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
0
0.5
probability
1
1
1
1
1
1
3
3
3
2
1
1
3
3
2
2
1
1
1
1
1
2
1
1
1
1
1
2
1
1
1
3
how a physical oceanographer might address the problem
crux: obtaining a large number of accurate trajectories
1
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?
Goals:
• present tools for observing the CCS circulation
• indicate the CCS general circulation
• demonstrate the importance of eddies
• show the “inshore” region has different physics
Message:
• need to know pathways prior to designating
ballast water dumping sites
• tools and knowledge exist so this can be done
with unprecedented accuracy
SVP drifter
Ø38 cm
• spherical plastic float, 38 cm diameter
• holey sock drogue (length ~ 5m)
SST
• 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
15 m
drifter tracks in the California Current
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
2 x 2 km grid cell
Satellite altimetry for measuring sea level
sea level and drifter tracks
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
26
6 + ML
40
20
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
none
SST, SSH
none
none
vertical
layers/levels
time step
assimilation
other
Low
computational
cost
open boundaries
All approaches to determining trajectories
have strengths and weaknesses
• drifters - most accurate trajectories
sampling bias
• altimetry – excellent time and space coverage
aliasing 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
number of 6-hr drifter observations in a 0.5º x 0.5º bin
mean velocity field at 15 m depth from drifter
observations
cm s-1
mean EKE0.5 at 15 m depth from drifter
observations
vector correlation and scatter plots of “geostrophic”
velocity residuals from drifters and AVISO
unbiased geostrophic velocity at 15 m from drifters and
altimetry
mean geostrophic EKE0.5 from corrected
altimetry
cm s-1
mean sea level (cm) from various ocean models
HYCOM
NLOM
POP
ROMS
EKE0.5 from various ocean models (0-20 cm
s-1)
HYCOM
POP
NLOM
ROMS
EKE0.5 comparison with data (0-20 cm s1)
ROMS
unbiased drifter data
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
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