The California Current System from a Lagrangian Perspective Carter Ohlmann Institute for Computational Earth System Science, University of California, Santa Barbara, CA Collaborators: Luca Centurioni and Peter Niiler

probability how a physical oceanographer might address the problem 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 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 spherical plastic float, 38 cm diameter holey sock drogue (length ~ 5m) SST (thermistor ° C) drogue on/off sensor (strain gauge, submergence) ARGOS position (150 – 1000 m; 3 – 4 hrs) drag area ratio ~ 40; slip = 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

HYCOMNLOMPOPROMS spatial domainglobal ~1000 x 2000 km (USWC) vertical coordinates hybridlayerslevelssigma (ETOPO5) horizontal resolution 1/12° (~7 km)1/32° (~3.5 km) 1/10° (~10 km)~5 km vertical layers/levels ML4020 time step6 hour 15 minute mixed layerKPPKraus-TurnerKPP wind forcingECMWFNOGAPS/HRNOGAPSCOADS (seasonal) heat forcingECMWFNOGAPSECMWFCOADS (seasonal) buoyancy forcingCOADS (restored to Levitus) Levitus (restoring) Levitus (restoring) COADS (seasonal); parameterization for Columbia River outflow integration time years assimilationnoneSST, SSHnone otherLow 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 0.5 º x 0.5º bin

mean velocity field at 15 m depth from drifter observations

mean EKE 0.5 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 EKE 0.5 from corrected altimetry cm s -1

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

EKE 0.5 from various ocean models (0-20 cm s -1 ) POP HYCOM NLOM ROMS

EKE 0.5 comparison with data (0-20 cm s -1 ) ROMSunbiased 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