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.