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WinDSSOcK: Winter Distribution and Success of Southern Ocean Krill: 2001 Drifter Measurements
R. Limeburner, R. Beardsley and B. Owens
Woods Hole Oceanographic Institution

 The study area has "a shelf circulation (cyclonic gyre) that retains the krill population in a favorable environment on the shelf for extended periods of time”.
 Characterize Western Antarctic Peninsula
shelf circulation processes and their spatial and temporal variability using satellite-tracked near-surface drifters, long term moorings and deep isobaric floats.

WOCE SVP Drifter
ice strengthened buoy hull
cold weather batteries

Gould Cruise Mar 18 – Apr 13, 2001
Deployed six current meter moorings near Marguerite Bay, Antarctica.
Deployed six drifters with drogues centered at 15 m. Eight drifters deployed by Palmer during May, 2001
Deployed 8 acoustic moorings to monitor whales.

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Seasonal Sea Ice Distribution

Large-scale Drifter Tracks
Solid blue circles are deployment positions.
Bay covered with ice by July, 2001.
Inner-shelf flow to SW
Outer-shelf flow to NW

Marguerite Bay Drifter Tracks
Blue deployment locations
Cyclonic flow in Bay
Clear inflow/outflow circulation in Bay
NW flow on outer shelf
Drifter animation next

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Rothera LP Windstress

Weak Wind Driven Response
The weak mid-shelf surface drifter velocities were surprising due to the strong winds observed during the 2001 cruises. The slow drifter speeds during large wind stress events may be due to the deep surface mixed layer (~ 50 m).  For an Ekman layer balance,
f * u * h = τ / ρ
for τ = 5 dynes/cm2, h = 50m, f = 1.3 10-4/s, u ~ 1.6 cm/s. Thus, the wind driven response on the open WAP shelf may be weak.

Lagrangian Time and Space Scales

The Lagrangian autocorrelation function was computed for each drifter velocity component, and then integrated from zero lag to the first zero crossing of the autocorrelation to give a Lagrangian integral time scale. The Lagrangian space scale was then found by multiplying the integral time scale by the rms velocity for each component.
Mean u Lagrangian time scale = 2.9 days
Mean v Lagrangian time scale = 1.8 days
Mean x Lagrangian space scale = 27 km
Mean y Lagrangian space scale = 18 km

Mean Flow
Inflow near Adelaide Island.
Outflow near Alexander Island.
Weaker mean flow at mid-shelf.
Greater variability near Alexander Island.

Maximum Velocity
April – July, 2001.
Cyclonic eddy in Marguerite Bay.
Weakest flow over the mid-shelf.
NW flow over the outer-shelf.

Rymill Bay
Located NE Marguerite Bay.
6 cyclones and 2 anticyclones over 40 days.
2 – 5 day periods.

High Frequency Motion
The inertial period is about 12.99 hours.
A simple model consisting of a mean current plus an inertial component was fit in a least-squares sense to the drifter position data.
One elliptical drifter track had a major axis of 16.8 cm/s and a minor axis of 10.5 cm/s, with the major axis oriented toward 28 ON.

2002 Field Program
 R/V GOULD cruise February 2002
 6 WHOI current meter moorings
 8 SIO acoustic whale moorings
 3 current meter moorings
 8 acoustic moorings
 6/10 drifters
 12 floats

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