0m

FINAL TECHNICAL REPORT

Submitted to

THE OFFICE OF NAVAL RESEARCH

byProf. David W. StuartPrincipal Investigator

Dept. of Meteorology (3034)Florida State University

Tallahassee, Florida 32306-3034Phone (904) 644-4056

DTICfor ELECTEONR Grant N00014-89-J-1578 AN141

FSU Budget 1338-596-27 N 9"

Titled"Analysis and Interpretation of the

Meteorology of the 1988 CoastalTransition Zone Experiment"

December 31, 1993

Aprl-v,,td for pu.fle Wem .

•>S 94-01560EII I . 13 076

RESEARCH GOALS:

During April-May-June 1987 and June-July-August 1988 the

Coastal Transition Zone (CTZ) Experiments were conducted off the

California Coast near Pt. Arena by the Office of Naval Research

(ONR). One of the basic goals of these experiments was : "To

understand the role of the atmosphere in driving coastal ocean

currents and to learn of possible ocean-atmosphere feedbacks."

This component of the CTZ program was the focus of the

meteorological conditions during these experiments including the

data collection, reduction, and interpretation. This grant covered

primarily the 1988 study.

OBJECTIVES:

This principal investigator acted as a focal person for the

meteorology of the CTZ experiments providing meteorological

expertise to the other investigators. Our basic objectives were:

1. Documentation of the meteorological conditions during

each field study.

2. Data reduction plus analysis-synthesis of the

meteorological conditions for each field study.

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ACCOMPLISHMENTS:

This P.I. was not in the field during the CTZ experiments, but

provided weather outlooks and wind data to the P.I.'s via OMNET

during the experiments. Our accomplishments are listed below.

DOCUMENTATION: We acquired extensive holdings of the

meteorological data for the field phase and these data have been

made available to other P.I.'s as needed. Once per day (OOGMT)

surface weather maps are on file for the period of 1 June-31 August

1988. Loops of 3atellite cloud images for the CTZ area for the

period of 1 June-23 August 1988 via GOES-West are readily available

on video tapes. The National Weather Service (NWS) daily show

known as A.M. Weather is also available for the period of 1 June-26

August. Twice weekly maps of SST, from a composite of data

sources, prepared by NWS at their San Francisco Forecast Office are

in our archives for the period of 7 June-30 August 1988 in hard

copy map format. Also hard copies of hourly weather reports for

the West Coast are in our files for the period 28 June-12 August

1988.

Our documentation is very extensive for the National Data Buoy

Center's (NDBC) coastal buoys and C-MAN stations. We have hard

copies of the hourly data for the period 1 March-31 August for

Buoys 46013 (Bodega Bay) and 46014 (Pt. Arena) and C-MAN PTACI (Pt.

Arena). We also have these and all the West Coast Buoy and C-MAN

station data on magnetic tape for the period 1 March-31 August

1988.

3

These buoy and C-MAN stations are shown in Fig. 1 plus NDBC-05

(Washington), 28 (Cape San Martin), 40 (Cape Foulweather), 42

(Monterey) and C-MAN CARO3 (Cape Arago) and NWPO3 (Newport).

In short, we were quite successful in obtaining the standard

data via the standard station spacing to document the broad

meteorological conditions during the field phase as well as before

and afterwards. Many of these data were shared with the CTZ P.I.'s

during and after the experiment.

DATA PLOTS: The NDBC buoy and C-MAN data have been prepared

into monthly time plots to help show the response of Sea Surface

Temperature (SST) to the local wind stress. These time plots use

hourly data but after it has been smoothed using a 25 point (hour)

running unweighted mean. We have prepared these monthly time plots

for all of the buoys and C-MAN stations cited above. Plots were

prepared for the sea level pressure, air temperature, sea surface

temperature (SST), wind direction and speed, u (E-W) and v (N-S)

components of the wind, and wind stress (magnitude and direction)

for each of these stations. We include a composite of stress

(magnitude and direction) and SST for NDBC Buoy 13 (Bodega Bay) for

June July August 1988 (Figure 2,3,4) to illustrate the data format

made available to the P.I.'s during the synthesis stage of this

work.

These plots show clear examples of SST response to increases

and decreases in the wind stress. This is especially true in the

June and July plots, but it is also evident in the August plots.

Such graphs were very valuable at the CTZ workshop in Corvallis

4

during 25-27 July. The "Filament Tracking Group" focused on the

period 4-11 July when there were three Research Vessels (WECOMA,

Pt. Sur, and Washington) in the area. The "Change in Filament

Group" focused on a period in late July.

MANUSCRIPTS: Many of the research results of the CTZ

Experiments in 1987 and 1988 appear in a special section of the

August 15, 1991 issue of the Journal of Geophysical Research. This

P.I. worked with many of the other P.I.'s particularly in the two

groups mentioned above. The wind data and general meteorological

conditions are woven throughout the numerous papers. However, this

P.I. was a coauthor only on the paper by Brink et al. A reprint of

that paper is attached.

MEETINGS: This P.I. attended the CTZ Workshops in September

1987 in Corvallis, OR, in November 1987 in LaJolla, CA, and again

in July 1989 in Corvallis, OR. This latter meeting was during this

grant period and was the meeting at which the groups cited above

were organized and the framework of the manuscripts took place.

The P.I. also attended the American Meteorological Society meeting

in New Orleans in January 1991 with visits to the ONR Laboratories

in Bay St. Louis, MS.

BUDGET: The main employees under this grant were the P.I.

(for limited support during the Summers of 1989, 1990, and 1991),

Robert Joyce (graduate student-Summer 1989), and Pejaver Rao (under

graduate student-Fall 1991). Expense items covered minor supplies,

phone services including OMNET, postage, xeroxing plus computer

supplies.

5

The total funds available were $39,995 over the full period of this

grant.

DISTRIBUTION LIST:

This final technical report is supplied as follows:

ADDRESSEES NUMBER OF COPIES

Scientific Officer Code: 1121CS 3Thomas KinderOffice of Naval Research800 North Quincy StreetArlington, Virginia 22217-5000

Administrative Grants Officer 1Office of Naval ResearchResident Representative N66020Administrative Contracting OfficerGeorgia Institute of Technology206 O'Keefe BuildingAtlanta, Georgia 30332-0490

Director, Naval Research Laboratory 1Attn: Code 2627Washington, D.C. 20357

Defense Technical Information Center 2Building 5, Cameron StationAlexandria, Virginia 22314

Robert M. Johnson, Vice PresidentSponsored ResearchFlorida State University

6

'hIeSW L* 134033"

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BUOYS AND LAND STATIONS125.W 124* 123*

Figure 1 - Meteorological Stations in the CTZ AreaData plots were prepared for the underlined buoy

and C-MAN stations

UWY 13 SQOCGA BAY JUNE 1908 UOOHED181 -v- , • , , . , , , , , , , , 19

369 - -- 363'

m 27u .- - 27i2

18 ' ---- * g * 9 ' * * I a 10

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TIME (GMT-DAYS)

Figure 2 Time Plots for NDBC Buoy 6 17 21 22 2 24 2 988

BUOY 13 BODEGA BAY JULY 1968 SMOOTHEO

-90 - 90w

TIME (GMTDAYS

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0 3

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 38 31

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TIME tGMT-DAYS)

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Figure 4 -Time Plots for NDBC Buoy 13 Bodega Bay - August 1988

JOURNAL OF GEOPHYSICAL RESEARCH. VOL 96. NO. CS, PAGES 14,693-14,706 AUGUST 1,. 0991

Statistical Properties of Near-Surface Flow in the CaliforniaCoastal Transition Zone

K. H. BRINK, 1 R. C. BEARDSLEY,1 P. P. NjILER,3 M. ABBoTT,3 A. HUYER, 3 S. RAMP, 4 T. STANTON,

4

AND D. STUART9

During the summers of 1987 and 1988, 77 near-surface saellite-tracked drifters were deployedin or near cold filaments near Point Arena, California (39"N), and tracked for up to 6 month.as part of the Coastal Transition Zone (CTZ) progrmn. The drifters had large drogues centeredat 15 m, and the resulting drifter trajectory data set has been analysed in terms of its Eulerianand Lagrangian statistics. The CTZ drifter results show that the California Current can becharacterized in summer and fall as a meandering coherent jet which on average flows southwardto at least 300N, the southern end of the study domain. From 390N south to about 330N, thetypical core velocities are of 0(50 cm s-1) and the current meanders have alongshore wavelengthsof 0(300 k]n) and onshore-offshore amplitude of 0(100-200 km). The lateral movement of thisjet leads to large eddy kinetic energies and large eddy diffusivities, especially north of 360N. Theinitial onshore-offshore component of diffusivity is always greater than the alongshore componentin the study domain, but at the southern end, the eddy diffusivity is more isotropic, with scalarsingle particle diffusivity (K-1 + Kyv) of 0(8 x 107 cm2 s-1). The eddy diffusivity increaseswith increasing eddy energy. Finally, a simale volume budget for the 1988 filament observednear 37"N off Point Arena suggests that subduction can occur in a filament at an average rateof 0(10 m d- 1 ) some 200 km offshore, thus allowing the cold water initially in the filamentcore to sink below the warmer ambient water by the time the surface velocity core hasturned back onshore. This process explains why satellite temperature and color imagery tendto "see" only flow proceeding offshore.

1. INTRODUCTION much broader spatial scale than that encompassed by therest of the CTZ measurements, which were confined to theOne central goal of the Coastal Transition Zone (CTZ) area north of roughly 37"N (Figure 1). The drifter data

program has been to understand the filaments of cold water used here cover latitudes from about 30 to 39N, that is

which project offshore from the coast of northern California tsay mtfe coa s of alori Suc bo ad s

during the summer upweUing season. To help achieve this t i s o metiest o Calinphas a d sccl-goal inensie pysial oeangrapic easremets ere descriptions will sometimes tend to deemphnsize specific fill-

goal, intensive physical oceanographic measurement. were amentary features, although these are of considerable inter-made in the upper ocean off California in the summers of est. A description of the individual CTZ drifter trajectories,

1987 and 1988 to provide new and more complete descrip-

tions of the water property and velocity structure of these including the tendency for the near-surface (15 m) flow to

cold filaments and their surrounding environment. Based in proceed offshore along the axis of a cold filaments, is pre-

part on the successful early exploration into the nature of fil- sented by M. Swenson et al. (Drifter observations of theaments by Davis [1985a] using radio-tracked surface drifters, dynamical and thermodynamical structures in a cold fila-amen by Daied prog siramincludedio-araked surfaedrifters, inment off Point Arena, California, in July 1988, submitted tothe CTZ field program included a large satellite-tracked Journal of Geopyscal Research, 1990, hereinafter referreddrifter program to observe and characterize the near-surface toaSwnneta,19)

flow and its dynamical properties. Since the filaments were th follwin et dea w a b f c pknow tobe urfce-ntesifed fatuestheCIT drftes -The following results deal with a number of specific top-

known to be surface-intensified features, the CTZ drifters ics, all addressed statistically. First, the 'Eulerian' meanwere all drogued at a common depth of 15 in, which fyr- flow and its accompanying variability will be treated. Next,quently placed the drogue within the surface mixed layer, the dispersion properties measured using the drifters will be

The objective of this study is to use the CTZ drifter inea- discussed. Both of these first two topics tend to deal withsurements to characterize statistically the kinematics anu the broader spatial scales. Next, the momentum balance fordynamics of the near-surface (15 m) flow. Since drifters near-surface flow will be treated in the CTZ intensive area.tend to spread out over a broad area with time, making Finally, the relation of drifter kinematics to frontal locationsfull use of the drifter data base often implies looking on a in the Point Arena vicinity will be considered.

'Woods Hole Oceanographic Institution, Woods Hole, Ma.- 2. T OBSERVATIONAL BASE"sadiusetta.2SCrippe Institution of Oceanography, La Jolla, California. The primary measurement system is the Tristar Mark II

3College of Oceanography, Oregon State University, Corvallis. p4Department of Oceanography, Naval Postgraduate School, drifter (Niler et at., 1987]. These instruments have large

Monterey, California. (6 m) radar reflector-shaped drogues located at a centralsDePartment of Meteorology, Florida State University, Talla- depth of 15 m. The drogue and surface float design has

hassee, been calibrated by in situ current measurements and follows

Copyright 1991 by the American Geophysical Union. horizontal currents to within about I cm/s under conditionsrepresentative of those encountered here. The drifters are

Paper number 91JC01072. tracked using the Argos satellite system. Typically, eight0148-0227/91/91JC-01072$05.00 fixes per day were obtained during the CTZ program, and

14,693

14,694 BmwI E~r AL.: N&Aw~r=J ( RO~W iN Cba~m TkAmi~)0t 7A2m

0a 1987 OR1VTERS

29

0 '9 0 !.l Fk ! !s-

& . .. .*• * ,. .oLRn cXI .'-0-.'.-0.....0 *0 0 Snrocrc

* 0.. 0

+ + ........ .0Box I

Box 2

lq-J

-,29.0-t2.O -12.0 -26.0 -,25.0 -12'1.0 -123.0 -122.0 -12'1.0 -12'0. -19.0

LONGITUDE 0

Fig. 1. Locator map. The open circle denotes the location of -129.0-1A.0 -1:2.0 -1A.0 -1A.0 -124.0 -1;3.0 -122.0 -121.0 -120.o -119.0

NDBC buoy 46013. The t•lid circles are locations of hydrographic LONGITUDEstations for a typical 1988 survey, defining the CTZ intensive re-gion. Boxes represent the areas within which diffusivities and 1988 ORIr'ERSLagrangian time scales were computed. Crosses represent the lo-cations of the Stabeno and SmitA (1987] moorings used to estimate bEulerian time scales. C

the accuracy per fix is generally about 300 m. After initial e.

editing of the drifter positions, cubic splines (each 4 days inlength) are used to smooth the tracks and remove inertialand tidal variance. The splines effectively represent a low- X,pass filter with a half power point of about 0.45 cpd.

The drifters were deployed during the late spring or sum- Rmer of 1987 and 1988 in small coherent clusters. Most of tue25 deployments in 1987 were in May and June, while mostof the 52 in 1988 were in July or early August. In 1987 A,the drifters provided good data for typically 25 days, whilein 1988, good data were typically available for 6 months ormore following launch. The drifters were usually launched i'in clusters of 3-17, with separations of 5-20 km from near- -'

est neighbors. Smaller separations were used in larger de-ployments. The deployment strategy was intended to al-low mapping of velocity and relative vorticity within the -

Point Arena filament (Swenson et al., 1990). The total setof drifter tracks (Figure 2) gives a measure of the amount ofdata available.

In addition to the drifter trajectories, several othersources of data were employed. Conductivity-temperature- .depth (CTD) surveys were made repeatedly on a regular grid(Figure 1) in the CTZ intensive area. Details on the hydro- 0graphic observations are given by Fleischbein et al. (19891 .and Huyer et al. [this issue]. Winds were measured at 10 m LO0-,G.TU-DE.0 -,2.0 -,25. -,24.0 -210 -,.0 -,1.0 -20.0 -11..elevation at NOAA Data Buoy Center (NDBC) environmen-tal buoy 46013, located at 38013'N, 123018'W. The wind Fig. 2. Complete (a) 1987 and (6) 1988 CTZ drifter tracks.

a Er AL.: NI•Aa-Jm Row IN Om=L IkANRflw Zot 14.695

velocity time series were converted into wind stresses using PRINCIPAL AXES 1987

the method of Large and Pond [1981], and then spline-fit in bthe same way as the drifter positions to provide a low-pass- a

filtered wind stress time series. Finally, advanced very high iresolution radiometer (AVHRR) images of surface tempera-ture were used. These were calibrated and registered using .the techniques described by Kelly and Dovu [1986], exceptthat no view angle correction was made.

3. EULERIAN STATISTCS

The general properties of the near-surface flow will first be - .. + +described in terms of "EulerianD flow patterns. We use the

term "Eulerian" cautiously because of the bias introduced "-e- .. 'Xby the tendency for drifters to avoid divergences and to enter •.an area only when the flow is in the right sense. In order

to make these estimates, 1-day averages of smoothed driftervelocities were sorted into 0.5" by 0.5* latitude-longitude • +bins. Bins containing less than 10 drifter days of data were

deleted from further consideration. For each bin, the vector o

mean flow and the standard deviations along principal axesdirections were computed. The results are shown in Figure 3

for 1987 and 1988 separately.In the bins where there was a large data base (greater

than 50 drifter days), standard deviations of currents were P

uEAN VELOCITY 1987a A,

I Kl/SCC

T, -1n.o-,12.o -127.0 -16.0 -1• 0.0 -124.0 -,23.0 -1h.0 -1iJ.0 - 1.0 -'.0LONG ITUOE

-• -NUMIBER OF SRIAPLCS 1987

C

O00 0 aa0 0 000 00 a00 0 0000

CI 0 0 0000000000000 000 0

00000004832 2 0 0 0 0 0 0 0 0

Sooooo44 40 000 00000

0 0 0 0 0 3 7 15 19252300 0 0 0 0 0 0 0

50 0000444 16 31 5 100 o o o' 0 0 0 0 0 4 0 9 20481833242 0 0 0 0 0 0

8 0 0 0000 133 151225 2 0 00 0 0 0

0000 677814 1112500 0 0 00

•0C 00000003247 1614120 0 000

0 0 05 0000000000 13430000

0 00000 000000360542 03 0 00 0

- 00000000000000030000

•t-n~-,•'o -,.o •:.o •,.o •,,o - .o •. -,'•. -• o-,'.o 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 3 0 0 0-1. - 42. 87.0 -126.0 -125.0 -1.24.0 -123.0 -122.0 -121.0 -1;0.0 -11,90

LONGITUDE 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 3 0 0 0

Fig. 3. Eulerianstatisticsfromdrifters. Results are only shown 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 2 1 0 0 0for boxes with more than 10 drifter days of data. Dots represent 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0box centers. (a) Mean velocity for 1987. (b) Standard deviations o.in principal axis system for1987. The half-length of each cross 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0representslstmndarddeviation. (c) Number of drifter days per 01 0 0 0 0 0o 0 0 00 0 0000 0 0 0box for 1987. (d) Mean velocity for 1988. (e) Standard deviations jin principal axis system for 1968. (J) Number of drifter days per -'In0- 12.-0 *12-.0 -2.0 -1•A.0 -1•4.0 -1A3.0 -1A.0 -121.0 -,0.0 -816.0

box for 1•98. LONGITUOC

14,696 Bz Er AL: NI AR-SUJCA F.ow IN Cbsr A M o t'i5 ZorIE

d MEA VELOCITY 1988 ,M.CC: NI R OF Sf ,PL.S 1988

1 I 0000000000 000000000

S. 0 0 0 0 0 0 0 0 0 63 0 0 0 0 0 0 0 0 0'• 0 0 0 0 0 0 0 2 25 61 0 0 0 0 0 0 0 0 0

/0 0 0 0 7 10935 12 0 00 00 00 0432 04•12302692560 3 0 0 0 0 0 0

/ ' 0173739665466264 261 8610 0 0 0 0 0 00A, a .- / -1 31 68 12111871 75 30 19 28 4 2 17 7 0 0 0 0 0 0 0

R ' --- 4 1885955642663521 222319 140 0 0 0 0 0 0

09412 172330602225 3 216 0 0 0 0 0

S,- - ,--- - 0 1314 1215 25 35 51 29 16 11 126 0 0 0 0 0

1 , 00538 3244747200 2 6 1 1 3 2 0 0".S" 5 8 4 2420532532427 4 3 6 160 3 0 0 0 0

0 /, - -. a.. -. • - 0 0 2044594928556232198 116 9 5 0 0 0 0

-N I P ". - - • • • 1 /'20 7 23 48 84 6756 56 94 35 21 14 249 1 0 0 0 0 0

0 t 0 0 2 19 13 1328793841 207 143 2 5 0 0 0 0

0 -?" • t \ 0 0 3 324265112484021 129 143 0 0 0 0 0

R '- 0 0 0 0 3 2 14 36 26 34 13 12 26 12 1 0 0 0 0 0A' 0 4 6 19 17 16 23 20 37 17 It 5 6 12 8 3 0 0 0 0

q 24 32 38 38 22 23 64 5938 58 44 4 2 7 30 26 38 16 5 0A __ __ _ _ _ _ _ _ __ _ _ _ _ _ __ _ _ _ _ _ _ _

- ,. o-1 . o -27.0 - ,26.0 -1 . o - 1•2 . 0 - ,2 .0 - ,2 .o -1;1.0 - ,2o.0 -119.0 -129.0-12 ,.0 - ,b .0 -22 .0 -125 .0 -424.0 -1 3.0 -1;2 .0 - • 'i.0 -220.0 -11 '.0LONGITUDE LONGITUDE

e PRINCIPAL AXES 1988 typically about 20-30 cm s-1. The Eulerian independenceI M/SEC time scale was calculated from moored current meter records

in the region (Figure 1) at depths of 145-200 m [Stabenoand Smith, 1987]. Although these instruments are far from

9 /the surface, they are the shallowest long records available.Although currents in the region are definitely baroclinic, re-versals of direction in the upper 200 m generally occur only

! when near-surface currents are weak (I10 cm s-1 ), [e.g.,x Kosro et at., this issue], so we believe that the 145-200 m

Y~( x X--,r-- x i Eulerian current independent time scales are at least rep-A- resentative of the near-surface flow. The average value oh-"" \" + (-•/ " A- ) "- • tained from these records was 12 days (obtained by integrat-

o X A,-r--+-- # -- ±--+ x , x ing the autocorrelation over only positive lags [e.g., Kundux /.+ 'A - . , ,, and Allen, 1976]. The drifter bins with the best data cover-

* ,x • + • / -- age span about 100 calendar days, or represent about 4 de-grees of freedom using the current meter time scale. Thus

+- X • X i - ' the 90% error bounds on velocity in the most data-rich binsq are typically about ±26 cm s-1 (about 1.5 times the length

_, * of an arrowhead in Figure 3). The most data-rich bins (Fig-ures 3c and 3J) tend to be those along the core of the mean. -a--- x x jets visible in Figure 3. Note that in a sense, this is a con-

. . . . . . " - -servative error estimate, since it is based on calendar dayso. ....- * * .. + , -,. of occupation rather than drifter days in a box.

- " .The means from both years (Figure 3) and from the twoyears combined (Figure 4) show a meandering core current

oA • - / - + which, on the whole, proceeds southward. Note that, de-

S" " ' X I spite similar release points, the 1987 and 1988 data setsR. .... • - -.- -,. y , .X have very different spatial coverages, with the 1987 drifters

-,.0---, " - . -..0 . - - -, .0 - .0 -1,9.0 being more concentrated inshore. In both years, the aver-LONGITUDE age core velocities are about 50 cm s- , and the meandering

BRlm Er AL.: NEAR-SUR1n(E Row IN Cbkn%. 'TRAtsio zDNE 14,697

a MEAN VELOCITY ALL patterns have alongshore wavelengths of 0(300 km). Thelocations of the cores differ south of about 36.5"N between

I fl/SEC the two years, and the differences are reflected in data dis-

tributions which barely overlap south of 35*N. For this rea-o son, only in the area north of about 36"N can the 1987 and

1988 data be compared meaningfully. In 1987, the offshore-directed branch near 37"N does not proceed as far offshoreas the similar feature near 37.5"N in 1988. This is qualita-

"7 // ' ' tively consistent with AVHRR images, which suggest that. --- - / / the filaments were shorter in 1987 than in 1988. Both years

A . , - t £ / / • show significant eastward velocity at about 36"N, suggest-ing that at least this part of the meander field may be geo-

;R" .graphically stable. It is possible that the differences between

S'• ' •' -the 1987 and 1988 mean current fields are related primar---- . / . ily to seasonal variability associated with the slightly earlier

1987 launches. Considerable caution, of course, must beused in interpreting the mean flow results because the Eu-

X !lerian sampling grid is dependent upon the flow itself and- '. ' I % the specific, tightly clustered deployment scheme used in

o 9. - A ' • • . the CTZ program. A further complicating factor is that,p •• - - . ,• , , , while the drifters were deployed in May-August, most of

the fixes in the southern region were obtained in Novem-• - - ' •I t ber and December. Thus there is a possibility that sea-

A 'k sonal variations in the California Current system can bealiased into apparent spatial variations of the mean flow.

- -Climatological 0/500 dbar flow patterns in this area are ori-

ented northwest-southeast (Hickey, 1979]. In the summer,:• .... P , *,flow is southeastward everywhere, while in the winter, flow

-29.0-.0 1-2A7.0 125.0 -25.0 -24.0 -123.0 -2.0 -121.0 -,2.0 -19.o within 100-200 km of shore reverses to flow northwestward.LONGITUDE In both seasons, maximum mean alongshore velocities are

about 15 cm s-1, and 5 cm s-' is typical. Thus a seasonalb PRINCIPAL AXES ALL aliasing of our mean flow seems possible, since the clima-

tological "California Current" as defined by the 0/500 dbaro I fl/SEC dynamic topography (Hickey, 1979] is indeed weaker in the

winter, although the combined mean drifter flow (Figure 4)o is both stronger and directed more southward than the cli-

matological version (Hickey, 1979] over the entire domain.

o The standard deviations of velocity present a rich pattern.Perhaps the most obvious aspect is a tendency for variance

x/•4*4X /to decrease from north to south (Figure 4). This could re-S X -AX"X-- k-XXY flect a true north-south gradient of the eddy kinetic energy,-*. .. ,-X-- + + seasonal variations in velocity variance or it could represent

"X . A'-' -A- X X X + a tendency for more evenly spread velocity sampling fartherto the south, since most of the drifters were deployed in or

"" / + ÷ -k-/ near energetic structures near 380-390N. A further tendencyS o " + / + / -) I.. •is for the major axis of variability to line up loosely with

X • g t --- x%-. the mean flow in the jet core regions, most notably in thenorthern part of the domain. South of about 33"N, where

X- the mean jet is no longer very pronounced, the principal axisk - +- i--1 ""orientations appear to be fairly random, although there is a

A X X - X i tendency for the axes to tilt toward the jet core, suggesting.. . ,- "..•-....,.. x xa convergence of southward momentum. The tendency for

current variability to decrease toward the south could repre-sent seasonal variability if currents were to become weaker

• f X , ,in the winter. This does not appear likely, though, since9 • , X -.- / +• 4 Poulain and Nailer (1989] found relatively small eddy kineticA, energies, representing all seasons, of about 110 cm2 -92 in

the area south of 34"N. This is consistent with a real south-R ward decrease in current variability. It therefore seems likely

129.0-128.0 -1i7.0 -1A.0 1-25.0 -1;4.0 -1.;.0 -1L2.0 -12•1.0 .20.0 -1,9.0 that the trend of lower energy toward the south is primarilyLON1G ITUI geographical, rather than seasonal.

Fig. 4. As in Figure 3, but based on the merged 1987 and 1988 All told, the Eulerian statistics confirm the highly vari-data sets. (4) Mean velocity. (S) Standard deviations, able nature of the California Current system. The core jet

14.698 BRa K Er AL.: NFAR-SUIUmE Flow IN Cb5LL flumwo, Z]NE

locations differed considerably between 1987 and 1988. Fur- 4. EDDY DiFrusivITtEsther, in most cases, the standard deviation of currents isgreater in magnitude than the mean. This, of course, implies 4.. .Sik Particle Statisticsthat an Eulerian mean California Current will look very lit-tle like an instantaneous realization. It is thus interesting to Single particle dispersion statistics were computed in theask whether the California Current has a sharp, meandering same manner used by Davis [1985b]. These measure thehigh-velocity core throughout the entire north-south extent tendency for an individual water parcel to deviate from theof the domain. This is clearly the case north of 369N, but ensemble average trajectory. Specifically, the eddy diffusiv-the weaker means and fluctuations toward the south could ity is defined asbe due either to the core becoming diffuse and weak, or to 1 dmeandering over a broad region of otherwise weak variabii- K""'(z) a) • d P(,t), (10)

ity. In order to gain some further insight about this point, we wherefound the strongest individual velocity measurement in each0.5' box (Figure 5). Large velocities are found at all lati- "'(f, t) =t) (I b)tudes, so the meandering jet model seems to apply through-out. There is, however, a tendency for the peak velocitiesto be about 20% weaker south of 330N than north of that where n and m can be either z or y, r"(i, t) is thelatitude. This north-south decrease in core velocities coin- location of a drifter in the n direction (since the time of itscides roughly with the location where both the mean and launch at t = 0 at position Z), and r" is the differencefluctuating currents (Figure 4) become noticeably weaker, in the n direction between an individual drifter's locationand Geosat estimates of sea surface height variability de- and the mean drift, r"' = r" - (I/N) • r". The average iscrease sharply (Flament et al., 1989]. Thus, we envision the over all N drifters which appear within a particular box.California Current as a meandering but coherent jet over Since the memory of drifters tends to be short (2-6 days; seethe domain of study, with a core velocity maximum which section 5), all drifters were treated as newly deployed afterdecreases south of about 33"N. The kinematic implication 10 days if they remained in the box. That is to say that,of this is that the core spreads out into a somewhat broader, for the purpose of calculating ,n, , a single drifter couldless energetic flow as the current proceeds southward, yield several trajectories, each starting 10 days after the

other if the drifter was in the box at its new starting time.This is simply a means of retrieving more information fromthe available data, e.g., Poulain and Nider (1989]. For all

MAX VELOCITY A.L diffusivity calculations, the coordinate system is rotated 28"counterclockwise so that z is perpendicular to the general

I f/SEC trend of the California coastline between 34"30'N and 39"N(Figure 1). This had the effect of decreasing the off-diagonalelements of the dispersion tensor K"y in some cases, while

R inot increasing it substantially for locations farther from the

coast.The single point dispersion calculations were carried out

/ for three boxes having particularly good data coverage (Fig-ure 1). The first box includes the filaments within whichmost drifters were deployed. The second box is farther off-shore and includes the region in which most 1988 drifters

""N " cease going offshore. The third box lies in the southern re-gion where the drifters tend to be well dispersed. The resultsfor the merged (1987 and 1988) measurements are shown in

-= \ Figure 6. In all cases, the zz (onshore-offshore) diffusivity/ is the largest component during the first 4 days. For the two

-• f// ~ ~>~ ~\ northern boxes, however, the diffusivity decreases by day 5X// V \ and then passes through zero. This behavior may reflect the

influence of long-lasting flow structures (e.g., the filament).O .\_ . For the southernmost box, no such persistent flow structures

were apparent and the zz diffusivity had more of a ten--- " \"\ " / dency to increase monotonically with time, as expected in a

"- " turbulent field [e.g., Davis, 1985b]., 4-.- , / , / •. In all cases, the initial alongshore yy diffusivity tended to

- -/increase with time, and not pass through zero. This suggestsA,/ j -" \ that the flow structures complicating the zz dispersion

i VI act mainly in the onshore-offshore direction, as might be, ,I ,,- , . expected for meanders oriented roughly perpendicular to the. -,.,.0 -4.0 -4S.0 -24.0 -1h.0 -,i.0 -,i'.0 -ji.0 -11.o coast. The off-diagonal diffusivity was small for t < 10 days

LONGITUOC but was of the same magnitude as the alongshore componentFig. 5. Velocity corresponding to the maximum speed observed of diffusivity.in each 0.s5 box (total data set). The results of the single point diffusivity calculations are

BRUN Er AL.: ,_AR-SuAmC FloW iN CATAL T:RA'• i'foN ZDNF 14.699

a Merged xx 0mCfusLv,.Lt summarized (Table 1) in terms of the average values over

the first 10 days. Except for the relatively sparse data basein 1987 (:I:40%), the 95% confidence intervals on the 10-day-averaged diffusivities are typically i20%. The meaningof traditional confidence intervals in this situation is a bitproblematic, especially in box 1, since the drifters are notobviously statistically independent. This follows because

o they are all launched into an energetic flow feature which waso. long-lived relative to drifter transit times of 5-10 days [e.g.,

-- --....----- --. -Koaro et al., this issue]. Because the drifters are all sampling. - the same feature with a short (I-2 months) time window,

.. it seems likely that individual drifters are not statisticallyindependent and that the means and diffusivities do not

o truly reflect an ideal "ensemble."It is worthwhile to ask whether the diffusivity can be pa-

o rameterized simply in terms of the eddy kinetic energy of

0.0 5.0 10.0 15.0 the system,TLme (O•s) , Z2

where 02 is the vector average velocity within a box. Noteb Merged SS OLCfusi-•v-S that since this measure accounts for deviations from botho. the spatial and temporal mean, it will tend to be larger than

the eddy kinetic energy computed at a single site from, say,a current meter mooring (since the single point variance is

o. computed relative to a temporal mean only). In order toreduce scatter, a "scalar diffusivity"

o. K = K"X + KIYY0

. . .......... averaged over 10 days lag is compared with E',, (Fig-> .ure 7). The open circles correspond to values calculated

in the larger boxes, while the dots correspond to estimatesSmade in smaller boxes (dashed boxes in Figure 1). The re-

no.. suit suggests a dependence of K on E' of the formn.) K = b(Ej, )11,2

0.0 5.0 10.0 where b = 42 km (by least squares fit). This result sug-TLme (Woas) gests [Krauss and Boning, 1987] that there is a constant

eddy length scale, but that the Lagrangian time scale isnot constant, a finding consistent with our time scale cal-culations (see below and Table 1). Our isotropic mixing

C Merged x ODifFusLvLLg length of 42 km can be compared to Krauss and Bining's

C! [1987] range of 31-39 km for the North Atlantic Ocean (us-ing their definition). Such comparable values suggest thatour measurements correspond to an open-ocean regime, notinfluenced by boundary effects in a gross sense.

The averaged single point diffusivity is always initiallyo largest (for short time lags) in the onshore-offshore direc-o- tion (the exception being in 1987 when the current meanders

Swere less pronounced). To interpret this result, K"" can.j> o be thought of as the diffusivity to be used in a steady state

"diffusive-advective model. The net flow is southward (or to-ward negative y) in general (Figure 4), while the onshore-

-i----.offshore component shows substantial variability but littleX net trend. Thus advection moves the drifters fairly consis-

tently toward the south, while the "diffusion" (parameter-S0.0 s.0 1.0 i. izing the various onshore and offshore flow patterns) leads

TLme (0ogs) mainly to a spreading in the onshore-offshore direction.

Fig. 6. Single point diffusivities, calculated with the merged 4.2. Taylor's Hypothesis(1987 and 1988) data sets. Solid line, box 1; dotted line, box2; dashed line, box 3. All diffusivity values are divided by If the flow field is homogeneous and stationary within107cm2 s- 1. (a) Onshore-offshore (xz) compone'ý.. (b) Along- a box, then by Taylor's hypothesis the eddy diffusivity isshore (yy,) component. (c) Off-diagonal (zx) component. identically equal to

14.700 BRIM Er" AL.: NFAJRI-au m Ro•.W IN Cbkm L TkAmom 7

TABLE 1. Eddy Diffusivities and Independence Time Scales

10-Day AverageSingle Point Kinetic IndependenceDiffusivities, Energy, Time Scales t

Ca 2 s-1 X 10- an2 0-2 days

Year Box n..., K.. K,1 Ksy Mean Eddy T T

1987 1 43 3.25 1.88 1.05 114 341 2.1 1.81968 1 83 10.04 2.66 2.32 361 811 4.2(2.8) 0.9

2 175 5.45 4.95 -1.66 56 719 2.5(1.0) 1.7(1.7)3 329 5.61 1.77 -1.21 100 228 5.8(3.3) 2.7

Merged 1 126 8.62 2.43 1.87 227 714 3.3(2.8) 0.9(0.8)2 181 5.34 4.95 -1.57 54 718 2.5(1.0) 1.8(1.6)3 329 5.61 1.77 -1.21 100 228 5.8(3.3) 2.7

" Maximum number of drifter days in the calculation.t From (3) and (6). Values in parentheses are time scales integrated to 50 days. Such values areonly given if the result stabilized for the last 20 days. Values not in parentheses are for integrationsterminated at the first zero crossing of the autocorrelation function.

n- _ a coast and the apparent north-south variation in EulerianKn' = Un urn' Lnm(t')dt', (2) statistics.

where L n, is the lagged correlation function of the La- 4.3. Particle Pair Dispersiongrangian n and m components of flow (u", um), and The rate at which two particles separate is the appropri-u Urn' is the zero lag covariance. The Lagrangian correla- ate eddy diffusivity to consider when examining the time-tion function L"' was computed as dependent behavior of a system [Davis, 1985b]. This particle

pair diffusivity is defined byN

L n,(t') = [Na'I 4 )auq(t)J ~dUn(0)U-'(t,), (3) anrn 1 d unm (4a)

where the standard deviations a'• are recomputed sep- wherearately for each time lag because the Lagrangian mean 1 ' (b)changes with lag, a finding consistent with Davis [1985b]. (4b)The sum is over all N drifter trajectories originating inthe box. Here we calculate the right-hand side of (2) foreach of the three boxes. Note that using this form of theautocorrelation function, which is necessary to account forthe inhomogeneity of the flow, means that the results of (1)and (2) need not be identical. Thus the extent to which 1this estimate agrees with the K""' estimated using (1) is 10a measure of the homogeneity of the flow.

Comparisons of the two estimates of diffusivity for boxes 1and 3 are shown in Figures 8. Generally, the two estimatesagree well for the first day .dt-r drifter launch, but substan- 0tial differences occur withip d:..ys. The divergence can beunderstood through examination of velocity covariance asa function of time since launch. The cases where the twoestimates diverge quickly correspond to case.ý where the co- 5variances (e.g., kinetic energy) change rapidly as a function +

of time since launch. That is, divergence of the estimatescorresponds to cases of strongly inhomogeneous flow statis-tics over scales comparable to, or smaller than, the box size.The cases where the two estimates diverge slowly, on theother hand, correspond to situations where the flow statis-tics remain relatively constant with time after launch. It isworth noting, too, that in box 3, Taylor's hypothesis worksfairly well, and this is a region of relatively uniform Ev'!er.n 0 05statistical properties (Figure 4). Soo

Taken together, the tests of Taylor's hypothesis lead to EKE (Ctif/S 2)the conclusion that the flow in the region sampled off Cali- Fig. 7. "Scalar diffusivity" as a function of eddy kinetic energy.fornia is rather inhomogeneous on scales as small as 200 km. Circles indicate large box estimates (solid boxes in Figure 1).This should perhaps not be surprising given the presence of Dots indicate small box estimates (dashed boxes in Figure 1).

BrmC zr AL.: Nw&ýA-JU( Row vi OmnL 'ThAImllm Z7t 14.701

a Mlerged Sax I To~Lo,- Test. b Mlerged Box ITa!#Loa Test.

16 ....... ...

0 0 ..

1 .. .......

x 0,

0.0 5.0 1. 5000;01.Time (Do~s) Time (Oo~s)

c Merged Box 3 TogjLoa- Test. d Mlerged Box 3 ToyLoa- Test.

> 0...............

.........

...........................................................................

001. ;0 005.0 10.0 1S.0Time IOo~js) Time (Oo.4s)

F"Ig.S. Coenparimons of single point diffumsivities computed directly (solid line) and usming Taylor's hypothemis (dottedline). All diffusivity values awe divided by 107an2 a-'. (a) Onshare-offahore (cc) component, box 1. (b) Along-shore (gy) component, box 1. (c) Onshore-offshore (cc) component, box 3. (4) Alongshore (ppy) component,box 3.

The ns component of separation between two drifters is for the nearshore region near 38*N. The alongshore ditrusiy-a", and s"' is the deviation of a given drifter pair from ities a-, far smaller than the onshor"e-fshore component,that of the mean at that time step. The average is over all apparently an a result of the alongshore low componentdrifter pairs, with so geographical binning. Following Davis tending always to have an equatorward component, as would[19851.), the calculation of P"' was binned by the initial be found in a meandering jet. The of-diagonal componentsseparation distance between drifters. No attempt was made of pair dispersion also tend to be small and show little netto carry out separate calculations for different geographical pattern. in all case, the dispersion values tend to decreaseareas due to the need to accumulate large numbers of de- toward zero, corresponding to a state where particle pairgrees of freedom, although nearly all paiirs started in either separations become relatively constant, at least for a time.box I or 2 (Figure 1). As in the single particle dispersion The pair diffusivilties tend to be the sae magnitude as thecalculations, all drifters were treated as newly launched af- single particle dispersions, although pair yj values, tend toter 10 days. The bins corresponded to initial separations of be somewhat smaller than mingle particle yy dispersions.less than 16 kmn, between 16 and 32 kin, and between 32 and64 km. 5. DYNA~mCS

The results of the calculations for the merged 1987-1969 51i Inega Time Scalesdata set, displayed as a function of pair separation (which in-creases with time for the results given) are shown in Figure 9. As Davis (19651b3 points out,.the autocorrelation functionThe initial separation bin can be identile~d by the location of velocity can be a useful diagnostic of the low. The Ernle-where a given curve passes through * -0. As in the case of Tian independence time scalesingle particle diluasivities, the onshore-offshore xs disper-sion is the largest, yieldng valnes substantially larger (for 7-s A-('= (5)the same separation) than those reported by Davis 1198651a

14,702 BRC Er AL.: NmA-SmC Row iN Ounkl, Tumn Zotii

a Po.r O•.USl.ts 71 = j LPo')dt' (6)

C!

. is a measure of how quickly the velocity changes for a particlebeing advected through the flow field. A highly nonlineart.ow field dominated by semipermanent flow features willlikely have TL < Tz, whereas a flow field dominated by

a". linear dynamics (flow speeds much less than typical "wave'speeds) will generally have TL 2ý To [Davis, 1985b].

In order to make such a comparison for the coastal tran-sition zone data base, the Eulerian time scale was estimatedfrom the current meter data obtained at depths of 145-200 m(data described by Stabeno and Smith (19871; see also Fig-

0 •ure 1). The moored current meters give a typical analogous0. (one-sided) value of 12 days. This value should be used with

caution, however, since the measurements are from a depth0.0 50.0 1m.0 5so.o 0c6.o range which is much deeper than the near-surface bound-

Seporot.Lon (Ka) ary layer (see section 3). Indeed, the current meter velocity

fluctuations are a good deal weaker than the surface cur-b • Poir 0LffusLvLtj rents (typically a factor of 4-5). One might expect that

o. near-surface velocity measurements would yield somewhatshorter time scales due to the presence of directly wind-driven flows, but estimates of momentum balances given be-

o. low suggest that this component of flow may not be large.Thus the 145-200 m current meter estimates may well bereasonable for comparison.

The Lagrangian time scales were estimated by calculat-ing, for all drifters originating in a given box, the Lagrangianautocorrelation function by (3). All calculations were car-

tied out in the same rotated coordinate system as was,*... , .. •.-.--.-.*....., o used for the diffusivity calculations. Given the short time

.... ....... scales involved, all drifters were treated as newly launched0. 10 days after their first appearance, leading to a greater

number of degrees of freedom. In many cases, the integral0.0 s6.0 ,. ,20.6o.0 (equation (6)) did not converge even after integrating over

SeporatLon (Km) 50 days, so the integration was instead truncated at the time

of the first zero crossing of the autocorrelation function.

C xs PoLr oLrfusLvLLS The results of the calculations (Table 1) yield Lagrangiantime scales shorter than the Eulerian time scale (12 days),and often much shorter. Thus the hypothesis of fundamen-tally nonlinear dynamics appears to be validated, at least inthe north where current meter data are available. Notice,however, that the idea of nonlinear dynamics does not meanthat the nonlinear terms in the momentum equation needto be large. The result means only that such terms must be

important relative to the other ageostrophic terms.

,. 5.2. Momentum Balances

..... . '-.. " ..... .... It is possible to use the drifter data to estimate the terms"W in the upper ocean momentum budget. Specifically, we write

. ... .." .. the horizontal momentum equations as

00C.0dil I air=S1paa.0o z;00 ~+fkxi)+ VP- 0T(7

Fig. 9. Pair diffusivities divided by 107cM2 9-1. Initial sep- where small-scale horizontal turbulent mixing has been ne-aration bins can be distinguished by where the curves have glected. The total derivative isa = 0. (a) Onshore-ofshore (zz) component. (b) Alongshore(V'Y) com ponent. (c) Off-diagonal (x•i) com ponent. d 1 1 6 (8)

d-ifay-i + uF +vF +,o , (e)(where A" is the autocorrelation function of the n com-ponent of velocity) is a measure of the typical time scale where f is the Coriolis parameter, • is the vertical unitof velocity changes for an observer located at a fixed point, vector, p is the density of water, and f is the verticalThe analogous Lagrangian time scale component of turbulent stress. If a slablike mixed layer is

BpWm Er AL: lARuJR-umI Row FIN (Oa %L TRANpnoN ZoNE 14,703

WIND STRESS AT 46013

rC.)

N-,In

1 2 3 4 5 6

170.0 140.0 190.0 280.0 216.0 2;0.0 230.0DAYS IN 1988

Fig. 10. Wind stress at buoy 46013. North is upward. For reference, July I is day 183. Horizontal lines representthe times of hydrographic surveys.

assumed (e.g., Davis ef al., 1981], then the vertical gradient The drifter estimate of pressure gradients (dashed arrow) isof turbulent stress in the mixed layer can be stated as compared to the pressure gradient based on 0/499 dbar dy-

namic height in Figure II c. There is a qualitative agreement=h , (9) between the two fields, but it is not very good. The momen-

T h 0 tum balance results from the three other surveys yielded

where h is the mixed layer depth and 5 is the wind stress comparable results.applied at the ocean's surface. The poor results of the regression approach should not be

Using the drifter data, it is straightforward to estimate the surprising. For a velocity within the filament of 50 cm s-1,Coriolis force term in (7). Further, the total derivative of a Coriolis term of 0(5 x 10-3 cm s-') would be antici-velocity is also straightforward to estimate, except that the pated. For a typical wind stress of I dyn cm- 2 and mixeddrifter can not account for the vertical derivative term in (8). layer depth of 20 m, a typical Ekman velocity would beThe problem then is to estimate the pressure gradient and about 5 cm s-1 and the stress gradient term in (7) wouldthe mixed layer depth. The approach taken was to consider be 0(0.5 x 10-3 cm s-2), small enough to play only aonly those drifters in or near the hydrographic sampling grid secondary effect relative to the dominating geostrophic bal-(Figure 1) during the time when a particular hydrographic uace, and making the regression approach difficult. Thesurvey was made, typically a period of 6-8 days. Then, poor agreement between the two pressure gradient esti-during this time, the first two terms in (7) were regressed mates could have two causes: (1) errors in computing theagainst the surface wind stress, so that the regression co- momentum balance from drifters (including the fact thatefficient a could be identified with (ph)-'. There were the drifters are not perfect Lagrangian instruments), andfour hydrographic surveys during which there were sufficient (2) 499 dbar not being the correct level of no motion fordrifters to carry out this calculation (surveys 3-6). The first the flow field. The moored current results of Stabeno andthree occurred during fairly strong winds, and the last dur- Smith [19871 suggest that a correct level of no motion in thising weak winds (Figure 10). In only one case was a reason- area would lie at a depth of 1000 dbar or greater and thatable value of h obtained: map 3, where h = 26 m but errors associated with the 494-dbar reference level would bethe z correlation is only 0.17. In all other caes, either a 0.0003-0.0012 cm 2 s8-, too small to account for all of thenegative or large 0(1-80 m) positive value was obtained for discrepancy in Figure llb, but still limiting our ability toh, in contrast to observed mixed layer depths of 0(20 m). assess the quality of the momentum balance estimates fromFinally, the pressure gradient was calculated as the resid- comparisons.ual of (7) and all results were averaged into 0.50 by 0.5" Nonetheless, some conclusions can be drawn from the mo-(latitude by longitude) bins to clarify presentation. mentum balance calculation. First, the lowest-order mo-

Typical results of a calculation are shown in Figure 11, mentum balance must be geostrophic. This finding is consis-representing survey 3 on days 187-194 (July 6-13), 1988. tent with the validity of quasigeostrophic dynamics. At theThe Coriolis (arrow) and acceleration (short stub) terms are next order, Ekman transport and other ageostrophic effectscompared in Figure 11, while the stress gradient and Cori- probably enter. It is not possible to say directly whetherolis ternm are compared in Figure 116. Clearly, the Coriolis the total time derivative is dominated by local or advectiveterm dominates, suggesting a nearly geostrophic balance. changes. Results of the time scale calculations (section 5.1)

14,704 lxw cr AL: WAsSuma FRoW w (hnL Wk IlGK ZoA

a V', B 167-19 i Io t 1 94 198s DELP CO

\ q

S,"-.....\.

ata

\'*. •s

q 0

-'12.0 -1t.0 -,s.0 -42.0 - I4.0 -122.. -,27.0 -126.0 -456.0 -44.0 -.1A.0 -1i2.0

LOWITITNC LONGITUIE

b M86-19S FV, TARKI Fig. II. (continued)

0.001 CMSCC suggest, however, that the nonlinear terms must be impor-

tant.

6. RELATION OF NEAR-SuRFAcE FLow To FRONTS

Very often, filaments have fronts along both their north-

ern and southern sides. Further, there is some evidence thatconvergence and subduction could be occurring, at least at

the southern front [Floment et at., 1985]. It thus seemso reasonable to attempt to use the drifter data to look for

evidence of such convergence.The approach used is straightforward. Frontal locations

are determined subjectively by using NOAA AVHRR satel-lite thermal imagery for 1988 only. These are overplotted

*' with drifter positions, so that the shortest distance of a given

drifter from both the southern side (q) and northern side(C) fronts can be determined. Note that this approach doesnot depend upon absolute surface temperatures, but ratheronly on the distance from fronts. Such features should not,

* over short time scales (days), depend greatly on surface heatft fluxes. Most drifters were deployed within the cold waters at

the nearshore end of the Point Arena filament. Those thatwere initially not between the fronts are dropped from the

calculation. Histograms of the distance of drifters from the-, ------- front versus time since launch were then computed (sum-

-'I.5 I.0 marised as means and standard deviations in Figure 12).1Ai'9• As the drifters proceed offshore in the filament, they draw

FIg. 11. Results of momemtun balance cakulatioss for day 168- closer to the south side front, i.e., q decreases. The av-196, 19W. (a) Coriolis (with arrowhead) and acceleration (with- erage distance to the north side front C remains roughlyout wvowhead) terms. Locatioss are given by dots. (b) Cr- constant at about 15 km. This is consistent with the filorl (with arowhead) and stress gradient (without arowhead)terms. (c) Camo s ptnure pradlmat computed as drif meat narrowing with distance offshore as is typically seen(dashedarrows)ad fromO/499dbwodya•mic height(solidlines), in satellite imagery. These results are also consistent with

BUN Er AL.: MAR-&wAcE Row IN CbAnL 'TAmmoN Zbw 14,705

used above. The result (Figure 12) shows that initially thedrifters did accelerate but that they slowed down by day 5from launch. Thus even though the filament becomes nar-

100 rower, the flow along its axis decelerates, implying down-welling. Most likely, this downwelling occurs at or near thesouthern side front, since the drifters tend to converge on it(Figure 12). It is straightforward to compute a volume bal-ance for the flow over the upper 15 m of the water column(assuming no flow through the filament edges) to arrive ata downwelling rate of 12 m d-' in the section within the

_X 50 filament 3 to 5 days after deployment (100-170 km from de-'1,. ployment). This rate of sinking is roughly consistent with

the 20 m d' estimate based on biological and chemicaltracers (Kadko et a1., this issue].

The drifter-based sinking velocity estimate is not veryprecise. It involves many assumptions, including that no

01 flow crosses the edge of a filament. Further, errors are likely0 10 to be introduced by the unavailability of satellite data dur-

ing cloudy periods, which lasted up to 5 days. Thus, the100 results need to be taken with caution. This is especially so

because Swenson et al. (1990) concluded that downwellingtakes place on the northern (not southern) edge of the fila-ment. While their result does not directly call into questionour estimate of vertical velocity, it does serve as a cautionabout the idea that downwelling tends to occur preferen-

50 tially along the southern side of the filament.

7. CONCLUSIONS

The CTZ drifter results strongly suggest that the Califor-nia Current between 30" and 40*N has a meandering, jetlikecurrent core. Some of the structure in the current (e.g., the

05 10 eastward flow near 36"N) appears to be reproducible from

1987 to 1988, although the fairly featureless mean patterns100 south of 34"N suggest that the core's position varies fairly

randomly in that area. The magnitude of the core velocity,the mean velocity and the eddy kinetic energy all tend todecrease toward the south, especially south of about 33"N.

Although the lowest-order momentum balance for thenear-surface currents is geostrophic, the governing, higher-

so order dynamics are more complex. Specifically, the disparityJ /•between Eulerian and Lagrangian current time scales sug-

gests that nonlinear effects are quite important. This finding0 is consistent with the interpretation of the California Cur-

"rent system as being hydrodynamically unstable, e.g., Ikedaand Emery (1984] and Haidvogel et al. [this issue].Accompanying the nonlinearity of the flow is a tendency

0 for rapid particle dispersion. The values for both the single0 5 10 point and pair dispersions are substantially larger than those

Time afler launch (d) estimated by Davis [1985b] for an area closer to shore near

Fig. 12. (s) Average distance of drifters from the south side front 38.5*N. Davis [1985b] also noted that dispersion tended to be40 (n) versus time since launch. Vertical bars represent *I standard isotropic by 50 km from shore, while alongshore dispersion

deviation. (b) Average and standard deviation of drifter distance dominated close to the coast. Our results, which representfrom the north side front (C). (c) Average velocity out the the region greater than 50 km from shore, show a strongfilament axis vems time from lamnch. tendency for dominance by the onshore-offshore component

of dispersion. This is the opposite sense of anisotropy to thatfound by Davis [19856]. Presumably this difference is related

convergence on the south side of the filament, but they do to the coastal inhibition of onshore flow over the shelf, andnot mean that downwelling (subduction) is occurring. For to the dominance of meandering jet patterns offshore.example, the offshore flow in tne filament could simply be The flow field offshore is sufficiently inhomogeneous as toaccelerating to compensate for the horizontal constriction, limit Taylor's hypothesis to only order of magnitude valid-

The acceleration hypothesis was tested by computing the ity. This inhomogeneity is also reflected in the substantialaverage velocity as a function of time for all of the drifters spatial variations in kinetic energy and eddy dispersion co-

14.706 1tN Er AL.: NEAR-&SwRACE Row IN CbArA1. TRu oN ZDoNE

effcients. This inhomogeneity is roughly consistent with Huyer, A., P. M. Kosro, J. Fleischbein, S. R. Ramp, T. Stanton,sea level variability estimated with Geosat [Flament et al., L. Washburn, F. P. Chaves, T. J. Cowles, S. D. Pierce, and R.

19891. L. Smith, Currents and water masses of the coastal transitionzone off northern California, June to August 1988, J. Geeopips.

It now seems very likely that subduction occurs in ila- Res., this issue.ments off northern California, based on chemical and hy- Ikeda, M., and W. J. Emery, Satellite observations and modelingdrographic tracers (Kadko et al., this issue]. The results of of meanders in the California Current system off Oregon andthe CTZ drifter study suggest that much of this occurs more northern California, J. Phys. Ocesnosr., 14, 1434-1450, 1984.

than 100 km offshore, and at an area-averaged rate within Kadko, D. C., L. Washburn, and B. Jones, Evidence of subduc-tion within cold filaments of the northern California coastal

the filament of 0(10 m d-'). Apparently the cold, dense transition zone, J. Geoph ps. Res., this issue.water of the filament core eventually has to sink below the Kelly, K. A., and R. E. Davis, An analysis of errors in sea surfacewarmer ambient water. By the time the meander causing temperature in a series of infrared images from NOAA 6, J.* the filament reverses back toward shore, the cold surface sig- Geopils. Res., 91, 2633-2644, 1986.Kosro, P. M., A. Huyer, S. R. Ramp, R. L. Smith, F. P. Chavez,nature is thus greatly reduced. This would account for why T. J. Cowles, M. R. Abbott, P. T. Strub, R. T. Barber, P.satellite temperature and color imagery only tend to "see" Jessen, and L. F. Small, The structure of the transition zoneflow proceeding offshore. between coastal waters and the open ocean off northern Cali-

fornia, winter and spring 1987, J. Geep ils. Res., this issue.Acknowledgments. This work was supported by the Office Krauss, W., and C. W. Bfning, Lagrangian properties of eddy

of Naval Research (ONR) through the Coastal Sciences section fields in the northern North Atlantic as deduced from satellite-(code 1122CS). tracked buoys, J. Mar. Res., 45, 259-291, 1987.

Kundu, P. K., and J. S. Allen, Some three-dimensional charac-teristics of low-frequency current fluctuations near the Oregon

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Davis, R. E., Drifter observations of coastal surface currents dur- surements in moderate to strong winds, J. PAy,. Oceanogr.,ing CODE: The method and descriptive view, J. Geophys. 11,324-336, 1981.Res., 90, 4741-4755, 1985a. Niiler, P. P., R. E. Davis, and H. J. White, Water-following char-

Davis, R.. E., Drifter observations of coastal surface currents dur- acteristics of a mixed layer drifter, Deep Sea Res., 34, 1867-ing CODE: The statistical and dynamical views, J. Geoplthy. 1881, 1987.Res., 90, 4756-4772, 1985b. Poulain, P.-M., and P. P. Niiler, Statistical analysis of the sur-

Davis, R. E., R. A. DeSzoeke, and P. P. Niiler, Variability in face circulation in the California current system using satellite-the upper ocean during MILE, II, Modeling the mixed-layer tracked drifters, J. PAhy. Occanogr., 19, 1588-1603, 1989.response, Deep Sea Res., 28, 1453-1476, 1981. Stabeno, P. J., and R.. L. Smith, Deep-sea currents off northern

Flament, P., L. Armi, and L. Washburn, The evolving structure California, J. Geophys. Res., 92, 755-771. 1987.of an upwelling filament, J. Geophya. Res., 90, 11,765-11,778,11,835-11,836, 1985.

Flament, P., P. M. Kosro, and A. Huyer, Mesoscale variability off M. Abbott and A. Huyer, College of Oceanography, OregonCalifornia as seen by the Geosat altimeter, in Proceedings of State University, Corvallis, OR 97331.the International Geoscience and Remote Sensing Symposium, R. C. Beardsley and K. H. Brink, Woods Hole OceanographicVancouver, 1989, vol. 2, I.E.E.E., New York, 1063-1068, 1989. Institution, Woods Hole, MA 02543.

Fleischbein, J., R.. E. Schramm, A. Huyer, P. M. Kosro, T. P. P. Niiler, Scripps Institution of Oceanography, University ofCowles, K. Krefft, and C. Paulson, CTD observations in the California, San Diego, La Jolla, CA 92093.coastal transition zone off northern California from R/V We- S. Ramp and T. Stanton, Department of Oceanography, Navalcoma, July to August 1988, Data Rep.144, Ref. 89-1, 242 pp, Postgraduate School, Monterey, CA 93943.Oreg. State Univ., Corvallis, 1989. D. Stuart, Department of Meteorology, Florida State Univer-

Haidvogel, D. B., A. Beckmann, and K. S. Hedstr6m, Dynamical sity, Tallahassee, FL 32306.simulations of filament formation and evolution in the coastaltransition zone. J. Geophys. Res., this issue.

Hickey, B. M., The California C,'wrent system - Hypothesis and (Received June 29, 1990;facts, Prog. Occanogr., 8, 191-279, 1979. accepted October 14, 1990.)

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