Vertical and cross-shelf flow in the coastal upwelling region off Oregon

Deep-SeaResearch,Vol.26A.pp. 399to 408 PergamonPressLtd 1979. Printedin Great Britain

Vertical and cross-shelf flow in the coastal upwelling region off Oregon DONALD R. JOHNSON* a n d WALTER R. JOHNSONt

(Received 14 March 1978; in revisedform 25 September 1978; accepted 17 October 1978) Abstract--In August 1973, an experiment was conducted over the Oregon continental shelf using an array of Cyclesondes. In August 1974, a similar experiment was conducted using a profiling current meter, which sampled repeatedly and sequentially at four locations. The results of these two profiling experiments were synthesized with emphasis on cross-shelfflow and vertical motions. A double cell type of cross-shelf circulation pattern was found. The double cell circulation occurs near a density front formed by upwarped isopycnals intersecting (or nearly intersecting) the sea surface and seems to be limited in horizontal extent. The vertical velocity profiles calculated for each experiment are similar, with maximum velocities of about 2 x 10- 2 cm s - 1 at mid-depth.

INTRODUCTION

THE CROSS-SHELFand vertical components of coastal upwelling circulation are important for physical and ecosystems dynamics, but these components are difficult to measure and, consequently, their values are uncertain. In the late 1960's, a few direct current measurements off the Oregon coast, coupled with evidence from hydrography and optics, resulted in a double cell cross-shelf circulation model (Fig. 1) (MOOERS, 1970; PAK, BEARDSLEYand SMITH, 1970; MOOERS,COLLINS and SMITH, 1976). This model served as an hypothesis in planning the subsequent CUEA (Coastal Upwelling Ecosystems Analysis) experiments. Fixed-level Aanderaa current meters were used as the basic current sensors for the (Coastal Upwelling Experiment) CUE-I and CUE-II experiments (summers of 1972 and 1973), and were supplemented by VACMs:~, Cyclesondes§, PCMslI, parachute drogues, and vane drogues. From the results of the fixed-level instruments (Aanderaa and VACM), the two-month mean cross-shelf flow appeared to be principally single celled (HuYER, 1974, 1976; HALPERN, 1976). However, using surface vane drogues for a few hours, a convergence zone was noted along a frontal region (STEVENSON, GARVINE and WYATT, 1974). Measurements by Cyclesondes (JOHNSON,VAN LEER and MOOERS, 1976) showed a weak, but definite, double cell pattern in 64-h means near midshelf, in good agreement with the proposed model. In the 1973 experiment, three Cyclesondes were placed in a triangular array on the * Rosentiei School of Marine and Atmospheric Sciences, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149, U.S.A. t College of Marine Studies, University of Delaware, Lewes, DE 19958, U.S.A. :~VACM = Vector Averaging Current Meter. § Cyclesonde = Autonomous Profiling Current Meter (operates on a mooring). II PCM = Profiling Current Meter (operates on a wire suspended from a ship).

399

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DONALD R. JOHNSON and WALTER R. JOHNSON

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Fig. 1. Schematic diagram of the double cell cross-shelf circulation model (after MOOERS, COLLINS and SMITH, 1976).

outer shelf. Horizontal flow measurements and calculations of vertical motions were made (JOHNSON, 1976). JOHNSON e t al. (1976) emphasized the alongshore and cross-shelf flow, but the vertical motions were not presented. In a similar experiment in 1974, a P C M was used in a rectangular pattern about 30km south of the 1973 array. The 24-h mean vertical motion was calculated by JOHNSON (1977), but the cross-shelf flow was not discussed in detail. This paper synthesizes the results of these two profiling experiments with emphasis on both vertical motions and cross-shelf flow. OBSERVATIONS

The centers of the two profiling arrays were separated by approximately 30 km (Fig. 2). The Cyclesonde array (28 to 30 August 1973) was centered on the 150-m isobath, oriented c a . 000°T, while the P C M array (25 to 29 August 1974) was centred further south on the 100-m isobath, oriented c a . 009°T. The corresponding winds and smoothed density (trt) sections (Fig. 3) indicate the state of the upwelling regime. August 1973 was characterized by bursts ('events') of upwelling-favourable winds; one of these events began 25 August and continued through the 64-h Cyclesonde array experiment. An upwelling-favourable wind of c a . 6 m s-1 on 29 August produced an abrupt decrease in surface temperature (JOHNSON, 1976). In August 1974, the winds were similarly variable. The first 24 h of P C M sampling was during an upwelling-favorable wind of c a . 5 to 6 m s-1; the second was during relative calm. The representative ~rt sections show a rise of the ~rt = 26.0 isopycnal from a depth of 60m at 20kin offshore to about 15m at 3 to 5km offshore in August 1973 and 1974. In the 28 to 29 August 1973 section, the permanent pycnocline (tr, --- 25.5 to 26.0, as defined by COLLINS, 1968) did not intersect the sea surface; however, there was a subsurface frontal zone c a . 15km offshore, as can be seen in the original density section (JOHNSON, 1976). HUYER (1974) and STEVENSON e t al. (1974) noted that the nearsurface density structure is sensitive to the local winds and that changes occur slowly at depth during the upwelling season, except nearshore.

Vertical and cross-shelf flow in the coastal upwelling region off Oregon

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Although the Cyclesonde and the PCM are both profiling instruments, they employed different design and deployment techniques. The Cyclesonde used a repackaged Aanderaa current meter with a horizontal rotor axis (VAN LEER, DUING, ERATT, KENNELLY and SPEIDEL, 1974). It was attached by rollers to a taut-wire mooring with a subsurface buoyancy element. It automatically cycled from 5m above the bottom to within 10m of the surface at northeast and within 30m at northwest and southwest (Fig. 2), and it sampled at 1-min intervals (equivalent to 1- to 3-m vertical resolution). The PCM (Du[NG and JOHNSON, 1972) used a modified Aanderaa current meter with a vertical rotor axis. It descended a taut wire suspended from a drifting ship and sampled at 2- to 3-m intervals; the data in the upper 10m were discarded because of possible contamination by wave motion and ship's influence. The data sets from each instrument were vertically filtered by 'Harming' and linearly interpolated to 5-m intervals. A more stable suspension was an advantage of the Cyclesonde. An advantage of the PCM was that only one instrument was used (in rotational sequence about the array), minimizing effects of calibration uncertainties. In the Cyclesonde experiment, a flux (mass balance) method was used to calculate the vertical velocity. The flux was approximated on each leg of the triangle by the average horizontal velocity vector perpendicular to that leg multiplied by its length. The flux estimates were then combined as a line integral for each 5-m level from 30 to 95 m. Below 95m, the mass divergence was approximated for a wedge-shaped region between northwest and southwest (Fig. 2), with the constraint that the net flux into the bottom was zero.

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integration of the continuity equation; a near-bottom moored current meter in the center of the array provided the bottom boundary condition w( - h) = (u 6h/fix + v 5h/Sy)*. The calculations were constrained by demanding that the vertical velocity vanish at the surface, w(0) = 0. A degree of arbitrariness was introduced because the profile for the period of upwelling-favorable winds had to be adjusted by removing suspect data (JoHNsoN, 1977). After the adjustment was made, however, however, both boundary conditions were matched. During a subsequent period of wind relaxation the two boundary conditions were well matched without profile adjustment. With the Cyclesondes, between 85 and 190 profiles were acquired and interpolated to * (u, v, w,) = (shoreward, poleward, upward velocities); h = water depth.

Vertical and cross-shelf flow in the coastal upwelling region off Oregon

403

hourly intervals at each station. With the PCM, eight profiles were acquired at each station over two 24-h periods. The computations presented are based upon averages over these periods. The measurement error of the Cyclesonde or P C M for an individual sample was _+ 1 cm s- 1. The error introduced by uncertainty in the PCM ship drift was less than + 1.4cms -1, (JOHNSON, 1977). Both data sets had standard deviations of ,-~ 10cm s -1, primarily contributed by semidiurnal tides and inertial motions (JOHNSON et al., 1976). Consequently, the several-day means of the cross-shore flow are masked in sequences of individual profiles. On a longer time scale, the onshore-offshore flow is characterized by large amplitude (,-~ 10 cm s - l ) , low frequency (less than 0.5 cpd) fluctuations (MOoERS et al., 1976), which are not resolved in these short duration experiments. Consequently, the means presented are expected to evolve on a several-day time scale. The positions of the observational stations relative to the upwelling frontal zone may greatly affect the circulation observed. Additional error may be introduced by uncertainties in the local onshore-offshore (cross-isobath) co-ordinate direction. The error in the calculated vertical velocity was about +1 × 1 0 - 2 c m s -1 for the Cyclesonde (JOHNSON, 1976), but was not estimated for the PCM although an error discussion was provided (JOHNSON, 1977). Additional information regarding the calibration of the Cyclesonde is given by VAN LEERet al. (1974) and of the PCM by DUING and JOHNSON (1972).

RESULTS

The strongest indications of the double cell pattern were seen at station northeast (100m) for the Cyclesonde and at station west (120m) for the PCM. Profiles of u, v, and temperature (T) at these stations are matched in Fig. 4, together with the respective calculations of w. Current profiles at the remaining stations can be found in JOHNSON et al. (1976) and JOHNSON (1977). The u, v components at station west were rotated by + 9 ° to match the direction of the isobaths; the vertical motion, w, was calculated from nonrotated components. The Cyclesonde mean temperature profile indicated the temperature inversion structure that is often observed off Oregon (MOoERs et al., 1976; HUVER and SMITH, 1974). Individual profiles often had more striking inversions. The P C M mean temperature profile had a weaker temperature inversion and appeared to be about 0.5 to 1.0°C warmer overall. Considering the differences in location, depth, and the one-year separation between experiments, it is remarkable how well the u components agree. The P C M station was 20m deeper and showed more structure in u near the bottom. This may have been due to differences in the direction of nearbottom alongshore flow, and, thus, inferred onshoreoffshore bottom Ekman flow. Specifically, during the Cyclesonde experiment, the mean near-bottom alongshore flow was equatorward at northeast (only), implying onshore Ekman transport. During the PCM experiment it was poleward, implying offshore Ekman transport. (The inferred bottom Ekman transports were consistent with the observations in both cases). The near-bottom alongshore flow alternated direction frequently (SMITH, 1974) and with cross-shelf position (JOHNSON, 1976). Thus, space and time variability is to be expected in the near-bottom cross-shelf flow. The warm temperature anomaly found in mid-column has been associated in the proposed model with an offshore flow (MOOERS et al., 1976). This seems to be the case for the Cyclesonde mean temperature profile. However, it is not at all clear for the PCM

404

DONALD R. JOHNSON and WALTER R. JOHNSON

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northeast by Cyclesonde (1973) and from station from the Cyclesonde array (@) and for the P C M array (O). Dotted lines represent extrapolations-interpolations. • Represents the bottom boundary condition for the PCM calculations using a near-bottom fixed-level Aanderaa current meter. ® represents the bottom boundary condition for the Cyclesonde calculations using a value of u = - 2 c m s - 1 estimated from the Cyclesonde profiles. W by P C M

T, u, a n d v profiles from station (1974). Lower: calculated w profiles

mean temperature profile. In the latter profile, although the maximum is not well defined, it seems to be associated with shoreward flow. Considering the differences in location and depths of the centers of each array and the large uncertainties in this type of calculation (JOHNSON, 1977), the w profiles show remarkable similarities in both magnitude and shape. The maximum vertical velocity is about 2 x 10 -2 cm S- 1 at mid-depth in each case. DISCUSSION

Off Oregon, a complex pattern occurs in the cross-shelf current profiles measured with profiling instruments and averaged over one to three days. The observed pattern resembles, to some degree, the proposed model of double cell circulation. To facilitate the discussion of observations relative to the double cell circulation model of MOOERS et al. (1976), the cross-shelf stream function, if, was evaluated, where u -- C~zffand w -- c~x~,. As the depth-integrated cross-shelf flow is assumed to vanish at each position on the variable depth continental shelf, ff = F(x, (), where x is the cross-shelf coordinate, z is the vertical co-ordinate, h(x) is the water depth, and ( = z/h. Thus, F(x, O) = F(x, - 1 ) = 0 for all values o f x . The mean u profile for northeast (Fig. 4) was extended to the surface with the constraint that the near-surface transport equal the estimated offshore surface Ekman transport. The profile w a s also somewhat smoothed, and it was

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further constrained to reproduce the depths of observed maxima, minima, and zerocrossings and to yield zero depth-integrated transport by adjustment of the n e a r - b o t t o m flow (Fig. 5a). F r o m this u profile, the stream function was evaluated with F(xt,()=

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where xl is a mid-shelf location, northeast in the 1973 case used as the simplest example. T h e w profile was evaluated from the following relation: 8~ dh w = 8x~ = 8xF + 8~F ~x = 8xF + dxx ~u(xl' ~)" The relation separates w into a part due to horizontal convergence (SxF) and a part due to lifting by a b o t t o m slope (dh/dx ~u). Because the internal radius of deformation, 2, is the d o m i n a n t cross-shelf scale in coastal upwelling, the first term is estimated as F/2, where 2 = 2 0 k m (Fig. 5b). The second term was evaluated with dh/dx = - 7 × 1 0 - 3 (Fig. 5c). Their sum, the estimated w, exhibits two zones of upwelling generally associated with onshore flow and two zones of downwelling associated with offshore flow (Fig. 5d). The first term in w, whose form is given by ¢, has two subsurface upwelling m a x i m a and vanishes at the surface and b o t t o m ; it resembles the areal average of w (Fig. 4) during the Cyclesonde experiment. The second term in w depends upon the b o t t o m slope and the local u profile; it is c o m p a r a b l e in m a g n i t u d e to the first term and gives two downwelling as well as two upwelling zones. [ N o t e that this w profile is based only on a representative mid-shelf u-profile, while the w profiles in Section 2 (Fig. 4) are based on data from an areal distribution of stations including at least one from mid-shelf.] Of course, the validity of the composite w (Fig. 5d) depends upon the accuracy of estimating ~xF as F/2. The u r~SRIA1264

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406

DONALD R. JOHNSON and WALTER R. JOHNSON

and composite w profiles do have the characteristics called for in the so-called double cell circulation model for coastal upwelling frontal zones. The w profile calculated by this technique (Fig. 5d) does not agree fully with the w profiles computed in Section 2 (Fig. 4), which suggest either the possible importance of alongshore convergence or that the double cell circulation is restricted to a limited zone. With the present data, the alongshore convergence cannot be isolated reliably because of the lack of measurement accuracy and because of uncertainties in the orientation of the onshore-offshore coordinate system (JOHNSON(1977). Further evidence for the double cell pattern off Oregon comes from optical (HARLETT and KULM, 1973; KOMAR, KULM and HARLETT, 1974) and phytoplankton (WRo~LEWSKL 1977) studies. The double-cell pattern has also been observed off Cabo Bojador, northwest Africa (JOHNSON, BARTON, HUGHES and MOOERS, 1975) and off Valparaiso, Chile (JOHNSON, FONSECA and SIEVERS, 1979). In both of these regions, profiling instruments were used. However, the double cell pattern was not observed with either fixed level or profiling instruments off Cap Blanc, northwest Africa (HALPERN, SMITH and MITTLESTAEDT, 1977). The Cap Blanc region has comparatively weak vertical density gradients and little indication of a frontal zone over the she!f (JOHNSON, 1979). As the original double cell model was predicated on the existence of a strong frontal zone, it is not surprising to find the double cell missing from the Cap Blanc study. (HALPERN (1976) also indicated that there was no evidence in combined Aanderaa-VACM data for the double cell circulation during a strong upwelling event (up to 5 dyn cm-2 wind stress) in July 1973 off Oregon. A more extensive comparison of these regions is presented by JOHNSON (1979); it is noted here that the double cell type of profile (1) seems to occur near a frontal zone (it either does not appear, or appears weakly at the shelf break) and (2) is associated with a layer of relatively large vertical shear in alongshore velocity at the base of the pycnocline. The frontal zone, when it exists, has limited vertical and horizontal extent (10 to 20m and 5 to 15km) and temporal variations with scales of 1 to 2 weeks. The interpretation of double cell circulation from several-day means of profiling measurements in the strong frontal region off Oregon is in conflict with the one cell pattern reported for two-month means from fixed-level current measurements (HuYER, 1976; HAkPERN, 1976). It is not clear why this is so. The 1973 fixed-level results have a discontinuity in the composite u-component profile between the VACM record at 18,3 m and the Aanderaa record at 20m. In both the two-month mean profile and in the wind event (ca. 1 day) mean profile (HALPERN, 1976, Figs. 7 and 12, respectively) the 18.3-m VACM exhibited seaward flow while the 20-m deep Aanderaa exhibited shoreward flow. The Cyclesonde measurements were in substantial agreement with the VACM measurements above 20 m, but they did not agree with the Aanderaa measurements below 20m (JOHNSONet al., 1976). HALPERN and PILLSBURY (1976)showed that pumping of the Aanderaa's Savonius rotor may be severe on a taut wire mooring when the main buoyancy element is near the surface, but this does not explain the differences in observed direction. Instead of instrumental uncertainties, it is possible that the differences may be due to strong spatial changes (definite double cell type profiles were observed at only two of the seven stations) aliasing and biasing by inertial oscillations, or to differences in times of deployment (in 1973 the fixed-level moorings were recovered just before the Cyclesonde experiment). In the fixed-level measurements the current meters were separated by about 2 m in the upper 20m and typically by 20m below. From Fig. 4, there is substantial

Vertical and cross-shelf flow in the coastal upwelling region off Oregon

407

structure in u on a vertical scale of 5 m. Thus, the fixed-level measurements below 20m may have been aliased in the vertical. As the cross-shelf circulation pattern and the position of the frontal zone are related and both vary on a 10-day time scale because of transient upwelling, a long-term (ca. month or longer) Eulerian average is likely to suppress a multi cell circulation field. As the double cell pattern has been observed in such widely diverse regions as northwest Africa and Chile, and as it was observed off Oregon in separate locations in separate years, it is not an unusual feature of upwelling circulation and it occurs in the upwelling frontal zone. In the originally proposed double cell model, the warm anomaly at mid-depth is formed by warm saline surface water that sinks along the base of the pycnocline and flows seaward in the lower cell. It also flows either equatorward or poleward depending upon the state of the upwelling regime. A tongue of warm water then extends seaward and downward. However, HUYER and SMITH (1974) established that the thermal structure is even more complex than the model indicates. A cold ribbon, presumably derived from Subarctic Water, appears at the base of the equatorward surface jet off Oregon. The warm tongue is observed beneath this cool anomaly late in the upwelling season. The double cell model does not require a warm anomaly associated with offshore flow, which would not be expected, for instance, in the case of weak nearsurface heating. Likewise, the temperature variability may reflect alongshore advection of 'relic' anomalies uncoupled from the local offshore flow. However, temperature can be used as a tracer, with due caution. Previous estimates of the vertical velocity off Oregon have been made from isopycnal displacements observed during upwelling spin-up (SMITH, PATULLO and LANE, 1966; WALSH, KELLY, WHITLEDGE, MACISAAC and HUNTSMAN, 1 9 7 4 ; HUYER, 1974; HALVERS 1976). As this method assumes no mixing across isopycnal surfaces, and because some smoothing of the data is required, these estimates probably represent lower limits on the vertical motion (HALPERN, 1976; JOHNSON, 1977). The present estimates using the continuity equation are in general agreement both with each other and the previous estimates. Considering the differences in instrumentation, calculation technique, and position (in space and time), this agreement is reassuring. However, there is an apparent contradiction: the calculated vertical velocities are upwards (Fig. 4) at middepth, where sinking along an upwarped isopycnal is inferred to occur in the double cell model. This may actually be additional evidence that the double cell type of circulation occurs in a limited region; hence, the w-component calculation, which is an average over the entire array, does not detect it. The results for a local estimate of w (Fig. 5) support this conjecture. Although considerable uncertainties remain in the application of the continuity equation to measurements and in the interpretation of cross-shelf flow, refinement in both instrumentation and experimental technique should improve the reliability of crossshelf and vertical flow estimates. Because of its significance for biological and chemical transports, the cross-shelf circulation must be understood before the more complex problems of ecosystems dynamics can be solved satisfactorily. Acknowledgements--The authors express gratitude to Drs L. SMALL,H. PAK, and R. ZANEVELDfor sharing their shiptime in 1974, to Dr. R. L. SMITH for the freedom to work on this interesting project, and to Dr J. C. VAN LEER for the success of the Cyclesonde experiment. Drs CHRISTOPHER N. K. MOOERSand RICHARD W. GARVINE generously edited and critically reviewed a draft. The research was supported by the National Science

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DONALD R. JOHNSON and WALTER R. JOHNSON

Foundation from its Office for the International Decade of Ocean Exploration under Grants GX33052 and GX28146. This paper is a contribution to the Coastal Upwelling Ecosystems Analysis Program. REFERENCES COLLINS C. A. (1968) Description of measurements of current velocity and temperature over the Oregon continental shelf, July 1965-February 1966. Ph.D. Thesis, Oregon State University, Corvallis, Oregon, 154 pp. DUING W. and D. R. JOHNSON (1972) High resolution current profiling in the Straits of Florida. Deep-Sea Research, 19, 259-274. HALPERN D. (1976) Structure of a coastal upwelling event observed off Oregon during July 1973. Deep-Sea Research, 23, 495-508. HALPERN D. and R. D. PILLSBURY (1976) Near-surface moored current meter measurements. Marine Technology Society Journal, 10, 32-38. HALPERN D., R. L. SMrrH and E. MITTELSTAEDT (1977) Cross-shelf currents on the continental shelf during coastal upwelling off northwest Africa. Submitted to Journal of Marine Research. HARLETr J. C. and L. D. KULM (1973) Suspended sediment transport on the northern Oregon continental shelf. Geological Society of America Bulletin, 84, 3815-3826. HUYER A. (1974) Observations of the coastal upwelling region off Oregon during 1972. Ph.D. Thesis, Oregon State University, Corvallis, Oregon, 149 pp. HUYER A. (1976) A comparison of upwelling events in two locations: Oregon and northwest Africa. Journal Marine Research, 34, 531-546. HUYER A. and R. L. SMITH (1974) A subsurface ribbon of cool water over the continental shelf off Oregon. Journal Physical Oceanooraphy, 4, 381-391. JOHNSON D. R. (1977) Determining vertical velocities during upwelling off the Oregon coast. Deep-Sea Research, 24, 171-180. JOHNSON D. R. (1978) Double-cell circulation during coastal upwelling. To be submitted to Deep-Sea Research. JOHNSON D. R., T. FONSECAand H. SIEVERS(1977) Upwelling in the Chile-Peru Coastal Current near Valparaiso, Chile. Submitted to Journal of Marine Research. JOHNSON D. R., E. D. BARTON, P. HUGHES and C. N. K. MOOERS (1975) Circulation in the Canary Current upwelling region off Cabo Bojador in August 1972. Deep-Sea Research, 22, 547-558 pp. JOHNSON W. R. (1976) Cyclesonde measurements in the upwelling region off Oregon. TR76-1 University of Miami, Florida, 123 pp. (Unpublished document). JOHNSON W. R., J. C. VAN LEER and C. N. K. MOOERS (1976) A Cyclesonde view of coastal upwelling. Journal of Physical Oceanooraphy, 6, 556-574. KOMAR P. D., L. D. KULM and J, C. HARLETT (1974) Observations and analyses of bottom turbid layers on the Oregon continental shelf. Journal of Geolooy, 82, 104-111. MOOERS C. N. K. (1970) The interaction of an internal tide with the frontal zone of a coastal upwelling region. Ph.D. Thesis, Oregon State University, Corvallis, Oregon, 480 pp. MOOERS C. N. K., C. A. COLLINS and R. L. SMITH (1976) The dynamic structure of the frontal zone in the coastal upwelling region off Oregon. Journal of Physical Oceanography, 6, 3-21. PAK H., G. F. BEARDSLEY, JR and R. L. SMITH (1970) An optical and hydrographic study of a temperature inversion off Oregon during upwelling. Journal of Geophysical Research, 4, 214-226. SMITH R. L. (1974) A description of current, wind and sea level variations during coastal upwelling off the Oregon coast, July-August, 1972. Journal of Goephysieal Research, 79, 435 443. SMITH R. L., J. G. PAX-rULLOand R. K. LANE (1966) An investigation of the early stage of upwelling along the Oregon coast. Journal of Geophysical Research, 71, 1135-1140. STEVENSON M. R., R. GARVINE and B. WYATT (1974) Lagrangian measurements in a coastal upwelling zone off Oregon. Journal of Physical Oceanooraphy, 4, 321-336. VAN LEER J., W. DUING, R. ERATq', E. KENNELLY and A. SPEXDEL(1974) The Cyclesone: an unattended vertical profiler for scalar and vector quantities in the upper ocean, Deep-Sea Research, 21, 385-400. WALSH J. J., J. C. KELLY, T. E. WHITLEDGE, J. J. MACISAAC and S. A. HUNTSMAN (1974) Spin-up of the Baja California upwelling ecosystem. Limnology and Oceanography, 19, 553 572. WROBLEWSKTJ. S. (1977) A model of phytoplankton plume formation during variable Oregon upwelling. Journal of Marine Research, 35, 357-393.