing on the Western Antarctic Peninsula Shelf: A Component of Peninsula Shelf: A Component of
Southern Ocean GLOBECSouthern Ocean GLOBEC
Susan L. Howard, Laurence Padman, Earth and Space ResearchEarth and Space Research
and Jason HyattWoods Hole Oceanographic InstituteWoods Hole Oceanographic Institute
Study RegionStudy Region
Our Goal Our Goal
We are looking at the physical processes that shape this environment and allow this region to be suitable for krill survival over winter.
CirculationCirculation
• Antarctic Circumpolar Current (ACC) at shelf break
• Fresh Southward Coastal Current
Hydrographic StructureHydrographic Structure
Our Goal Our Goal
• identify processes affecting the vertical heat, salt, and nutrient transport in this region; and
• estimate the vertical diffusivities and fluxes through the pycnocline.
We are looking at the physical processes that shape this environment and allow this region to be suitable for krill survival over winter.
We seek to:
OutlineOutline• Summarize our data
• Discuss 2 potentially important vertical flux processes:
– Shear-induced turbulent mixing
– Double diffusion
• Estimate total diffusivity and heat flux
Data SummaryData Summary
• 5 cruises in Year 1 (Feb-Sept 2001)
– Feb-Mar: R/V L.M. Gould Mooring Deployment Cruise
– Apr-Jun: R/V L.M. Gould Process Cruise(Fall) R/V N.B. Palmer Survey Cruise
– July-Sept: R/V Gould: Process Cruise (Winter) R/V N.B. Palmer Survey Cruise
• ADCP and CTD from all cruises• Mooring data also collected (not yet analyzed)
ADCP DataADCP Data
Mooring
Fall Process
Fall Survey
Winter Process
Winter Survey
CTD DataCTD DataFall Survey Cruise
Process 1: Shear-Driven MixingProcess 1: Shear-Driven Mixing
Evidence of Shear in the NBP0103 - Fall Survey Cruise
Distance (km)
More evidence of ShearMore evidence of ShearWinter Process Cruise
Fh 25 W m-2
Kv 1 x 10-4 m2s-1
Process 2: Double-Diffusive ConvectionProcess 2: Double-Diffusive Convection
Previous Studies => Double Diffusion important on WAP
Smith and Klinck, 2002 (LTER program)
Heat Budget (simple model study):Heat Budget (simple model study): • need 5-10 W m-2 diapycnal heating• and need different diffusivity of heat and salt.
Observations:Observations:• Found evidence that heat flux from double diffusion
was often between 3-8 W m-2.
=>vertical structure in data near coast
Winter survey
More layering
Less layering
Process 2: Double-Diffusive Convection ?Process 2: Double-Diffusive Convection ?
• R ~ 1.5 => Double Diffusion
• Most R > 3 => Weaker Double Diffusion
Winter Survey Cruise
R ~ 1.5
Fh 2-4 W m-2
ResultsResultsProcess 1: Shear Driven Mixing Process 1: Shear Driven Mixing (Pacanowski and Philander, 1981)
• Average Heat Flux – 1-2 1-2 W m-2
• Large events provide 10-2510-25 W m-2
Process 2: Double DiffusionProcess 2: Double Diffusion(Kelly, 1984; Kelly, 1990)
• Average Heat Flux <1<1 W m-2
• High values of 2-42-4 W m-2
Total Average Heat Flux: Total Average Heat Flux: 1-31-3 W m W m-2-2
Major Sources of Major Sources of UncertaintyUncertainty
• How frequent are strong shear events?
• Why don’t we see double diffusion? Is double-diffusion disrupted by shear?
• Heat flux algorithms are only estimates. Microstructure measurements are needed to more accurately calculate fluxes.
ConclusionsConclusions
• Shear appears to be important to mixing in this area.
• Double diffusion appears to play minor role.
• Estimated Vertical Diffusivity: ~1 x 10-5 m2 s-1
• Average diapycnal heat flux is 1-3 W m-2 .
AcknowledgementsAcknowledgements
Eric Firing, Jules Hummon, and Teri Chereskin have provided us with invaluable assistance in the configuration and support of the shipboard systems, as well as initial processing.