JOURNAL OF GEOPHYSICAL
RESEARCH, VOL. 104, NO. C7, PAGES 15,713-15,726, JULY 15, 1999
Nearssurface trajectories
off central and southern
California
C.D. Winant, D.J. Alden, E.P. Dever, K.A. Edwards, and M.C. Hendershott
Center for Coastal Studies, ScrippsInstitution of Oceanography,La Jolla, California
Abstract.
The near-surface
coast of central and southern
circulation
California
in the Santa Barbara
Channel and off the
is described based on 20 releases of drifters
drogued I m beneath the surfacefrom 12 sites within the channel at bimonthly
intervals. This description includes small-scalefeatures of the circulation which are
not part of descriptionsbased on moored observationsor of the statistics of the
drifter releases.The eventual fate of drifters at long time intervals compared to the
residencetime in the channel(about 7 days) is alsoincluded. In the channelthe
trajectories document a persistent cyclonic circulation with a typical recirculation
period between 3 and 5 days. In the spring, currents near the mainland are
weaker than near the Channel Islands, and the overall flow is toward the southeast.
Trajectories document the possibility for water parcels to leave the channel through
the interisland passes. In the late fall and winter a poleward flow with velocities
oftenexceeding
0.5 m s-1 is confinedwithin 20 km of the mainland. Between
these two seasonsthe cyclonictendency is enhanced, although most of the drifters
eventually migrate westward. The trajectories of drifters released at the same
time from sites only 20 km apart can be remarkably different. Once the drifters
migrate out of the channel, their trajectories can be grouped into a few patterns.
In spring and summer, drifters tend to remain in the Southern California Bight.
Their trajectories often remain closeover extended periods, as if they were caught
in convergencezones. In fall the drifters often are caught in a poleward current.
1. Introduction
The maps also suggesta complex pattern of small eddies betweenthesetwo areas. The Organizationof PerThe SantaBarbaraChannel(SBC, Figure1) is boundsistentUpwellingSystems(OPUS) experimentand the
ed to the north by the mainland coast of California,
Santa Barbara Channel CirculationStudy (1983-1984)
which is oriented from east to west in this area. The
were designed to study the responseof the circulation
four Channel Islands, San Miguel, Santa Rosa, Santa
to strong and spatially variable wind forcing. In the
Cruz, and Anacapa, constitute the southern boundary.
vicinity of Point Arguello and Point Conception, obserThe passagesbetween the islandsare typically 40 m
vations suggestthat currents and water properties often
deep. The channelis about 100 km long and 40 km
respondto the wind in the classicalsense: Increasing
wide. The central basin extends to 500 m, and there
equatorward winds lead to colder surface temperatures
are narrow, 3 to 10 km wide shelveson either side. The
and higherequatorwardcurrents[Brink, 1983;Huyer,
easternsill has a depth of 220 m, and the depth at the
western
The
1983; Winant et al., 1987]. In the SBC the flow con-
sill is 400 m.
circulation
of coastal waters in the SBC
and
neighboringareaswas first describedin 1969, as part
of surveys done in the aftermath of the Santa Barbara Channel oil spill. On the basis of three hydrographiccruisesconductedin May, August,and December 1969and eightdrifter cardsurveys,Kolpack[1971]
presentedmaps of the near-surfacecirculation in the
SBC. The most important features of these maps are a
cycloniccell locatedin the westernhalf of the channel
and northwestward flow present in the eastern section.
Copyright1999 by the AmericanGeophysical
Union.
Papernumber1999JC900083.
0148-0227/99/1999JC900083
$09.00
sistsof eddies,jets, and fronts which are not persistently
correlatedwith the local winds [Atkinsonet al., 1986;
Barth and Brink, 1987; Brink and Muench, 1986; Gunn
et al., 1987;Lagerloefand Bernstein,1988].
On a yearly average,winds in the channelare strong
and equatorward southwestof a line running approximately between Point Conception and Santa Cruz Is-
land, and weakeverywhereelsein the channel[Brink et
al., 1984; Caldwell et al., 1986; Dotman and Winant,
1995;Winant and Dotman, 1997]. The SBC is alsopart
of the transition zone where cold upwelled waters meet
warm Southern California Bight waters of subtropical
origin[Chelton,1984; Lynn and Simpson,1987]. The
different water properties can lead to surface pressure
differenceswhich may have a dynamical role in forcing
15,713
15,714
WINANT ET AL.' NEAR-SURFACE TRAJECTORIES OFF CALIFORNIA
35 ø 007V
'i•onte}ey
Los
34ø30'N
NDBC23
SMIN-••N
;MOV
34 ø007V
•
--J
ROOF------•OOF
.
San
Miguel
Is.
120 ø3 0 'W
ß
•..
•O
•.100•,_.,1-,._.•,CAO•
"--{.•Chinese
•
121 øO0 'W
CAIN
ß
.-I ¸ NDBC54•OMI
r" ß GOMI•CNDBC53
•CAMI
'•'•••Anacapa
SantaCruz Is.
is/(
San
"• -
Santa
Rosa
.
120 øO0 'W
119 ø30 'W
! ! 9 øO0 'W
Figure 1. SantaBarbaraChannel- SantaMaria BasinCoastalCirculationStudy(SBC-SMB
CCS) locationmap. NDBC buoysare designated
by NDBCxx,wherexx refersto the last two
digitsof the buoynumber.The boxed-inareain the centerof the maprepresents
the areaused
for residence time calculations. Contour depths are in meters.
the circulation[Hickey,1992;Lentzand Winant,1986]. which the statisticallysignificantempiricalorthogonal
Radio-tracked drifters were used during the Coastal functions(EOFs) capture only a fraction of the total
Ocean DynamicsExperiment (CODE) to identify for variance
[Harmsand Winant,1998](hereafter
HW98)
the first time featuressuchas jets and squirts[Davis, or on statistics of the drifter measurements. The second
1985]asimportantprocesses
in the cross-shelf
exchange is to documentthe eventualfate of drifters after they
of water. A central advantage of drifters is that they
can unambiguouslyshowclosedhorizontalcircuits.
In 1992 a multiyear program, the Santa Barbara
Channel - Santa Maria Basin Coastal Circulation Study
leavethe study region,thus augmentingthe available
descriptionsof the parcel motion along the California
coast. The following section briefly describesthe instruments,the configurationof the deploymentarray,
(SBC-SMB CCS) sponsoredby the MineralsManage- and the scheduleof deployments.The observedtrajecment Service(MMS), wasinitiated with two objectives. tories are presented in section 3. Residencetimes in the
The first was to describethe flow and developsimple dy- SBC and groundingevents,whendrifterswent ashore,
namical relationshipsbetweenit and the forcingmecha- are described in section 4. The results are summarized
nismsto guidethe implementationof numericalmodels. in section 5. Throughout the paper, "poleward" and
The secondobjective was to summarizethese descrip- "equatorward" are used to denote the current directed
tions in a manner useful to analysts who are charged along the coastwith the mainland either on the right
with assessing
what might happenin the eventof an oil or on the left.
spill.
There are two motivations for this paper. The first
2.
Observations
is to completethe descriptionsof the SBC circulation
The observationalperiod in the SBC extended from
by documentingsmall-scalefeatureswhich are lost in
1992
until 1996. The experiment consistedof five obserdescriptionsbased solely on moored observations,for
WINANTET AL.:NEAR-SURFACE
TRAJECTORIES
OFFCALIFORNIA
15,715
vational components. The first provided a description the following address: http://www-ccs.ucsd.edu/
of the atmosphereover the area of interest; results are research
/ sbcsmb/ drifters.
Details of the drifter releases,dates, and instrumensummarizedby C.E. Dormanand C.D. Winant [Dotman and Winant, 1999]. Anothercomponentsynthe- tation are included. There is alsothe capability to plot
sizedmooredobservationsof currentsand related phys- trajectories of any drifter or group of drifters. Ani-
ical parameters[Harms and Winant, 1998]. A third
mations of each drifter
componentwas designedto provide spatially intensive
snapshotsof the circulation basedon acousticdoppler
followingaddress:http://www-ccs.ucsd.edu/research/
sbcsmb/ sat_images
/ movies.
html.
currentprofiler(ADCP), expendable
bathythermograph
(XBT) andconductivity-temperature-depth
(CTD) sur- 3. Description
veysconductedat regularintervals. The daily archiving
of advanced,very high resolutionradiometer(AVHRR)
release can be viewed at the
of Trajectories
For each release,the date, the number of drifters resatelliteimagery[Hendershott
and Winant, 1996]con- leasedand beached, and the prevailing synoptic state
stituted the fourth component. The fifth component, of the circulation(as explainedbelow)are listedin Tawhich forms the subject of this paper, provided a de- ble 1. There were about 5 releaseseach year, and the
scriptionof near-surfaceparticle trajectories as a means releaseswere approximately evenly spaced over the 4
of achievingboth objectivesof the experiment. A statis- year period.
tical and dynamical interpretation of these observations
3.1.
is presentedin a separatepaper [Deveret al, 1998].
The drifters used in this study were similar to those
described
by Davis[1985].Briefly,the drifterconsists
of
a submergedvertical tube inside which reside the electronics and batteries. An antenna protrudes from the
top of the tube and extendsup through the water sur-
face. Four submerged
cloth vanesof total area 1.8 m2
extend radially from the tube. The entire assemblyis
attached to four flotation elementsby short lengths of
nylon line. Two modificationswere made to the original design. The original cubic floats used to buoy the
drifter to the surfacewere replacedby spheresof similar
volume, and the buoys were equipped with a transmitter to locate the buoy using orbiting satellites and the
Argossystem. This systemlocatedthe drifters up to six
times each day, with a positional accuracy that varied
between 150 m and 1000 m, dependingon the quality
of the fix. In a few casesthe frequencyof locationswas
reducedto a fix every few days.
Drifters were programmedto transmit for a period of
40 days followingtheir release. Typically, trajectories
consistof two stages:an initial period during which the
drifter stayed inside the SBC area; and a final stage,
when the path moved either into the Southern California Bight or off the coast of central California. These
stagesare describedseparatelyin the followingsection.
Drifters were releasedapproximately every 2 months at
the locations shown in Figure 1. The locations were
selected
to coincide
in most
cases with
current
meter
moorings, as describedin HW98. Additional release
sites were located between mooringsso that the overall
deployment scheme was spatially reasonably uniform.
Deploymentswere not made within 20 km of the channel's eastern entrance, between Anacapa Island and
Port Hueneme, since those drifters would have quickly
left the area of interest
at times
when
the current
is
equatorward.
The position of each drifter releasedas a function of
time is available
over the internet
and can be viewed at
In
the
Channel
Trajectories for each releasewithin the SBC are compared in Figure 2. While no two releaseswere exactly
the same,two generalfeatures can be identified in this
comparison. The trajectories frequently suggesta cyclonictendencywithin the channel. At times a net flow,
averagedover the width of the channel, is superposed
on the cyclonic circulation. In the spring the net flow
is equatorward, and in the fall and winter it is in the
dppositedirection.Thesefeaturescombineto produce
the followingthree patterns.
In severalreleases(for instance,2, 6, 7, l l, 12, 13,
15, 17, and 19), after remainingfor sometime in the
channel,the drifters migrate into the Southern CaliforniaBight, either throughthe easternentrance,betweenAnacapaIslandand Port Hueneme,or through
Table 1. Drifter ReleaseDates and Descriptions
Release
Release
Date
2
3
4
5
6
7
8
9
l0
ll
12
13
14
15
16
17
18
19
20
21
May 11, 1993
July 26, 1993
Oct. 6, 1993
Dec. 8, 1993
Feb. 15, 1994
April 13, 1994
June 15, 1994
Sept. 13, 1994
Nov. l, 1994
Jan. 31, 1995
March 16, 1995
May 16, 1995
July 7, 1995
Aug. 24, 1995
Oct. 31, 1995
Jan. 10, 1996
May 1, 1996
Aug. 3, 1996
Sept. 13, 1996
Dec. 17, 1996
Number
Beached
Circulation
of Drifters Drifters Synoptic State
12
15
11
14
12
12
13
14
ll
10
ll
12
ll
10
12
14
ll
10
9
11
6
5
6
4
4
8
6
6
2
1
1
4
5
4
1
2
4
4
3
2
upwelling
cyclonic
relaxation
relaxation
upwelling
upwelling
cyclonic
cyclonic
relaxation
upwelling
upwelling
upwelling
cyclonic
upwelling
relaxation
upwelling
cyclonic
upwelling
cyclonic
relaxation
15,716
WINANT
ET AL.: NEAR-SURFACE
TRAJECTORIES
OFF CALIFORNIA
•Release
2 ii! • Release•..
:iii• Release,..•
i!: •Release!..•
ß
• I I'II'
I I I I I I I I I , I I
I I I I I I I I I I I I I I I I I
I I I I I I I I I I I I I I I I I
• Release
(.•.
Release
3
.•,,ly
93
!i
Cyclonic
•,'• '• !i
May
95
'
I
Upwelling'::::
:]
•
May
90
hie
I
•t '• .R..•.•.•ati.O.•
•; ' :::::::::::::::::::::::
...........
:::::::::::::::::::::::::::
Figure 2. A comparison
of driftertrajectories
in eachof the 20 releases.In the upwelling
pattern,mostdriftersleavethe channeleitherthroughthe easternentranceor throughthe
interislandpasses.In the relaxationpattern,manydrifterstravelalongthe coastpolewardafter
leavingthe channel.In the cyclonic
pattern,driftersbecome
entrainedin a gyrein the western
channel.Subsequently,
theydrift southtowardthe SouthernCaliforniaBight.
the passeswhich separate the Channel Islands. This
pattern is referred to as the upwelling pattern since it
occursmore often in the spring and early summer,when
the winds in the western portion of the channel and off
central California become persistent and equatorward,
conditionswhich are associatedwith upwellingoff Point
Conception.
In other releases(for instance,4, 5, 10, 16, and 21)
drifters seededin the northeasternpart of the channel
are sweptinto a polewardcurrent. Most of thesetrajectories are over the northern shelf. Beyond Point Conception,severalof the drifters remain closeto the coast,
continuingpolewardalong the central California coast.
Other driftersleavethe channelthroughthe westernen-
WINANT
ET AL.: NEAR-SURFACE
TRAJECTORIES
trance, then drift toward the south. This pattern is thus
not simply a reversalof the upwellingpattern, sincetrajectoriesoften bifurcate upon leaving the channel. The
relaxationpattern tendsto occurmore frequentlyin late
fall and early winter, after upwellingepisodes,when the
equatorward winds have relaxed, and, for this reason,
is called the relaxation pattern.
Somereleases(for instance,3, 8, 9, 14, 18, and 20) are
characterizedby an intense cyclonic circulation, with
only weak flow out of the channel. This pattern is
described as the cyclonic pattern. In some instances
(notablyreleases3 and 9) the trajectoriessuggestthe
entrance.
Grouping releasesin terms of three principal patterns
is consistentwith the EOF analysis of the same obser-
vationsdescribedby Dever et al. [1998].The first EOF
15,717
upwelling,relaxation,cyclonic,propagating
cyclones,
floodeast,and flood west. The three patternsdescribed
herehaveclearcorrespondence
with the synopticviews
presentedin HW98: the upwelling,relaxation,and cyclonicpatternsare the same. The cyclonicand propagatingcyclonicpatternsare difficultto distinguishfrom
eachother in the drifter observations,
and are grouped
togetherin this descriptionsimply as the cyclonicpattern. The upwelling and flood east patterns described
in HW98 also fall into a singlecategoryin this drifter
description(upwelling),as do the relaxationand flood
westpatterns (relaxation).
Fluctuations
presenceof a cycloniceddy which migrates toward the
western
OFF CALIFORNIA
in the wind
stress at National
Data
Buoy Centerbuoy 46054 for the 4 year period during
whichthe releasestook placeare illustratedin Figure3.
The periods during which each of the 20 releasestook
place are also shown. There is a tendencyfor different
patterns to occur at different times of the year. The
mode, with a positive weight, correspondsto the upwelling pattern, and with a negativeweight corresponds first deploymentsin eachyear usuallycorresponded
to
to the relaxation pattern. The second EOF mode cor- the upwellingpattern. The cyclonicpattern usually
respondsto the cyclonic pattern described here.
occurs during the summer. In the late fall and winHW98 summarizes the near-surface circulation
in the
ter the relaxation pattern is more frequentlyobserved.
SBC in terms of synoptic views or characteristicflow HW98 suggeststhat the upwellingpattern occurswhen
patterns, initially deduced from a subjective classifica- the equatorwardwind stressis large. This is consistent
tion of daily averagesof moored current measurements, with the wind stressillustrated in Figure 3. Cyclonic
and subsequently identified with particular combina- patterns tend to occur when the wind stressis large
tions of EOFs of the moored observations.
In that
and equatorward, in agreementwith moored observarepresentation,about 60% of variance in the observa- tions. Finally, the relaxation pattern takes place after
tions can be categorizedin terms of six flow regimes: the wind stress has become weaker.
2
3
4
5
6
7
8
9 10
1112
13 14 15
16
17
18
19 20
21
0.25 Pa
Wind
Stress
-0.25 Pa
1993
1994
1995
1996
Figure 3. Low-passfiltered time seriesof wind stressat NDBC buoy 46054 along the major axis
(122ø). The signconvention
is suchthat positivevaluescorrespond
to poleward(towardthe NW)
stresses.The vertical bars represent 14 day periods beginning on the releasedate. Each bar is
coded(open,shaded,solid)accordingto the stateof circulation(upwelling,cyclonic,relaxation).
The release number
is noted above each bar.
15,718
3.2.
WINANT ET AL.- NEAR-SURFACE TRAJECTORIES OFF CALIFORNIA
Characteristic
Trajectories of the individual drifters are illustrated in
Figure 4.
Releases
Each drifter trajectory includescirculationfeatures
Noneof the 11 driftersreleasedleft the channelthrough
which reveal interestingcharacteristics
of the circula- the eastern entrance, and six were carried north along
tion but are of sufficientlysmall scalethat they do not the central California coast beyond Point Conception.
appearin statisticaldescriptions
or descriptions
based
Most
on moored observations. For example, there are sev-
the channel. Once around the point, the drifters trav-
of these were released
in the northeast
section of
eral trajectorieswhichexit the SBC throughthe passes elednorth with velocitiesof order 0.2 m s-l; three of
whichseparatethe ChannelIslands.The followingde- theseran aground. Of the five remainingdrifters, three
scriptionof threeindividualreleases
is intendedto high- went south, into the Southern California Bight, where
light suchfeatures. AVHRR satelliteimagesare fre- one eventually ran aground near San Diego, and two
quentlyusedto identifyfeaturesof the surfacecircu- beached, one on the south side of San Miguel Island
lation[Lagerloef
andBernstein,1988]and havebeen and the other on the south side of Santa Rosa Island.
compared
to driftertrajectories
in similarstudies[e.g., Trajectories of three drifters, releasedat ROIN, ROMI,
Chereskinand Niiler, 1994; Davis, 1985; Poulain and and GOOF, provide evidence of the cyclonic compo-
Niiler, 1989]. Accordingly,
the trajectoriesillustrated
nent
of the circulation
near
the center
of the channel
in this sectionare superimposed
on mapsof seasurface even during this time of weak winds.
Each drifter track is superposedon the same AVHRR
image,
representingthe average temperature over the 2
3.2.1. Relaxation pattern (October 6, 1993).
week
period
foilrovingthe release,in an attempt to repEquatorward winds relaxed in fall 1993, and release
resent
mean
conditions for the period the drifters were
4, beginningon October6, was characterizedby weak
winds. Drifters were released from all sites, although in the channel. The temperature extremesin the image
the drifter released at SMIN failed to report positions. differ by more than 4øC. The temperature gradient is
temperature.
Temperature
13.8
14.2
14.6
15.0
15.4
15.8
16.2
t6,6
17.0
17,4
17.8
.
.
...
:!:.:,..
"•'.-.-.:;i:?
-:<&,,%•i.--
GOMI
6 days
CAMI
:"'
11days
;:• %..,.•:..7.,.-•%.:
................
.........
......
....
.
:.
..:.....
.
ß :.
5;;?
...ii
............
..•
;:'. •-;':';:?.•;½:•::L
............................
-................
:.
...
SMOF
11 days
.ii• 7--L•
'"'•,
...............
:::.:;'•::..-'
.......:
.....
•'••].•••
ROOF GOOF CAOF
4 days
.::%
&,•i½-.:;?.',.:•,:.:½'
ß
-
36days
•
......
'. .....................................
"•5•!]•:
30days
.
, ½/,')}•:•;;•...
' ............
Figure 4. Individualdriftertrajectories
for release4. Driftersweredeployed
on October6,
1993. The drifter releasedat SMIN did not report positions.The time duringwhicheachdrifter
remainedin the area illustratedin Figure I is noted beneaththe releasesite. The underlying
imagerepresents
average
temperatures
(in degrees
Celsius)overthe periodOctober6-20,1993.
The averages
werecomputedfrom availableAVHRR images.
15,719
WINANT ET AL.- NEAR-SURFACETRAJECTORIES OFF CALIFORNIA
Temperature
12
13
2days
14
:
14•ys
15
16
• •'=""•' I day
17
• ......
5days
....... ....
....
SMMI
3,23days
,:'-':'•
ROMI
....
2 days
23 days
G.OMI
CAMI
8,5 days
.:.: -..'...{.•:.:.:%,½
...... ..
•'.5 ;•
,.'
.
.
.
.
.
½%:
,a;:•
ROOF'
4.days
,,:{i•:i;.;.. GOOF
!,,;!:,
5 days
•:2J••i..11•
':;11• •:::::::'
.:-:;:;:::
CAOF
11 days
•iii %:'•ir .
...
ß
,
'::.0
"
Figure 5. Individual drifter trajectories for release9. Drifters were deployed on September 13,
1994. The time during which each drifter remained in the area illustrated in Figure 1 is noted
beneath the releasesite. Pairs of drifters were releasedsi•nultaneouslyat ROMI and CAMI. The
underlyingimagerepresentsaveragetemperatures(in degreesCelsius)for all imagesreceivedon
September 14, 1994.
as expectedin this area and in this season,with warmer
waters in the southeastern portion of the figure. The
image showsa small eddy located near the middle of
the channeljust above Santa Rosa island. The drifter
3.2.2. Cyclonic pattern (September 13, 1994).
An episodeof strongwindsfavorableto upwellingended
on September13, 1994, when the drifters in release9
where deployed. Drifters were releasedat all the lo-
releasedat ROIN appears to circle around this feature.
The variety of behaviors exhibited by drifters deployedat adjacent locationsalong the Channel Islands
in this release demonstrates the difficulty of classifying
releasesin simple statistical terms. While generalities
can be stated, for instance, that drifters released near
the mainland coast tend to move poleward, such characterizations do not account for the rapid change in
particle trajectories which can be dependent on specific release site or time of release. For example, the
cations within the channel, and a pair of drifters was
drifter
alongtheislandchaintowardtheeast.Havingreached
released at ROOF
crossed the channel and ran
released at ROMI and at CAMI. This release was typi-
cal of the cycloniccirculationpattern,as severalof the
drifters were caughtup in closedcyclonictrajectories
as illustratedin Figure5. Noneof the driftersleft the
channel through the eastern entrance.
Two of the drifters released at the western entrance
(at SMINandSMMI) initiallymoved
towardthesouthwest,neverenteringthe channel.The drifterreleased
at
SMOF,northoftheSanMiguelIsland,initiallytraveled
agroundbetweenPoint Conceptionand Point Arguello
soonafter release;the drifter releasedat GOOF, 28 km
eastof ROOF, milled around cyclonicallyin the channel
for 20 days accomplishingfour full circuits before running agroundon the south sideof Santa Rosa Island;
SantaCruz,the drifterbeganmillingaroundfor several
and the drifter releasedat CAOF, another 20 km to the
sevenfull revolutionswith a periodof about 3 daysfor
east,initially movedto the north shoreand then drifted
onecircuit,at the sametime asit slowlydriftedtoward
westward out of the channel, then south.
the west.
daysbeforerunningaground
in Chinese
Harbor,onthe
northeastern side of the island.
The drifter releasedat ROIN was caughtin a cyclonic
gyreassoonasit wasreleased.
The driftercompleted
15,720
WINANT
ET AL.- NEAR-SURFACE TRAJECTORIES
OFF CALIFORNIA
Temperam.re
11.4
11,8
12,2
12.6
13.0
13.4
13.8
iI•:.11:':';':;:'
.......
';i•ii!•
$MIN
• *':;!}/•
ROIN
f'•""""•C
'""'•/•
GOIN
• '*:';;•
CAIN
SM• *a/•••••
ROMI GOMI CAMI
4 d•vs
?:
:•
..........................
:...:,..,..•..................
SMOF
1 day
:
7 days
........
11days
;
.....
,%:,** :.
ROOF
GOOF
CAOF
4 days
8 days
7 days
.>
Figure 6. Individualdriftertrajectories
for release
12. Driftersweredeployed
on March16,
1995.The drifterreleased
at SMMI didnotreportpositions.
Thetimeduringwhicheachdrifter
remained
in theareaillustrated
in FigureI is notedbeneath
therelease
site. Theunderlying
imagerepresents
average
temperatures
(in degrees
Celsius)
overthe 2 weekperiodMarch16-30,
1995.The averages
werecomputed
fromavailableAVHRRimages.
Two drifters were released at ROMI, at the same
time. Initially both drifters were caught up in the same
cyclonicfeature as the drifter releasedat ROIN. The ra-
The drifter releasedat GOIN initially traveledtowardthe northwestand ran agroundwithin 1 day of
itsrelease.The drifterreleased
at GOMI wascaughtin
dius of the revolutions of the drifters released at ROMI
the samegyreasdriftersreleased
in the vicinity.After
was smallerthan for the drifter releasedat ROIN, sug- fiverevolutions,
whenits positionwasverycloseto its
gestingthat the centerof the cyclonicfeature was closer deployment
point, the drifter movednorth,then west.
to ROMI. After approximately 5 days the two drifters Uponreachingthe longitudeof Point Conception,
the
deployed at ROMI separated. One continued to follow the drifter
released
at ROIN
drifter movedto the north, milled around for several
and left the channel
daysbetweenPoint Arguelloand Point Sal, and then
through the western entrance. The other drifter crossed wascaughtin the generalsouthwardflowingcurrent.
the channel to the south where it became part of the The drifter releasedat GOOF initially traveledeasteastward current and drifted all the way to the east- ward,then reversed
course,crossed
the channel,and
ern entrance. It then reversed direction, crossedthe beachednear Point Conception.
channel to the north and ran aground on the mainland
The drifter releasedat CAIN initiallyfollowedthe
coast. The behavior of this drifter pair is one of the most coastpoleward,
andthenran aground
approximately
5
conspicuousexamples of dispersion associatedwith the
days later near Santa Barbara. Two drifters were re-
shear across the Santa Barbara
leased
at CAMI;bothinitiallymoved
to thewest,stay-
The
drifter
released
Channel.
at ROOF
started
toward
the
northeast. Once beyond the middle of the channel,it
reversed
course and left the channel
at the western
en-
trance following a similar course to the ROIN drifter
and one of the ROMI
drifters.
ing closetogetheruntil reachingthe westernentrance
of the channel. At this point, one turned southand
the othercontinuedtowardthe west,and then accom-
plished
a broadcyclonic
turn(notshown
in Figure5) at
the endof whichits trajectoryfell in with thoseof other
WINANT ET AL' NEAR-SURFACETRAJECTORIESOFF CALIFORNIA
drifters which left the channel. Finally, the drifter deployedat CAOF remainedin the vicinity of the eastern
tip of Santa Cruz Island until it ran aground.
Two weeklongaveragesof the SST mask the presence
of the cyclonicfeature representedin the middle of each
frame in Figure 5, presumablybecausethis feature travels sui•ciently far during this period that its identity is
15,721
well beyond the SBC. Plate 1 illustrates the releaseson
a larger scale,superposedon averageAVHRR maps for
a 30 day period. When viewed from this larger perspective, trajectories also appear to fall into a few characteristic patterns.
In springand early summer,the upwellingseason,the
drifters leavethe SBC through the easternexit. Once in
smearedby the averagingprocess.Accordingly,the un- the Southern California Bight, the trajectories occupya
derlying image representsthe averageSST determined broad region, a pattern usually associatedwith a coastal
from images available for a single day, September 14, jet.
1994. In this feature a cyclonic swirl of cold water is
During the rest of the summer and early fall, the
located
north
of Santa Rosa Island.
A warm
filament
drifters
often
comingfrom the northern shelf is wrapped around the
western and southern side of this eddy. The 5øC difference between highest and lowest temperatures on the
map is characteristic for the late summer.
Of the 14 drifters released,six ran aground and eight
exited through the western entrance. Those which left
drifted south, with severalreachingMexican waters be-
channel
and drift
exit
from
contrast across the channel is smaller than in either of
observations
[Chereskinand Niiler, 1994;Poulain and
Niiler, 1989]. Maps of the dynamicheight referenced
toward
the
western
the south.
entrance
to the
A remarkable
fea-
ture of their subsequenttrajectories is that they often
remain groupedclosetogether. In this pattern, usually
associated with the cyclonic circulation in the channel, the trajectories appear to conglomerate into distinct filaments. Comparison with features of the underlying AVHRR images suggeststhat the filaments coforethe end of transmission(40 daysafter deployment). incide with cooler water plumes extending south, per3.2.3. Upwelling pattern (March 16, 1995). haps deriving from the California current. This behavRelease12 occurredat the beginningof the 1995 season ior is analogousto that exhibited by drifters during the
for winds favorable to upwelling. Trajectories are illus- CaliforniaCoastalTransitionZone(CTZ) [Brink et al.,
trated in Figure 6. Drifters releasedin the southwestern 1991].
part of the channel drifted into the Southern California
In late fall and early winter releases,drifters exit the
Bight betweenSanta Rosaand Santa Cruz Islands. The western entrance of the channel as they usually do in
remainingdrifters left the channelthrough the eastern summer. However, after exiting the westernchannelentrance, drifter trajectories can behave quite differently
entrance,between Anacapa and the mainland coast.
than in summer. Drifters releasedin fall and early winDrifters releasedalong the northern shelf, at GOIN,
ROIN, and CAIN, first migrated westwardand toward ter can remain over the shelf and be caught up in polethe center of the channel, in the oppositedirection from ward flow to the north. This is observed in October
the wind. After a day the westernmostof these veered 1993, December 1993, and December 1996. In October
toward the southeast,joining the drifters releasedto the 1993 and December 1993, drifters remain over the shelf
west, while the other two reverseddirection and exited and flow north to approximately Point Sur, where they
the channelthrough the easternentrance. Severalother are carried offshore. In December 1996, drifters remain
drifters proceededin a direct coursetoward the eastern over the shelf beyond Point Sur. The majority beach
exit, with speedsof order0.2 m s-1.
in Monterey Bay; however, one drifter continued past
Three drifters left the SBC through the Santa Rosa- Point Reyes before its 40 day tracking period ended,
Santa Cruz passage.Each then first turned toward the and another was later recovered off Astoria, Oregon,
west and followed the southern coast of Santa Cruz Isindicating at least the possibility of continuous poleland, suggestingthat the flow through the passagesmay ward flow between the Santa Barbara Channel and the
wrap around the islands. One drifter beachedon the northern California coast in late fall and early winter.
south shore of Santa Cruz. Two that exited from the
The question naturally arises as to how these trajecchannel's eastern end then turned to the west once they tories, viewed on the large scale, can be reconciledwith
cleared the south end of Anacapa Island.
existing descriptionsof circulation in the Southern CaliThe underlyingimage representsa 2 week long aver- fornia Bight, basedeither on hydrographicobservations
age, beginningon the day of release.The temperature [Hickey,1979; Lynn and Simpson,1987],or on drifter
the other two cases,as is expectedfor this period of the
year. In the channelthe main featureconsistsof a warm
area near the mainland in the generalvicinity of Santa
Barbara. Drifters deployedat GOIN and CAMI, which
initially migratedwest, may have been caughtin this
feature before enteringthe generallysoutheastcurrent.
3.3 Beyond the Channel
to 500 db consistentlysuggestthe presenceof a closed
cycloniccirculation in the Southern California Bight in
summer
and fall.
That
feature
is on a scale substan-
tially larger than the cyclone which is present within
the SBC.
Yet
of the 235 drifter
releases described
in
this report, only one trajectory, release 15, shows evidence of a closed circulation
in the Southern
California
Sincethe lifetime of the drifters usually exceedstheir Bight. This result could be due to the relatively small
residencetime in the channel,the trajectories extended area over which drifters were seeded. The prevailing
15,722
WINANT
ET AL.- NEAR-SURFACE
Release No. 2
May 1993
spas
TRAJECTORIES
ReleaseNo. 6
OFF CALIFORNIA
ReleaseNo. 7
Feb.1994
ReleaseNo. 12
April1994
Sprin,
.
Mar. 1995
Sp•ns
Sp•in•
ß,
Release No. 13
May 1995
Spring
Release No. 3
July1993
Summer
ReleaseNo. 15
ReleaseNo. 17
Aug.1995
Spring
Jan.1996
Spring
Release No. 8
Release No. 9
June1994
Sept.1994
Summer
ß
ReleaseNo. 19
Aug.1996
Spring
Release No. 14
July1995
Summer
Summer
Release No. 4
Release No. 5
Oct 1993
Dec. 1993
Winter
Winter
ß
Release No. 18
Release No. 20
May 1996
Sept 1996
Summer
$unu•er
Release No. 10
Release No. 11
Release No. 16
Release No. 21
Nov. 1994
Jar• 1995
Ocr 1995
Dec. 1996
Winter
Winter
Winter
Winter
Plate 1. Comparisonof drifter trajectoriesfor individual releaseson the scaleof the Southern
CaliforniaBight. Underlying
colorimagesrepresent
the average
temperature
overthe 30 day
period beginningon the date of the release. The temperaturescaleis adjustedto maximize
the contrastovereachimage,Warmertemperatures
correspond
to red-orange
colorsand colder
temperatures
correspond
to blue-purplecolors.Imagesare groupedaccordingto the large-scale
flow pattern, designatedas spring,summeror winter.
WINANT
ET AL.: NEAR-SURFACE
TRAJECTORIES
OFF CALIFORNIA
15,723
pattern of surfacetrajectories during summer and fall
is more consistentwith the idea of a filament, similar to
and winter, groundingsalso occurredalong the central
California coast, well north of the channel. Locations
the pattern describedby Chereskinand Niiler [1994].
of all the sites where drifters ran ashore in the SBC and
vicinity are illustrated in Figure 7. The sites are distributed almost continuouslyalong the mainland coast
4. Residence Times and Groundings
within the SBC, as well as alongthe north facingcoast
In order to describe the time period during which of the Channel Islands. A few drifters ran agroundon
drifters remain in the channel, a residencetime is de- the south side of the Channel Islands. ChineseHarbor,
fined as the averageperiod for which a groupof drifters on the north side of Santa Cruz Island, appearsto be a
remain in a regionextendingfrom 33.75øN to 34.65øN site where drifters run aground more often.
and from 120.7øW to 119.2øW. This area extends from
In view of the cyclonic circulation in the SBC, it
Port Huenemeto Point Arguello and includesthe south might be expected that drifters released at different
shore of the Channel Islands.
The area was chosen
siteswould run aground in specificareasof the channel.
based on the observation that once a drifter is outside
The location where drifters ran aground is illustrated
these boundaries, it rarely returns.
in Figure 8 for each releasesite. Drifters launchedfrom
The averageresidencetime for all drifters which do the northern end of the line between Point Conception
not ground is 8 days. This time does not depend and San Miguel (SMIN and SMMI) had the shortest
stronglyon the prevailingcirculationpattern: For up- residencetimes and were the least likely to ground.
welling/relaxation/cyclonic
patternsthe corresponding Times until groundingwere also quite short. No drifters
residence
timesare8/7/6 days. Oneexplanationfor this launched from these sites grounded on the mainland
result may be that all the circulationpatterns include coast east of the launch point. The Channel Islands
the cycloniccomponent.The cycloniccirculationtends were generallyfavored as groundingpoints. For drifters
to prolongresidencetimes independentof whetherthe launchedon the SM line, groundingson San Miguel ocflowaveragedacrossa sectionof the channelis poleward curred for all synoptic conditions,while groundingson
or equatorward.
Santa Rosaand Santa Cruz occurredfor upwellingand
The results summarized in Table I show that 78 of the
cyclonic conditions.
235 drifters(or over 30%) releasedin the channelran
Drifters launchedalong the RO line extending north
aground, and the majority of these incidents occurred from Santa Rosa alsotendedto groundalongthe Chanwithin the channel. Outside the SBC, Santa Monica nel Islands, generally during upwelling or cyclonic conBay was a commongroundingsite. During the late fall ditions. The exceptionis San Miguel Island, which had
35W
34W
121 øW
120 øW
119 øW
118 øW
Figure 7. Locationwhere drifters released]n the courseof this experimentran aground.
15,724
WINANT
ET AL.' NEAR-SURFACE
TRAJECTORIES
OFF CALIFORNIA
!
SMO F
ROOF
7 2 33%
7417%
iliiiii:iiiiiiii:ii?iiii?::i?:
GO0 F
177 60%
1063Z:•
......
CA 0 F
iliiiiiiii!iiiii:!::?ii:ii:ii!i:11il
118 41%
Figure 8. Location where drifters releasedin the courseof this experimentran aground, segregated by the position of release. The releaseposition is shownby the open circle. The first inset
numberrepresentsthe medianresidencetime for all drifters launchedfrom eachsite (in days).
The secondnumberrepresentsthe mediantime (in days)beforedriftersthat beachrun aground.
The third number is the percentageof all drifters launchedat each site which ran aground.
several drifters ground on it during relaxation condi-
Drifters
launched in the northwestern
SBC where the
tions as well.
persistent cyclonic circulation keeps the velocity high
Drifters launched along the GO line extending north tend to exit the channelrapidly and are the least likely
from Santa Cruz often beached along the mainland to beach. Conversely,drifters launched north of Santa
as well as the Channel Islands.
Drifters launched at
Cruz where average speedsare slower remain in the
GOIN were particularly likely to beach along the main- channel longer and are more likely to beach. Crossland coast near their launch point for the upwelling channel exchangeof drifters is generally highest in the
state. Drifters launched at GOMI also occasionally western Santa Barbara Channel, with several of the
beached on the mainland
coast between Santa Bardrifters launchedat SMIN and ROIN beachingon the
bara and Port Hueneme during upwelling conditions. Channel Islands. In the eastern channel, many drifter
Drifters launched at GOOF had the highest likelihood releaselocations(GOIN, GOOF, CAIN, CAMI, and
of grounding of all deployment locations. Under cy- CAOF) favor beachingrelatively near the releaseloclonic or upwelling conditions the beachingpoint was cation.
often at Chinese Harbor
on Santa Cruz.
beached on either the mainland
Drifters
which
or Channel Island coasts
west of the GO line did so under cyclonicor relaxation
5. Summary
conditions.
The central result of this descriptionis to document
Drifters launchedalongthe CA line generallybeached the existenceof a persistentcycloniccirculationin the
east of their launchpoint under upwellingor cyclonic SBC. Beyondthis ubiquitousfeature, the circulation
conditions. The few which beached west did so under
can usually be describedin terms of three possible
cyclonic or relaxation conditions. Drifters launched at patterns. In the upwelling pattern, drifters exit the
CAIN and CAMI tendedto groundoverthe relatively channelwithina few days,eitherthroughthe passages
broad shelfextendingbetweenSanta Barbara and Port whichseparatethe ChannelIslands,or morefrequently
Hueneme(Figure 1). These groundingsgenerallyoc- throughthe easternentrancebetweenAnacapaIsland
curred under upwellingor cyclonicconditions.Drifters and Port Hueneme. In the cyclonicpattern, drifters
launchedat CAOF often groundedon Santa Cruz under tend to remainin the SBC for morethan a week,parcyclonicor upwelling conditions.
ticularlythosewhichhavebeenreleasedawayfrom the
WINANTET AL' NEAR-SURFACE
TRAJECTORIES
OFFCALIFORNIA
15,725
entrances. Drifters describeanticlockwisetrajectories thanksgoto thestaffof theScripps
Institutionof Oceanogwith a period of 3 to 5 days. HW98 has shown how raphy Hydraulics Laboratory, to the Center for Coastal
in somecasesthesecyclonespropagatetoward the east, Studiescomputersupport group, and to the officersand
with in oneinstancea velocityof 0.06m s-1. After only crewof the R/V Sprouland New Horizon. The intellectual
supportprovidedby David Browneof the PacificRegionof
a short residencetime in the channel, drifters released MMS is gratefully acknowledged.Support for this work is
during relaxation patterns migrate out of the channel providedby CooperativeAgreement14-35-0001-30571bethrough the westernentrance,betweenPoint Concep- tweenthe MineralsManagement
Serviceandthe University
tion and San Miguel Island. Subsequently,
many of the of Californiaat San Diego.
drifters(and mostof thosereleasedalongthe northern
sideof the channel)migratepolewardalongthe central References
California coast.
Atkinson,L. P., K. H. Brink,R. E. Davis,B. H. Jones,
These three patterns tend to occur at different times
T. Paluszkiewicz,
and D. W. Stuart, Mesoscale
hydrographicvariabilityin the vicinityof PointConception
and
of the year. The upwelling pattern usually occursin
Arguello
duringApril-May1983:TheOPUS1983Experthe early spring,after winds becomepersistentlyequaiment,J. Geophys.
Res., 91(Cll), 12,899-12,918,
1986.
torward. The cyclonicpattern appearsmost frequently Barth,J. A., andK. H. Brink,Shipboard
Acoustic
Doppler
duringthe late spring,the summer,and early fall, when
Profilervelocity
observations
nearPointConception:
Spring
winds are still persistentand equatorward and when the
1983,J. Geophys.
Res., 92(C4), 3925-3943,1987.
dynamicsof coastalupsealeveldifferenceis in a directionoppositeto the wind Brink, K. H., The near-surface
welling,Prog. Oceanogr.,12, 223-257,1983.
stress. Finally the relaxation pattern usually occursin
Brink, K. H., and R. D. Muench, Circulation in the Point
late fall and through the winter, when the winds are
Conception-Santa
BarbaraChannelregion,J. Geophys.
no longer persistently equatorward, but the sea level
Res., 91(C1), 877-895, 1986.
remains higher to the south.
Brink, K. H., D. W. Stuart, and J. C. Van Leer, Observations of the coastal upwellingregion near 34ø 30'N off
It is significantfor oil spill risk analysis that about
California:Spring1981,J. Phys.Oceanogr.,
1J, 378-391,
one third of the 235 drifters releasedin this study ran
1984.
ashore,for the most part in the SBC, either along the
mainland
coast or on the Channel
leased on the northern
Islands.
Drifters
re-
side of the western entrance had
Brink, K. H., R. C. Beardsley,
P. P. Niiler, M. Abbott, A.
Huyer, S. Ramp, T. Stanton,and D. Stuart, Statistical
properties of near-surface flow in the California coastal
transitionzone, J. Geophys.Res., 96(C1), 14,693-14,706,
the least likelihood of beaching, presumably because
1991.
of the persistent westward current in this area, which
Caldwell,P. C., D. W. Stuart, and K. H. Brink, Mesoscale
tends to sweepthe drifters offshore,where they become
windvariabilitynearPointConception,
Californiaduring
entrained into the California current system.
spring1983, J. Clim. Appl. Meteorol.,25, 1241-1254,
Drifters released at CAIN
and GOOF
were the most
1986.
variabilityof alongshore
geostrophic
likely to run aground. Over half the drifters releasedat Chelton,D. B., Seasonal
velocityoff centralCalifornia,J. Geophys.
Res., 89(C3),
CAIN beachedalong the mainland California coast,to
3473-3486, 1984.
the west of their releasesite. It is noteworthy that this Chereskin,T. K., and P. P. Niiler, Circulation in the Enlocation is closeto the site of the oil spill which occurred
senadafront-September
1988, DeepSea Res. II, Jl(8),
1251-1287, 1994.
in 1969 and resulted in extensiveoiling of beachesand
coastal
areas
in the
channel.
Drifters
released
from
GOOF tended to beachalongthe Channel Islands,with
multiple groundingsin ChineseHarbor, on Santa Cruz
Island. Sites from which a large number of drifters ran
aground are sites where the averagecurrent is weak.
Beyond the Santa Barbara Channel, drifters take a
variety of paths. Drifters launchedin springtend to exit
Davis, R. E., Drifter observationsof coastal surface currents
duringCODE: The methodand descriptiveview, J. Geophys. Res., 90(C3), 4741-4755,1985.
Dever, E.P., M.C. Hendershott,and C.D. Winant, Statistical aspects of surface drifter observations of circulation in
the SantaBarbaraChannel,J. Geophys.
Res.,103(Cll),
24,781-24,797, 1998.
Dor•nan,C. E., and C. D. Winant, Buoy observations
of the
atmosphere
along
the
west
coast
of
the
U.S.,
1981-1990,
the eastern entrance of the channel and continue south
J. Geophys.Res., 100(C8), 16,029-16,044,1995.
into the SouthernCalifornia Bight. In summer,drifters Dorman,C. E., and C. D. Winant, The marinelayerin and
tend to exit the western entrance to the channel and be
aroundthe Santa Barbara Channel,Mon. WeatherRev.,
in press, 1999.
carried south in patterns analogousto those deployed
in the Coastal Transition Zone (CTZ) study. Drifters Gunn,J. T., P. Hamilton,H. J. Herring,K. L. Kantha,G.
S. Lagerloef,G. L. Mellor, R. D. Muench,and G. R. Stelaunched in fall and early winter exit the western engen, Santa Barbara Channel circulation model and field
trance to the Santa Barbara Channel. After exiting,
study,final report, DynalisisPrinceton,Rep. 92.1, 92.2,
Princeton, N.J., 1987.
they can remain on the shelf and move poleward for periods of weeks. Offshore flow can occur near Point Sur, Harms,S., and C. D. Winant, Characteristicpatternsof the
circulationin the Santa Barbara Channel,J. Geophys.
or drifters may continueflowingpolewardinto Monterey
Res., 103(C2), 3041-3065,1998.
Bay and on to the northern California coast.
Hendershott,M. C., and C. D.Winant, The circulationin
the SantaBarbaraChannel,Oceanography,
9(2), 114-121,
1996.
Acknowledgments.
The successfulacquisition of the
large variety of data utilized in this study was made possi- Hickey,B. M., The Californiacurrent system-Hypotheses
ble only throughthe effortsof a great many people.Special
and facts, Prog. Oceanogr.,8, 191-279, 1979.
15,726
WINANT
ET AL.: NEAR-SURFACE
Hickey, B. M., Circulation over the Santa Monica-San Pedro
basinand shelf,Prog. Oceanogr.,30, 37-115, 1992.
Huyer, A., Coastal upwellingin the California current system, Prog. Oceanogr., 12, 259-284, 1983.
Kolpack, R. J., Biological and OceanographicalSurvey in
the Santa Barbara Channel Oil Spill 1969-1970, vol. 2,
chap. 4, pp. 90-180, Allan Hanock Foundation, Univ. of
Southern Calif., 1971.
TRAJECTORIES
OFF CALIFORNIA
Winant, C. D., and C. E. Dorman, Seasonalpatternsof surface wind stress and heat flux over the Southern California
Bight, J. Geophys.Res., 102(C3), 5641-5653,1997.
Winant, C. D., R. C. Beardsley,and R. E. Davis, Moored
wind, temperature,and currentobservations
madeduring
Coastal Ocean Dynamics Experiments I and 2 over the
northern California continentalshelf and upper slope, J.
G½ophys.
Res., 92(C2), 1569-1604,1987.
Lagerloef, G. S., and R. L. Bernstein, Empirical orthogonal function analysis of advanced very high resolution radiometer surface temperature patterns in the Santa Bar-
bara Channel, J. Geophys.Res., 93(C6), 6863-6873,1988.
Lentz, S. J., and C. D. Winant, Subinertial currents on the
southern California shelf, J. Phys. Oceanogr., 16, 17371750, 1986.
Lynn, R. J., and J. J. Simpson, The California current system: The seasonal variability of its physical characteris-
tics, J. Geophys.Res., 92(C12), 12,947-12,966,1987.
Poulain, P.-M., and P. P. Niiler, Statistical analysis of the
surface circulation in the California current system using
satellite-tracked drifters, J. Phys. Oceanogr., 19, 15881603, 1989.
D. J. Alden, E. P. Dever, K. A. Edwards,M. C. Hender-
shott,andC. D. Winant,Centerfor CoastalStudies,Scripps
Institution of Oceanography,
Universityof Californiaat San
Diego, 9500 Gilman Drive, La Jolla, CA 92093-0209.
(email:dja@coast.ucsd.edu;
dever@coast.ucsd.edu;
kate@coast.
ucsd.edu;mch@coast.ucsd.
edu;
cdw@ucsd.edu)
(ReceivedJuly 13, 1998; revisedFebruary11, 1999;
acceptedFebruary 24, 1999.)