ICES STATUTORY MEETING 1
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weIl with the Elanan transport inferred from the local wind stress. Indeed, comparisonsofthe
Ekman transport computed forthe surface layer agreed weH with observatio~s in all ~oast;al
upwelling regions studied (Lentz, 1992). Although the hottom Ekman layer IS of eqUivalent
importance in many models of coastal up\velling and cross-shClf transport, measurements in it have
not been adequate to allow equivalent analyses (Lentz arid Trowbridge, 1991). The CODE,
experiment also showed that flow patterns could be much more complex than inferred during
earlier studies of coastal upwelling (e.g.; Davis, 1985; Kosro, 1987; Send, 1989). A corripanion
experiment, SuperCODE, looked at the large alongshore seale, from 35°N to sooN, defined the ,
seasonal cycle, and found that the tendency for subsurface poleward flow over the continental shelf
was ubiquitous; the mean poleward flow actually increased at lower latitudes in spite of the
.inereased equatorward meau wind (Strub et al., 1987).
Thc Coastal Trarisition Zone (Crz) experiment, motivated in part by the results from CODE•.
investigated the cool 'filaments' or Jets' that are so conspicuous in satellite images of SST within a
couple of hundred kilometers of the coast off northern California. Simihlr features are seen in the
satellite iri13ges of other coastal upwelling regions, eSpCcially in thc Benguela Currcnt region off
South Africa. Tbe erz experiments were centered off northern Califomia at about 39°N, and took
pIace during the summers of 1987 and 1988 (Strub et al., 1991), the serison wben the alongshore
wind-stress exceeds 1 dyne cm-2 anct the resultirig scaward Ekman transport exceeds 1 Sv (i.c.,
106 m3s- 1) per 1000 kIn.The 1987 experiment looked at the larger scale (42 t037°N) and
provided evidence that the structures seen in the satellite images were not simply upwelling
'squirts' or' a field of oceanic mesoscale eddies iuteracting with the coastal ocean, but the result of a
strong alongshore jet meandering equatorward, perhaps starting as a coastal jet and eventuaIly
meandering as far as 300 km seaward. The jet flowed along the front separating the cool,
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productive, recently upwelled ,vater from the warmer, relatively barren waters offsnore (Kosro ct
a1., 1991; Hood et a1., 1990). Thesejets are of sufficient depth (and transport) to be a major
influenee on the surrounding oceari; the upwelling front and its associatedjet meander quasicontinously equatorward and offshore,allowirig the upwelled water to extend rrmch farther
seaward (by almost an order of magnitude) than the shelf width. Figure 1 is a satellite image of
SST upon which near-surface (25 m depth) currents, measured with ship-oome ADCP. have been,
overlaid (Smith, 1992). Figure 2 shows the"temperature and alongshore velocity in sections normal
to the coast at 43°, 41.5°, and 400 N about the time ofthe image. The section at 43°N shows a
'classic' coastal jet ~lDd upwelling front. This jet, and thc front; meandered farther sea\vard as the
two southern sections show. The two southern sections had additional measurements that support
the interpretation that the jet and front are the boundary between water upwelled at the coast and
oceanic water, (Figure3, basCd on Hood et al.; 1991). Thejet and front continued equatorward to
at least 37°N; maps ofvarious properties, including satellite and ship SST, dynamic topography,
and cWorophyIl concentration, are given in CTZ Group (1988) and Hood et al. (1990, 1991).
The 1988 experiment, iooking in detail at a smaller region (37.5 to 39.5°N), confirmed that the jet
flows along the front, then 100-200 km seaward of the shelf break, separating upwelled water
from the nutrient poor water seaward (Chavez et al.; 1991). Thejet is narrow, 30-75 km, rapid (at
times> 1 m s-l) ät the sunace, arid extends to >200 m, although with diminished velocity (10 cm
s-I). Since tbe jet flows rapidly equatorward, it efficiently ädvects water wiih upstream properties.
Off northern California the jet carries water of lower salinity than that present either offshore or
onshore at that latitude (Huyer et aL, 1991). Indeed, the large volume of water ti-ansported within
the jet (3-5 Sv) suggests that it may be a major eonstituent of the Califomia Current. Satellite SST
images are not useful in determining whether the current velocity structure is seasonal; since the
cold upwelled water which serves as a "dye" or tracer is prescnt only during seasons of coastal
upwelling. On the basis of limited winter cruises. these strang narrow currcnts seem to occur only
during the seasons'ofpersistent eoastal upwelling, Le., strong equatorward winds (Kosro et al,
1991).
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The results of the erz program show that thc upweiIing front may advanee seaward a eoupie of
hundred km beyond the eontinental shdf as the jet eontinues to fIow equatorward for several
hundred lari without breaking up. The jet itself is not a region of abundant iUltrients and
phytoplankton (Chavez et al., 1991). Thejet and front remain a botind:.iry, albeit rin aetiveorie with
some exchange across thc front. perhaps by up\vcllirig arid dowriwelling along thejet's cdges. As
the upwelling front meanders much farthcr seaward, the upwelling 'signal' extends much farther,
seaward than earlier thought likely; this must have a significant effect on fishcriesrecruitment arid
the sediment record.
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Presciti: Ä major study is now (1992-4) taking place off northein Califomia. The goal of this
study, sponsored by the Office of Naval Research, is to iInderstand the physieal arid biologieal
dyriamies of the mesoscalc intcraetions in what is assumed to be the weakly nonlincar fIow regime
of castem boundary eurrent regimes. Althoughthe Califomia Currerit is the most studied of the .
eastem boundary eurrcntS, itS large variability aiId thc diffieultyof quasi-synoptic srirripling leave
many qtiestions about the mesoscale f10\V field imanswcrcd. The CfZ experiment showed tliat
'spectacular' features are superimpased on the long tCIm average cireulation of the Califomia
CuiTent, which is eharacterized by a southward drift near the surface of the order of 10 ern/s. ,
Superimposed on the broad f10w are mesoscale eddies andjetlikc fIows of25 to 100 cffils~that
dcforrn the main thermcicline by 100 to 200 m vertically and have horizontal scah~s of 10 to 1000
km; Amorig thc hYPothese are that these features have thcir origin on the continental shelf or inrier
slope (eddies are perhaps 'spun ofr the mearideringjet as it transits the slope) and that th6
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cireidation aeeompanining the meanders has asignifieaßt vertical eomponent. No direet
measui-ementS of these features häve bCen rriädeover thCir,'lifetimes'(the duration of the erz
experiments was on the order of orie riionth) so we have little kllowledge of their dynamical arid
energetic iriteraetions. Moored arrays of eurrent meters have bCen deployed for two ycars,
extendirig fromjust seaward of the coritinental shelf break to about 300 km offshore overthe
abyssal plain. Lagrarigian drifte ci, towed CfD systems, and satellite derived SST arid sea surfaee
being inade over the shelf and inner slope by
altimctry will be employed. Some merisurements
thc,U. S. National Fisheries Service because the habitat ofthe U. S.west eoast groundfishis
betwceri thc 50 and 1500 m isobath iri a narrow band along nearly thc entire U.S. west cmist.' The
adults spawn in this region but the young must return to the shelfto begiIl d6rriersallife.
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Future: Future studies, I expeet, will again foeus on th~ shelf: the inner shelf where the erossshelf divergenee of th6 Ekman transport is maximum, the fIow in the bottom boundary layer
(where because of the fe'Y measurements in the bottorn boundary laycr in coastal upwelling regions
even the Ekrilan transport is untested), and the behavior of eoastal upwellirig fronts and their' .
pcissible transition from the shelf to thc open occan (how does tllC front ovcr the continental margin
respond to wind variability, and how does it 'regulate' thc distribution and transport of planktonic
forms?). Another topic deserving of study are thc ubiquitoüs poleward currents. usually .
subsurfacc over thc inner slope and outer shelf, that havc been observed in all coastal upwelling
regions iri spite ofequatorWard wiöds (Neshyba ei al., eds., 1989). There is not yet a satisfaetory
model for poleward undercurrents. They inay be a source of upwelling water, and they may .
provide a 'recycling conveyor belt' for nutrientS and biota. Unlike the jets of the coastal transition
zone. the jjoleward uridereurrents seem trapped to the eontinental margin. Th6 time scale of the
undercurrents variability over the slopc is greater than that of typic3.l shelf currentS resPünding to
the local wind~ Although there is evidence for the existence of poleward undercuiTents along the
eastem bOundaries of the Atlantic and Prieific Ocean in both hemispheres. thcre is 00 definitive
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study of their eoritinuity ovcr even a few huridrcd kilometers.
REFERENCES
Chi:lVez, F. P., et al., 1991. Horizontal transport and the distribution of nutrlerits in surface waterS
in the eoastal transition zone off northern California: Effects on primary production; phytoplanktori
. bioinass and species composition. J.. Geophys. Res. 96, 14833-14848.
Coastal Transitiori Zone GroUI>' 1988. The Coastal Transition Zone Program, EOS, Trans. Am.
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Geophys. Union 69(27), 698-699.
Davis, R. E., 1985. Drifter observations of eoastal suiface eurrents duririg CODE. J. GeophYs.
Res. 90,4741-4772.
an
. Hood, R.R. et al., 1990. Physieal rind bioiogieal structtire alorig upwelling fiorit off northern
California: surface patterns in temperature, flow, phytoplankton biomass arid species
eomposition, J. Geophys. Res. 95, 18081-18094.
Hood, R.R., ci ai., 1991. Phytoplankton rind photosyrithetic light response in thc eoastal
transition zone off northern California in June 1987. J. Geophys. Res. 96, 14769-14780..
Huyer, A:, et al., 1991. Currents arid water rriasses of thceoastal transition iorie off northern
California, Jurie to August 1988, J. Geophys. Res. 96, 14809-14831.
Kosro, P. M., 1987. Structure of the eoastal eurrent field off northern Californhi dJring thc
Coastal Ocean Dyriamics Experiment, J. Gcophys. Res. 92, 1655-1681. See other papers/rom
CODE and SuperCODE in the same issue.
Kosro, P.M.et al., 1991. The structure ofthc transition zone l>Ctween eoastal wäte~ anel the operi
ocean off northern California, Winter and Spring 1987, J. GeophYs. Res. 96, 14707-14730.
Lentz, S. J., 1992. Thc surfaee lJoundary layer in coastal upwelling regions. J. PhYs.
1517-1539.
06. 22,
Lentz, S. J., and J. H. Trowbridge, 1991. The bottoin boundary layer over the northern
California shelf. J. Phys. Oe. 21, 1186-1201.
Ncshyba, SJ., C.N.K. Mooers, R.L: Smith, and R.T. Barber, eds., 1989. Poieward flows along
eastern oeean boundaries, Coastal and Esturarine Studies No. 34, Springer-Verläg, New York..
This collection o/papers has SOlne discussion 0/ every eastern boundary currelit region. See
review by Warren. B. A., 1990. Lirimology ~nd Oceanography, 35, 1219-1220..
Send, U., 1989. The origin of eddy heat f1uxes in the northcm Califomia upwelling regime. J.
Geophys. Res. 94, 871-876.
Smith, R.L., 199i Coastal upwelling in thc modem ocean. In: Summerhayes, C.P. et al., eds.,
Upwelling Systems: Evolution since the early Miocene. Geologieal Soe. Spec. Pub. No 64, 9-28.
Strub, P~T.,et al., 1987. Seasonal eycles ofcurrents, temperatures, winds rind sea level over the
northeast Pacific eontinental shclf: 35° to 48°N, J. Geophys. Res. 92, 1507-1526.
Strub, P. T., et al., 1991. The nature of the cold filaments in the Califomhi Current system, 1.
Geophys.Res.96, 14743-14768, 1991. See other papers/rom crz in the same issue.
Sverdrup, H. U., 1938. On the process of upwelling. J. Maririe Res. 1, 155-163..
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ABOVE: Figure 1. Satellite sea sunace temperat'ure (SST) and shipbome acoustic doppler current
measurements at 25 m during erz experiment 12-18 June 1987. Arrow bclow Cape Blanco
represents 0.5 iTIJs; crosses indicate 1 degree latitudellongitude gnd (cross just offshore ofCape
Blaricois at 43°N, 125°\V; cross just offshore of Point Arena is at 39°N, 124°W). Data and figure
provided by P. M. Kosro and P. T. Strub.
NEXT PAGE: Figure 2. Sections normal to the coast at about 43°, 41.5°, and 400 N(see Figure I)
showing teniperature from erD stations and alongshore velocity (north\vard is positive, units are
m/s) from acoustic doppler cUITent profiler (ADCP). Distances are from the coast; the inriermost
stations of the southem 1\vo sections are seaward of the shelf break. Note the similarity of the
equatorward vclocity structure Get) iri the vicintiy of thc front (I3°C).
Figure 3. Potential denshy (sigma-theta), chlorophyJl-a, alongshore geostrophic vclocity, and
particle volume along the cross-niargin section at 400 N (just sOllth of Cape Mendocino) shown in
Figllres 1 and 2; station mlmbers as in Figllre 2 but distance scale in Figure 3 is relative to most
inshore station, which is 25 km offshore. Figure is adaptcd from Hood ct aI., (1991).
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