Mixing on the Western Antarctic Peninsula Shelf: A Component of Southern Ocean GLOBEC Susan L. Howard, Laurence Padman, Earth and Space Research and Jason Hyatt Woods Hole Oceanographic Institute

Study Region

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.

Circulation Antarctic Circumpolar Current (ACC) at shelf break Fresh Southward Coastal Current

Hydrographic Structure

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:

Outline Summarize our data Discuss 2 potentially important vertical flux processes: –Shear-induced turbulent mixing –Double diffusion Estimate total diffusivity and heat flux

Data 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 Data Mooring Fall Process Fall Survey Winter Process Winter Survey

CTD Data Fall Survey Cruise

Process 1: Shear-Driven Mixing Evidence of Shear in the NBP Fall Survey Cruise Distance (km)

More evidence of Shear Winter Process Cruise F h  25 W m -2 K v  1 x m 2 s -1

Process 2: Double-Diffusive Convection Previous Studies => Double Diffusion important on WAP Smith and Klinck, 2002 (LTER program) Heat Budget (simple model study): need 5-10 W m -2 diapycnal heating and need different diffusivity of heat and salt.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 ? R  ~ 1.5 => Double Diffusion Most R  > 3 => Weaker Double Diffusion Winter Survey Cruise R  ~ 1.5 F h  2-4 W m -2

Results Process 1: Shear Driven Mixing (Pacanowski and Philander, 1981) 1-2Average Heat Flux – 1-2 W m Large events provide W m -2 Process 2: Double Diffusion (Kelly, 1984; Kelly, 1990) <1Average Heat Flux <1 W m High values of 2-4 W m -2 Total Average Heat Flux: 1-3 W m -2

Major Sources of Uncertainty 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.

Conclusions Shear appears to be important to mixing in this area. Double diffusion appears to play minor role. Estimated Vertical Diffusivity: ~1 x m 2 s -1 Average diapycnal heat flux is 1-3 W m -2.

Acknowledgements 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.

Intrusions Mixing at edge of coastal current Station 16 Station 99

Heat Fluxes Required average heat flux of -75 W m -2 Fall Winter Heat loss

Salt Fluxes Required average salt flux of 1.6 mg m -2 s -1 Fall Winter Salt gain

Ri as proxy for mixing Richardson number ( Ri ) obtained from ADCP and CTD can indicate potential for mixing.

Vertical Dffusivities & Heat Flux Pacanowski and Philander (1981) parameterization: Kv=(5x (1+5Ri)2)/(1+5Ri) Heat Flux FH=  0CpKv  /  z 

NBP0103 Station 76