Calhoun: The NPS Institutional Archive DSpace Repository Faculty and Researchers Faculty and Researchers Collection 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.; Garfield, Newell; Paquette, Robert G.; Carter, Everett Geophysical Research Letters, Vol. 23, No. 18, pp. 2461-2464, September 1, 1996 http://hdl.handle.net/10945/45730 Downloaded from NPS Archive: Calhoun
. 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 Curtis A. Collins, Newell Garfield, Robert G. Paquette, and Everett Carter 1 Department of Oceanography, Naval Postgraduate School, Monterey, California Abstract. Subsurface Lagrangian measurements at about 140 m showed that the path of the California Undercurrent lay next to the continental slope between San Francisco (37.80N) and St. George Reef (41.8øN) during mid-summer 1993. The mean speed along this 500 km path was 8 cms -1. The flow at this depth was not disturbed by upwelling centers at Point Reyes or Cape Mendocino. Restfits also demonstrate the ability to acoustically track floats located well above the sound channel axis along the California coast. Introduction Barriers to continuou subsurface alongshore flow are most likely to exist at capes. During summer, upwelling is especially persistent and well developed at Pt. Arena and Cape Mendocino (Bray and Greengrove, 1993), and is evidenced by minimum sea surface temperature. Instabilities in the alongshore flow develop, forming filaments and eddies. Filaments of cold water are subsequently observed flowing in an offshore direction (sometimes extending for hundreds of kilometers) from these upwelling centers, restilting in the transport of water from the shelf into the deep ocean (Brink and Cowles, 1991). These filaments have offshore-directed velocities that may exceed 50 cms -1 at the surface and have been observed to extend from the surface to greater than 500 m off Point Arena (Ramp et al., 1991). We have recently begun a program of Lagrangian measurements to study the continuity and structure of the California 1Now at Taygeta Scientific Inc., Monterey, California This paper is not subject to U.S. copyright. Published in 1996 by the American Geophysical Union. Paper number 96GL0213 8 Undercurrent along the coasts of California and Oregon. We are using quasi-isobaric (float depth controlled primarily by the pressur effect on density) RAFOS floats (Rossby et al., 1986) to make these measurements. A RAFOS float consists of a hydrophone mounted in a glass tube that is about 2 meters long. These hydrophones receive signals from three sound sources that were moored 400 km offshore between 34.3øN and 40.4øN. The sound sources emit 15 W, 80 s signals a t 260 Hz three times per day. The sources are moored at the center of the SOFAR channel, which occurs at depths ranging :from 500 to 600 m off the California Coast. Beginning this program, we were unsure how' shallow we could place the floats. The observed depth of the maximum poleward flow off Central California appears to be about 150 m (Rischmiller, 1993), so this would be the ideal depth. But optimum sound propagation, and hence position fixing, occurs near the depth of the axis of the SOFAR channel. Deploying floats in the SOFAR channel (550 to 600 m) would The existence of poleward subsurface flow along the West Coast of the United States is well known. Along the continental shelf and upper slope, poleward flow has been observed using both direct measurements, including current meters (e.g., Wickham et al., 1987, Huyer et al., 1989) and short term drogue deployments (Reid, 1962), and indirect methods based upon geostrophy (Chelton, 1984). The avoid the "shadow" zones that are created by fronts associated poleward flow is called the California Undercurrent and with mesoscale features in shallower waters. Our floats have transports equatorial waters poleward, resulting in a wedge of been set at a variety of depths in the upper half of the SOFAR warm, high salinity and low oxygen water at intermediate channel, from 140 m to 700 m. The purpose of this letter is to depth next to the coast (Lynn and Simpson, 1987). These describe the behavior of the shallowest float, not only because waters contrast with the equatorward flowing Subarctic waters this float trajectory was similar to that for the deeper floats that lie offshore and are relatively cool, fresh and highly that were in the Undercurrent, but also because robust tracking oxygenated. As pointed out by Mooers (1989), questions remain about the character of the flow in the California at shallow depths may allow the use of quasi-isopycnal (or Undercurrent: is the current a continuous flow or a series of pressure-effect compensated)rafos technology to better connected eddies? sample thermodynamic processes associated with upwelling. 2461 Results RAFOS float NPS#5 was launched on July 7, 1993, in the California Undercurrent above the 1000 m isobath due west of San Francisco and surfaced on September 5, 1993, about 60 km off Cape Blanco, Oregon. During its subsurface mission, NPS#5 was able to hear at least two of the three offshore sound sources during 162 of the 180 listening periods, and all three sources were heard during 92 of the listening periods. Since a minimum of two sources is required to fix the position of the float, we were able to construct a trajectory for the float. The longest period when no or only one source was heard was 18 hours. This occurred four times. The track of the float is indicated in Figure 1 as a series of daily positions plotted on an advanced very high resolution radiometer (AVHRR) sea surface temperature image from a NOAA polar-orbiting satellite for September 2, 1993. (Vertical motion of the float is also indicated in Figure 1: open dots represent sinking, while solid dots represent shoaling.) After launch the float first drifted northwestward and then northeastward, turning to the northwest again upon reaching the 200 m isobath. The float remained in the Undercurrent,
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 '%. Figure 1. Chart of the trajectory of RAFOS float NPS#5. The trajectory begins off San Francisco at 37'-50.4'N, 123'-27.3'W on July 7, 1993, and ends just south of Cape Blanco at 42ø-43.6'N, 125ø-06.4'W on September 5, 1993. Positions are indicated by circles and are given daily. Open (closed) circles indicate regions where the float sinks (rises). Isotherms are derived from AVHRR imagery for September 2, 1993. moving steadily poleward with a mean speed of 8 cms -1, along the 200 m isobath. During this poleward drift, the float passed trader two major upwelling centers: Pt. Arena on July 18 and Cape Mendocino on July 28. (The existence of upwelling at these locations on these dates was confirmed by examination of AVHRR imagery.) After passing to the west of St. George Reef at 41.80N on August 10, the float drifted offshore into 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 entrained in an anticyclonic eddy, moving around the eddy three times with a circular motion of 35 km diameter. During its third circuit around the eddy, NPS#5 was displaced farther to the west and north. The trajectory ends with the surfacing of the float on September 5, 1995. The speed of the float is proportional to the distance between the daily positions given in Figure 1, i.e., the faster the float moves the greater the distance between the positions. (The distance between the daily positions is actually the minimum distance that the float moves, because small meanders, tidal motions, etc. cause small deviations along this path). Along the coast, minimum speeds occur at Pt. Reyes, just to the south of Pt. Arena and St. George Reef, and just to Three regions of shoaling (the float encountering denser water) were observed: from just north of Pt. Reyes to Pt. Arena, a midway between Cape Mendocino and St. George Reef, and during the northward and westward flow from just south of St. George Reef to 125.2øW. Sinking occurred before, between and after these periods. At Point Arena and Cape Mendocino, minimum pressure was observed just to the south of the Point/Cape, while off St. George Reef minimum pressure was observed offshore as the float crossed 42øN. Each of these positions coincided with the general location of an upwelling center as defined by temperature minima in the AVHRR image (Figure 1). While the float was entrained in the anticyclonic eddy off Cape Blanco, the float rose as it moved the north of Cape Mendocino. The largest speeds, 15 to 20 offshore and sank as it moved toward the coast. At the end of cms -1, occurreduring offshore movement north of St. George its mission, the float was about 20 decibars deeper than when Reef, while along the coast speeds greater than 10 cms 4 occurred just to the north of Pt. Reyes, just south of Cape Mendocino, and about halfway between Pt. Arena and Cape Mendocino. The float also measured pressure and temperature during its mission, which is shown in Figure 2. The pressure of the float varied from 160 decibars (dbar, I dbar = 104Pa ~ 1 m) to 110 it was launched. Within the region shown in Figure 1, temperature generally decreases to the north, inshore (during summer, due to upwelling) and with increasing pressure (a typical CTD cast shows temperature decreases with increasing pressure at -0.01 to -0.02øCdbar 4 at 140 dbar). The float cooled both as it shoaled and sank, and temperature at the end of the mission dbar during the mission. Variability in pressure was had decreased by 0.55øC. About half (0.3øC) of the observed associated with changes in water properties. These affect the float due to differences in the thermal expansion and the compressibility of the float and seawater: NPS#5 was 65% as cooling occurred while the float was shoaling between Pt. Reyes and Pt. Arena, and a similar cooling occurred during shoaling to the south of St. George Reef and as the float compressible as sea water and had only 8% of the thermal moved offshore. Relatively little temperature decrease expansion of seawater. This means that 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 descends, the float cannot descend as far and will enter less dense water. Similarly, ascending isopycnals will move the anticyclonic motion in Gorda Basin. Just north of Cape Mendocino, a different water mass was encountered, and a period of sustained warming was experienced by the float, float into denser water. The resulting float displacement is beginning at 130 dbar, 7.85øC in Figure 2. The float limited by the vertical gradients of temperature and salinity at the float. temperature reaches 8.1øC south of St. George Reef, and then the pressure-temperature relationship nearly retraces its path to 130 dbar, 7.85øC, indicating a mirror image to the north of 105 the water structure to the south. At the end of its mission, NPS#5 was 20 dbar deeper and 0.55øC colder than at the launch point. A CTD cast at launch yielded S=34 for the observed temperature of 8.37øC. CTD 120 casts for the region during summer months yield ds/dp -- 0.016 Sdbar -1 and dt/dp = -0.081 øcdbar -1. Using these quantifies, the observed 20 dbar pressure change would result from a freshening of the water to S=33.8. This salinity is reasonable based upon reported data (Kosro, et al., 1995) and climatology 135 for the region (Churgin and Halminski, 1974). Discussion 150 165 7.7 7.8 7.9 8 8.1 8.2 8.3 8.4 Temperature, øc Figure 2. Pressure-temperature measurements from RAFOS float NPS#5. Values are plotted daily for the period July 8 - September 4, 1993. "S" indicates the launch point and "E" indicates the end of the trajectory. "A", "M", and "SG" indicate the pressure and temperature of the float when it is at the latitude of Pt. Arena (38.9'N), Cape Mendocino (40.4'N) and St. George Reef (41.8'N), respectively. The character of the subsurface poleward flow along central and northern California appears to be markedly different from the surface flow. The latter is marked by divergent flow, upwelling, cold filaments, jets and squirts associated with offshore flow. The trajectory of NPS#5 indicated a continuity of poleward subsurface flow over four degrees of latitude. The major reason for this is the divergence at the surface associated with upwelling. This requires subsurface convergence for continuity. For a coordinate system where distance and velocity to the north, east, and upward are given by x, y, z and u, v, w, respectively, continuity requires -' Z'Z--- 4- where the quanity in brackets is the divergence. At the surface and bottom, w = 0, and coastal upwelling 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 divergence occurs. Beneath this layer, 0w/0z > 0 and convergence results. The northward flow from NPS#5 indicates that 0v/0y ~ 0 so 0u/0x < 0. Since u = 0 at the coast, onshore flow occurs next to the coast at depth. The float leaves the coast at 41.8øN, north of St. George Reef, when its poleward velocity is low and it is shoaling. As noted earlier, the shoaling is because the float has encountered a denser water mass. If we assume that these density changes are representative of the water column, historical surveys (Fig. 5, Bray and Greengrove, 1993) for the region can be used to estimate corresponding changes in geopotential, about 2 to 4 dyn. cm (surface referenced to 500 dbar). The shoaling of the float then corresponds to a poleward-directed horizontal pressure gradient, normally associated with eastward geostrophic flow of a few cms -1, which would oppose the observed flow exhibited by NPS#5. Another dynamical process must occur. Note that Largier et al. (1993) report a mesoscale anticyclonic eddy near the shelf edge at 41.6øN in May and June, 1988. Such a feature could transport a float offshore. The northward flow around capes appears markedly different from that predicted by theoretical models. Barotropic models (Arthur, 1965, Freeland, 1990) predict maximum northward speed at the cape, where the float experienced minimum speeds. These models also predict a maximum convergence to the north of the cape and maximum upwelling to the south of the cape. To the south (north) of Pt. Reyes and Cape Mendocino, NPS#5 sank (shoaled) due to less (more) dense water. Since upwelling to the south of capes should result in denser water, this behavior is also inconsistent with barotropic model results. We are clearly able to track shallow floats next to the coast. At depths as shallow as 90 m, shadowing (associated with the refraction of sound) was not a problem. We are unsure how much shallower we can push the RAFOS system. Floats at the surface are not able to fix the time of arrival of signals from the sources, and temperature inversions are not uncommon in near surface waters due to subduction and mixing of Subarctic and upwelled waters. Nevertheless, we feel that these floats can contribute to studies of the dynamics and thermodynamics of upwelling along the California coast. Acknowledgments. We are indebted to Mr. Tarry Rago for the preparation of NPS#5 and the sound sources, to Ms. Maria Stone and Mr. Paul Jessen for mooring the sound sources, to Mr. Brian Miller for assembling the AVHR imagery used in Figure 1, to Prof. C. N. K. Mooers for comments and corrections to this manuscript, and to the master and crew of the R/V Point Sur for their assistance with at-sea operations. This research was supported by the Naval Postgraduate School, the Office of Naval Research, and the Oceanographer of the Navy. References Arthur, R. S., On the calculation of vertical motion in Eastern boundary currents from determinations of horizontal motion, J. Geophys. Res. 70(12), 2799-2803, 1965. Bray, N. A. and C. L. Greengrove, Circulation over the shelf and slope off Northern California, J. Geophys. Res. 98(10), 18,119-18,145, 1993. Brink, K. H., and T. J. Cowles, The Coastal Transition Zone Program, J. Geophys. Res. 96(8), 14,637-14,647, 1991. Chelton, D. B., Seasonal variability of alongshore geostrophic velocity off Central California, J. Geophys. Res. 89, 3473-3486, 1984. Churgin, J. and S. J. Halminski, Temperature, salinity, oxygen and phosphate in waters off the United States, Eastern North Pacific, 259 pp., National Oceanic and Atmospheric Administration, U.S. Dept.. of Commerce, Wash., D.C., 1974. Freeland, H. J, The flow of a coastal current past a blunt headland, Atmosphere-Ocean 28(3), 288-302, 1990. Huyer, A., P.M. Kosro, S. J. Lentz, and R. C. Beardsley, Poleward flow in the California Current system, in Poleward flows along Eastern ocean boundaries, edited by S. J. Neshyba, C. N. K. Mooers, R. L. Smith and R. T. Barber, pp. 142-156, Springer-Verlag, New York, 1989. Kosro, P.M., J. A. BartIt, J. Fleischbein, A. Huyer, R. O'Malley, K. Shearnan and R. L. Smith, SEASOAR and CTD observations during EBC cruises W9306A and W9308B, June to September 1993, Data Report 160, Ref95-2, 128 pp, College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331-5503, 1995. Largier, J. L, B. A. Magnell, and C. D. Winant, Subtidal Circulation Over the Northern California Shelf, J. Geophys. Res., 98, 18,147-18,179, 1993. Lynn, R. J. and J. J. Simpson, The California current system: the seasonal variability of its physical characteristics, J. Geophys. Res. 92, 12,947-12,966, 1987. Mooers, C. N.K., Workshop summary: poleward flow - observational and theoretical issues, in Poleward fiows along Eastern ocean boundaries, edited by S. J. Neshyba, C. N. K. Mooers, R. L. Smith and R. T. Barber, pp. 2-16, Springer-Verlag, New York, 1989. Ramp, S. R., P. F. Jessen, K. H. Brink, P. P. Niiler, F. L. Daggett, and J. S. Best, The Physical Structure of Cold Filaments Near Point Arena, California, During June 1987, J. Geophys. Res. 96(8), 14,859-14,883, 1991. Reid, J. L., Jr., Measurements of the California Countercurrent at a Depth of 250 meters, J. Mar. Res., 20, 134-137, 1962. Rischmiller, F. W., Variability of the California Current system off Point Sur, California, from April 1988 to December 1990, M.S. Thesis, 157 pp., Naval Postgraduate School, Monterey, California, 1993. Rossby, T., D. Dorson, and J. Fontaine, The RAFOS system, J. Atmos. Oceanic Technol. 3, 672-679, 1986. Wickham, J. B., A. A. Bird and C. N. K. Mooers, Mean and variable flow over the central California continental margin, 1978-1980. Cont. Shelf. Res., 7, 827-849, 1987. C. Collins, N. Garfield, R. Paquette, Department of Oceanography, Naval Postgraduate School, 833 Dyer Road, Room 328, Monterey, CA 93943-5122 (e-mail: collins@oc.nps.navy. mil, garfield@oc.nps.navy.mil, paquette@oc.nps.navy.mil) E. Carter, Taygeta Scientific Inc., 1340 Munms Ave., Suite 223, Monterey, CA 93940 (e-mail: skip@taygeta. com) (Received May 1, 1996, revised June 21, 1996, accepted July 1, 1996)