Lagrangian Measurement of Subsurface Poleward Flow Between 38 Degrees N and 43 Degrees N Along the West Coast of the United States During Summer, 1993

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1996-09-01 Lagrangian Measurement of subsurface poleward Flow between 38 degrees N and 43 degrees N along the West Coast of the United States during Summer, 1993

Collins, Curtis A.

Geophysical Research Letters, Vol. 23, No. 18, pp. 2461-2464, September 1, 1996 http://hdl.handle.net/10945/45730 GEOPHYSICAL RESEARCH LETTERS, VOL. 23, NO. 18, PAGES 2461-2464, SEPTEMBER 1, 1996

Lagrangian Measurement of subsurface poleward Flow between 38øN and 43øN along the West Coast of the United States during Summer, 1993

CurtisA. Collins,Newell Garfield, Robert G. Paquette,and Everett Carter 1 Departmentof Oceanography,Naval Postgraduate School, Monterey, California

Abstract. SubsurfaceLagrangian measurementsat about Undercurrentalong the coastsof California and Oregon. We 140 m showedthat the pathof the CaliforniaUndercurrent lay are using quasi-isobaric(float depth controlled primarily by next to the continentalslope betweenSan Francisco(37.80N) the pressureeffect on density)RAFOS floats (Rossby et al., and St. GeorgeReef (41.8øN) duringmid-summer 1993. The 1986) to make these measurements. A RAFOS float consists meanspeed along this 500 km pathwas 8 cms-1. Theflow at of a hydrophonemounted in a glasstube that is about2 meters this depth was not disturbedby upwelling centersat Point long. These hydrophonesreceive signals from three sound Reyesor CapeMendocino. Restfits also demonstratethe abil- sources that were moored 400 km offshore between 34.3øN and ity to acousticallytrack floats located well above the sound 40.4øN.The sound sources emit 15 W, 80 s signalsa•t 260 Hz channelaxis along the California coast. threetimes per day. The sourcesare mooredat the centerof the SOFARchannel, which occursat depthsranging :from 500 to Introduction 600 m off the California Coast. Beginningthis program,we wereunsure hoW' shallow we The existenceof polewardsubsurface flow along the West . could place the floats. The observeddepth of the maximum Coast of the United States is well known. Along the polewardflow off Central California appearsto be about 150 continentalshelf and upper slope, polewardflow has been m (Rischmiller,1993), so this would be the ideal depth. But observedusing both direct measurements,including current optimum sound propagation, and hence position fixing, meters(e.g., Wickhamet al., 1987, Huyer et al., 1989) and occurs near the depth of the axis of the SOFAR channel. short term drogue deployments(Reid, 1962), and indirect Deploying floats in the SOFAR channel(550 to 600 m) would methods based upon geostrophy (Chelton, 1984). The avoid the "shadow"zones that are createdby fronts associated poleward flow is called the California Undercurrentand with mesoscale features in shallower waters. Our floats have transportsequatorial waters poleward, resulting in a wedgeof beenset at a varietyof depthsin the upperhalf of the SOFAR warm, high salinity and low oxygen water at intermediate channel,from 140 m to 700 m. The purposeof this letteris to depthnext to the coast(Lynn and Simpson,1987). These describethe behaviorof the shallowestfloat, not only because waterscontrast with the equatorwardflowing Subarcticwaters this float trajectorywas similarto that for the deeperfloats that lie offshoreand are relatively cool, fresh and highly thatwere in the Undercurrent,but also becauserobust tracking oxygenated. As pointed out by Mooers (1989), questions remain about the character of the flow in the California at shallow depthsmay allow the use of quasi-isopycnal(or Undercurrent: is the current a continuous flow or a series of pressure-effectcompensated)RAFOS technology to better connected eddies? samplethermodynamic processes associated with upwelling. Barriersto continuoussubsurface alongshore flow are most likely to exist at capes. During summer,upwelling is espe- Results cially persistentand well developedat Pt. Arena and Cape Mendocino(Bray andGreengrove, 1993), and is evidencedby RAFOS float NPS#5 was launchedon July 7, 1993, in the California Undercurrent above the 1000 m isobath due west of minimum sea surface temperature. Instabilities in the alongshoreflow develop, forming filaments and eddies. San Franciscoand surfacedon September5, 1993, about 60 Filamentsof cold water are subsequentlyobserved flowing in km off Cape Blanco, Oregon. During its subsurfacemission, NPS#5 was able to hear at least two of the three offshore sound an offshore direction (sometimesextending for hundredsof kilometers) from these upwelling centers, restilting in the sourcesduring 162 of the 180 listening periods,and all three transportof water from the shelf into the deep ocean(Brink sourceswere heard during 92 of the listeningperiods. Since a and Cowles, 1991). These filaments have offshore-directed minimumof two sourcesis requiredto fix the positionof the velocitiesthat may exceed 50 cms-1 at the surfaceand have float, we were able to constructa trajectoryfor the float. The beenobserved to extendfrom the surfaceto greaterthan 500 m longest period when no or only one sourcewas heard was 18 hours. This occurred four times. off Point Arena (Ramp et al., 1991). We have recently begun a programof Lagrangianmeasure- The trackof the float is indicatedin Figure 1 as a seriesof mentsto study the continuityand structureof the California daily positionsplotted on an advanced very high resolution radiometer(AVHRR) sea surfacetemperature image from a 1NowatTaygeta Scientific Inc.,•Monterey, California NOAA polar-orbiting satellite for September 2, 1993. (Vertical motionof the float is alsoindicated in Figure 1: open Thispaper is not subjectto U.S. copyright.Pub- dots representsinking, while solid dots represent shoaling.) lishedin 1996by theAmerican Geophysical Union. After launch the float first drifted northwestward and then northeastward,turning to the northwestagain upon reaching Papernumber 96GL0213 8 the 200 m isobath. The float remained in the Undercurrent,

2461 2462 COI.I.INS, ET AL: POLEWARD FLOW ALDNG THE U.S. WEST COAST IN SUMMER 1993

I 17.00 I

Pt Arena 9. O0

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Figure1. Chartof the trajectoryof RAFOSfloat NPS#5. The trajectorybegins off SanFrancisco at 37'-50.4'N, 123'-27.3'W on July 7, 1993, andends just southof CapeBlanco at 42ø-43.6'N, 125ø-06.4'Won September5, 1993. Positionsare indicatedby circlesand are given daily. Open (closed)circles indicate regions where the float sinks (rises). Isothermsare derived from AVHRR imageryfor September2, 1993. movingsteadily poleward with a meanspeed of 8 cms-1, along these locationson these dates was confirmed by examination the 200 m isobath. During this polewarddrift, the float passed of AVHRR imagery.) After passingto the west of St. George tradertwo major upwelling centers: Pt. Arena on July 18 and Reef at 41.80N on August 10, the float drifted offshore into Cape Mendocinoon July 28. (The existenceof upwellingat water 4000 m deep in the Gorda basin. Here it became COl J!.INS, ET AL.: POLEWARD FLOW ALONG THE U.S. WEST COAST IN SUMMER 1993 2463

entrainedin an anticycloniceddy, moving around the eddy Three regions of shoaling(the float encounteringdenser three times with a circular motion of 35 km diameter. During water) were observed: from just north of Pt. Reyes to Pt. its third circuitaround the eddy,NPS#5 was displacedfarther to Arena, a midway betweenCape Mendocino and St. George the westand north. The trajectoryends with the surfacingof Reef, and during the northwardand westwardflow from just the float on September5, 1995. south of St. George Reef to 125.2øW. Sinking occurred The speed of the float is proportionalto the distance before, betweenand after theseperiods. At Point Arena and betweenthe daily positionsgiven in Figure 1, i.e., the faster CapeMendocino, minimum pressurewas observedjust to the the float movesthe greaterthe distancebetween the positions. southof the Point/Cape,while off St. George Reef minimum (The distance between the daily positions is actually the pressurewas observedoffshore as the float crossed42øN. Each minimum distance that the float moves, because small me- of these positionscoincided with the general location of an anders,tidal motions, etc. cause small deviations along this upwelling center as defined by temperatureminima in the path). Along the coast,minimum speedsoccur at Pt. Reyes, AVHRR image(Figure 1). While the float was entrainedin the just to the southof Pt. Arena and St. GeorgeReef, and just to anticycloniceddy off Cape Blanco, the float rose as it moved the north of Cape Mendocino. The largestspeeds, 15 to 20 offshore and sank as it moved toward the coast. At the end of cms-1, occurred during offshore movement north of St. George its mission,the float was about20 decibarsdeeper than when Reef,while along the coastspeeds greater than 10 cms4 it was launched. occurredjust to the north of Pt. Reyes,just south of Cape Within the regionshown in Figure 1, temperaturegenerally Mendocino, and about halfway betweenPt. Arena and Cape decreasesto the north, inshore(during summer,due to up- Mendocino. welling) and with increasingpressure (a typical CTD cast The float also measuredpressure and temperatureduring its showstemperature decreases with increasingpressure at -0.01 mission,which is shownin Figure 2. The pressureof the float to -0.02øCdbar4 at 140 dbar). The float cooledboth as it variedfrom 160 decibars (dbar, I dbar= 104Pa~ 1 m) to 110 shoaledand sank, and temperatureat the end of the mission dbar during the mission. Variability in pressure was had decreasedby 0.55øC. About half (0.3øC) of the observed associatedwith changesin water properties. These affect the cooling occurredwhile the float was shoaling between Pt. float due to differences in the thermal expansion and the Reyes and Pt. Arena, and a similar cooling occurredduring compressibilityof the float and seawater:NPS#5 was 65% as shoalingto the south of St. George Reef and as the float compressibleas sea water and had only 8% of the thermal moved offshore. Relatively little temperature decrease expansionof seawater.This meansthat NPS#5 is not able to occurred when the float sank 30 m while entrained in follow the vertical motion of the water: when an isopycnal anticyclonic motion in Gorda Basin. Just north of Cape descends, the float cannot descend as far and will enter less Mendocino, a different water mass was encountered, and a densewater. Similarly, ascendingisopycnals will move the period of sustainedwarming was experienced by the float, float into denserwater. The resulting float displacementis beginning at 130 dbar, 7.85øC in Figure 2. The float limited by the vertical gradientsof temperatureand salinity at temperaturereaches 8.1øC southof St. GeorgeReef, and then the float. the pressure-temperaturerelationship nearly retraces its path to 130 dbar, 7.85øC, indicatinga mirror image to the north of 105 the water structure to the south. At the end of its mission,NPS#5 was 20 dbar deeperand 0.55øCcolder than at the launchpoint. A CTD castat launch yielded S=34 for the observedtemperature of 8.37øC. CTD 120 castsfor the regionduring summer months yield dS/dp-- 0.016 Sdbar-1 anddT/dp = -0.081øCdbar -1. Usingthese quantifies, the observed20 dbar pressurechange would result from a fresheningof the waterto S=33.8. This salinityis reasonable basedupon reported data (Kosro,et al., 1995) and climatology 135 for the region (Churgin and Halminski, 1974).

Discussion

The characterof the subsurfacepoleward flow along central 150 and northernCalifornia appears to be markedlydifferent from the surfaceflow. The latteris markedby divergentflow, upwelling, cold filaments,jets and squirtsassociated with off- shoreflow. The trajectoryof NPS#5 indicateda continuityof 165 polewardsubsurface flow overfour degreesof latitude. The ma- 7.7 7.8 7.9 8 8.1 8.2 8.3 8.4 jor reasonfor this is the divergenceat the surface associated Temperature,øC with upwelling. This requiressubsurface convergence for continuity. For a coordinatesystem where distanceand velocity Figure 2. Pressure-temperaturemeasurements from RAFOS to the north,east, and upward are givenby x, y, z and u, v, w, float NPS#5. Valuesare plotted daily for the periodJuly 8 - respectively,continuity requires September4, 1993. "S" indicatesthe launchpoint and "E" indicatesthe end of the trajectory. "A", "M", and "SG" indicate the pressureand temperatureof the float when it is at the lati- -'•Z'Z--- 4- tudeof Pt. Arena (38.9'N), Cape Mendocino(40.4'N) and St. wherethe quanityin bracketsis the divergence.At the surface GeorgeReef (41.8'N), respectively. andbottom, w = 0, and coastalupwelling results in positive w 2464 COLIJNS, Er AL: POLEWARD FLOW ALONG THE U.S. WEST COAST IN SUMMER 1993 in the upper ocean, so Ow/Oz< 0 and divergenceoccurs. References Beneaththis layer, 0w/0z > 0 and convergenceresults. The Arthur,R. S., On thecalculation of verticalmotion in Easternboundary northwardflow from NPS#5 indicatesthat 0v/0y ~ 0 so 0u/0x currentsfrom determinationsof horizontalmotion, J. Geophys.Res. < 0. Since u = 0 at the coast, onshore flow occurs next to the 70(12), 2799-2803, 1965. coastat depth. Bray,N. A. andC. L. Greengrove,Circulation over the shelfand slope off Northern California, J. Geophys.Res. 98(10), 18,119-18,145, The float leavesthe coastat 41.8øN, north of St. George 1993. Reef, when its polewardvelocity is low and it is shoaling. As Brink,K. H., andT. J. Cowles,The CoastalTransition Zone Program,J. noted earlier, the shoalingis becausethe float has encountered Geophys.Res. 96(8), 14,637-14,647,1991. a denserwater mass. If we assumethat thesedensity changes Chelton,D. B., Seasonalvariability of alongshoregeostrophic velocity are representativeof the water column,historical surveys (Fig. off CentralCalifornia, J. Geophys.Res. 89, 3473-3486, 1984. Churgin,J. andS. J. Halminski,Temperature, salinity, oxygen and phos- 5, Bray and Greengrove,1993) for the region can be used to phatein watersoff the UnitedStates, Eastern North Pacific, 259 pp., estimatecorresponding changes in geopotential,about 2 to 4 NationalOceanic and AtmosphericAdministration, U.S. Dept.. of dyn. cm (surfacereferenced to 500 dbar). The shoalingof the Commerce, Wash., D.C., 1974. float then correspondsto a poleward-directed horizontal Freeland,H. J, The flow of a coastalcurrent past a blunt headland, pressure gradient, normally associated with eastward Atmosphere-Ocean28(3), 288-302, 1990. Huyer,A., P.M. Kosro,S. J. Lentz,and R. C. Beardsley,Poleward flow geostrophicflow of a few cms-1, whichwould oppose the in the CaliforniaCurrent system, in Polewardflows alongEastern observedflow exhibited by NPS#5. Another dynamical oceanboundaries, edited by S. J. Neshyba,C. N. K. Mooers,R. L. processmust occur. Note that Largier et al. (1993) report a Smithand R. T. Barber, pp. 142-156,Springer-Verlag, New York, mesoscaleanticyclonic eddy near the shelf edge at 41.6øN in 1989. Kosro,P.M., J. A. BartIt,J. Fleischbein,A. Huyer, R. O'Malley, K. May and June, 1988. Such a feature could transporta float Shearnanand R. L. Smith, SEASOARand CTD observationsduring offshore. EBC cruisesW9306A and W9308B, June to September1993, Data The northwardflow aroundcapes appears markedly different Report160, Ref95-2, 128 pp, Collegeof Oceanicand Atmospheric from that predictedby theoreticalmodels. Barotropicmodels Sciences,Oregon State University, Corvallis, Oregon 97331-5503, 1995. (Arthur, 1965, Freeland, 1990) predict maximum northward Largier,J. L, B. A. Magnell, and C. D. Winant, SubtidalCirculation speed at the cape, where the float experiencedminimum Over the NorthernCalifornia Shelf, J. Geophys.Res., 98, 18,147- speeds.These models also predict a maximum convergenceto 18,179, 1993. the northof the cape and maximumupwelling to the southof Lynn,R. J. andJ. J. Simpson,The Californiacurrent system: the sea- the cape. To the south (north) of Pt. Reyes and Cape sonalvariability of itsphysical characteristics, J. Geophys.Res. 92, Mendocino, NPS#5 sank (shoaled) due to less (more) dense 12,947-12,966, 1987. Mooers,C. N.K., Workshopsummary: poleward flow - observational water. Sinceupwelling to the southof capesshould result in andtheoretical issues, in Polewardfiows along Eastern ocean bound- denser water, this behavior is also inconsistent with aries,edited by S. J. Neshyba,C. N. K. Mooers,R. L. Smithand R. T. barotropicmodel results. Barber, pp. 2-16, Springer-Verlag,New York, 1989. We are clearlyable to track shallowfloats next to the coast. Ramp,S. R., P. F. Jessen,K. H. Brink,P. P. Niiler, F. L. Daggett,and J. At depthsas shallowas 90 m, shadowing(associated with the S. Best, The PhysicalStructure of Cold FilamentsNear Point Arena, California,During June 1987, J. Geophys.Res. 96(8), 14,859-14,883, refractionof sound)was not a problem. We are unsurehow 1991. muchshallower we canpush the RAFOS system. Floats at the Reid, J. L., Jr., Measurements of the California Countercurrent at a surfaceare not able to fix the time of arrival of signalsfrom Depthof 250 meters,J. Mar. Res.,20, 134-137, 1962. the sources,and temperatureinversions are not uncommonin Rischmiller,F. W., Variabilityof theCalifornia Current system off Point Sur,California, from April 1988to December1990, M.S. Thesis,157 nearsurface waters due to subductionand mixing of Subarctic pp., Naval PostgraduateSchool, Monterey, California, 1993. and upwelledwaters. Nevertheless,we feel that these floats Rossby,T., D. Dorson,and J. Fontaine,The RAFOSsystem, J. Atmos. can contributeto studiesof the dynamics and thermodynamics Oceanic Technol. 3, 672-679, 1986. of upwelling along the California coast. Wickham, J. B., A. A. Bird and C. N. K. Mooers, Mean and variable flow over the central Californiacontinental margin, 1978-1980. Cont.Shelf. Res., 7, 827-849, 1987. Acknowledgments. We are indebtedto Mr. Tarry Ragofor the preparationof NPS#5 and the sound sources,to Ms. Maria C. Collins,N. Garfield,R. Paquette,Department of Oceanography, Stone and Mr. Paul Jessenfor mooring the soundsources, to NavalPostgraduate School, 833 DyerRoad, Room 328, Monterey,CA Mr. Brian Miller for assemblingthe AVHR imageryused in 93943-5122(e-mail: [email protected]. mil, Figure 1, to Prof. C. N. K. Mooers for commentsand correc- [email protected],[email protected]) tions to this manuscript,and to the masterand crew of the R/V E. Carter,Taygeta Scientific Inc., 1340Munms Ave., Suite223, Point Sur for their assistancewith at-sea operations. This re- Monterey,CA 93940(e-mail: skip@taygeta. com) searchwas supportedby the Naval PostgraduateSchool, the Office of Naval Research,and the Oceanographerof the Navy. (ReceivedMay 1, 1996, revisedJune 21, 1996, acceptedJuly 1, 1996)