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Lagrangian trajectories on the Oregon shelf during upwelling
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Continental Shelf Research 24 (2004) 1421–1436 www.elsevier.com/locate/csr
Lagrangian trajectories on the Oregon shelf during upwelling Eric A. D’Asaro Applied Physics Laboratory and School of Oceanography, University of Washington, 1013 NE 40th Street, Seattle, WA, 98105, USA Received 12 November 2003; accepted 1 June 2004
Abstract Neutrally buoyant, isopycnal-following floats were deployed on the Oregon continental shelf during the upwelling seasons of 2000 and 2001 and were carried southward by the mean current. The floats made CTD profiles and obtained GPS fixes twice daily, thus providing a hydrographic section along a known track. The floats followed the water accurately while at depth, but were displaced from the trajectories of the deep water during semidiurnal surfacings. These effects were large for water depths shallower than 100 m, but small on the rest of the shelf. Float trajectories, corrected for advection while on the surface, showed significant error when near the shore, but little net effect offshore. Some floats moved onshore and upward along the sloping isopycnals as expected during upwelling. Although the position of the isopycnal could be predicted accurately from the wind, the motion along the isopycnal showed significant fluctuations unrelated to the wind. These may be due to barotropic shelf waves. Some floats moved southward, roughly following the isobaths around Heceta Bank to Cape Blanco. Here they underwent large vertical and cross-shelf excursions and eventually moved offshore. Two floats passed through this region 25 days apart following different trajectories, indicating an unsteady flow. Overall, these data show the expected mix of a classical upwelling circulation in the north, an offshore jet with eddies in the south, and a strong influence of topography on both the mean flow and its fluctuations. r 2004 Published by Elsevier Ltd. PACS: 92.10.Sx Keywords: Coastal upwelling; Lagrangian; North America; California current; Oregon
1. Introduction Continental shelves act as a barrier and conduit between biological and chemical sources on the land and the open ocean. Typically, conditions on E-mail address: [email protected] (E.A. D’Asaro). 0278-4343/$ - see front matter r 2004 Published by Elsevier Ltd. doi:10.1016/j.csr.2004.06.003
the shelf are different than those in the nearby open ocean (e.g. Barber and Smith, 1981). This implies that the exchange of water between the shelf and the open ocean is slow enough that these differences can develop. Accordingly an understanding of the rates and processes of the exchange is crucial to understanding the shelf environment.
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This paper describes measurements of the crossshelf exchange on the Oregon shelf made using water-following, neutrally buoyant floats deployed on the shelf and tracked for up to 72 days. The data are too sparse to produce a robust statistical picture of water trajectories. However, approximate time scales for both cross-shelf and vertical motion, as well as some of the processes causing them, can be explored. Section 2 describes the instrumentation and the accuracy with which water parcels are tracked. Section 3 describes observations of nearshore upwelling, flow response to a widening of the shelf at Heceta Bank, and the trajectories associated with the offshore jet at Cape Blanco. Section 4 summarizes the results of the experiment.
2. Instrumentation 2.1. Lagrangian floats Lagrangian floats were deployed on the Oregon shelf during the upwelling seasons of 2000 and 2001. These floats (see inset pictures in Fig. 1) are designed and built at the Applied Physics Laboratory, University of Washington (APL-UW). They consist of a cylindrical hull 0.85 m long with sensors on each endcap and a buoyancy control piston extruding from the bottom endcap. An approximately 1-m2 folding cloth drogue provides drag. The drogue configuration changed between 2000 and 2001. Many details on float construction and performance can be found in D’Asaro (2003). For the measurements reported here, the floats operated in two distinct modes: Lagrangian drift and profiling. During a Lagrangian drift the floats attempt to remain on a target isopycnal. The compressibility of the float was designed to be close to that of seawater; its effective compressibility is made to match exactly by varying the float’s buoyancy in response to changes in pressure. A float with the same compressibility as seawater will remain on the same isopycnal as the isopycnal moves vertically and follow the horizonal motion of water on that isopycnal (Rossby et al., 1985). Conductivity/temperature sensors mounted on the bottom or on both ends of the
float are used to compensate for possible slow changes in the float’s density so as to maintain it on the reference isopycnal. The drag of the drogue causes the float to accurately follow water, not isopycnals, at high frequencies (D’Asaro, 2003). The net accuracy with which the floats followed the water is discussed in Section 2.3. Every 12 h the floats were programmed to profile vertically to the surface. Upon reaching the surface a GPS receiver obtained fixes accurate to roughly 10 m. A subset of the collected data was telemetered using the Orbcomm satellite system. Commands were relayed to the float using the same system. After roughly 20 min on the surface, the float profiled down to a specified depth 20–50 m below that of the reference isopycnal, and then adjusted itself to begin another Lagrangian drift. Lagrangian drifts typically lasted 10.5 h with the remaining 1.5 h of each cycle spent profiling and communicating. All floats carried a radio transmitter that could be tracked by the ARGOS satellite system that was activated whenever the float was on the surface. This acted as a backup navigation system and recovery beacon. At the end of the float’s mission, or if the software detected an error, a weight was dropped to bring the float to the surface. 2.2. Missions and sensors Float sensor and mission information is listed in Table 1. All floats measured pressure at the top endcap with an accuracy of about 20 cm. Some floats measured distance from the seabed using a Tritech PA200 acoustic altimeter. All floats measured temperature and salinity at the bottom endcap using a SeaBird Electronics conductivity/ temperature module with an integral pump designed specifically for floats. Some floats had a second such sensor on the top endcap. Comparison of pre- and post-cruise calibrations for floats 2, 4, and 10 indicated a shift in the salinity calibration of about 0.005 PSU. These sensors have sometimes undergone more dramatic calibration shifts due to ingestion of biota into the cell by the pump. The signature is first a noisy salinity signal with fresh spikes and second, a permanent shift in calibration of up to 0.4 PSU. Most of the
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Table 1 Float deployment information Float ID
Dates
Duration (days)
Sensors
Comments
2 4A 4B 3 8 9 10
5/28–6/10, 2000 5/25–6/7, 2000 6/15–7/8, 2000 7/20–8/15 2000 7/23–10/4 2001 7/30–8/8 2001 7/30–10/3 2001
17 27 24 27 73 8 65
P, P, P, P, P, P, P,
Isohaline, oscillations, vol. errors Isohaline, oscillations, vol. errors Isohaline, oscillations, vol. errors Isopycnal, vol. errors Isopycnal Isopycnal Isopycnal
TS, A, GPS, Argos TS, A, GPS, Argos TS, A, GPS, Argos TS, A, GPS TS, Argos, Chl, I TS(2), UVW, GPS, Argos TS(2), UVW, GPS, Argos
Sensors: P—Pressure; TS—Temperature and salinity on bottom or bottom and top; A—Altitude, i.e., distance from the bottom; GPS—GPS satellite fixes; Argos—Argos satellite beacon; Chl—chlorophyll fluorescence; I—490 nm downward irradiance; UVW— acoustic Doppler velocity.
Fig. 1. Float tracks from (a) 2000 and (b) 2001. Color indicates float number. An ‘S’ is placed near the start of each trajectory. In each panel tracks from other panel are shown in brown for reference. Time along each track is labelled by yearday. Bottom topography (black) shown by 50, 100, 150, and 200 m contours. ‘‘NDBC 46050’’ and ‘‘NDBC 46029’’ mark the locations of these two meteorological buoys. Photographs of Lagrangian floats are shown for each year.
floats used here underwent some activity of this first type. These data were removed based on visual inspection. There was no obvious evidence of associated calibration shifts.
Fig. 1 shows the track of each float; Fig. 2 shows the depth and density data. Floats were deployed by chartered fishing boat at approximately the 100-m depth contour on the northern part of the
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North Wind / m s-1
10 5 0 -5 -10 0
(b) b
(a)
2000
Float 2 3 4
2001
Float
8 9 10
10 20 30 40 50
Float Pressure / db
60 70
(d)
(c)
80
Potential Density/kg m-3
26.5 26 25.5 25 160
180
200
(e) 220
Yearday 2000
(f) 210 220 230 240 250 260 270
Yearday 2001
Fig. 2. Float data and wind for 2000 (a, c, e) and 2001 (b, d, f). (ab) North component of wind from NDBC buoy 46050. Positive numbers imply a downwelling favorable wind coming from the south. (cd) Pressure and (ef) potential density measured on each of the floats during the Lagrangian drifts. Potential density for float 8 is plotted both as measured and offset by 1 km m3 in panel f.
Oregon shelf (Fig. 1, deployment locations marked by ‘S’). The first two floats, 2 and 4, were programmed to follow an isohaline, rather than an isopycnal. This, plus a control system that assumed the wrong drag law (see D’Asaro (2003) for details), led to large oscillations in the floats’ depth. The mission of float 2 was terminated by a hardware error. Control problems in float 4 caused it to sink to the bottom starting on day 160 and slowly bounce down the shelf into deeper water until day 167. This segment of data is not shown in Figs. 1 and 2. A strong surface layer of freshwater from the Columbia River prevented floats 2 and 4 from surfacing fully during the first part of their missions and diminished the number and quality of GPS fixes and satellite data telemetry. Both, however, made excellent measurements of altitude off the bottom. Accordingly, the tracks for these
floats were adjusted by linearly interpolating the latitude between the fixes and choosing the longitude so that the water depth from the topography matched that inferred from the altimeter. This worked well because the topography of the shelf usually slopes monotonically down to the west. Communications for float 4 improved as it moved south. It was moved off the bottom on day 167. The control algorithm parameters were adjusted on day 174 to minimize the oscillations. The resulting reduction of the density variability is seen in Fig. 2e. Frequent GPS fixes resumed on day 176. Due to these problems, data from float 4 has been divided into two segments: 4A, which ends on day 160, and 4B, which resumes on day 167. As float 4 crossed the shelf break near Cape Blanco, concern about a possible, but nonexistent, leak caused us to terminate its mission by satellite command.
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Float 3 was deployed during the recovery cruise for float 4. Isopycnal tracking and a proper drag law were used. Its Orbcomm antenna was damaged on the second surfacing, probably by a boat, and it was presumed lost. The float continued operating well without telemetry, eventually upwelling onto a beach. It was found by a beachcomber, dragged up a cliff, and recovered by APL-UW. The control of floats 2, 3, and 4 suffered from uncertainty of about 5 g in buoyancy as indicated by the ‘‘vol. error’’ in Table 1. This was caused by errors in the tracking of the ballasting piston, which controls the float buoyancy. The piston position was known exactly once every 1.5 days when it was moved to a known reference position as it left the surface. Subsequent motions led to the accumulation of errors in the piston position. These errors are shown by the periodic ‘‘sawtooth’’ pattern of density for floats 3 and 4 (Fig. 2e). This problem was solved for floats 8, 9, and 10. The variation in density is accordingly much smaller for these floats (2f). Float 8 was a renamed float 3 with additional biological sensors. A new GPS antenna design did not work. According, the track is determined only from the ARGOS fixes and is much noisier than that of other floats. The target isopycnal for float 8 was adjusted twice during the mission (Fig. 2f) in order to maintain it within the euphotic zone. All the analysis presented here is from observations before day 240, when the first adjustment was made. Floats 9 and 10 were equipped with a Doppler sonar (is not used here) and two CTD’s. The float 9 mission was cut short by a software error. It was found by a beachcomber and carried to the local police station where APL-UW retrieved it. 2.3. How Lagrangian are the trajectories? 2.3.1. Surfacing and profiling effects Basic physics dictate that the floats will accurately follow the velocity of the water surrounding them in the direction parallel to the local neutral surface (D’Asaro et al., 1996) as long as they have the same density as the surrounding water. The large drogue further reinforces this tendency
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(D’Asaro, 2003). Scientifically, however, the most desirable quantity is a Lagrangian trajectory. The float’s trajectory differs from this for a variety of reasons discussed below. The floats spend approximately 12% of their time profiling on the surface. If the currents are different at these depths than at the reference isopycnal, the float’s overall trajectory will not reflect the motion of the water at the reference isopycnal. Floats 8, 9, and 10 made GPS measurements every 30 s while on the surface. ~S during From these, the average surface current U each surfacing was computed. The average velo~G , estimated from the city between surfacings U first GPS position from each surfacing minus the last from the previous surfacing, was taken to be the time-weighted sum of the current at the ~I and during the profiles reference isopycnal U ~P . Three different models of the current profile U ~ as a function of depth were used to compute UðPÞ, the float velocity at the pressure P of the float. The first is a linear interpolation between the surface and isopycnal velocity ~1 ðPÞ ¼ U ~ I þ ðU ~S U ~I Þ PI P ; U PI
ð1Þ
where PI is the pressure during the Lagrangian drifts interpolated to all other times. The second uses a linear fit, but only shallower than the Lagrangian drifts. ~2 ðPÞ ¼ U ~1 ðPÞ ðPoPI Þ; U ~I ðP4 ¼ PI Þ: ¼U
(2)
The third has an Ekman-layer-like profile. ~3 ðPÞ ¼ U ~S ðPo10 dbÞ U ~I ðP4 ¼ PI Þ: ¼U
(3)
~G . The ~I was initially set to U The value of U velocity profiles (1)–(3) were then used to recur~P sively obtain successively better estimates of U ~I . A few recursions lead to rapid converand U gence. The three models yielded similar results, so the results do not appear to be sensitive to the details of the model. For the long trajectory of float 10 (Fig. 1b), the corrections (1)–(3) to the trajectory were small as long as the float was on the shelf. During the first
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10 days the float was advected about 3 km offshore; by day 240 this switched to 1–2 km onshore. By the time the float left the shelf on day 249, the net displacement increased to 4–5 km offshore. After the float upwelled to the surface on day 258, the calculations became meaningless. In contrast, the corrections to the float 3 trajectory were significant and persistently offshore; float 3 was kept away from the coast by offshore advection when it was on the surface. This played an important part in the data interpretation (see Section 3.2.1). 2.3.2. Shear dispersion Deviations of the float trajectory from an isopycnal trajectory were also caused by the deviation of the float from the target isopycnal during the Lagrangian drift. If the float moves above the target isopycnal and there is a vertical (more properly ‘diapycnal’) shear of the horizontal (really ‘isopycnal’) velocity, then the float trajectory will be horizontally displaced from the water trajectory at the isopycnal. This effect can be estimated directly for float 10. This float had CTD’s on both endcaps with a separation of dZ ¼ 1:4 m. The deviation of the float’s center from the sref ¼ 25:95 kg m3 isopycnal was computed from the two CTD’s using ðsb þ st Þ=2 sref DZ ¼ dz; hsb st i
ð4Þ
where the subscripts t and b refer to the top and bottom CTD’s. Occasionally, the stratification sb st becomes very small or negative, resulting in unstable values of DZ. A small amount of time averaging, indicated by the hi in (4), stabilized the values. The results are insensitive to the amount of averaging as long as it is large enough. A value of 104 s was used. DZ has a standard deviation of 0.3 m; the isopycnal is usually between the two ~ the CTD’s. The sonar on float 10 measured dU, horizontal velocity of the water near the top CTD relative to the float’s center, about dZ=2 below. The velocity of the float relative to the velocity of the isopycnal is estimated as ~¼2 DU
~ dU Dz: dz
ð5Þ
The net horizontal displacement of float 10 relative to the sref ¼ 25:95 kg m3 isopycnal was 1 km offshore and 3 km southward from days 214 to 255. This corresponds to a mean velocity of about 1 mm s1 , a very small effect. It should be several times larger for floats 2 and 4 because they followed an isopycnal less accurately. 2.3.3. Isopycnal vs. Lagrangian At the time scales of interest here, these floats are not Lagrangian but isopycnal. If there is diapycnal mixing and the diapycnal density flux profile is curved, water will flow through isopycnals at a rate e ¼ ðDsz Þz =sz , where D is the scalar diffusivity, s is the potential density, and the subscript z denotes differentiation. Neither the magnitude nor sign of this effect can be estimated accurately. With a constant D ¼ 104 m2 s1 and an exponential vertical decay of sz with a scale of 30 m, e ¼ 0:29 m day1 or 14 m over 50 days. In this case, the isopycnal diffuses downward relative to the water as diffusion decreases the water density locally. Equivalently, the water flows upward past an isopycnal-following float.
3. Observations and results 3.1. Southward flow The floats were deployed within the expected position of the southward-going upwelling jet. Fig. 3 shows the magnitude of the southward float velocity. Most of the floats stayed within the jet. When the floats were over the central or outer shelf, they moved southward at 0.05–0:15 m s1 . These are comparable to average mid-depth velocities reported by Huyer (1983) and Federuik and Allen (1995) from mooring observations. Although the data are limited, some patterns are evident within this overall southward flow. The southward velocity decreased for float 3 when it moved inshore after day 212. This is consistent with the offshore position of the upwelling jet shown, for example, in Huyer (1974) and by the radar surface current maps in Oke et al. (2002b). The southward velocity also decreased when floats 8 and 10 moved off the shelf near Cape Blanco.
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North Wind / m s-1
5 0 -5
5 day running mean
(a)
-10
2000
Float
2 3 4
2001
North Velocity / m s-1
Float
(b)
8 9 10 offshore offshore
0.1
0 re
ho
ins
-0.1
-0.2
5 day running mean 160
180
(c) 200
220
Yearday 2000
(d) 210 220 230 240 250 260 270
Yearday 2001
Fig. 3. North wind component for (a) 2000 and (b) 2001 as in Fig. 2 but averaged with 5-day running mean. Northward velocity for floats from (c) 2000 and (d) 2001 averaged with 5-day running mean. Curves are dashed when float is outside of main southward jet.
These periods are dashed in Fig. 3. Excluding these, the southward speed increased with time over each season and increased for each float as it moved south. Periods of northward flow in both seasons occurred early in the records and when the floats were on the northern part of the shelf. The southward increase in speed is consistent with the rapid southward strengthening of the wind stress and its curl (Huyer, 1983). The increase with time is partially an artifact of the southward motion of the floats and the southward increase in speed, but also consistent with a maximum temporal increase in wind stress and transport in July as found, for example, by Kosro (2003). 3.2. Nearshore upwelling 3.2.1. Overview The ability of the floats to follow directly the vertical motion of isopycnals offers a potentially unique view into upwelling. The vertical motion consists of both energetic high frequency motions, the subject of a later paper, and low frequency motions, which are considered here. Floats 2, 3, and both segments of 4 (Fig. 2c) moved upward at
rates of roughly 3 m day1 ð35 mm s1 ). In contrast, floats 8 and 10 moved downward at roughly 10 mm s1 while on the shelf (Fig. 2d). Trajectories are considered in more detail below. Floats 2 and 4 were deployed at the 100-m isobath, near the head of Astoria Canyon. In retrospect, this was a bad choice both because the flow in this region is influenced by the canyon and the nearfield of the Columbia River outflow, and because the float could not surface through the plume and thus could not navigate or communicate. These floats also had the control problems described above. Thus although they both upwelled and moved toward the coast, these various problems make a more quantitative analysis problematic. The float 3 trajectory is detailed in Fig. 4. The float was deployed at mid-shelf (90-m isobath) and moved southward and onshore during a period of downwelling favorable winds, reaching the 25-m isobath approximately 2 km offshore of Cape Foulweather in 10 days. It then moved offshore and upward (days 212–217) and then onshore again, reaching a depth of only 2 m on day 219. Its southward motion then slowed and the float
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Fig. 4. Float 3 data. (a) Northward wind component from NDBC buoy 46050. (b) Time-depth section along the track of float 3. Individual twice-daily CTD profiles are averaged into 2-m vertical bins and the resulting array averaged with a 5 5 box filter. This smoothed field is colored and contoured with an interval of 0:25 kg m3 . Float depth is smoothed with a 20 000-s running mean filter and plotted as the heavy blue line. (c) Float trajectory (red) plotted over 10-m interval isobaths (black and gray) and annotated with date (blue circles). Raw GPS fixes are shown by red ‘x’ symbols. Blue and green lines show the float trajectory corrected using the average of (1)–(3) in (5). These trajectories start on days 203, 208, 213, 218, and 223. A star is plotted at the start of each day on each trajectory. Trajectory is terminated after it crosses the shore. The duration of each trajectory in days is plotted at its end.
underwent another offshore excursion before it finally hit the bottom (day 228) and upwelled to the beach within the bottom boundary layer. The float’s motion does not accurately reflect the water motion because the float was advected by surface currents during its twice-daily profiles (see Section 2.3.1). Trajectories corrected for the surface currents are shown by the blue and green lines in Fig. 4c. The three correction models (1)–(3) yield similar results, so their mean is used. The
corrected tracks are persistently more onshore than the true track. The average of onshore velocities along tracks beginning each day at the float position and ending at the coast is 0:03 m s1 , with a range of 0.014–0:09 m s1 and a standard deviation of 0:017 m s1 . The speeds were greatest when the float was very close to the shore. Excluding trajectories that start less than 7 km from shore, the mean speed was 0:027 m s1 and the range was 0.013–0:052 m s1 . For these
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motion of the isopycnal from that of the float. The isopycnal depth HðxÞ is assumed to be a function of distance from the coast x as
trajectories the average time to reach the coast was 6.0 days with a standard deviation of 2.6 days and a range of 1.7 to 11.5 days. Fig. 5 shows the low-passed float trajectory as a function of depth and distance from the coast, and the average isopycnals from days 213 to 229. The isopycnals are clearly tilted upward toward the shore, as expected. For the first part of the data (days 202–212), the float and its isopycnal (25:8 kg m3 ) are generally deeper than during the second part (days 212–229). During both parts, however, the float slides up and down the tilted isopycnal and it moves on and offshore. The trajectory is therefore a complex combination of the vertical motion of the isopycnal and horizontal motion of the float along the isopycnal.
HðxÞ ¼ H 1 þ ðH 0 H 1 Þex=R ;
ð6Þ
where H 1 is the depth far offshore and H 0 is the depth at the coast. Austin and Barth (2002) fit (6) to CTD sections and moorings off Newport, Oregon, (44:65 N) during the 1999 upwelling season. The value of H 1 was fixed at 125 m. The value of H 0 was found to be well predicted by H 0 ¼ 23 þ 0:85 T 8 ;
ð7Þ
where Z
t
T 8 ðtÞ ¼ 0
3.2.2. Vertical motion of the isopycnal The model of Austin and Barth (2002) is used to predict the vertical motion of the isopycnal as a function of wind stress and thus separate the
tn ðt0 tÞ=dt 0 e dt r0
ð8Þ
is an average alongshore wind stress tn . The average is taken over previous times only with an exponential weighting of dt ¼ 8 days. r0 is the water density. This fit the depth of the 25:8 kg m3
0 5
25.5
219 228 227
Isopycnal Depth / m
10
213
221
226
15
216
20 202
25
222
205 207
210
30
.0
Drift Trajectory
35
26
yearday
40
0
2
4
6
8
10
12
14
16
18
Offshore Distance / km Fig. 5. Cross-shelf view of the trajectory of float 3. CTD data from float profiles on days 213–229 (light grey dots) are averaged in bins of offshore distance and depth. The resulting isopycnal contours are plotted as solid and dashed lines. The depth of the 25:8 kg m3 isopycnal during the Lagrangian drifts is computed from the potential density at the float and the vertical density gradient computed on nearby profiles. This depth is usually within 5 m of the float depth. This depth is low passed (cutoff frequency is 1 cycle per day) and plotted as the heavy shaded line. White circles indicate the time along this trajectory.
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isopycnal in the 1999 repeat CTD section data with an rms deviation of about 5 m. Fig. 6 shows the application of this model to the float 3 data. The isopycnal depth was computed from the float depth and density during the Lagrangian drifts, and the vertical density gradient from the profiles and low-pass filtered to remove semi-diurnal tidal variations. The resulting depth varies from 25 m at the start of the mission to the surface at its end (Figs. 5 and 6). Wind data from NDBC buoys 46 050 and 46 050 (see Fig. 1) were used to compute tn , the northward component of wind stress using Fairall et al. (1996). Unless stated otherwise data from buoy 46 050 is used. A relative humidity of 90% was assumed. When no surface temperature was available, a neutral boundary layer was assumed. These assumptions had little effect. The value of R was estimated by minimizing the difference between H predicted by the model and measured by the float. The values of R and H 1 could not be determined independently. The
best-fit value R ¼ 82 km was found assuming the Austin and Barth (2002) value of H 1 ¼ 125 m. The predicted isopycnal height at the coast H 0 varied from 20 m depth to 10 m height (Fig. 6, grey lines). The predicted depth at the float (dashed lines) differs from this as the float moved onshore and offshore. Understandably, because the model was calibrated from data near Newport, Oregon, it works best when the float is in this vicinity. The most persistent deviations occur at the start of the data, days 202–212. Although the float is closer to buoy 46 029 than 46 050 during this time, the effect of using 46 029 instead of 46 050 is small (grey solid lines) so this cannot explain the error. The model fits better during this time using a somewhat smaller value of R ¼ 50 km (light dashed line). However, the stratification is stronger here than further south so the radius of deformation should be larger rather than smaller. The shelf is narrower, which could be important. Despite this, the Austin and Barth (2002) model predicts the
20
Isopycnal Depth /m
10
Isopycnal Depth at Coast Model 46050 wind 46029 wind
Isopycnal Depth at Float Data Model 46050 wind, R=82 km R=50 km
0
-10
-20
-30
-40 200
205
210
215
220
225
230
Yearday of 2000 Fig. 6. Depth of the 25:8 kg m3 isopycnal at float 3 (heavy line) during its Lagrangian drifts compared with the prediction of the Austin and Barth (2002) model. Predicted isopycnal depth at the coast H 0 (light grey lines) is shown for two different weather buoys. Predicted depth at the float is shown using one buoy and two different values of R, the offshore decay length (dashed lines).
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vertical motion of the isopycnal well; the rms deviation of the model from the data is 6 m, nearly the same as found by Austin and Barth (2002).
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reflects the otherwise unsubstantiated choice of D/2 for the return flow thickness; different choices of this depth would lead to different values for X e . More importantly, the fluctuations in offshore distance from days 210 to 229 are clearly not predicted by the wind-forced Ekman model (9) nor are they correlated with the wind-forced isopycnal displacements predicted by Austin and Barth (2002). Instead, their structure (Fig. 4c) marks them as nearly barotropic shelf waves (Gill, 1982, Chapter 10.12), with a cyclonic curvature offshore and an anti-cyclonic curvature nearshore consistent with the conservation of barotropic potential vorticity. Thus, the net onshore motion of water on the 25:8 kg m3 isopycnal reflects the overall upwelling circulation driven by the net upwelling favorable wind. The fluctuations in cross-shelf motion reflect the influence of finite amplitude shelf waves of unknown origin. The fluctuations in isopycnal depth along the trajectory reflect both the temporal variations in the isopycnal depth caused by the varying along-shelf wind stress and the cross-shelf
3.2.3. Motion along the isopycnal The motion of water along the isopycnal is distinct from the vertical motion of the isopycnal. In a simple, two-dimensional, steady-state upwelling model, the offshore Ekman transport ts =f r0 is balanced by a deep onshore return flow. The float’s isopycnal was chosen to be within this onshore flow based on historical data (Huyer, 1983); the trajectories in Fig. 4 clearly show that it is. The onshore velocity is equal to the transport divided by a depth. A value of half the water depth D is used. The net eastward displacement due to this is Z t 2ts Xe ¼ dt: ð9Þ r 0 0 fD Fig. 7 compares X e to the eastward displacement of float 3, corrected as in Fig. 4c. Although X e predicts the net total displacement well, this only 0
Offshore distance / km
-5
-10
-15
-20
Float Ekman model
-25
-30
205
210
215
220
225
Yearday 2000 Fig. 7. Eastward (cross-shelf) displacement of float 3, i.e., the corrected eastward displacement of the float from its initial position (solid) compared to the prediction of an Ekman transport model (dashed).
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motion of the water parcels along the cross-shelf sloping isopycnals. 3.3. Heceta Bank and Cape Blanco
220
Cape
Blanc
o
Most of the float trajectories (Fig. 1) follow the topographic contours around Heceta Bank. This offshore path is consistent with the high resolution modelling and the radar surface current maps described in Oke et al. (2002a, b). Two floats, 8 and 10, reached Cape Blanco and moved offshore (Figs. 8–10). The motion of float 10 corresponds closely to the cartoon of the flow in this region of Barth et al. (2000). After rounding Heceta Bank the float traveled southward along the coast within the baroclinic upwelling jet. Just upstream of Cape Blanco it moved offshore and descended. This is evident in the 3-D trajectory (Fig. 8, days 248–252 ). A sea surface temperature image taken at this time (Fig. 9) shows that cold water streamed offshore with the float. This cold tongue warmed offshore; the float trajectory suggests that some of this surface warming may have been due to the subduction of the cold water
beneath warmer water. Solar heating is the other obvious possibility. The float did not follow the path of the cold water, but looped cyclonically, so that by day 257 it was being carried north. On day 257 it executed a second 180 turn, equivalent to a vorticity of close to f , and rose to the surface. On day 258 it sank again while executing a slight cyclonic turn, and then, on days 259 and 260, rose to the surface during an anticyclonic turn. The float then accelerated offshore on the surface in a large pool of cold upwelled water, eventually circulating in an offshore eddy. Float 8 traversed this same region about 25 days earlier. Note that its GPS did not work so its track is of poorer quality. Between Heceta Bank and Cape Blanco floats 8 and 10 travelled on similar paths and at a similar speeds. At Cape Blanco, however, float 8 did not loop offshore, but tracked steadily past the cape. Here, where float 10 surfaced, sank, and then moved offshore on the surface, float 8 surfaced, sank, and moved offshore at depth (see Figs. 2d and 10a) and meandered in an offshore eddy. Although SST data during this time were mostly obscured by clouds, they suggest
246
124
258
248
260
50m
232 236
Float 8 0 20 40 60 80 44.5
, la (longitude titude, surface) , latitude, Float 10 depth)
43.5
262
250
(longitude , latitude, surface) (longitude , latitude, depth)
44
124.5
100m
(longitude
252
125
43
42.5
42
125.5
Fig. 8. Three-dimensional view of the trajectories of floats 8 and 10 near Cape Blanco. The trajectory of each float is plotted both at the surface (red and black lines, respectively) and at the float depth (gray and magenta lines, respectively). Light vertical lines connecting these are plotted five times per day. Selected times along the trajectory are annotated. The coastline and depth contours to 100 m are drawn in the background.
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Sea Surface Temperature - day 251 2130Z 18
12
14
16
10
246 32m 260
258 0m
248 28m
18m
0m
32m 256 254
125 W
250
50m
252
Float 10
N
42.5N
43N
Fig. 9. Trajectory of float 10 near Cape Blanco superimposed on bottom topography (black lines to 200 m) and sea surface temperature image from day 251. Yearday of 2000 and selected float depths are annotated along trajectory.
0 Float 8
Float 10
40 min 60 (a) 220
225
230
235
0 10
Depth / m
20
240
245
250
255
260
Yearday 2001 270 260
25.55 isopycnal depth Float depth
Yearday 2001
Depth / m
max 20
30 40
250
240
50
230
60 (b) 90
80
70
60
50
40
30
20
10
0
220
Distance from Shore / km Fig. 10. (a) Depth-time trajectories of floats 8 and 10 near Cape Blanco. Depths are averaged over 12 h to remove tidal fluctuations. Heavy line and circles (plotted every 6 h) show the depth of the 25:5 kg m3 isopycnal at the location of each float. Small stars indicate the depth of the floats. The line color indicates the time as indicated by the colorbar. Thin lines, colored by time, indicate minimum and maximum isopycnal depths during each interval. (b) Same depths plotted against distance from shore.
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that the rapid vertical motion of float 8 occurred near a tongue of cold water similar to that in Fig. 9. Fig. 10b shows the position of both floats’ isopycnal as a function of distance from the coast. The isopycnal sloped upward toward the coast as expected. Remarkably, the slopes were nearly the same for floats 8 and 10 despite their very different trajectories. This is particularly true if the times of rapid upwelling (days 233–235 for float 8, after day 257 for float 10) are removed. Notice also that the offshore/downward and onshore/upward legs of the float 10 offshore loop (days 248–257) follow almost exactly the same path. Barth et al. (2000) hypothesize that the flow in this region is strongly controlled by topographic stretching of the water column. The data here clearly support this hypothesis. The track of float 10 shows a clear relationship between offshore displacement and the curvature of the track; offshore segments curve cyclonically and inshore segments curve anticyclonically. The vertical density gradient measured at float 10 is also weakest when it is furthest offshore. The isopycnal slope seen in Fig. 10b is most easily explained by the stretching of the water column as it moves offshore; the isopycnal depth is the same on the offshore-going and the onshore-going parts of the trajectory because the stretching is reversed as the water column moves back onshore. The process is the same for floats 8 and 10 so the slopes are similar. This implies that these cross-slope motions are primarily barotropic, and thus influenced by the bottom. More peculiar is that this cross-slope exchange sets up a cross-slope baroclinic structure and thus an along-slope baroclinic flow. This dynamic merits further investigation.
4. Summary and discussion Isopycnal-following, neutrally buoyant floats were deployed on the Oregon continental shelf at mid-shelf and mid-depth in the summers of 2000 and 2001. All measured temperature, salinity, and pressure; some measured velocity shear and biological variables. The floats alternated between twice-daily vertical profiles from 40–90 m to the
surface and Lagrangian drifts within a few meters of an isopycnal in the range of 25.5–25:8 kg m3 . The float tracks were determined from GPS and ARGOS fixes made when the floats were on the surface. Data and control commands were relayed between the float and shore using the ORBCOMM satellite telemetry system. Advection of the floats during profiling and when they were on the surface caused their tracks to differ from the Lagrangian trajectories of water on their target isopycnal. This could be evaluated for floats that made rapid GPS measurements on the surface. A nearshore float showed a large offshore bias, while a mid-shelf float showed little bias. Other biases of the float motion from a true water-following trajectory appeared small. All floats moved southward with the prevailing summertime flow and with a velocity increasing further south. Some moved inshore following the expected upwelling circulation. One float (float 3) upwelled to the beach. Its track was strongly biased offshore by the offshore surface flow. After correction for this effect, the mean onshore velocity at the 25:8 kg m3 isopycnal was 2:7 cm s1 , corresponding to an average time from mid-shelf to the beach of 6 days. Cross-shelf motion of the float allowed the structure of the isopycnal surface to be investigated. The 25:8 kg m3 isopycnal sloped upward toward the coast, as expected. The extrapolated height at the coast could be accurately modelled as a function of 8-day-averaged wind stress following Austin and Barth (2002). The cross-shelf motion along the isopycnal, however, showed energetic fluctuations unrelated to the wind and suggestive of barotropic shelf waves. Other floats deployed at mid-shelf did not move shoreward but instead followed the isobaths southward around Heceta Bank to Cape Blanco. Two floats (floats 8 and 10) proceeded further. Near Cape Blanco both underwent rapid (up to 50 m day1 ) upward and downward displacements of about 30 m from a starting depth near 30 m. Both eventually moved offshore, float 10 on the surface and float 8 at depth. The float 10 trajectory is remarkably similar to that hypothesized by Barth et al. (2000); that of float 8 shows much less horizontal excursion.
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Overall, the dominant influences on the float trajectories are wind and topography. The net southward flow and inshore upwelling are the expected results of the prevailing north wind. Equally important, however, is the strong constraint of topography; the southward flow follows the isobaths. Significant cross isobath motion is strongly associated with track curvature, indicating the role of topographic stretching and potential vorticity conservation in controlling the flow direction. These observations probably represent the first use of long-term profiling floats in a coastal environment. These observations clearly showed that such floats can operate and provide useful information without too many losses due to grounding. However, the time spent profiling clearly degraded the accuracy of the Lagrangian trajectories. This loss was offset by the gain in the ability to track isopycnals and their spatial structure and, of course, track and control the floats in nearly real time. In the strongly upwelling nearshore environment the time spent on the surface had the fortunate effect of keeping the float from moving onshore in the upwelling flow and ending its mission on the beach or rocks. With improved satellite telemetry now available the time spent on the surface could be greatly reduced, thus making the floats more Lagrangian but, unfortunately, more prone to reach the shore. This could be mitigated by restricting the floats to short missions with a rapid ability to rescue the floats, or by using only inexpensive, expendable floats. More profoundly, truly Lagrangian floats will go where the water goes. Thus, unless most water parcels travel through the surf zone, itself a fascinating hypothesis, accurately Lagrangian floats would also not commonly beach. The technical challenge for the safe use of floats in the nearshore environment may thus lie in the improvement of their water-following ability. Although the number of floats and the quality of their trajectories is limited, these data show several distinct pathways from mid-shelf. One, followed by floats 2, 3, and 4A, is the classical baroclinic upwelling circulation: mid-shelf water moves onshore and upward along the sloping isopycnals and then southward in the nearshore
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upwelling jet. The other, followed by floats 4B, 8, and 10, is an alongshore flow, closely following the isobaths around Heceta Bank and eventually moving offshore at Cape Blanco and into the eddy field of the California Current. Our evidence is limited, but it appears that mid-shelf water from the northern Oregon shelf can take either of these paths. Although the average summertime velocity is strongly onshore at mid-depth, apparently not all particles move rapidly inshore. However, all of the floats reached to within a few meters of the surface at least once, suggesting that the probability that water will upwell at some point is high. More fundamentally, it is not clear what controls the path that water will take. One might expect that for a given wind forcing, particles that are sufficiently far offshore would follow the offshore track and those sufficiently inshore would follow the nearshore upwelling path. The location of the boundary between these two regimes would likely be a strong function of the wind forcing. A better understanding of these Lagrangian properties of the circulation would seem a worthy goal with clear implications for characterization of the physical environment experienced by organisms.
References Austin, J., Barth, J.A., 2002. Variation in the position of the upwelling front on the Oregon shelf. Journal of Geophysical Research 107 doi:10.1029/2001JC000585. Barber, R., Smith, R.L., 1981. Coastal upwelling ecosystems. In: Lonhurst, A.R. (Ed.), Analysis of Marine Ecosystems. Academic Press, New York, pp. 31–68. Barth, J., Pierce, S., Smith, R., 2000. A separating coastal upwelling jet at Cape Blanco, Oregon and its connection to the California current system. Deep-Sea Research II 47, 783–810. D’Asaro, E.A., 2003. Performance of autonomous Lagrangian floats. Journal of Atmospheric and Oceanic Technology 20 (6), 896–911. D’Asaro, E.A., Farmer, D.M., Osse, J.T., Dairiki, G.T., 1996. A Lagrangian float. Journal of Atmospheric and Oceanic Technology 13 (6), 1230–1246. Fairall, C., Bradley, E., Rogers, D., Edson, J., Young, G., 1996. Bulk parameterization of air-sea fluxes for tropical ocean global atmosphere coupled ocean atmosphere response experiment. Journal of Geophysical Research 101 (C2), 3747–3764.
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Federuik, J., Allen, J., 1995. Upwelling circulation on the Oregon continental shelf II: Comparison with observations. Journal of Physical Oceanography 25, 1867–1889. Gill, A.E., 1982. Atmosphere-Ocean Dynamics. Academic Press, New York, 662pp. Huyer, A., 1974. Observations of the coastal upwelling region off Oregon during 1972. Ph.D. Thesis, Oregon State University, Corvallis, Oregon. Huyer, A., 1983. Coastal upwelling in the California current system. Progress in Oceanography 12, 259–284. Kosro, P., 2003. Enhanced southward flow over the Oregon shelf in 2002: a conduit for subarctic water. Geophysical Research Letters 30 (15) doi:10.1029/2003GL017436.
Oke, P., Allen, J., Miller, R., Egbert, G., 2002a. A modelling study of the three-dimensional continental shelf circulation off Oregon, part II: dynamical analysis. Journal of Physical Oceanography 32 (5), 1383–1403. Oke, P., Allen, J., Miller, R., Egbert, G., Austin, J., Barth, J., Boyd, T.J., Kosro, P., Levine, J., 2002b. A modelling study of the three-dimensional continental shelf circulation off Oregon, part I: model-data comparisons. Journal of Physical Oceanography 32 (5), 1360–1382. Rossby, T., Levine, E.R., Connors, D.N., 1985. The isopycnal Swallow float—a simple device for tracking water parcels in the ocean. Progress in Oceanography 14 (1–4), 511–525.
Continental Shelf Research 24 (2004) 1421–1436 www.elsevier.com/locate/csr
Lagrangian trajectories on the Oregon shelf during upwelling Eric A. D’Asaro Applied Physics Laboratory and School of Oceanography, University of Washington, 1013 NE 40th Street, Seattle, WA, 98105, USA Received 12 November 2003; accepted 1 June 2004
Abstract Neutrally buoyant, isopycnal-following floats were deployed on the Oregon continental shelf during the upwelling seasons of 2000 and 2001 and were carried southward by the mean current. The floats made CTD profiles and obtained GPS fixes twice daily, thus providing a hydrographic section along a known track. The floats followed the water accurately while at depth, but were displaced from the trajectories of the deep water during semidiurnal surfacings. These effects were large for water depths shallower than 100 m, but small on the rest of the shelf. Float trajectories, corrected for advection while on the surface, showed significant error when near the shore, but little net effect offshore. Some floats moved onshore and upward along the sloping isopycnals as expected during upwelling. Although the position of the isopycnal could be predicted accurately from the wind, the motion along the isopycnal showed significant fluctuations unrelated to the wind. These may be due to barotropic shelf waves. Some floats moved southward, roughly following the isobaths around Heceta Bank to Cape Blanco. Here they underwent large vertical and cross-shelf excursions and eventually moved offshore. Two floats passed through this region 25 days apart following different trajectories, indicating an unsteady flow. Overall, these data show the expected mix of a classical upwelling circulation in the north, an offshore jet with eddies in the south, and a strong influence of topography on both the mean flow and its fluctuations. r 2004 Published by Elsevier Ltd. PACS: 92.10.Sx Keywords: Coastal upwelling; Lagrangian; North America; California current; Oregon
1. Introduction Continental shelves act as a barrier and conduit between biological and chemical sources on the land and the open ocean. Typically, conditions on E-mail address: [email protected] (E.A. D’Asaro). 0278-4343/$ - see front matter r 2004 Published by Elsevier Ltd. doi:10.1016/j.csr.2004.06.003
the shelf are different than those in the nearby open ocean (e.g. Barber and Smith, 1981). This implies that the exchange of water between the shelf and the open ocean is slow enough that these differences can develop. Accordingly an understanding of the rates and processes of the exchange is crucial to understanding the shelf environment.
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This paper describes measurements of the crossshelf exchange on the Oregon shelf made using water-following, neutrally buoyant floats deployed on the shelf and tracked for up to 72 days. The data are too sparse to produce a robust statistical picture of water trajectories. However, approximate time scales for both cross-shelf and vertical motion, as well as some of the processes causing them, can be explored. Section 2 describes the instrumentation and the accuracy with which water parcels are tracked. Section 3 describes observations of nearshore upwelling, flow response to a widening of the shelf at Heceta Bank, and the trajectories associated with the offshore jet at Cape Blanco. Section 4 summarizes the results of the experiment.
2. Instrumentation 2.1. Lagrangian floats Lagrangian floats were deployed on the Oregon shelf during the upwelling seasons of 2000 and 2001. These floats (see inset pictures in Fig. 1) are designed and built at the Applied Physics Laboratory, University of Washington (APL-UW). They consist of a cylindrical hull 0.85 m long with sensors on each endcap and a buoyancy control piston extruding from the bottom endcap. An approximately 1-m2 folding cloth drogue provides drag. The drogue configuration changed between 2000 and 2001. Many details on float construction and performance can be found in D’Asaro (2003). For the measurements reported here, the floats operated in two distinct modes: Lagrangian drift and profiling. During a Lagrangian drift the floats attempt to remain on a target isopycnal. The compressibility of the float was designed to be close to that of seawater; its effective compressibility is made to match exactly by varying the float’s buoyancy in response to changes in pressure. A float with the same compressibility as seawater will remain on the same isopycnal as the isopycnal moves vertically and follow the horizonal motion of water on that isopycnal (Rossby et al., 1985). Conductivity/temperature sensors mounted on the bottom or on both ends of the
float are used to compensate for possible slow changes in the float’s density so as to maintain it on the reference isopycnal. The drag of the drogue causes the float to accurately follow water, not isopycnals, at high frequencies (D’Asaro, 2003). The net accuracy with which the floats followed the water is discussed in Section 2.3. Every 12 h the floats were programmed to profile vertically to the surface. Upon reaching the surface a GPS receiver obtained fixes accurate to roughly 10 m. A subset of the collected data was telemetered using the Orbcomm satellite system. Commands were relayed to the float using the same system. After roughly 20 min on the surface, the float profiled down to a specified depth 20–50 m below that of the reference isopycnal, and then adjusted itself to begin another Lagrangian drift. Lagrangian drifts typically lasted 10.5 h with the remaining 1.5 h of each cycle spent profiling and communicating. All floats carried a radio transmitter that could be tracked by the ARGOS satellite system that was activated whenever the float was on the surface. This acted as a backup navigation system and recovery beacon. At the end of the float’s mission, or if the software detected an error, a weight was dropped to bring the float to the surface. 2.2. Missions and sensors Float sensor and mission information is listed in Table 1. All floats measured pressure at the top endcap with an accuracy of about 20 cm. Some floats measured distance from the seabed using a Tritech PA200 acoustic altimeter. All floats measured temperature and salinity at the bottom endcap using a SeaBird Electronics conductivity/ temperature module with an integral pump designed specifically for floats. Some floats had a second such sensor on the top endcap. Comparison of pre- and post-cruise calibrations for floats 2, 4, and 10 indicated a shift in the salinity calibration of about 0.005 PSU. These sensors have sometimes undergone more dramatic calibration shifts due to ingestion of biota into the cell by the pump. The signature is first a noisy salinity signal with fresh spikes and second, a permanent shift in calibration of up to 0.4 PSU. Most of the
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Table 1 Float deployment information Float ID
Dates
Duration (days)
Sensors
Comments
2 4A 4B 3 8 9 10
5/28–6/10, 2000 5/25–6/7, 2000 6/15–7/8, 2000 7/20–8/15 2000 7/23–10/4 2001 7/30–8/8 2001 7/30–10/3 2001
17 27 24 27 73 8 65
P, P, P, P, P, P, P,
Isohaline, oscillations, vol. errors Isohaline, oscillations, vol. errors Isohaline, oscillations, vol. errors Isopycnal, vol. errors Isopycnal Isopycnal Isopycnal
TS, A, GPS, Argos TS, A, GPS, Argos TS, A, GPS, Argos TS, A, GPS TS, Argos, Chl, I TS(2), UVW, GPS, Argos TS(2), UVW, GPS, Argos
Sensors: P—Pressure; TS—Temperature and salinity on bottom or bottom and top; A—Altitude, i.e., distance from the bottom; GPS—GPS satellite fixes; Argos—Argos satellite beacon; Chl—chlorophyll fluorescence; I—490 nm downward irradiance; UVW— acoustic Doppler velocity.
Fig. 1. Float tracks from (a) 2000 and (b) 2001. Color indicates float number. An ‘S’ is placed near the start of each trajectory. In each panel tracks from other panel are shown in brown for reference. Time along each track is labelled by yearday. Bottom topography (black) shown by 50, 100, 150, and 200 m contours. ‘‘NDBC 46050’’ and ‘‘NDBC 46029’’ mark the locations of these two meteorological buoys. Photographs of Lagrangian floats are shown for each year.
floats used here underwent some activity of this first type. These data were removed based on visual inspection. There was no obvious evidence of associated calibration shifts.
Fig. 1 shows the track of each float; Fig. 2 shows the depth and density data. Floats were deployed by chartered fishing boat at approximately the 100-m depth contour on the northern part of the
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North Wind / m s-1
10 5 0 -5 -10 0
(b) b
(a)
2000
Float 2 3 4
2001
Float
8 9 10
10 20 30 40 50
Float Pressure / db
60 70
(d)
(c)
80
Potential Density/kg m-3
26.5 26 25.5 25 160
180
200
(e) 220
Yearday 2000
(f) 210 220 230 240 250 260 270
Yearday 2001
Fig. 2. Float data and wind for 2000 (a, c, e) and 2001 (b, d, f). (ab) North component of wind from NDBC buoy 46050. Positive numbers imply a downwelling favorable wind coming from the south. (cd) Pressure and (ef) potential density measured on each of the floats during the Lagrangian drifts. Potential density for float 8 is plotted both as measured and offset by 1 km m3 in panel f.
Oregon shelf (Fig. 1, deployment locations marked by ‘S’). The first two floats, 2 and 4, were programmed to follow an isohaline, rather than an isopycnal. This, plus a control system that assumed the wrong drag law (see D’Asaro (2003) for details), led to large oscillations in the floats’ depth. The mission of float 2 was terminated by a hardware error. Control problems in float 4 caused it to sink to the bottom starting on day 160 and slowly bounce down the shelf into deeper water until day 167. This segment of data is not shown in Figs. 1 and 2. A strong surface layer of freshwater from the Columbia River prevented floats 2 and 4 from surfacing fully during the first part of their missions and diminished the number and quality of GPS fixes and satellite data telemetry. Both, however, made excellent measurements of altitude off the bottom. Accordingly, the tracks for these
floats were adjusted by linearly interpolating the latitude between the fixes and choosing the longitude so that the water depth from the topography matched that inferred from the altimeter. This worked well because the topography of the shelf usually slopes monotonically down to the west. Communications for float 4 improved as it moved south. It was moved off the bottom on day 167. The control algorithm parameters were adjusted on day 174 to minimize the oscillations. The resulting reduction of the density variability is seen in Fig. 2e. Frequent GPS fixes resumed on day 176. Due to these problems, data from float 4 has been divided into two segments: 4A, which ends on day 160, and 4B, which resumes on day 167. As float 4 crossed the shelf break near Cape Blanco, concern about a possible, but nonexistent, leak caused us to terminate its mission by satellite command.
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Float 3 was deployed during the recovery cruise for float 4. Isopycnal tracking and a proper drag law were used. Its Orbcomm antenna was damaged on the second surfacing, probably by a boat, and it was presumed lost. The float continued operating well without telemetry, eventually upwelling onto a beach. It was found by a beachcomber, dragged up a cliff, and recovered by APL-UW. The control of floats 2, 3, and 4 suffered from uncertainty of about 5 g in buoyancy as indicated by the ‘‘vol. error’’ in Table 1. This was caused by errors in the tracking of the ballasting piston, which controls the float buoyancy. The piston position was known exactly once every 1.5 days when it was moved to a known reference position as it left the surface. Subsequent motions led to the accumulation of errors in the piston position. These errors are shown by the periodic ‘‘sawtooth’’ pattern of density for floats 3 and 4 (Fig. 2e). This problem was solved for floats 8, 9, and 10. The variation in density is accordingly much smaller for these floats (2f). Float 8 was a renamed float 3 with additional biological sensors. A new GPS antenna design did not work. According, the track is determined only from the ARGOS fixes and is much noisier than that of other floats. The target isopycnal for float 8 was adjusted twice during the mission (Fig. 2f) in order to maintain it within the euphotic zone. All the analysis presented here is from observations before day 240, when the first adjustment was made. Floats 9 and 10 were equipped with a Doppler sonar (is not used here) and two CTD’s. The float 9 mission was cut short by a software error. It was found by a beachcomber and carried to the local police station where APL-UW retrieved it. 2.3. How Lagrangian are the trajectories? 2.3.1. Surfacing and profiling effects Basic physics dictate that the floats will accurately follow the velocity of the water surrounding them in the direction parallel to the local neutral surface (D’Asaro et al., 1996) as long as they have the same density as the surrounding water. The large drogue further reinforces this tendency
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(D’Asaro, 2003). Scientifically, however, the most desirable quantity is a Lagrangian trajectory. The float’s trajectory differs from this for a variety of reasons discussed below. The floats spend approximately 12% of their time profiling on the surface. If the currents are different at these depths than at the reference isopycnal, the float’s overall trajectory will not reflect the motion of the water at the reference isopycnal. Floats 8, 9, and 10 made GPS measurements every 30 s while on the surface. ~S during From these, the average surface current U each surfacing was computed. The average velo~G , estimated from the city between surfacings U first GPS position from each surfacing minus the last from the previous surfacing, was taken to be the time-weighted sum of the current at the ~I and during the profiles reference isopycnal U ~P . Three different models of the current profile U ~ as a function of depth were used to compute UðPÞ, the float velocity at the pressure P of the float. The first is a linear interpolation between the surface and isopycnal velocity ~1 ðPÞ ¼ U ~ I þ ðU ~S U ~I Þ PI P ; U PI
ð1Þ
where PI is the pressure during the Lagrangian drifts interpolated to all other times. The second uses a linear fit, but only shallower than the Lagrangian drifts. ~2 ðPÞ ¼ U ~1 ðPÞ ðPoPI Þ; U ~I ðP4 ¼ PI Þ: ¼U
(2)
The third has an Ekman-layer-like profile. ~3 ðPÞ ¼ U ~S ðPo10 dbÞ U ~I ðP4 ¼ PI Þ: ¼U
(3)
~G . The ~I was initially set to U The value of U velocity profiles (1)–(3) were then used to recur~P sively obtain successively better estimates of U ~I . A few recursions lead to rapid converand U gence. The three models yielded similar results, so the results do not appear to be sensitive to the details of the model. For the long trajectory of float 10 (Fig. 1b), the corrections (1)–(3) to the trajectory were small as long as the float was on the shelf. During the first
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10 days the float was advected about 3 km offshore; by day 240 this switched to 1–2 km onshore. By the time the float left the shelf on day 249, the net displacement increased to 4–5 km offshore. After the float upwelled to the surface on day 258, the calculations became meaningless. In contrast, the corrections to the float 3 trajectory were significant and persistently offshore; float 3 was kept away from the coast by offshore advection when it was on the surface. This played an important part in the data interpretation (see Section 3.2.1). 2.3.2. Shear dispersion Deviations of the float trajectory from an isopycnal trajectory were also caused by the deviation of the float from the target isopycnal during the Lagrangian drift. If the float moves above the target isopycnal and there is a vertical (more properly ‘diapycnal’) shear of the horizontal (really ‘isopycnal’) velocity, then the float trajectory will be horizontally displaced from the water trajectory at the isopycnal. This effect can be estimated directly for float 10. This float had CTD’s on both endcaps with a separation of dZ ¼ 1:4 m. The deviation of the float’s center from the sref ¼ 25:95 kg m3 isopycnal was computed from the two CTD’s using ðsb þ st Þ=2 sref DZ ¼ dz; hsb st i
ð4Þ
where the subscripts t and b refer to the top and bottom CTD’s. Occasionally, the stratification sb st becomes very small or negative, resulting in unstable values of DZ. A small amount of time averaging, indicated by the hi in (4), stabilized the values. The results are insensitive to the amount of averaging as long as it is large enough. A value of 104 s was used. DZ has a standard deviation of 0.3 m; the isopycnal is usually between the two ~ the CTD’s. The sonar on float 10 measured dU, horizontal velocity of the water near the top CTD relative to the float’s center, about dZ=2 below. The velocity of the float relative to the velocity of the isopycnal is estimated as ~¼2 DU
~ dU Dz: dz
ð5Þ
The net horizontal displacement of float 10 relative to the sref ¼ 25:95 kg m3 isopycnal was 1 km offshore and 3 km southward from days 214 to 255. This corresponds to a mean velocity of about 1 mm s1 , a very small effect. It should be several times larger for floats 2 and 4 because they followed an isopycnal less accurately. 2.3.3. Isopycnal vs. Lagrangian At the time scales of interest here, these floats are not Lagrangian but isopycnal. If there is diapycnal mixing and the diapycnal density flux profile is curved, water will flow through isopycnals at a rate e ¼ ðDsz Þz =sz , where D is the scalar diffusivity, s is the potential density, and the subscript z denotes differentiation. Neither the magnitude nor sign of this effect can be estimated accurately. With a constant D ¼ 104 m2 s1 and an exponential vertical decay of sz with a scale of 30 m, e ¼ 0:29 m day1 or 14 m over 50 days. In this case, the isopycnal diffuses downward relative to the water as diffusion decreases the water density locally. Equivalently, the water flows upward past an isopycnal-following float.
3. Observations and results 3.1. Southward flow The floats were deployed within the expected position of the southward-going upwelling jet. Fig. 3 shows the magnitude of the southward float velocity. Most of the floats stayed within the jet. When the floats were over the central or outer shelf, they moved southward at 0.05–0:15 m s1 . These are comparable to average mid-depth velocities reported by Huyer (1983) and Federuik and Allen (1995) from mooring observations. Although the data are limited, some patterns are evident within this overall southward flow. The southward velocity decreased for float 3 when it moved inshore after day 212. This is consistent with the offshore position of the upwelling jet shown, for example, in Huyer (1974) and by the radar surface current maps in Oke et al. (2002b). The southward velocity also decreased when floats 8 and 10 moved off the shelf near Cape Blanco.
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North Wind / m s-1
5 0 -5
5 day running mean
(a)
-10
2000
Float
2 3 4
2001
North Velocity / m s-1
Float
(b)
8 9 10 offshore offshore
0.1
0 re
ho
ins
-0.1
-0.2
5 day running mean 160
180
(c) 200
220
Yearday 2000
(d) 210 220 230 240 250 260 270
Yearday 2001
Fig. 3. North wind component for (a) 2000 and (b) 2001 as in Fig. 2 but averaged with 5-day running mean. Northward velocity for floats from (c) 2000 and (d) 2001 averaged with 5-day running mean. Curves are dashed when float is outside of main southward jet.
These periods are dashed in Fig. 3. Excluding these, the southward speed increased with time over each season and increased for each float as it moved south. Periods of northward flow in both seasons occurred early in the records and when the floats were on the northern part of the shelf. The southward increase in speed is consistent with the rapid southward strengthening of the wind stress and its curl (Huyer, 1983). The increase with time is partially an artifact of the southward motion of the floats and the southward increase in speed, but also consistent with a maximum temporal increase in wind stress and transport in July as found, for example, by Kosro (2003). 3.2. Nearshore upwelling 3.2.1. Overview The ability of the floats to follow directly the vertical motion of isopycnals offers a potentially unique view into upwelling. The vertical motion consists of both energetic high frequency motions, the subject of a later paper, and low frequency motions, which are considered here. Floats 2, 3, and both segments of 4 (Fig. 2c) moved upward at
rates of roughly 3 m day1 ð35 mm s1 ). In contrast, floats 8 and 10 moved downward at roughly 10 mm s1 while on the shelf (Fig. 2d). Trajectories are considered in more detail below. Floats 2 and 4 were deployed at the 100-m isobath, near the head of Astoria Canyon. In retrospect, this was a bad choice both because the flow in this region is influenced by the canyon and the nearfield of the Columbia River outflow, and because the float could not surface through the plume and thus could not navigate or communicate. These floats also had the control problems described above. Thus although they both upwelled and moved toward the coast, these various problems make a more quantitative analysis problematic. The float 3 trajectory is detailed in Fig. 4. The float was deployed at mid-shelf (90-m isobath) and moved southward and onshore during a period of downwelling favorable winds, reaching the 25-m isobath approximately 2 km offshore of Cape Foulweather in 10 days. It then moved offshore and upward (days 212–217) and then onshore again, reaching a depth of only 2 m on day 219. Its southward motion then slowed and the float
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Fig. 4. Float 3 data. (a) Northward wind component from NDBC buoy 46050. (b) Time-depth section along the track of float 3. Individual twice-daily CTD profiles are averaged into 2-m vertical bins and the resulting array averaged with a 5 5 box filter. This smoothed field is colored and contoured with an interval of 0:25 kg m3 . Float depth is smoothed with a 20 000-s running mean filter and plotted as the heavy blue line. (c) Float trajectory (red) plotted over 10-m interval isobaths (black and gray) and annotated with date (blue circles). Raw GPS fixes are shown by red ‘x’ symbols. Blue and green lines show the float trajectory corrected using the average of (1)–(3) in (5). These trajectories start on days 203, 208, 213, 218, and 223. A star is plotted at the start of each day on each trajectory. Trajectory is terminated after it crosses the shore. The duration of each trajectory in days is plotted at its end.
underwent another offshore excursion before it finally hit the bottom (day 228) and upwelled to the beach within the bottom boundary layer. The float’s motion does not accurately reflect the water motion because the float was advected by surface currents during its twice-daily profiles (see Section 2.3.1). Trajectories corrected for the surface currents are shown by the blue and green lines in Fig. 4c. The three correction models (1)–(3) yield similar results, so their mean is used. The
corrected tracks are persistently more onshore than the true track. The average of onshore velocities along tracks beginning each day at the float position and ending at the coast is 0:03 m s1 , with a range of 0.014–0:09 m s1 and a standard deviation of 0:017 m s1 . The speeds were greatest when the float was very close to the shore. Excluding trajectories that start less than 7 km from shore, the mean speed was 0:027 m s1 and the range was 0.013–0:052 m s1 . For these
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motion of the isopycnal from that of the float. The isopycnal depth HðxÞ is assumed to be a function of distance from the coast x as
trajectories the average time to reach the coast was 6.0 days with a standard deviation of 2.6 days and a range of 1.7 to 11.5 days. Fig. 5 shows the low-passed float trajectory as a function of depth and distance from the coast, and the average isopycnals from days 213 to 229. The isopycnals are clearly tilted upward toward the shore, as expected. For the first part of the data (days 202–212), the float and its isopycnal (25:8 kg m3 ) are generally deeper than during the second part (days 212–229). During both parts, however, the float slides up and down the tilted isopycnal and it moves on and offshore. The trajectory is therefore a complex combination of the vertical motion of the isopycnal and horizontal motion of the float along the isopycnal.
HðxÞ ¼ H 1 þ ðH 0 H 1 Þex=R ;
ð6Þ
where H 1 is the depth far offshore and H 0 is the depth at the coast. Austin and Barth (2002) fit (6) to CTD sections and moorings off Newport, Oregon, (44:65 N) during the 1999 upwelling season. The value of H 1 was fixed at 125 m. The value of H 0 was found to be well predicted by H 0 ¼ 23 þ 0:85 T 8 ;
ð7Þ
where Z
t
T 8 ðtÞ ¼ 0
3.2.2. Vertical motion of the isopycnal The model of Austin and Barth (2002) is used to predict the vertical motion of the isopycnal as a function of wind stress and thus separate the
tn ðt0 tÞ=dt 0 e dt r0
ð8Þ
is an average alongshore wind stress tn . The average is taken over previous times only with an exponential weighting of dt ¼ 8 days. r0 is the water density. This fit the depth of the 25:8 kg m3
0 5
25.5
219 228 227
Isopycnal Depth / m
10
213
221
226
15
216
20 202
25
222
205 207
210
30
.0
Drift Trajectory
35
26
yearday
40
0
2
4
6
8
10
12
14
16
18
Offshore Distance / km Fig. 5. Cross-shelf view of the trajectory of float 3. CTD data from float profiles on days 213–229 (light grey dots) are averaged in bins of offshore distance and depth. The resulting isopycnal contours are plotted as solid and dashed lines. The depth of the 25:8 kg m3 isopycnal during the Lagrangian drifts is computed from the potential density at the float and the vertical density gradient computed on nearby profiles. This depth is usually within 5 m of the float depth. This depth is low passed (cutoff frequency is 1 cycle per day) and plotted as the heavy shaded line. White circles indicate the time along this trajectory.
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isopycnal in the 1999 repeat CTD section data with an rms deviation of about 5 m. Fig. 6 shows the application of this model to the float 3 data. The isopycnal depth was computed from the float depth and density during the Lagrangian drifts, and the vertical density gradient from the profiles and low-pass filtered to remove semi-diurnal tidal variations. The resulting depth varies from 25 m at the start of the mission to the surface at its end (Figs. 5 and 6). Wind data from NDBC buoys 46 050 and 46 050 (see Fig. 1) were used to compute tn , the northward component of wind stress using Fairall et al. (1996). Unless stated otherwise data from buoy 46 050 is used. A relative humidity of 90% was assumed. When no surface temperature was available, a neutral boundary layer was assumed. These assumptions had little effect. The value of R was estimated by minimizing the difference between H predicted by the model and measured by the float. The values of R and H 1 could not be determined independently. The
best-fit value R ¼ 82 km was found assuming the Austin and Barth (2002) value of H 1 ¼ 125 m. The predicted isopycnal height at the coast H 0 varied from 20 m depth to 10 m height (Fig. 6, grey lines). The predicted depth at the float (dashed lines) differs from this as the float moved onshore and offshore. Understandably, because the model was calibrated from data near Newport, Oregon, it works best when the float is in this vicinity. The most persistent deviations occur at the start of the data, days 202–212. Although the float is closer to buoy 46 029 than 46 050 during this time, the effect of using 46 029 instead of 46 050 is small (grey solid lines) so this cannot explain the error. The model fits better during this time using a somewhat smaller value of R ¼ 50 km (light dashed line). However, the stratification is stronger here than further south so the radius of deformation should be larger rather than smaller. The shelf is narrower, which could be important. Despite this, the Austin and Barth (2002) model predicts the
20
Isopycnal Depth /m
10
Isopycnal Depth at Coast Model 46050 wind 46029 wind
Isopycnal Depth at Float Data Model 46050 wind, R=82 km R=50 km
0
-10
-20
-30
-40 200
205
210
215
220
225
230
Yearday of 2000 Fig. 6. Depth of the 25:8 kg m3 isopycnal at float 3 (heavy line) during its Lagrangian drifts compared with the prediction of the Austin and Barth (2002) model. Predicted isopycnal depth at the coast H 0 (light grey lines) is shown for two different weather buoys. Predicted depth at the float is shown using one buoy and two different values of R, the offshore decay length (dashed lines).
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vertical motion of the isopycnal well; the rms deviation of the model from the data is 6 m, nearly the same as found by Austin and Barth (2002).
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reflects the otherwise unsubstantiated choice of D/2 for the return flow thickness; different choices of this depth would lead to different values for X e . More importantly, the fluctuations in offshore distance from days 210 to 229 are clearly not predicted by the wind-forced Ekman model (9) nor are they correlated with the wind-forced isopycnal displacements predicted by Austin and Barth (2002). Instead, their structure (Fig. 4c) marks them as nearly barotropic shelf waves (Gill, 1982, Chapter 10.12), with a cyclonic curvature offshore and an anti-cyclonic curvature nearshore consistent with the conservation of barotropic potential vorticity. Thus, the net onshore motion of water on the 25:8 kg m3 isopycnal reflects the overall upwelling circulation driven by the net upwelling favorable wind. The fluctuations in cross-shelf motion reflect the influence of finite amplitude shelf waves of unknown origin. The fluctuations in isopycnal depth along the trajectory reflect both the temporal variations in the isopycnal depth caused by the varying along-shelf wind stress and the cross-shelf
3.2.3. Motion along the isopycnal The motion of water along the isopycnal is distinct from the vertical motion of the isopycnal. In a simple, two-dimensional, steady-state upwelling model, the offshore Ekman transport ts =f r0 is balanced by a deep onshore return flow. The float’s isopycnal was chosen to be within this onshore flow based on historical data (Huyer, 1983); the trajectories in Fig. 4 clearly show that it is. The onshore velocity is equal to the transport divided by a depth. A value of half the water depth D is used. The net eastward displacement due to this is Z t 2ts Xe ¼ dt: ð9Þ r 0 0 fD Fig. 7 compares X e to the eastward displacement of float 3, corrected as in Fig. 4c. Although X e predicts the net total displacement well, this only 0
Offshore distance / km
-5
-10
-15
-20
Float Ekman model
-25
-30
205
210
215
220
225
Yearday 2000 Fig. 7. Eastward (cross-shelf) displacement of float 3, i.e., the corrected eastward displacement of the float from its initial position (solid) compared to the prediction of an Ekman transport model (dashed).
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motion of the water parcels along the cross-shelf sloping isopycnals. 3.3. Heceta Bank and Cape Blanco
220
Cape
Blanc
o
Most of the float trajectories (Fig. 1) follow the topographic contours around Heceta Bank. This offshore path is consistent with the high resolution modelling and the radar surface current maps described in Oke et al. (2002a, b). Two floats, 8 and 10, reached Cape Blanco and moved offshore (Figs. 8–10). The motion of float 10 corresponds closely to the cartoon of the flow in this region of Barth et al. (2000). After rounding Heceta Bank the float traveled southward along the coast within the baroclinic upwelling jet. Just upstream of Cape Blanco it moved offshore and descended. This is evident in the 3-D trajectory (Fig. 8, days 248–252 ). A sea surface temperature image taken at this time (Fig. 9) shows that cold water streamed offshore with the float. This cold tongue warmed offshore; the float trajectory suggests that some of this surface warming may have been due to the subduction of the cold water
beneath warmer water. Solar heating is the other obvious possibility. The float did not follow the path of the cold water, but looped cyclonically, so that by day 257 it was being carried north. On day 257 it executed a second 180 turn, equivalent to a vorticity of close to f , and rose to the surface. On day 258 it sank again while executing a slight cyclonic turn, and then, on days 259 and 260, rose to the surface during an anticyclonic turn. The float then accelerated offshore on the surface in a large pool of cold upwelled water, eventually circulating in an offshore eddy. Float 8 traversed this same region about 25 days earlier. Note that its GPS did not work so its track is of poorer quality. Between Heceta Bank and Cape Blanco floats 8 and 10 travelled on similar paths and at a similar speeds. At Cape Blanco, however, float 8 did not loop offshore, but tracked steadily past the cape. Here, where float 10 surfaced, sank, and then moved offshore on the surface, float 8 surfaced, sank, and moved offshore at depth (see Figs. 2d and 10a) and meandered in an offshore eddy. Although SST data during this time were mostly obscured by clouds, they suggest
246
124
258
248
260
50m
232 236
Float 8 0 20 40 60 80 44.5
, la (longitude titude, surface) , latitude, Float 10 depth)
43.5
262
250
(longitude , latitude, surface) (longitude , latitude, depth)
44
124.5
100m
(longitude
252
125
43
42.5
42
125.5
Fig. 8. Three-dimensional view of the trajectories of floats 8 and 10 near Cape Blanco. The trajectory of each float is plotted both at the surface (red and black lines, respectively) and at the float depth (gray and magenta lines, respectively). Light vertical lines connecting these are plotted five times per day. Selected times along the trajectory are annotated. The coastline and depth contours to 100 m are drawn in the background.
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Sea Surface Temperature - day 251 2130Z 18
12
14
16
10
246 32m 260
258 0m
248 28m
18m
0m
32m 256 254
125 W
250
50m
252
Float 10
N
42.5N
43N
Fig. 9. Trajectory of float 10 near Cape Blanco superimposed on bottom topography (black lines to 200 m) and sea surface temperature image from day 251. Yearday of 2000 and selected float depths are annotated along trajectory.
0 Float 8
Float 10
40 min 60 (a) 220
225
230
235
0 10
Depth / m
20
240
245
250
255
260
Yearday 2001 270 260
25.55 isopycnal depth Float depth
Yearday 2001
Depth / m
max 20
30 40
250
240
50
230
60 (b) 90
80
70
60
50
40
30
20
10
0
220
Distance from Shore / km Fig. 10. (a) Depth-time trajectories of floats 8 and 10 near Cape Blanco. Depths are averaged over 12 h to remove tidal fluctuations. Heavy line and circles (plotted every 6 h) show the depth of the 25:5 kg m3 isopycnal at the location of each float. Small stars indicate the depth of the floats. The line color indicates the time as indicated by the colorbar. Thin lines, colored by time, indicate minimum and maximum isopycnal depths during each interval. (b) Same depths plotted against distance from shore.
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that the rapid vertical motion of float 8 occurred near a tongue of cold water similar to that in Fig. 9. Fig. 10b shows the position of both floats’ isopycnal as a function of distance from the coast. The isopycnal sloped upward toward the coast as expected. Remarkably, the slopes were nearly the same for floats 8 and 10 despite their very different trajectories. This is particularly true if the times of rapid upwelling (days 233–235 for float 8, after day 257 for float 10) are removed. Notice also that the offshore/downward and onshore/upward legs of the float 10 offshore loop (days 248–257) follow almost exactly the same path. Barth et al. (2000) hypothesize that the flow in this region is strongly controlled by topographic stretching of the water column. The data here clearly support this hypothesis. The track of float 10 shows a clear relationship between offshore displacement and the curvature of the track; offshore segments curve cyclonically and inshore segments curve anticyclonically. The vertical density gradient measured at float 10 is also weakest when it is furthest offshore. The isopycnal slope seen in Fig. 10b is most easily explained by the stretching of the water column as it moves offshore; the isopycnal depth is the same on the offshore-going and the onshore-going parts of the trajectory because the stretching is reversed as the water column moves back onshore. The process is the same for floats 8 and 10 so the slopes are similar. This implies that these cross-slope motions are primarily barotropic, and thus influenced by the bottom. More peculiar is that this cross-slope exchange sets up a cross-slope baroclinic structure and thus an along-slope baroclinic flow. This dynamic merits further investigation.
4. Summary and discussion Isopycnal-following, neutrally buoyant floats were deployed on the Oregon continental shelf at mid-shelf and mid-depth in the summers of 2000 and 2001. All measured temperature, salinity, and pressure; some measured velocity shear and biological variables. The floats alternated between twice-daily vertical profiles from 40–90 m to the
surface and Lagrangian drifts within a few meters of an isopycnal in the range of 25.5–25:8 kg m3 . The float tracks were determined from GPS and ARGOS fixes made when the floats were on the surface. Data and control commands were relayed between the float and shore using the ORBCOMM satellite telemetry system. Advection of the floats during profiling and when they were on the surface caused their tracks to differ from the Lagrangian trajectories of water on their target isopycnal. This could be evaluated for floats that made rapid GPS measurements on the surface. A nearshore float showed a large offshore bias, while a mid-shelf float showed little bias. Other biases of the float motion from a true water-following trajectory appeared small. All floats moved southward with the prevailing summertime flow and with a velocity increasing further south. Some moved inshore following the expected upwelling circulation. One float (float 3) upwelled to the beach. Its track was strongly biased offshore by the offshore surface flow. After correction for this effect, the mean onshore velocity at the 25:8 kg m3 isopycnal was 2:7 cm s1 , corresponding to an average time from mid-shelf to the beach of 6 days. Cross-shelf motion of the float allowed the structure of the isopycnal surface to be investigated. The 25:8 kg m3 isopycnal sloped upward toward the coast, as expected. The extrapolated height at the coast could be accurately modelled as a function of 8-day-averaged wind stress following Austin and Barth (2002). The cross-shelf motion along the isopycnal, however, showed energetic fluctuations unrelated to the wind and suggestive of barotropic shelf waves. Other floats deployed at mid-shelf did not move shoreward but instead followed the isobaths southward around Heceta Bank to Cape Blanco. Two floats (floats 8 and 10) proceeded further. Near Cape Blanco both underwent rapid (up to 50 m day1 ) upward and downward displacements of about 30 m from a starting depth near 30 m. Both eventually moved offshore, float 10 on the surface and float 8 at depth. The float 10 trajectory is remarkably similar to that hypothesized by Barth et al. (2000); that of float 8 shows much less horizontal excursion.
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Overall, the dominant influences on the float trajectories are wind and topography. The net southward flow and inshore upwelling are the expected results of the prevailing north wind. Equally important, however, is the strong constraint of topography; the southward flow follows the isobaths. Significant cross isobath motion is strongly associated with track curvature, indicating the role of topographic stretching and potential vorticity conservation in controlling the flow direction. These observations probably represent the first use of long-term profiling floats in a coastal environment. These observations clearly showed that such floats can operate and provide useful information without too many losses due to grounding. However, the time spent profiling clearly degraded the accuracy of the Lagrangian trajectories. This loss was offset by the gain in the ability to track isopycnals and their spatial structure and, of course, track and control the floats in nearly real time. In the strongly upwelling nearshore environment the time spent on the surface had the fortunate effect of keeping the float from moving onshore in the upwelling flow and ending its mission on the beach or rocks. With improved satellite telemetry now available the time spent on the surface could be greatly reduced, thus making the floats more Lagrangian but, unfortunately, more prone to reach the shore. This could be mitigated by restricting the floats to short missions with a rapid ability to rescue the floats, or by using only inexpensive, expendable floats. More profoundly, truly Lagrangian floats will go where the water goes. Thus, unless most water parcels travel through the surf zone, itself a fascinating hypothesis, accurately Lagrangian floats would also not commonly beach. The technical challenge for the safe use of floats in the nearshore environment may thus lie in the improvement of their water-following ability. Although the number of floats and the quality of their trajectories is limited, these data show several distinct pathways from mid-shelf. One, followed by floats 2, 3, and 4A, is the classical baroclinic upwelling circulation: mid-shelf water moves onshore and upward along the sloping isopycnals and then southward in the nearshore
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upwelling jet. The other, followed by floats 4B, 8, and 10, is an alongshore flow, closely following the isobaths around Heceta Bank and eventually moving offshore at Cape Blanco and into the eddy field of the California Current. Our evidence is limited, but it appears that mid-shelf water from the northern Oregon shelf can take either of these paths. Although the average summertime velocity is strongly onshore at mid-depth, apparently not all particles move rapidly inshore. However, all of the floats reached to within a few meters of the surface at least once, suggesting that the probability that water will upwell at some point is high. More fundamentally, it is not clear what controls the path that water will take. One might expect that for a given wind forcing, particles that are sufficiently far offshore would follow the offshore track and those sufficiently inshore would follow the nearshore upwelling path. The location of the boundary between these two regimes would likely be a strong function of the wind forcing. A better understanding of these Lagrangian properties of the circulation would seem a worthy goal with clear implications for characterization of the physical environment experienced by organisms.
References Austin, J., Barth, J.A., 2002. Variation in the position of the upwelling front on the Oregon shelf. Journal of Geophysical Research 107 doi:10.1029/2001JC000585. Barber, R., Smith, R.L., 1981. Coastal upwelling ecosystems. In: Lonhurst, A.R. (Ed.), Analysis of Marine Ecosystems. Academic Press, New York, pp. 31–68. Barth, J., Pierce, S., Smith, R., 2000. A separating coastal upwelling jet at Cape Blanco, Oregon and its connection to the California current system. Deep-Sea Research II 47, 783–810. D’Asaro, E.A., 2003. Performance of autonomous Lagrangian floats. Journal of Atmospheric and Oceanic Technology 20 (6), 896–911. D’Asaro, E.A., Farmer, D.M., Osse, J.T., Dairiki, G.T., 1996. A Lagrangian float. Journal of Atmospheric and Oceanic Technology 13 (6), 1230–1246. Fairall, C., Bradley, E., Rogers, D., Edson, J., Young, G., 1996. Bulk parameterization of air-sea fluxes for tropical ocean global atmosphere coupled ocean atmosphere response experiment. Journal of Geophysical Research 101 (C2), 3747–3764.
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Federuik, J., Allen, J., 1995. Upwelling circulation on the Oregon continental shelf II: Comparison with observations. Journal of Physical Oceanography 25, 1867–1889. Gill, A.E., 1982. Atmosphere-Ocean Dynamics. Academic Press, New York, 662pp. Huyer, A., 1974. Observations of the coastal upwelling region off Oregon during 1972. Ph.D. Thesis, Oregon State University, Corvallis, Oregon. Huyer, A., 1983. Coastal upwelling in the California current system. Progress in Oceanography 12, 259–284. Kosro, P., 2003. Enhanced southward flow over the Oregon shelf in 2002: a conduit for subarctic water. Geophysical Research Letters 30 (15) doi:10.1029/2003GL017436.
Oke, P., Allen, J., Miller, R., Egbert, G., 2002a. A modelling study of the three-dimensional continental shelf circulation off Oregon, part II: dynamical analysis. Journal of Physical Oceanography 32 (5), 1383–1403. Oke, P., Allen, J., Miller, R., Egbert, G., Austin, J., Barth, J., Boyd, T.J., Kosro, P., Levine, J., 2002b. A modelling study of the three-dimensional continental shelf circulation off Oregon, part I: model-data comparisons. Journal of Physical Oceanography 32 (5), 1360–1382. Rossby, T., Levine, E.R., Connors, D.N., 1985. The isopycnal Swallow float—a simple device for tracking water parcels in the ocean. Progress in Oceanography 14 (1–4), 511–525.