L. Padman, R. Muench, A. Orsi; Presentation OS41H-06 at the

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Observations of Dense Off-Slope Flow With
Energetic Tidal Forcing

• Laurence Padman 1, Robin Muench 1
• Alejandro Orsi 2, Arnold Gordon 3
• 1 Earth Space Research, Seattle WA
• 2 Texas AM, College Station TX
• 3 Lamont-Doherty Earth Observatory, NY

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Setting for AnSlope the NW Ross Sea
Ant. Slope Current Warm inflows Cold
outflows Dense outflows off the shelf Shallow
banks 1000 m isobath
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AnSlope Cruise1 (Feb-Apr 2003) track
and station locations
Ross Ice Shelf
Drygalski Trough
Focus Region
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Antarctic Slope Front
?

• Mean current flow is along the shelf break (into
  screen).
• Warm deep water (MCDW) gets onto shelf.
• Cold, dense water escapes from shelf.
• Dense Plume contributes to AABW

Mixing

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Tidal Environment
?U? gt 50 cm s-1 along break Spring currents
2 ?U? Diurnal, topo vorticity waves short
space scales strong cross-slope
currents RossTIM model validated w. current
meters and ADCP.
Yo-yo CTD/CMiPS ?
RossTIM model
Mean tidal speed ?U? (cm s-1)
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Hydrographic variability
CTD/CMiPS Locations
HSSW
CTD-179/180 CTD-181
Plume on slope
T S T-S
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Hypothesis ...
A combination of small- and micro-scale processes
associated in large part with tidal currents
influence the flux and T-S properties of dense
water moving seaward over the Ross Sea shelf
break.
Test using ...
- Tidal currents derived from models and
observations - Small-scale and microscale field
data - Moored, time-series current observations.
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Yo-yo CTD/CMiPS sequence, 900 m isobath
CTD/CMiPS
Alongslope tidal current (cm s-1) Cross-slope
tidal current (cm s-1) Cross-slope tidal
displacement (km)

• CTDs 179 180 (thick benthic plumes),
• coincident with
• max. offshore disp.
• max. westward flow (augments ASF)

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CTD-Mounted Microstructure Profiling System
(CMiPS)
CMiPS Overview Pmax 3500 m Internal
recording Sample rate512 Hz Resolution 1-10
cm Requires monotonic descent at moderate speed
no large vessel heave on CTD cable. (Good in ice.)
Pressure Thermistor (x2) Conductivity Cell
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Plume structure on slope
CTD-180 CMiPS
Density T Tz T
Interface 10 cm thick!
Kz?0.004 m2s-1 FH?103 W m-2
LSDW HSSW
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Mixing on sill
CTD-181 CMiPS (on sill)
Density T Tz T
Kz?0.003 m2s-1 FH?250 W m-2
MCDW LSDW HSSW
HSSW warms by 0.05o/day
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High frequency internal waves?
EK-500 38 kHz
10 min.
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Summary of near-frontal structures
ASF (outer edge of V) has along-slope speeds of
50 cm s-1 This front is very sharp (2-5
km?) Tidal cross-slope advection 10-20
km Plume thickness ? 100 m Plume outflow
at sill 50 cm s-1 Interface at top of plume
very sharp (10 cm) Mixing rates at top of
plume Kz gt10-3 m2s-1. Tidal advection (?U?
50 cm s-1) brings Warm Water onto outer shelf
where it can mix directly with HSSW.
?
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Tidal rectification (Lagrangian)
15 days of particle tracking
10 km/day
Start
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Preliminary conclusions ...

• Tides are a significant source of cross-slope
  advection of water masses in AABW source region
  in the NW Ross Sea. Tides here are both
  irritating noise and fascinating signal.
• Benthic stirring/mixing by tides contributes to
  reduction of HSSW signal as plume flows over the
  sill.
• Comparisons to major AABW source region in
  Weddell Sea.