The California Current System from a Lagrangian Perspective Carter Ohlmann
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Transcript of The California Current System from a Lagrangian Perspective Carter Ohlmann
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The California Current System from a Lagrangian Perspective
Carter Ohlmann
Institute for Computational Earth System Science, University of California, Santa Barbara, CA 93106
Collaborators: Luca Centurioni and Peter Niiler
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how a physical oceanographer might address the problem
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crux: obtaining a large number of accurate trajectories
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Outline:
• tools to describe the ocean pathways- surface drifters for various scales- satellite altimetry- numerical models
• summary of CCS drifter observations
• CCS shown with combined data sets
• comparison between data and OGCM results
• how would ballast water move?
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Goals:
• present tools for observing the CCS circulation
• indicate the CCS general circulation
• demonstrate the importance of eddies
• show the “inshore” region has different physicsMessage:
• need to know pathways prior to designating ballast water dumping sites
• tools and knowledge exist so this can be done with unprecedented accuracy
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Ø38 cm
SST
15 m
SVP drifter•spherical plastic float, 38 cm diameter
•holey sock drogue (length ~ 5m)
•SST (thermistor +- 0.1° C)
•drogue on/off sensor (strain gauge, submergence)
•ARGOS position (150 – 1000 m; 3 – 4 hrs)
•drag area ratio ~ 40; slip = 1 - 2 cm s-1
•mean half life >400 days
•Kriging of fixes (6 hour intervals)
•Correction for wind slip
•Recovery of “drogue off” data
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drifter tracks in the California Current
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Microstar drifter• tri-star drogue (length ~1m) • GPS position accurate to 10 m
• position updates every 10 minutes• data transmitted via Mobitex™ digital, data-only, cellular network• near real-time data and thus recoverable• drag-area-ratio = 41.3• slip 1 – 2 cm s-1
• 1 – 2 day deployment time
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2 x 2 km grid cell
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Satellite altimetry for measuring sea level
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sea level and drifter tracks
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HYCOM NLOM POP ROMS
spatial domain global global global ~1000 x 2000 km (USWC)
vertical coordinates hybrid layers levels sigma (ETOPO5)
horizontal resolution
1/12° (~7 km) 1/32° (~3.5 km)
1/10° (~10 km) ~5 km
vertical layers/levels
26 6 + ML 40 20
time step 6 hour 6 hour 6 hour 15 minute
mixed layer KPP Kraus-Turner KPP KPP
wind forcing ECMWF NOGAPS/HR NOGAPS COADS (seasonal)
heat forcing ECMWF NOGAPS ECMWF COADS (seasonal)
buoyancy forcing COADS(restored to
Levitus)
Levitus(restoring)
Levitus (restoring)
COADS (seasonal);
parameterization for Columbia River outflow
integration time 1990-2001 1991-2000 1990-2000 9 years
assimilation none SST, SSH none none
other Low computational
cost
open boundaries
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All approaches to determining trajectories have strengths and weaknesses
• drifters - most accurate trajectoriessampling bias
• altimetry – excellent time and space coveragealiasing issues
• models – models are models
• HF radar – excellent time and space coverage
range limitations
An understanding of ballast water transport will come from a combination of approaches
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number of 6-hr drifter observations in a 0.5º x 0.5º bin
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mean velocity field at 15 m depth from drifter observations
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mean EKE0.5 at 15 m depth from drifter observations
cm s-1
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vector correlation and scatter plots of “geostrophic” velocity residuals from drifters and AVISO
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unbiased geostrophic velocity at 15 m from drifters and altimetry
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mean geostrophic EKE0.5 from corrected altimetry cm s-1
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POP
HYCOM NLOM
ROMS
mean sea level (cm) from various ocean models
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EKE0.5 from various ocean models (0-20 cm s-1)
POP
HYCOM NLOM
ROMS
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EKE0.5 comparison with data (0-20 cm s-1)
ROMS unbiased drifter data
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Question: How would dumped ballast water be transported through the CCS?
Answer: Don’t know exactly, yet; but know how to figure it out.
• large quantities of trajectories are needed• connectivity matrices can be computed• many observational capabilities exist • combination of data sets is powerful
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Key point summary:
• a variety of observational techniques can be combined for leveraging (including models)
• eddy energy is many times larger than the mean beyond the shelf break (altimetry + drifters)
• shelf flow is neither in geostrophic nor Ekman balance; Lagrangian observations are lacking; need work here
• new drifter technology and HF radar are available for observing shelf circulation
• accurate pathways are not presently available, but the data and methods for determining them are