JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 102, NO. C4, PAGES 8555-8571, APRIL 15, 1997
Subtidal velocity correlation scales
on the northern
California
shelf
E. P. Dever •
Woods Hole OceanographicInstitution,Woods Hole, Massachusetts
Abstract. Along- and cross-shelfcorrelationscalesof subtidalcross-shelf(u) and alongshelf (v) velocitiesare estimatedusingmoored recordsfrom severalfield programsover
the northern California shelf. Over record lengthsof 4-6 months, along-shelfcorrelation
scalesof v are greater than maximummooringseparations(60 km). In the cross-shelf
direction,
v isgenerally
correlated
between
the60 and130rnisobaths
(10-15km
separation).Along-shelfcorrelationscalesof u are much smallerthan thoseof v and are
often not resolvedby minimum mooring separations.Time seriesbetweenNovember 1988
and May 1989 do resolvealong-shelfcorrelationscalesof near-surfaceu and indicate that
they are 15-20 km. During this time the along-shelfcorrelationscaleof near-surfaceu
showsvariabilityon a monthlyscale.It is generallylong (30 km or more) when correlation
of u with wind stressis highand short(15 km or less)whencorrelationwith wind stressis
low. On at least one occasion,short along-shelfcorrelation scalescoincidewith the
intrusion
of an offshore mesoscale feature
onto the shelf. Cross-shelf
correlation
scales of
u are resolvedfor typicalmooringseparations.In general,u is correlatedbetween the 90
and 130 rn isobaths(7-13 km separation)and betweenthe 60 and 90 rn isobaths(-5 km).
1.
Introduction
There existsa considerablebody of theory concerningthe
dynamicsof subtidalvelocity on wind-drivenshelves[Allen,
1980].This includessurfaceand bottom boundarylayer theory
[Brink, 1983; Lentz, 1992; Trowbridgeand Lentz, 1991], twodimensionalupwelling models [Csanady,1982; Janowitzand
Pietrafesa, 1980; Mitchurn and Clarke, 1986], and coastaltrapped waves (CTW) theory [Chapman, 1987; Brink, 1991].
These theories
often assume the circulation
is two-dimensional
or, at most, slowlyvarying in the along-shelfdirection.
Observationalestimatesof along-shelfvelocity(v) correlation lengthsover wind-forcedshelves[KunduandAllen, 1976;
Winant et al., 1987] generally agree with this theoretical assumption, and subtidal v can be correlated over along-shelf
distancesgreater than maximum mooring separations(60 km
and more). The long v correlationscalesobservedwhen wind
stressforcing dominates do not preclude shorter correlation
scaleswhen wind stressforcing is less important [Winant,
1983].
Understandingcross-shelfflow remainsa major challengein
coastal oceanography[Smith, 1995]. However, along-shelf
scalesof subtidalcross-shelf
velocity(u) are not only shorter
than those predicted by wind-forced theory but have usually
not been resolved[e.g.,Kundu and Allen, 1976; 145'nant
et al.,
1987;145'nant,
1983].Successful
estimatesshouldthereforeprovide insightinto the dynamicsof u and how observedvelocities
differ from simplewind-forcedtheory.They may alsocontribute to a better understandingof the spatial scalesover which
point measurementsapplyto u on wind-drivenshelveswhich
should be useful to interpretation of existing measurements
and to future experiment design.
The objectivesof this study are to summarize the spatial
scales of both subtidal
v and u on the northern
California
shelf
and to gain insight into the processeswhich affect u spatial
scalesin particular.The spatial scaleswill be defined here in
terms of correlation lengths, the length over which subtidal
fluctuationsare correlatedat somesignificancelevel. They will
be estimated from time serieslasting several months and encompassingall seasons.
The remainder of the paper is divided into five sections.In
section2 I give a brief overviewof the general conditionsover
the northern
California
shelf and an introduction
to the field
programsused.In section3 I list the proceduresusedto estimate u and v correlation scalesand give resultsfor the various
experiments. In section 4 the u correlation scalesbetween
November 1988 and May 1989 are further examined with a
view to identifyingthe processeswhich affect them. In section
5 I compare the observedcorrelation scaleswith those expected from wind-forced theory. In section6 the results are
summarized.
2.
2.1.
Background and Observations
Background
The region examinedin this studyextendsfrom Point Reyes
to Point Arena (Figures 1 and 2). It is noted for its strong
along-shelfwind forcing and relatively straight narrow shelf
[Beardsley
and Lentz, 1987],and in theserespectsit is perhaps
an archetypeof wind-forcedcirculationpresentalong much of
the U.S. Pacific coast and elsewhere. Wind stress here exhibits
•Now at Centerfor CoastalStudies,ScrippsInstitutionof Ocean- strong seasonalvariability. In winter and spring it is distin-
ography,La Jolla, California.
guishedby weak monthlymeansand polewardand equator-
Copyright1997 by the American GeophysicalUnion.
wardfluctuations
ontimescales
ofdays.
In summer
i{ isdistin-
Paper number 96JC03451.
guishedby strongmonthly meanswith periods of upwelling
favorablestresswhich persistfor severalweeks [Halliwell and
0148-0227/97/96 JC-03451 $09.00
8555
8556
DEVER:
124 ø
VELOCITY
SCALES
ON THE
NORTHERN
123 ø
CALIFORNIA
SHELF
124 ø
123 ø
ß
NDBCbuoys
ß STRESS
Moorings
o
ß
39 ø
NCCCSMoorings
SMILEMoorings
39 ø
39 o
39 ø
38 ø
38 ø
38 ø
Pt. Arena
38 ø
124 ø
123 ø
Figure 1. Map of northernCaliforniashelfshowingnominal
Coastal Ocean DynamicsExperiment(CODE) (circles)and
National Data Buoy Center (NDBC) (solidsquares)mooring
locations.Exact locationsvaried slightlyfrom deploymentto
deployment,and most locationsincludednearbysurfaceand
subsurfacecomponents.Solid circlesdenote locationsoccupied duringCODE-2, and open circlesdenotelocationsoccupied duringCODE-1. The centralC line (with the exceptionof
C1) was present during both CODE-1 and CODE-2. The
southerlyR3 locationwas occupiedduringCODE-1 and fall
and winter CODE deploymentsbut was later moved north
during CODE-2.
124 ø
123 ø
Figure 2. Map of northernCaliforniashelfshowingnominal
Northern California Coastal Circulation Study (NCCCS)
(opencircles),ShelfMixed LayerExperiment(SMILE) (solid
circles),SedimentTransportEventsover the Shelfand Slope
(STRESS)(solidtriangles),and NDBC (solidsquares)mooring locations.NCCCS mooringlocationsvaried slightlyfrom
deploymentto deployment.
3. Velocity Correlation Length Scales
3.1.
Procedures
Correlationswere estimatedfrom low-passfiltered velocity
records(Table 2). The PL64 filter usedhas a 38 hour halfpower point and is describedby Limeburner[1985]. Record
Allen, 1987; Strubet al., 1987;Beardsleyet al., 1987]. Figure 3
showsthat the low-frequencyalong-shelfwind stresshas long
spatialscalesin all seasons[see alsoBeardsleyet al., 1987].
4:• + O•o+
+½ +
Data used to estimate the correlation scalesof along-shelf
wind
stress are shown in Table
2.2.
Field Programs
o
o)
o
1.
0.5
In part becauseof the characteristics
listedabove,the northern California shelf has been the site of several large field
programs.Theseincludethe CoastalOceanDynamicsExperiment (CODE) [see,e.g.,Beardsley
and Lentz, 1987;Winantet
al., 1987; Lentz and Chapman, 1989], Northern California
CoastalCirculationStudy(NCCCS) [Largieret al., 1993],Shelf
Mixed Layer Experiment(SMILE) [Deverand Lentz, 1994],
and Sediment Transport Events over the Shelf and Slope
(STRESS) study.Nominal mooringlocationsused here are
shownfor CODE in Figure 1 and for SMILE, STRESS, and
NCCCS in Figure 2. Table 2 liststhe exactlocationsand start
andstoptimesof time seriesusedin thisstudy.Comprehensive
informationconcerningthesemooredmeasurementsare given
by RosenfeM[1983], Limeburner[1985], EG&G, Inc. [1989,
1990a,b], Alessiet al. [1991],and Fredericks
et al. [1993].
o
-0.5
0
5'0
1•0
along-shelf separation in km
Figure 3. Correlationsof low-passedalong-shelfwind stress,
•-Y,asa functionof along-shelfmooringseparationfor summer
(CODE-2) NDBC 14,N3, C3, R3, andNDBC 13 (opencircles)
and winter and spring(SMILE) NDBC 14, G3, C3, M3, and
NDBC 13 (pluses).The along-shelfdirectionis defined as
317øTfor all locationsexceptfor the NDBC 14 locationwhere
it is defined as 341øT. Correlation
scales of •-y are not sensitive
to the precisedefinitionof along-shelfdirectionand exceed
100 km.
DEVER:
VELOCITY
SCALES
ON THE
NORTHERN
CALIFORNIA
SHELF
8557
Table 1. Data Used to Estimate Along-Shelf Wind StressCorrelation Scales
Position
Mooring
Latitude
Longitude
NDBC14
C3
R3
NDBC13
39011.98
38036.40
38025.33
38011.98
124000.00
123027.70
123016.36
123018.00
N3
38048.07 '
123ø41.71 '
G3
38044.40 '
123036.40 '
C3
M3
NDBC13
38ø38.71 '
38032.67 '
38011.98 '
123029.56 '
123022.97 '
123018.00 '
NDBC14
39011.98 '
124000.00 '
Principle
Axis,
øT
•-7,
dyncm-2
•'•,
dyncm-2
O.x,
dyncm-2
O-y,
dyncm-2
rx,
hours
Ty,
hours
CODE-2:0400 UT, April 5, 1982, to 2100 UT, July 25, 1982
'
'
'
'
'
'
'
'
333
313
313
312
-0.1
0.1
0.1
0.2
-0.9
- 1.1
-1.0
-1.0
0.2
0.1
0.1
0.2
1.3
1.3
1.2
1.2
47
58
53
53
44
58
55
57
1.1
46
46
0.8
38
38
0.9
0.8
0.9
38
38
38
38
38
43
0.9
38
38
CODE-2:0400 UT, April 11, 1982, to 2100 UT, July 25, 1982
335
-0.3
-1.0
0.1
SMILE: 0700 UT, Nov. 16, 1988, to 1100 UT, April 6, 1989
342
-0.3
-0.2
0.3
SMILE.' 0700 UT, Nov. 16, 1988, to 1900 UT, May 13, 1989
311
312
303
0.0
-0.0
0.1
-0.4
-0.3
-0.4
0.1
0.1
0.3
SMILE: 1200 UT, Jan. 15, 1989, to 0100 UT, May 4, 1989
342
-0.0
-0.0
0.1
Start and stoptimesgivenare thoseusedfor correlationcalculations.The along-shelfdirectionis taken as 317øTexceptat NDBC 14 where
it is taken as 34 løT.
start and stop times were chosensuchthat completerecords
existedat most mooringsin any given field program deployment. To compareNCCCS and SMILE, the NCCCS records
were dividedinto three periods,the secondof whichcoincided
with SMILE. Record lengthsrangedfrom 90 to 200 days(Table 2). The (one-sided)autocorrelationtimescalesprovide an
estimateof the number of degreesof freedom for crosscorrelations
between
two time
series. Autocorrelation
timescales
were generally3-4 daysfor v and 2-3 daysfor u. Longer time
seriesalso tend to have longer autocorrelationtimes as they
start to pick up seasonalvariability.The autocorrelationtimescalesgenerally suggestat least 30 degreesof freedom. This
estimate
is conservative
in that cross-correlation
timescales
are
these recordswere in the surfaceboundarylayer, though several are probablybelow it duringweak wind stressconditions,
especiallyin summer[Lentz, 1992]. Middepth recordswere as
near as possibleto half the water depth at each mooring.
Near-bottom velocityrecordswere 10 to 20 m abovethe bottom. Though 20 m is near the upper limit of bottom boundary
layer [Lentzand Trowbridge,1991], it was felt the larger number of records gained was preferable to the limited records
within
3.2.
10 m of the bottom.
Along-Shelf Correlation Scales
Correlations as a function of along-shelf separation were
estimated primarily along the 90 m isobath.Additional estimates along the 60 and 130 m isobathswere made using the
CODE-2 data. Minimum along-shelfseparationsranged from
4 km during the common SMILE/NCCCS time period to 26
alwayslessthan autocorrelationtimescales.For 30 degreesof
freedom,correlationsof 0.30 (0.35) or greaterare significantat
the 90% (95%) level.
Velocity recordswere rotated into local isobath reference km in CODE-2.
Along-shelfvelocities(Figure 4 and Table 3) were signififramesat eachmooringlocation(Table 2). The CODE-2 local
isobathdirections[Winant et al., 1987] were also used for the cantly correlated at all along-shelfmooring separationsfor
analogousCODE-i, NCCCS, SMILE, and STRESS locations. near-surface, middepth, and near-bottom depths; hence v
The CODE-1 C1 isobathorientationis givenby Lentz [1994], along-shelfcorrelation scalesare greater than 60 km in this
while the CODE-1
R3 and M3 and the SMILE
M3 and G3
area. Correlationsappearto showlittle variationbetweenfield
isobathorientationswere estimatedfrom charts.As noted by programswhich occurred during different seasons.Though
Smith [1981],u is sensitiveto the referenceframe. To test this, correlated at all depths, correlations of interior and nearcorrelations were calculated in which all records were rotated
bottom v were slightlylarger than near-surfacev (Table 3 and
ñ2 ø and ñ5 ø from the local isobaths. The v correlation
coefFigure 4). Along-shelfcorrelationsof v are not a strongfuncficients varied by only ñ0.02. Correlation coefficientsfor u tion of total water depth thoughCODE-2 observationssuggest
showedmore scatter(about ñ0.10). Sensitivityof u correla- some decrease in near-surface v correlation at the 130 m isotions to coordinate rotation was greatestfor near-bottom in- bath (N4, C4, and R4) relativeto the 60 (N2, C2, and R2) and
struments and those located at the 30 or 60 m isobath. How90 m (N3, C3, and R3) isobaths(Table 3).
ever, qualitativeresultswere unaffectedin that u correlations
Along-shelfcorrelationscalesof u (Figure 5 and Table 3)
which were significantin the local isobath reference frame were resolved only for near-surfaceu during SMILE and
remained significantin the rotated referenceframes.
NCCCS (and possiblyby CODE-1 C3 and M3) alongthe 90 m
Correlations were calculated for near-surface, middepth, isobath. SMILE and NCCCS correlations indicate that alongand near-bottomrecords.These depthswere chosenbasedon shelf correlation scales of near-surface u are from 15 to 20 km.
the divisionof flow into a surfaceboundary layer, an inviscid Little information is available about subsurfaceu along-shelf
interior (exceptat the 30 m C1 site), and a bottom boundary correlationscales.There is the suggestionthat middepth and
layer. Near-surfacerecordswere at 10 m, or failing that, at the near-bottom u are correlated at separationsof 4-10 km but
shallowestavailable depth (always20 m or less). Generally, that correlation lengths are lessthan 25 km.
8558
DEVER:
VELOCITY
SCALES ON THE NORTHERN
CALIFORNIA
SHELF
Table 2. VelocityData Used to EstimateCorrelationScales
Position
Instrument
Water
Mooring Latitude Longitude Depth, m Depth, m
Isobath,
øT
•,
10-2 m s-•
•,
10-2 m s-•
o-.,
10-2 m s-•
o-•,,
10-2 m s-•
T.,
T,,, Principle
hours hours Axis, øT
CODE-I: 0400 UT, April 17, 1981, to 1500 UT, July10, 1981
'
'
'
'
'
'
'
'
'
38ø36.11
38ø36.19
38ø36.19
38ø31.30
38021.80
38021.70
38021.70
'
'
'
'
'
'
'
123ø27.18
123ø27.18
123ø27.18
123ø40.10
123013.00
123ø13.10
123ø13.10
'
'
'
'
'
'
'
38ø36.11
38ø36.10
38ø36.10
38031.30
38ø31.30
38021.50
38ø21.95
38ø21.95
'
'
'
'
'
'
'
'
123ø27.18
123027.40
123027.40
123040.60
123ø40.10
123013.00
123ø13.15
123ø13.15
'
'
'
'
'
'
'
'
9
55
75
9
150
9
55
75
123ø25.32
123025.32
123025.32
123ø27.71
123032.70
123032.70
'
'
'
'
'
'
R4
38ø38.16 '
38ø38.16
38ø38.16
38ø38.38
38ø34.30
38ø34.30
38ø33.26
38ø33.26
38ø33.26
38ø30.80
38ø30.88
38ø49.50
38ø49.50
38ø48.07
38ø48.09
38ø48.09
38ø45.79
38ø45.71
38ø45.71
38ø27.17
38ø27.14
38ø27.14
38ø25.38
38ø25.33
38ø25.38
38ø20.36
38ø20.84
38ø20.84
'
'
'
'
'
'
'
'
'
'
'
10
35
53
10
53
83
10
70
121
20
150
10
35
10
53
83
10
70
121
20
35
53
20
53
70
10
70
110
C2
38038.33 '
123024.60 '
10
C2
C3
C3
C3
C4
C4
C4
C5
C5
M3
M3
M3
R3
R3
R3
'
30
30
63
90
90
90
133
133
133
402
402
90
90
90
90
90
90
'
'
'
'
'
'
'
'
'
'
'
'
'
'
'
'
'
C1
38039.80
38ø39.80
38ø39.20
38ø36.38
38ø37.20
38ø37.20
38ø34.50
38ø34.50
38034.50
38031.27
38031.27
38031.60
38031.60
38031.60
38ø21.60
38ø21.65
38ø21.65
4
123ø25.10
123ø25.10
123025.60
123ø27.71
123028.30
123028.30
123032.60
123032.60
123032.60
123ø40.41
123ø40.41
123023.20
123023.30
123023.30
123ø13.00
123ø13.00
123013.00
C1
27
4
9
39
83
19
65
123
9
152
9
55
74
9
55
75
335
335
325
317
317
317
319
319
319
330
330
317
317
317
336
336
336
-1.6
0.4
-4.4
-9.8
-2.7
-0.5
-6.3
-1.2
-1.1
-6.5
1.9
-4.0
2.7
1.3
-3.8
2.6
2.2
-3.4
1.0
-2.4
-12.7
-2.3
0.4
-22.8
-9.1
-1.6
-23.5
-0.6
-1.4
2.8
4.3
-5.6
1.3
2.5
1.2
1.0
3.8
4.5
4.5
2.7
9.1
5.7
4.5
14.4
5.0
4.8
4.1
3.6
4.2
3.2
2.7
6.1
4.1
12.1
24.0
19.5
13.7
21.2
14.4
10.3
21.1
14.5
22.9
18.5
17.2
15.2
12.1
10.9
38
38
38
38
40
42
45
46
39
54
53
44
50
43
38
43
51
38
40
54
46
51
45
58
56
46
52
91
61
56
54
66
58
51
344
344
340
330
342
315
330
322
312
16
340
331
330
317
347
324
316
18.9
13.1
11.1
9.6
12.8
9.0
8.2
76
75
38
52
38
38
38
121
87
76
88
80
78
66
337
333
319
337
356
332
327
21.3
11.6
8.8
15.2
12.2
14.3
9.9
9.1
42
38
38
77
63
38
38
38
58
51
39
104
105
51
40
38
332
324
320
12
337
345
328
323
19.3
13.3
8.2
24.9
15.7
11.6
21.9
12.3
10.7
19.6
11.7
25.9
15.7
28.9
17.1
13.9
24.4
15.0
14.0
13.7
10.6
6.3
17.8
12.4
11.9
18.4
9.9
7.9
38
90
44
53
65
38
38
38
38
54
56
46
43
73
43
55
81
92
49
42
38
38
45
48
50
40
38
45
62
61
60
76
74
65
98
69
49
160
133
61
60
60
60
53
110
80
55
49
50
50
61
59
57
85
70
59
323
325
321
327
324
314
332
320
314
337
332
316
308
325
315
302
338
322
305
316
310
310
333
323
317
347
333
322
101
103
317
CODEW2: 0400 UT, Aug. 7, 1981, to 2100 UT, Dec. 7, 1981
C3
C3
C3
C5
R3
R3
R3
90
90
90
400
90
90
90
9
55
75
150
9
55
75
317
317
317
330
336
336
336
-1.9
4.6
0.3
0.5
-1.4
1.3
-0.7
7.6
9.4
8.1
4.4
6.4
7.2
5.1
4.2
2.1
1.5
4.1
5.2
2.1
1.7
CODEW3: 0400 UT, Dec. 16, 1981, to 2100 UT, March 19, 1982
C3
C3
C3
C5
C5
R3
R3
R3
90
90
90
400
400
90
90
90
317
317
317
330
330
336
336
336
-1.6
2.0
0.6
-0.8
1.7
-0.3
1.3
0.4
3.5
5.6
5.4
-7.3
3.6
0.2
3.4
3.5
6.3
2.9
2.4
8.8
5.0
6.5
2.4
2.3
CODE-2:0400 UT, April 5, 1982,to 2100 UT, July25, 1982
C2
C2
C2
C3
C3
C3
C4
C4
C4
C5
C5
N2
N2
N3
N3
N3
N4
N4
N4
R2
R2
R2
R3
R3
R3
R4
R4
123ø31.68 '
123ø31.56 '
123ø31.56 '
123ø40.25
123ø40.41
123ø40.11
123ø40.11
123ø41.71
123ø41.77
123ø41.77
123ø45.60
123045.55
123045.55
123ø13.97
123ø13.94
123ø13.94
123ø16.40
123ø16.36
123ø16.40
123022.94
123022.95
123022.95
'
60
60
60
93
90
90
130
130
130
400
400
60
60
90
90
90
129
130
130
60
60
60
90
90
90
130
130
130
325
325
325
317
317
317
319
319
319
330
330
316
316
319
319
319
319
319
319
319
319
319
329
329
329
339
339
339
-1.8
0.4
-0.4
-3.8
0.7
0.0
-7.3
0.7
0.1
0.7
1.9
-2.1
1.1
-3.8
1.9
-0.8
-5.6
1.6
1.0
1.4
1.4
0.2
0.2
0.6
0.5
-0.7
1.8
-0.1
3.6
3.5
1.9
-6.4
0.5
0.9
-20.7
-4.7
1.4
-16.9
4.8
-2.7
1.7
-8.3
2.1
3.5
-17.3
-2.3
4.3
6.5
5.8
3.1
-0.0
1.2
1.6
- 18.3
-4.9
-0.1
2.5
2.7
1.1
6.6
2.7
1.8
9.2
4.6
3.4
9.5
3.8
4.0
1.7
6.0
2.6
3.3
9.0
4.6
3.3
3.0
1.9
1.7
4.1
2.3
1.9
6.8
5.2
4.0
NCCCS: 0100 UT, March 23, 1988, to 0000 UT, Aug. 16, 1988
60
325
-0.0
-1.3
2.1
17.5
DEVER:
VELOCITY
SCALES
ON THE
NORTHERN
CALIFORNIA
SHELF
8559
Table 2. (continued)
Position
Instrument
Mooring Latitude Longitude Depth, m
Water
Depth, m
Isobath,
øT
•,
10-2 m s-•
•,
10-2 m s-•
rru,
10-2 m s-•
rr•,,
Tu,
T•,, Principle
10-2 m s-• hours hours Axis, øT
NCCCS: 0100 UT, March 23, 1988, to 0600 UT, Nov. 16, 1988
C3
C3
C3
C4
C5
38ø36.77 '
38ø36.77 '
38ø36.77 '
38ø33.60 '
38ø30.00 '
123ø26.93 '
123ø27.48 '
123ø26.93 '
123ø30.78 '
123ø38.55 '
10
45
75
10
150
90
90
90
130
400
C5
38ø30.35 '
123ø39.73 '
10
C5
38ø30.00 '
123ø38.55 '
150
C3
C3
C3
C4
38ø36.77 '
38ø36.77 '
38ø36.77 '
38ø33.60 '
123ø26.93
123ø27.48
123ø26.93
123ø30.78
'
'
'
'
10
45
75
10
C2
38ø38.33 '
123ø24.60 '
10
C2
C3
C3
C3
C4
38ø38.33 '
38ø36.77 '
38ø36.77 '
38ø36.77 '
38ø33.60 '
123ø24.60 '
123ø26.93 '
123ø27.48 '
123ø26.93 '
123ø30.78 '
10
10
45
75
10
C3
38ø38.71 '
123029.56 '
11.5
C3
C4
C4
G3
M3
38ø38.71 '
38036.78 '
38036.78 '
38044.40 '
38032.67 '
123029.56
123031.87
123031.87
123ø36.40
123ø22.97
44.5
10
52
10
10
C2
38039.80 '
123027.82 '
10
C3
C3
38038.44 '
38038.44 '
123029.64 '
123029.64 '
79
91
C3
38ø38.14 '
123029.97 '
77
C3'
38036.89 '
123ø27.18 '
86
C2
C3
C3
C4
C4
38039.50 '
38ø38.14 '
38ø38.14 '
38ø35.64 '
38035.64 '
123ø25.64 '
123ø28.31 '
123ø28.31 '
123032.52 '
123032.52 '
39
59
80
59
120
317
317
317
319
330
-3.0
2.1
1.3
-8.0
0.8
-6.1
2.7
3.4
-17.9
7.0
5.5
2.7
3.1
7.1
3.3
23.4
14.4
7.2
21.5
6.6
65
156
55
215
92
434
194
76
268
102
321
324
300
348
330
22.0
105
101
360
8.8
47
92
318
17.4
11.4
8.1
17.5
47
85
55
110
108
92
56
162
325
319
310
329
17.1
47
62
339
13.2
27.0
19.3
11.5
21.6
68
66
38
54
49
75
103
60
72
213
339
323
320
307
327
14.4
38
104
325
10.9
17.3
13.3
16.1
17.4
52
95
109
95
38
92
149
134
118
115
320
337
327
322
326
18.8
38
59
326
7.6
6.8
38
38
43
38
313
309
9.9
38
50
314
6.7
38
38
313
7.1
12.1
8.9
15.8
9.6
38
38
38
46
38
47
64
51
105
49
323
317
310
328
312
NCCCS: 0900 UT, May 4, 1988, to 0700 UT, Aug. 17, 1988
400
330
-1.3
-19.6
13.0
NCCCS: 0700 UT, Nov. 16, 1988, to 0100 UT, Feb. 12, 1989
400
330
-1.2
6.9
3.3
NCCCS: 0700 UT, Nov. 16, 1988, to 1900 UT, May 13, 1989
90
90
90
130
317
317
317
319
-1.5
2.8
0.2
-3.2
-3.9
3.7
3.7
-13.5
5.2
2.7
2.3
7.3
NCCCS: 0200 UT, Feb. 21, 1989, to 1900 UT, May 13, 1989
60
325
-1.9
-2.9
2.1
NCCCS*: 2000 UT, May 13, 1989, to 2300 UT, Oct. 17, 1989
60
90
90
90
130
325
317
317
317
319
1.2
-4.3
0.6
1.0
-5.6
4.6
-6.8
-5.2
4.1
- 19.1
2.0
4.4
3.1
2.8
6.4
SMILE: 0700 UT, Nov. 16, 1988, to 1400 UT, April 21, 1989
93
317
-2.2
-3.3
4.3
SMILE: 0700 UT, Nov. 16, 1988, to 1900 UT, May 13, 1989
'
'
'
'
'
93
117
117
93
93
317
319
319
317
317
1.7
-5.5
0.4
-3.6
-4.2
1.8
-12.0
-4.1
-5.9
-4.0
2.3
7.8
3.8
5.7
4.6
SMILE: 0400 UT, Feb. 28, 1989, to 1900 UT, May 13, 1989
80
325
-2.6
-8.5
4.9
STRESS-I: 1800 UT, Dec. 8, 1988, to 0700 UT, Feb. 25, 1989
97
97
317
317
-0.1
-1.3
5.7
3.5
1.8
1.9
STRESS-l: 1200 UT, March 6, 1989, to 0100 UT, May 5, 1989
95
317
-0.3
1.5
1.9
STRESS-I: 1000 UT, Dec. 9, 1988, to 0300 UT, Feb. 24, 1989
92
317
-0.6
4.2
1.9
STRESS-2:1900 UT, Nov. 23, 1990, to 0600 UT, March 7, 1991
49
90
90
130
130
325
317
317
319
319
0.7
1.9
-1.0
1.3
-0.3
4.8
9.8
5.9
3.1
3.8
1.4
2.7
1.9
4.0
2.4
Start and stop times given are thoseusedfor correlationcalculations.
*From overlappingmultipledeployments,
positiongivenis recordedpositionin ScrippsData Zoo files(deployment1).
3.3.
Cross-Shelf
Correlation
Scales
large-scalewind forcing,interpretationof along-shelfcorrelaCorrelations as a function of cross-shelfseparationwere tion scalesis fairly straightforwardin that we expect alongestimatedprimarilyalongthe C line. Mooringsalongthe C line shelfcorrelationsto be primarilya functionof separationand
rangedfrom the inner shelfto the outer shelf,and separations not along-shelfposition. In contrast,there is no reason to
were from i to 12 km becomingprogressively
larger offshore. expect cross-shelfcorrelation scalesto be independentof
Additional
estimates of cross-shelf correlations
were made
cross-shelfposition. Coastal oceanographersoften divide
shelvesinto three regions[see, e.g., Lentz, 1995;Allen et al.,
alongthe CODE-2 N and R lines.
Cross-shelfscalesin general are more difficult to interpret 1983]:an inner shelfwhere surfaceand bottom boundarylaythan along-shelfcorrelationscales.For a straight shelf with ers interact, a midshelf where surface and bottom boundary
8560
DEVER:
VELOCITY
(a)
'-
ON THE
near-surface
v
1
•)
SCALES
(b)
+
•o
o0.5
CALIFORNIA
8'
SHELF
middepthv
1
-•++
•
NORTHERN
0
ß
•
(D + o x
+o e
o
0.5
o
•
0 ...............
0
o
o
• -0'50
2'0
40
60
along-shelfseparationin km
(c)
•q)
1
o
o
0.5
o
•
6O
o
ß
o
4O
near-bottom
v
--k
•-
o
2O
along-shelfseparation in km
e
ß
x
o
+
CODE-1
CODEW2
CODEW3
CODE-2
SMILE/NCCCS-2
O ...............
o
o
-0.5
20
40
60
along-shelfseparationin km
Figure 4. Correlationsof along-shelfvelocity v as a function of along-shelfmooring separationfor (a)
near-surface,(b) middepth,and (c) near-bottominstruments.Lagsat maximumcorrelationare usuallyvery
closeto thoseat 0 lag, and for consistency,
0 laggedcorrelationsare plotted throughout.Correlationsare also
presentedin Table 3. Along-shelfcorrelationscalesof v exceedthe maximummooringseparation(--•60km).
layersare thin comparedto the total water depthbut wherethe
shelf is generally isolated from off-shelf variability, and an
outer shelfwhere open oceanvariability is important.Because
the processeswhich govern the velocity field on the shelf are
themselvesoften a strong function of cross-shelfposition,
cross-shelfcorrelationsestimatedhere will be discussed
in light
of cross-shelfposition as well as separation.
Cross-shelf
correlationscalesof v (Figure6 andTable 4) are
10-15 km. Despite showingmore scatter than v along-shelf
correlations,they are well resolvedin the sensethat observations are significantlycorrelated to their nearest neighbors.
Scatterbetweenexperimentsagainshowsno seasonalvariability. Most observationsare over the 60, 90, and 130 m isobaths.
Within this region, v correlationsare not a strongfunctionof
position; that is, correlationsbetween v at 60 and 90 m (a
distanceof about 5 km) appear to be about the sameas correlationsbetweenv at 90 and 130m (a distanceof about8 km).
Similarly, cross-shelfcorrelationsof v exhibit little systematic
variabilitywith instrumentdepth.
Like the along-shelfu correlations,the cross-shelfu correlations
where
were
best resolved
in the near-surface
the cross-shelf correlation
observations
scale is about 10 km. In con-
trast to along-shelfcorrelationsof u, cross-shelfcorrelationsof
u (Figure 7 and Table 4) were resolvedby typical mooring
separations.Comparisonof u correlationsbetweenthe 60 and
90 isobathswith thosebetween90 and 130 m (mostlyfrom
summerobservations)givessomesuggestion
that near-surface
u is more highlycorrelatedbetweenthe outer-shelfpair despite
the greater mooringseparation.Though the CODE-2 observationsgivesomeindicationthat u correlationsare higherfor nearsurface and near-bottom
instruments
than for interior
instru-
ments,the large scatterand limited numberof time seriesmake
definitivestatementsaboutdepth dependenceimpossible.
4. Interpreting u Along-Shelf Correlation
Length Scales During the Winter and
Spring 1988-1989
Both along-shelfand cross-shelfcorrelation scalesof v and
cross-shelf
correlationscalesof u were resolvedadequatelyby
most field programsexamined in section3. However, alongshelf u correlationswere not resolvedby typical along-shelf
mooringseparationsof 30 km, and short mooring separations
(5-15 km) are required.This is muchshorterthan the scaleof
the along-shelfwind stress(see Figure 3) which is generally
acknowledgedto be a dominant driving force of near-surface
cross-shelf
circulation[e.g.,Dever, 1995;Lentz, 1992] and warrants a closer examination.
To gain insightinto the processeswhich reduce along-shelf
correlation scales of u, observations from the combined
SMILE, STRESS,and NCCCS moorings(Figure2) are examined here in further detail. These mooringscome closestto
meetingthe requirementsfor resolvingalong-shelfscalesof u,
though spatial coverageis only extensivenear the surface.
They provide correlation scale estimatesfor near-surfaceu
from 4 to 30 km along the 90 m isobathbetween November
1988 and May 1989.
Time seriesof low-passfiltered along-shelfwind stress(r y)
and u (Figure 8) showvisuallythat numerouswind forcing
events(e.g., December 6-7, December 22-24, March 1, and
April 10) are reflected to some extent at all mooring sites.
However, there is additionalu variabilitywhich is not evident
DEVER: VELOCITY
SCALES ON THE NORTHERN
CALIFORNIA
SHELF
8561
Table 3. Correlations
of u and v asa Functionof Along-ShelfSeparation
Max Lag
Distance,
Moorings
Hours
km
Max Lag
0 Lag
u Correlations
,
0 Lag
u Correlations
Hours
v Correlations
v Correlations
Hours
Near-SurfaceCorrelations
CODE-1
C3(009):M3(009)
M3(009):R3(009)
C3(009):R3(009)
2028
2028
2028
11.61
25.00
36.45
0.47
0.34
0.22
0.52
0.34
0.23
- 11
-2
-5
0.67
0.82
0.56
0.73
0.85
0.62
-13
-10
-14
2946
33.56
0.52
0.52
-4
0.78
0.83
-14
2250
34.00
0.28
0.28
-2
0.63
0.64
-6
2682
2682
2682
2682
2682
2682
2682
2682
2682
26.18
29.98
56.07
26.29
29.64
55.89
26.42
30.76
56.83
0.03
0.24
0.06
0.03
0.07
-0.01
-0.03
0.08
-0.03
-0.17
0.26
0.13
-0.08
-0.15
0.19
0.26
0.21
-0.20
-89
+ 100
+ 100
-43
- 100
+49
+ 56
+55
+49
0.87
0.78
0.75
0.87
0.72
0.71
0.68
0.50
0.55
0.87
0.78
0.75
0.87
0.74
0.72
0.68
0.50
0.57
+1
-5
+0
-3
-9
-5
- 14
+0
-16
3752
4285
3752
3752
4285
4285
4285
4.67
10.04
14.46
14.71
19.12
29.16
5.02
0.73
0.54
0.35
0.48
0.39
0.11
0.89
0.73
0.54
0.37
0.48
0.39
-0.27
0.89
+0
+2
-4
+0
-2
-53
- 1
0.93
0.90
0.86
0.77
0.70
0.55
0.97
0.93
0.90
0.86
0.77
0.71
0.55
0.97
+0
+0
-2
-2
-4
-4
+0
-9
-4
-9
CODEW2
C3(009):R3(009)
CODEW3
C3(009):R3(009)
CODE-2
C2(010):R2(020)
N2(010):C2(010)
N2(010):R2(020)
C3(010):R3(020)
N3(010):C3(010)
N3(010):R3(020)
C4(010):R4(010)
N4(010):C4(010)
N4(010):R4(010)
SMILE
C3(010):NC(010)
NC(010):M3(010)
G3(010):C3(010)
C3(010):M3(010)
G3(010):NC(010)
G3(010):M3(010)
C4(010):NC(010)
Middepth Correlations
CODE-1
C3(039):M3(055)
M3(055):R3(055)
C3(039):R3(055)
2028
2028
2028
10.42
25.15
35.36
0.66
-0.12
-0.23
0.67
--0.28
-0.30
-2
-43
- 17
0.81
0.85
0.64
0.85
0.86
0.67
2946
33.74
-0.06
-0.20
-22
0.92
0.93
2250
33.12
-0.06
-0.22
- 19
0.67
0.68
2682
2682
2682
2682
2682
2682
2682
2682
2682
26.25
29.98
56.14
26.29
29.72
55.97
26.28
30.60
56.54
0.22
0.28
0.64
-0.12
-0.35
0.28
0.01
-0.08
0.03
0.24
0.29
0.64
-0.12
-0.37
0.34
0.30
-0.28
0.23
+ 15
-9
- 1
+7
+ 14
- 16
+ 52
+ 88
-57
0.79
0.67
0.72
0.92
0.79
0.79
0.82
0.77
0.73
0.80
0.92
O.80
+0
0.80
0.82
0.77
0.73
-3
+ 1
-3
-3
4285
4.67
0.56
0.56
+0
0.96
0.96
+0
-14
+ 17
+ 10
0.84
0.85
0.65
0.86
0.86
0.68
-6
-5
-8
-40
0.92
0.93
-5
CODEW2
C3(055):R3(055)
CODEW3
C3(055):R3(055)
-3
CODE-2
C2(035):R2(035)
N2(035):C2(035)
N2(035):R2(035)
C3(053):R3(053)
N3(053):C3(053)
N3(053):R3(053)
C4(070):R4(070)
N4(070):C4(070)
N4(070):R4(070)
0.69
0.72
+4
-10
- 1
SMILE
C3(045):NC(045)
Near-Bottom
Correlations
CODE-1
C3(083):M3(074)
M3(074):R3(075)
C3(083):R3(075)
2028
2028
2028
10.42
25.15
35.36
0.35
0.25
0.44
0.45
0.36
0.49
2946
33.74
0.16
2250
33.12
0.10
0.26
+ 15
0.78
0.78
+0
2682
2682
2682
2682
2682
2682
2682
26.25
26.29
29.72
55.97
26.28
30.60
56.54
0.41
0.44
0.31
0.73
0.03
0.34
0.26
0.41
0.44
0.32
0.75
0.16
0.38
0.39
-2
- 1
-6
-8
+56
-14
-24
0.80
0.95
0.86
0.84
0.85
0.88
0.75
0.80
0.95
0.86
0.84
0.85
0.88
0.75
+2
+1
-3
-1
-2
+0
-3
3268
1842
4.38
4.58
0.51
0.66
0.51
0.66
+2
-2
0.94
0.96
0.94
0.96
+1
- 1
CODEW2
C3(075):R3(075)
-0.31
CODEW3
C3(075):R3(075)
CODE-2
C2(053):R2(053)
C3(083):R3(070)
N3(083):C3(083)
N3(083):R3(070)
C4(121):R4(110)
N4(121):C4(121)
N4(121):R4(110)
SMILE
C3(079):NC(075)
C3(091):C3'(086)
Positivelagsdenotethe first listedseriesleadingthe second.
8562
DEVER: VELOCITY SCALESON THE NORTHERN CALIFORNIA SHELF
(a)
c
o
o
c
(D+
0.5
middepth u
1
øo0.5
'
+ + (9
o
ß-
(b)
near-surface u
1
,
+
o
O X(9
o
e
0
0
o
O-
(•DOX
o
o
.
o
20
40
20
60
(c)
c
near-bottom
o
u
0.5
o
ß-
60
1
...-,
o
40
along-shelfseparationin km
along-shelfseparationin km
e
CODE-1
ß
x
CODEW2
CODEW3
o
+
CODE-2
SMILE/NCCCS-2
0-
o
• -0.5•
20
40
60
along-shelfseparationin km
Figure5. Correlations
of cross-shelf
velocity
u asa function
of along-shelf
mooring
separation
for (a)
near-surface,
(b) middepth,
and(c) near-bottom
instruments.
Correlations
arealsopresented
in Table3.
Along-shelf
correlation
scales
ofnear-surface
u are15-20km.Along-shelf
scales
ofsubsurface
u arenotwell
resolved but are less than 25 km.
(a)
c
(b)
near-surface v
middepth v
1
1
o•
.m
o
o
o
0.5
0.5
o
o
o
o+ ©
©
,-
0
0
o
10
2o
3o
cross-shelf separation in km
(c)
c
near-bottom
10
2o
3o
cross-shelfseparation in km
v
1
'•
ß
o
o 0.5
©
ß
x
o
ß
+
[]
ß
o
o
e
o
©
'.,•
•
-0-50
o
o
o '0.50
10
20
CODE-1
CODEW2
CODEW3
CODE-2
NCCCS-1
SMILE/NCCCS-2
NCCCS-3
STRESS-2
30
cross-shelf separation in km
Figure6. Correlations
of along-shelf
velocity
v asa function
of cross-shelf
mooring
separation
for (a)
near-surface,
(b) middepth,
and(c) near-bottom
instruments.
Correlations
arealsopresented
in Table4.
Cross-shelf correlation scales of v are 10-15 km.
DEVER: VELOCITY
SCALES ON THE NORTHERN
CALIFORNIA
SHELF
8563
Table 4. Correlations
of u and v asa Functionof Cross-Shelf
Separation
Maximum Lag
Moorings
Hours
Distance,
km
0 Lag
u Correlations
u Correlations
Maximum Lag
0 Lag
Hours
v Correlations
+1
+ 10
+ 12
+8
-76
-29
+35
-38
-74
+3
0.82
0.70
0.50
0.61
-0.02
0.22
0.13
-0.06
0.02
0.07
0.86
0.79
0.71
0.68
0.36
0.39
0.41
0.15
-0.14
0.14
+8
+13
+22
+20
+ 100
+44
+53
+94
+43
-81
-3
0.13
0.22
+ 89
+2
+2
-2
+2
+2
+ 100
+ 12
+86
-78
+95
+83
+3
0.88
0.86
0.87
0.45
0.53
0.57
0.21
0.36
0.46
0.13
0.01
0.47
0.89
0.87
0.87
0.54
0.58
0.57
0.32
0.42
0.53
0.18
-0.21
0.47
v Correlations
Hours
Near-Surface Correlations
CODE-1
C1(004):C2(004)
C2(004):C3(009)
C1(004):C3(009)
C3(009):C4(019)
C4(019):C5(009)
C2(004):C4(019)
C1(004):C4(019)
C3(009):C5(009)
C2(004):C5(009)
C1(004):C5(009)
2028
2028
2028
2028
2028
2028
2028
2028
2028
2028
1.18
5.52
6.68
7.99
11.46
13.49
14.64
18.53
23.56
24.60
0.73
0.42
0.36
0.38
0.15
-0.08
0.09
-0.03
0.13
0.33
0.73
0.50
0.46
0.39
-0.42
-0.21
0.17
-0.21
0.17
0.33
2250
21.39
0.49
0.49
2682
2682
2682
2682
2682
2682
2682
2682
2682
2682
2682
2682
3.52
4.74
4.86
7.03
8.06
12.77
10.50
12.80
13.37
20.94
25.57
17.63
0.57
0.30
0.51
0.75
0.66
0.02
0.31
0.04
0.11
0.00
0.07
-0.20
0.57
0.30
0.51
0.75
0.66
0.17
0.35
0.14
-0.23
0.18
0.23
-0.20
3504
5718
3504
2519
2519
2488
4.52
8.06
12.57
13.21
21.11
25.63
0.38
0.55
0.05
0.44
0.48
0.35
0.39
0.55
0.11
0.45
0.49
0.35
-4
+0
-53
+6
+5
-3
0.76
0.75
0.40
0.47
0.34
0.14
1962
4285
1962
4.36
7.37
11.73
0.44
0.69
0.00
0.48
0.69
-0.30
-6
-1
+34
0.83
0.58
0.20
0.83
+2
0.58
0.21
+ 3
+9
3772
3772
3772
4.57
7.47
12.03
0.47
0.64
0.24
0.48
0.65
0.29
-5
- 1
- 16
0.75
0.80
0.54
0.76
+8
0.81
+ 12
0.57
+ 27
1259
3752
1792
3.23
4.90
8.11
0.81
0.85
0.75
0.81
0.85
0.75
+0
+1
+0
0.90
0.82
0.39
0.90
0.82
0.39
+0
+3
+6
-72
-56
+31
0.74
0.32
0.05
0.76
0.45
0.27
+8
+86
- 100
CODEW3
C3(009):C5(009)
CODE-2
N2(010):N3(010)
C2(010):C3(010)
R2(020):R3(020)
N3(010):N4(010)
C3(010):C4(010)
R3(020):R4(010)
N2(010):N4(010)
C2(010):C4(010)
C4(010):C5(020)
C3(010):C5(020)
C2(010):C5(020)
R2(020):R4(010)
+6
+8
+4
+30
+23
+4
+33
+24
+94
+68
-99
+5
NCCCS-1
C2(010):C3(010)
C3(010):C4(010)
C2(010):C4(010)
C4(010):C5(010)
C3(010):C5(010)
C2(010):C5(010)
0.77
+8
0.76
+9
0.46
0.57
0.36
0.21
+60
+ 100
- 100
- 100
NCCCS-2
C2(010):C3(010)
C3(010):C4(010)
C2(010):C4(010)
NCCCS-3
C2(010):C3(010)
C3(010):C4(010)
C2(010):C4(010)
SMILE
C2(010):C3(010)
C3(010):C4(010)
C2(010):C4(010)
Middepth Correlations
CODE-I
C3(039):C4(065) 2028
C4(065):C5(152) 2028
C3(039):C5(152) 2028
8.13
12.64
20.75
0.02
0.06
-0.36
-0.30
-0.41
-0.40
2946
20.80
0.13
0.14
-78
0.17
0.17
-3
2250
20.83
0.03
0.16
+ 75
0.25
0.25
+3
2682
2682
2682
2682
2682
2682
2682
2682
2682
2682
2682
2682
3.55
4.74
4.86
7.02
8.06
12.68
10.54
12.80
13.37
20.94
25.57
17.54
0.63
0.02
0.67
0.65
0.29
0.37
0.41
0.11
0.06
-0.19
0.03
0.23
0.63
-0.04
0.68
0.65
-0.29
0.44
0.41
0.14
-0.24
-0.22
-0.12
0.24
+0
-62
-3
+1
- 60
-13
+0
-22
-97
- 14
+ 100
-4
0.91
0.86
0.88
0.73
0.66
0.65
0.51
0.38
0.33
0.24
0.15
0.54
0.92
0.87
0.88
0.74
0.67
0.65
0.53
0.44
0.34
0.24
0.27
0.54
+5
+6
+3
+6
+7
+2
+ 10
+84
-8
+1
+94
+4
5718
21.11
-0.25
-0.38
-36
0.23
0.24
+ 100
CODEW2
C3(055):C5(150)
CODEW3
C3(055):C5(150)
CODE-2
N2(035):N3(053)
C2(035):C3(053)
R2(035):R3(053)
N3(053):N4(070)
C3(053):C4(070)
R3(053):R4(070)
N2(035):N4(070)
C2(035):C4(070)
C4(070):C5(150)
C3(053):C5(150)
C2(035):C5(150)
R2(035):R4(070)
NCCCS-1
C3(045):C5(150)
8564
DEVER:
VELOCITY
SCALES ON THE NORTHERN
CALIFORNIA
SHELF
Table 4. (continued)
Maximum Lag
Maximum Lag
Distance,
Moorings
0 Lag
km
Hours
0 Lag
u Correlations
u Correlations
Hours
v Correlations
v Correlations
Hours
-0.02
-0.14
+26
MiddepthCorrelations(continued)
NCCCS-2
C3(045):C5(150)
2107
21.44
0.19
0.19
+2
4285
4.90
0.50
0.52
-4
0.83
0.83
+0
2484
7.66
0.47
0.48
+3
0.78
0.78
+0
SMILE
C3(045):C4(052)
STRESS-2
C3(059):C4(059)
Near-Bottom
Correlations
CODE-1
C1(027):C3(083) 2028
C3(083):C4(123) 2028
C1(027):C4(123) 2028
7.12
8.13
15.00
0.14
0.67
0.17
0.23
0.68
0.18
-12
+3
-6
0.49
0.81
0.30
0.66
0.81
0.44
+ 19
+ 1
+ 18
4.74
0.36
0.37
-3
0.77
4.86
7.02
8.06
12.68
12.80
17.54
0.70
0.76
0.70
0.50
0.35
0.31
0.70
0.76
0.70
0.53
0.35
0.34
+1
+1
-1
-7
-4
-7
0.84
0.85
0.76
0.66
0.44
0.54
0.79
0.85
0.86
0.76
0.66
0.46
0.54
+6
+6
+ 1
+2
-3
+9
+3
4.61
7.66
12.26
0.47
0.58
0.38
0.47
0.58
0.38
+2
+0
- 1
0.66
0.86
0.48
0.66
0.86
0.48
+3
- 1
+0
CODE-2
C2(053):C3(083)
R2(053):R3(070)
N3(083):N4(121)
C3(083):C4(121)
R3(070):R4(110)
C2(053):C4(121)
R2(053):R4(110)
2682
2682
2682
2682
2682
2682
2682
STRESS-2
C2(039):C3(080
) 2484
C3(080):C4(120)
C2(039):C4(120)
2484
2484
Positivelagsdenotethe first listedseriesleadingthe second.
whichreducecorrelationlengthsof neareverywhere.
For example,from earlyto mid-Februaryoffshore standthe processes
surfaceu, 28 day subsetsof the 6 monthtime period are next
absentat G3. Similarly,in late April, offshoreflow is observed considered.Twenty-eightdayswas chosenin order to give
10 degreesof freedomper subset.Correlations
at M3, while onshoreflow is observedat G3. To better under- approximately
flow at 10 m is observed at C3, NCCCS C3, and M3 but is
(a)
near-surface
(b)
u
c
1
middepth u
1
++
oo o
0.5
0.5
+
o
•
o o
o
o
..•
0(9
0
(90
,o
0
10
0
20
30
cross-shelfseparation in km
(c)
near-bottom
---0-•
10
20
30
cross-shelfseparation in km
u
1
e
ß
x
o
ß
+
[]
ß
o%
0.5
•
o
o
•
o
0
• -0.%
10
20
CODE-1
CODEW2
CODEW3
CODE-2
NCCCS-1
SMILE/NCCCS-2
NCCCS-3
STRESS-2
30
cross-shelfseparation in km
Figure7. Correlations
of cross-shelf
velocityu as a functionof cross-shelf
mooringseparation
for (a)
near-surface,
(b) middepth,and (c) near-bottom
instruments.
Correlations
are alsopresented
in Table4.
Cross-shelfcorrelationscalesof u are approximately10 km.
DEVER:
VELOCITY
NOV
SCALES
DEC
ON THE
JAN
NORTHERN
CALIFORNIA
SHELF
APR
8565
FEB
MAR
20
30
10
20
30
10
20
30
10
20
10
20
30
10
20
30
MAY
10
20
30
10
20
30
10
20
30
10
20
10
20
30
10
20
30
10
3
-4
2O
10
0
-10
-20
-30
20
10
0
-20
-30
20
10
0
-10
-20
-30
20
10
0
-10
-20
-,30
NOV
DEC
JAN
FEB
MAR
APR
MAY
Figure8. Timeseriesof low-pass
filteredr y (dyncm-2) andu (cms-1) at 10m. Mooringlocations
areas
denotedin Figure2. Correlationsof u with r y rangefrom 0.5 to 0.6. Periodsof strongalong-shelfvariability
in early February and late April are bracketedby solid lines.
of 0.50 (0.58) are significantat the 90% (95%) level for 10
degreesof freedom.
4.1. Monthly Variation in Correlation Scales of
Near-Surface Cross-Shelf Velocity
Monthly plots of near-surfaceu correlationas a functionof
along-shelfseparation(Figure9) showa greatdeal of variability. From mid-Novemberto mid-January,and againfrom midFebruary to early April, along-shelfcorrelationsshow little
drop-off as a function of separation.However, from midJanuaryto early February and againfrom early April to early
May, shorter along-shelfcorrelationscalesexist. Correlation
scalesare always_>4km as the SMILE C3 and NCCCS C3
mooringsremain well correlatedover all months.
Monthly variabilityin •'"g •h• ..... ,..,;..... ,• • ....
surfaceu appearsto be related to the importanceof alongshelf wind stressforcing relative to other processes.During
monthsin which correlation length scalesare long, all mooringstend to be highlycorrelatedwith localwind stress(Table
5). Conversely,
when correlationlengthscalesare short,some
or mostmooringsare lesswell correlatedwith along-shelfwind
stress.Wind stressvariability, as indicated by its standard deviation, remains similar through most of the experiment.
Therefore lower correlationsof near-surfaceu with along-shelf
wind stressare primarilyattributednot to a weakeningof wind
stressbut to the greater importanceof other processes.However,there is a slightweakeningof wind stressvariabilityfrom
mid-Januarythroughearly February,and this playsa role in
reducingcorrelationwith wind stressand hence along-shelf
correlation
scales.
Betweenmid-Januaryand early February,near-surfaceu is
we!!corr,q•t,•dbetwe......
i-gs NCCCS C3 and SMILE C3
and M3 but not between G3 and the other moorings. This
suggests
a break in cross-shelfcirculationbetweenthe northern and southernpartsof the studyarea rather than a general
increasein short-scaleu fluctuations.This along-shelfvariability is alsomarkedby a three-dimensional
heatbalancebetween
January30 and February12 [Deverand Lentz, 1994,Figure 9]
and by a stronglythree-dimensionalmassbalanceas indicated
by depth-averaged
cross-shelf
flow at C3 betweenJanuary30
and February20 [Dever,1995].This depth-averaged
cross-shelf
flow, which is uncorrelated with the wind, also probably re-
ducesalong-shelfcorrelationsof near-surface
u betweenearly
Februaryand early March (Figure 9) as shiftingthe start and
stop times of the 28 day periodsshowsreducedalong-shelf
8566
DEVER:
VELOCITY
SCALES
ON THE
NORTHERN
Nov 16, 1988 to Dec 14, 1988
o
o
CALIFORNIA
Dec 14, 1988 to Jan 11, 1989
o
o
SHELF
o
o
o
0.5
O.5
o
o
-0.5
-0.5
-1
o¸
o
o
-1
10
0
20
30
10
0
20
30
along-shelfseparation in km
along-shelfseparation in km
Jan 11, 1989 to Feb 8, 1989
Feb 8, 1989 to Mar 8, 1989
o
o
o
0.5
o
0.5
o
o
o
o
1'0
2'0
3'0
o
o
o
-0.5
-0.5
-1
1'0
0
2'0
-1
3'0
0
along-shelfseparation in km
along-shelfseparation in km
Mar 8, 1989 to Apr 5, 1989
Apr 5, 1989 to May 3, 1989
,
o
o
o
o
0.5
0.5
o
o
o
0
-0.5
-o.5
-10
1'0
2'0
3'0
-10
along-shelfseparationin km
1'0
2'0
3'0
along-shelfseparationin km
Figure 9. Correlationsof near-surfacecross-shelf
velocityu as a functionof along-shelfmooringseparation
for six 1-month periods between November 1988 and May 1989. Correlations with wind stressare also
presentedin Table 5.
Table 5. Correlationsof Near-Surfaceu With Along-ShelfWind StressOver 1-Month Periods
Period
Nov. 16, 1988, to
Dec. 14, 1988
Wind o-,dyncm-2
u scale, km
0.74
30
Dec. 14, 1988, to
Jan. 11, 1989
0.72
30
Jan. 11, 1989, to
Feb. 8, 1989
0.65
15
Feb. 8, 1989, to
March 8, 1989
1.02
30
March 8, 1989, to
April 5, 1989
0.95
30
April 5, 1989, to
May 3, 1989
0.71
15
Mooring
G3
C3
NCCCS
M3
C3
0.77
0.78
0.68
0.71
0.82
0.63
0.67
0.73
0.44
0.41
0.38
0.69
0.61
0.60
0.79
0.75
0.81
0.63
0.79
0.73
Rough estimatesof u correlation scalesand the standarddeviations,o-,of r y for each month are also shown.
*Short
record.
0.56
0.76*
0.70
0.18
DEVER:
(a)1
VELOCITY
SCALES
i
i
ON THE
NORTHERN
i
CALIFORNIA
SHELF
8567
i
'.--0.,5
o
I
0
0
I
0
3
O.4
O'
.6
0.5
frequency,cpd
(b)
15
10
E
C3
•
1
½.
•
.--
-5 -NCCCS
C3
0.4
_1o[
0.2
M3
I
15
.1
0
02
O.3
O.4
O.5
]
O.6
frequency, cpd
Figure 10. (a) Coherenceof near-surfaceu at C3 with •-Y. Coherenceof other near-surfaceu recordswith
•-Yalso declinewith decreasingfrequency.(b) Coherenceof near-surfaceu with distancefrom C3. The 90%
confidence level is 0.45, and the 95% confidence level is 0.50.
correlation scales (and correlations with along-shelf wind
stress)which are most evident from late January to midFebruary and increaseagain in late February.
During April, correlationscalesare reducedprobablyby the
presenceof a mesoscalefeature over much of the shelf. This
feature, describedby Largieret al. [1993], originatesfrom offshore as indicated by its high temperature and low salinity
[Alessiet al., 1991]. It is responsiblefor strongequatorward
flow over the studyarea despitethe absenceof persistentequatorwardwind forcing(Figure 8), and it affectsboth the nearsurfacecirculationand the heat and salt balances[Dever and
Lentz,1994].Similarfeatureshavealsobeenobservedat different
timesin previousyears[Lentz,1987;Washburnet al., 1993].
Largier et al. [1993] postulate that oceanic mesoscalefeatures commonlyoccur over the northern California shelf and
that they are a significantsourceof forcingfor shelfcirculation
at periodslongerthan 10 daysfor whichwind stressvarianceis
decline for longer periods where oceanic mesoscaleforcing
may become more important.
The associationof the longestspatial scalesof near-surface
u with wind stressforcing is also supportedby empirical orthogonalfunctions(EOFs) of near-surfaceu during SMILE
and NCCCS (Figures 11 and 12). The lowest mode, which
accountsfor 57% of the variance,is highly correlated(0.76)
with along-shelfwind stress,is important throughoutthe November to May period, and exhibitslittle along-shelfstructure.
Higher modes have more complex spatial structuresand are
uncorrelatedwith the wind stress.They becomeimportant on
timescales
of weeks
when
correlation
scales
are
reduced.
Modes 2 and 3 become important during January and early
February. Together they represent u fluctuations at G3 (in
January)and C3 and NCCCS C3 (in February).Mode 2 again
becomesimportant in late April when it representsoffshore
flow
1-2_1
.......
at M3
_1__
and onshore
,,_1
ß
flow
at G3.
The
timescales
of the
mgnm muuc•, tnmr intermittent nature, and the lack of correancepeaksat periodsbetween2.5 and 5 days[Dever,1995]. If lation with wind stressall suggestthat they are not associated
wind stressforcing dominates at short periods and oceanic with wind forcing.
mesoscalevariabilityis the primary sourceof forcingfor longer
periods,then spatial coherenceof near-surfaceu should be 4.2. Monthly Variation in Cross-Shelf Velocity Correlation
Between SMILE
C3 and NCCCS
C3
highestin the wind band asthere is no reasonto expectoceanic
Velocity measurementat most SMILE and NCCCS moormesoscaleforcingto be two-dimensional.Figure 10 lendssome
supportto this idea. Both coherencewith the wind stressand ingswas limited to the near surface(10 m). However, C3 was
along-shelfcoherenceare largestat the samefrequenciesand instrumentedthroughout the water column. The C3 sites of
--
WlllU
8568
DEVER:
15
VELOCITY
SCALES
E
NORTHERN
CALIFORNIA
SHELF
Therefore, in the absenceof other processes,theoretical u
correlation scaleswithin the surface and bottom boundary
layersshouldreflect thoseof along-shelfwind stressand interior v, respectively.Over the northern California shelf these
along-shelfcorrelationscalesare 100 km or more (Figure 3)
and over 60 km (Figure 4).
The theoretical correlation scales above are larger than
thoseobservedon the northern California shelf. This is especially true for u. Observedv correlation scalesare next dis-
G3% e•
E 10
o
ON THE
5
o
cussed,and a more extensive discussionof u correlation scales
c
0
C3
follows.
o....,`
Correlations of v estimated in this study and previously
[Winant et al., 1987] over the northern California shelf and
'1,,i
elsewhere[KunduandAllen, 1976]are in manywaysconsistent
..c -5
NCCCS
C3
with those expectedfrom CTWs. The correlation scale of v
estimatedhere, though almost certainly less than the limits
o
impliedby CTWs, is greaterthan 60 km (Figure 4). Maximum
< -lO
lagged v along-shelfcorrelation coefficientstend to exhibit
behavior consistentwith CTWs in that southern velocity
recordsgenerallylead northern velocityrecordsby about 3 to
-15
12 hours (Table 3). Application of CTWs is limited to the
-5
0
5
midshelfand outer shelfwhere surfaceand bottom boundary
Mode Amplitudein cm/s
layerscan be consideredthin. The increasingimportanceof
Figure 11. Along-shelfstructureof near-surfaceu empirical mesoscaleprocessesoffshoreand the cross-shelfstructureof
orthogonalfunctions(EOFs). The solidline indicatesmode 1 CTW modeson the northernCaliforniashelf[Chapman,1987]
which accounts for 57% of the total variance. Modes 2, 3, and would suggestthat along-shelfcorrelationscalesdecreaseoff4, indicated by the dashed,dash-dotted,and dotted lines, ac- shore. In this study, examination of along-shelfcorrelation
countfor 24%, 13%, and 5% of the total variance,respectively. scales of v was limited to CODE-2 observations between 60
and 130 m. Within this region, little cross-shelfvariation in
along-shelfv correlationsis evident except for near-surface
NCCCS, SMILE, and STRESS were locatedapproximately4 observationswhere along-shelf v correlations at the 130 m
km apart in the along-shelfdirection.They providean oppor- isobathare reducedslightlyrelative to thoseat the 60 and 90 m
tunity to examinecorrelation in the along-shelfdirection for isobaths.This is a possibleindicationthat mesoscaleprocesses
near-surface,middepth,and near-bottomdepths.Correlations are more important near the surface.
In the cross-shelfdirection, v is generallycorrelatedfrom 60
betweenthesetwo sites(Figure 13) showedthat near-surface
correlationswere the highestand that monthly trends in cor- to 130m. Despiteits smallermagnitudeand possibleeffectsof
relationswere the same at all depths.This indicatesthat re- localbathymetry,inner-shelfv (30 m) is alsosignificantlycormarksconcerningcorrelationscalesof near-surfaceu may also relatedout to the midshelf(90 m), arguingfor a similarresponsein v to wind forcingover muchof the shelf.Maximum
be applicableto interior and near-bottomu.
laggedv cross-shelfcorrelationstend to exhibitbehaviorconsistentwith two-dimensionalupwellingmodelsin that inshore
5.
Discussion
velocityrecordsgenerallylead offshorevelocityrecords(Table
On wind-forced shelves,CTWs and Ekman theory have 4), suggesting
a morerapidresponse
to windforcingin shallow
been compared quantitatively with observed interior [e.g., water.
Along-shelf correlation scalesof u are much shorter than
Chapman, 1987] and boundarylayer flows [e.g.,Dever, 1995;
Lentz, 1992]. CTWs and Ekman theory both imply correlation those of v and were best resolvedby near-surfacemeasurescalesfor u and v. For v, velocities associatedwith CTWs are ments during SMILE and NCCCS (Figure 5). Near-surface
much larger than surfaceand bottom Ekman velocitiesso that and near-bottomalong-shelfscalesof u are substantiallyless
the wind-forcedv is dominatedby the CTW responsethrough- than those of along-shelfwind stressand interior along-shelf
out the water column.Brink et al. [1987, 1994] have examined velocitywhich set the scalesof u in models of surfaceand
the scalesof motionimpliedby full (i.e., no long-waveassump- bottomboundarylayers.Figures8, 9, 10, and 12 and Table 5 all
tion) CTWs and found that v is resonantfor low wavenumber suggestthat wind forcingis associatedwith the longestscalesof
along-shelfwind stressvariabilityimplyingcorrelationscalesof near-surfaceu in winter and springand that other processes
100-200 km.
act to reducecorrelationscaleson periodsof severalweeksto
Brink et al. [1987, 1994] also found u velocitiesassociated i month (Figures8 and 9 and Table 5). The timescalesand
with CTWs to be more sensitiveto high wavenumberalong- spatialstructuresof variabilityin short-scaleu are consistent
shelfwind stresswith scalesof 15 km or less,implyingsimilarly with those of mesoscale features. The similar time variation of
shortcorrelationscalesfor u forcedby CTWs. However,unlike correlationsbetween two nearby moorings at near-surface,
v the character of the total wind-forced
u is not determined
middepth,and near-bottomlocations(Figure 13) suggests
the
mainly by the CTW response. Cross-shelfboundary layer processes
which act to reduce near-surfaceu correlationare
transportsare the samemagnitudeas CTW cross-shelftrans- also important throughoutthe water column.
Examinationof u along-shelfcorrelationscaleswas limited
ports, and becausethey are confinedto relativelythin layers,
the velocitiesassociatedwith them are greater [Dever,1995]. to the period between November 1988 and May 1989. This
M3
i
.,"½1"
;/
i
DEVER:
VELOCITY
NOV
20
30
SCALES ON THE NORTHERN
DEC
10 20
JAN
30
10 20
30
CALIFORNIA
FEB
MAR
10 20
10 20
SHELF
APR
30
10 20
8569
MAY
30
10
O -3
-4
15
i
i
-lO -
-
-15
15-
-
10-
-
"'-'-'V
E- 1
-15
v
10
E- 1
-15
10
-5
-lO
-- I 5 ••j•••••••••••••
20
,30 10
NOV
20
DEC
/
,30
10
20
JAN
30
10
20
FEB
10
20
MAR
30
10
20
APR
,30
10
MAY
Figure12. Timeseries
of •-Y(dyncm-2) andnear-surface
u (cms-l) EOF modes.
To lengthen
theEOF
timeseriesintolateApril, theSMILE timeseriesat C3 hasbeenextended
bypatchingthe8.5 and11.5m time
seriestogether.The correlationcoeflScient
of mode1 with •-Yat C3 is 0.76.Higher modesare uncorrelated
with
•-Y.
period coveredthe winter and springstormseasonand ended with this front, and with later fronts causedby wind stress
daysafter the springtransitionto upwelling;hencethe appli- relaxationand subsequentupwelling,are a ready sourceof
cability of the above observationsto u correlation scalesin the three-dimensional
variabilityin summer.In thisway,upwelling
summer(upwelling)seasonis uncertain.Section4.1 showed favorablewindswhichpersistlong enoughto causethe develthat for someperiodsof 1 month or more, along-shelfcorre- opment of an upwellingfront may be an indirect sourceof
lation scaleswere at least 30 km, the maximumalong-shelf short correlationlength scales.
mooringseparationin SMILE. Though30 km is the minimum
The importanceof short-scalewind stressin reducingcoralong-shelf
mooringseparationduringCODE-2, makingit dif- relation scalesof interior u remainsuncertain.Thoughalongficult to comparespatialscales,a similartreatment of summer shelfwindstressis highlycorrelatedoverdistances
of up to 130
CODE-2
data revealed no time when correlation scales were as
km for both SMILE and CODE-2 (Figure 3), aircraftobsergreat as 30 km. This suggests
the possibilityof seasonal(or vationsmake it clear that short-scalewind stressis present
interannual)variation in along-shelfscalesof near-surfaceu. southof PointArena [Winantet al., 1988;Dotmanet al., 1995].
Seasonalvariabilitycouldbe due to greateroffshoremesoscale Neither the aircraftobservations
nor the availablebuoyobseractivityin the summer[Kosroet al., 1991]whichwould tend to vations allow the estimationof the completewavenumber
shorten correlation length scalesin summer. Offshore me- spectrum.Thoughinclusionof an estimatedshort-scale
(down
soscalevariability in summermay or may not be related to to 10 km) wind stressin a stochastic
model[Brinket al., 1994]
instabilityof an upwellingfront. Barth [1994] examinedthe reduces interior u scales, observed u fluctuations are more
developmentof frontal instabilitiesand found that the time- energeticthan predictionsof the linear model.
Cross-shelf correlation scales of u are about 10 km. These
scalefor development
of a short-wavelength
(20 km) baroclinic
instabilitywas about 1.5 days.In winter, wind forcingevents correlationscalesextend over a substantialportion of the
tendto be relativelybrief anddo not lead to the development northernCaliforniashelfand are only about5 km lessthan the
of an upwellingfront. After the springtransitionto upwelling, cross-shelf correlation scales of v. There is some indication
a front developsand moves offshore.Instabilities associated that u is more weakly correlatedfrom the inner shelf to the
8570
DEVER:
VELOCITY
SCALES
ON THE
NORTHERN
CALIFORNIA
SHELF
i
0.8-
00.6o
•0.4o
!
!
0.2-
0
-do
i
o
i
do
do
i
do
i
1989 yearday
Figure 13. Correlationsof cross-shelf
velocityu betweenSMILE and NCCCS C3 moorings.Near-surface
(solid),middepth(dashed),and near-bottom(dotted) instrumentsare consideredfor sixone-monthperiods
betweenNovember 1988 and May 1989. Though correlationsare highestbetweennear-surfaceinstruments,
monthlytrends are similar for all instrumentdepths.
midshelf(60 to 90 m, a distanceof approximately5 km) than
from the midshelfto the outer shelf(90 to 130 m, a distanceof
---8 km). Becausewe expect offshoremesoscalefeaturesto
but are less than 25 km. In the cross-shelf direction, u corre-
reduce
lation
correlation
scales over the outer
shelf rather
than the
and indicate that they are 15-20 km. Along-shelf correlation
scalesof subsurfaceu are not well resolvedby availabledata
scales are ---10 km. There
is some indication
that u is
inner shelf, it is likely some other processis responsiblefor
morehighlycorrelatedbetweenthe 90 and 130m isobaths
than
this. As most of the observations
between
used in Table
4 come from
summer, one reasonableexplanationis near-shorevariability
causedby relaxationfrom upwelling[Sendet al., 1987].During
the 60 and 90 rn isobaths.
To investigatefurther the processes
whichdeterminealong-
shelf correlation scalesof u, SMILE and NCCCS recordsfrom
November 1988 to May 1989 were examinedin greater detail.
from Point Reyesoccursin a wedgeconfinedprimarilyto the Monthly variation of correlationscaleswas comparedto corinner-shelf and midshelf locations. This could cause a break in
relation with along-shelfwind stress,the heat balance, and
the character of the flow field between the 60 and 90 m isoother descriptiveinformation about shelf circulation.During
baths. This type of three-dimensionalvariability could over- several months the along-shelf correlation scale of nearwhelm the wind-drivensignalespeciallyover the inner shelf surfaceu was at least 30 km, the maximummooring separawhere models suggestwind-driven u is reduced relative to tion, and it was alwaysgreater than 4 km, the minimum moordeeper water by the overlap of surfaceand bottom stresses ing separation.Along-shelf correlation scaleswere generally
[Mitchurnand Clarke, 1986;Lentz, 1994].
longwhencorrelationof velocitywith wind stresswashighand
relaxation events, northward advection of warm saline water
short when correlation
6.
Summary
Correlationsof subtidalcross-shelf(u) and along-shelf(v)
velocitiesare estimatedas a functionof along-shelfand crossshelfdistanceusingmooredtime seriesfrom severalfield programsover the northernCalifornia shelf.Distancesoverwhich
velocities are significantlycorrelated are termed correlation
with wind stress was low. Short corre-
lation scalescoincided with three-dimensional heat, salt, and
volumebalances.During April 1989, short correlationscales
coincided with the intrusion
of an offshore mesoscale feature
onto the shelf. Along-shelf correlationsof near-surface,middepth, and near-bottomu between two nearby (4 km) sites
showed near-surface correlations were highest but that
monthlytrendsin correlationswere similar at all depths.
scales.
Over periods of 4-6 months, correlation scalesof v are
resolvedin both the along-shelfand cross-shelfdirections.In
the along-shelfdirection, v is significantlycorrelatedfor distancesgreaterthan 60 km, the maximummooringseparation.
In the cross-shelfdirection, v is generallycorrelatedbetween
the 60 and 130 m isobaths(10-15 km). Correlationsof v show
little dependenceon instrumentdepth;subsurface
v is perhaps
more highly correlatedthan surfacev.
Correlationscalesof u overperiodsof 4-6 monthsare much
smallerthan thoseof v and are often not resolvedby minimum
mooringseparations.Mooringsdeployedin SMILE and NCCCS do resolvealong-shelfcorrelationscalesof near-surfaceu
Acknowledgments. This work was done as part of my thesisin the
MIT/WHOI Joint Program in Oceanography.Financial supportwas
providedby NSF grantOCE 91-15713.I thankespeciallymy advisorS.
Lentz and alsocommitteemembersJ. Price, K. Brink, R. Beardsley,P.
Malanotte-Rizzoli, and R. Weller for helpful discussions
and comments.The careful commentsof two reviewersare also appreciated.
Specialthanksare alsoextendedto B. Butman at USGS WoodsHole
andJ. Trowbridge(WHOI) for makingavailableSTRESSdata and to
J. L. Largier (SIO, Universityof Cape Town, South Africa), B. A.
Magnell (EG&G), and C. D. Winant (SIO), who generouslymade
availableNCCCS data. Supportfor preparationof the manuscriptat
SIO was providedby The Mellon Foundationand by the Minerals
ManagementServiceunder cooperativeagreement14-35-0001-30571.
Woods Hole OceanographicInstitution contribution9187.
DEVER:
VELOCITY
SCALES
ON THE
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