The California Current System from a Lagrangian Perspective Carter Ohlmann

1. 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

2. 0 0.5 1 probability how a physical oceanographer might address the problem crux: obtaining a large number of accurate trajectories

3. 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?

4. 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

5. 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

6. drifter tracks in the California Current

7. 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

8. 2 x 2 km grid cell

9. Satellite altimetry for measuring sea level

10. sea level and drifter tracks

13. 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

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

15. mean velocity field at 15 m depth from drifter observations

16. cm s-1 mean EKE0.5 at 15 m depth from drifter observations

17. vector correlation and scatter plots of “geostrophic” velocity residuals from drifters and AVISO

18. unbiased geostrophic velocity at 15 m from drifters and altimetry

19. mean geostrophic EKE0.5 from corrected altimetry cm s-1

20. mean sea level (cm) from various ocean models HYCOM NLOM POP ROMS

21. HYCOM POP NLOM EKE0.5 from various ocean models (0-20 cm s-1) ROMS

22. EKE0.5 comparison with data (0-20 cm s-1) ROMS unbiased drifter data

23. 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

24. 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