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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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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 CORE Metadata, citation and similar papers at core.ac.uk Provided by Calhoun, Institutional Archive of the Naval Postgraduate School Calhoun: The NPS Institutional Archive Faculty and Researcher Publications Faculty and Researcher Publications 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 similar to 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 '%. 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)
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