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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probability

how a physical oceanographer might address the problem

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crux: obtaining a large number of accurate trajectories

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 physicsMessage:

• need to know pathways prior to designating ballast water dumping sites

• tools and knowledge exist so this can be done with unprecedented accuracy

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

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

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

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

number of 6-hr drifter observations in a 0.5º x 0.5º bin

mean velocity field at 15 m depth from drifter observations

mean EKE0.5 at 15 m depth from drifter observations

cm s-1

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

POP

HYCOM NLOM

ROMS

mean sea level (cm) from various ocean models

EKE0.5 from various ocean models (0-20 cm s-1)

POP

HYCOM NLOM

ROMS

EKE0.5 comparison with data (0-20 cm s-1)

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