Ocean circulation off the northwest coast of the United States is driven by a variety of mechanisms, the most important of which is the seasonally varying wind stress. The response of the coastal ocean to strong equatorward ("upwelling-favorable") winds during the summer, described for example off the Oregon coast based on a number of past studies (e.g., Huyer 1983), consists of net offshore transport in the surface Ekman layer, upwelling of cold, saline water near the coast, and the formation of a strong alongshore coastal jet that is in geostrophic balance with the upwarped isopycnals. Winds become predominantly upwelling-favorable after the "spring transition" (Huyer et al. 1979), and the upwelling regime persists through the summer and early fall before returning to winter conditions after a fall transition. During the upwelling regime, the vertical structure of the alongshore velocity field consists of a southward coastal jet in the upper water column extending down to 50-75 m depth, with maximum speeds near the surface of up to 1 knot, and a more sluggish poleward undercurrent near the bottom over the outer continental shelf (Huyer et al. 1978).
The summer upwelling circulation is affected on short time scales by changes in the wind stress on 2- to 10-day time scales (Huyer 1983). On longer time scales, the timing and intensity of the upwelling circulation varies from year to year (Huyer et al. 1979, Strub and James 1988) under the influence of interannual variability in the atmospheric driving and in coupled ocean-atmosphere phenomena, for example El Niño (Huyer and Smith 1985). The following discussion will not focus on interannual variability of coastal ocean circulation, except to point out differences between two particular years along the Oregon coast (1994 and 1995), concentrating instead on a description of spatial variation of the upwelling circulation associated with the interaction of a strong upwelling jet with a coastal promontory. Spatial variability in the coastal upwelling circulation is readily apparent in satellite sea surface temperature (SST) maps, an example of which is shown in Figure 1. Spatial variability is an important factor to consider, together with temporal variability, when assessing the influence of the coastal ocean on biological productivity and the transport of biogeochemical and anthropogenic material.
Recently, scientists at Oregon State University (OSU) have been studying the region near Cape Blanco, Oregon in an effort to understand how and why the strong alongshore coastal upwelling jet turns offshore, crosses the steep topography of the continental margin, and becomes an oceanic jet (Barth et al. 1994, Barth and Smith 1996, Smith et al. 1996). The Cape Blanco region was chosen because historical observations (Huyer 1990, Smith 1992) and satellite imagery (Fig. 1) suggest that this is a dividing point between a region to the north of the cape where upwelling is fairly well confined to inshore of the continental shelf break (approximately the 200-m isobath) and a region to the south of the cape where a meandering equatorward jet and upwelled water extend well seaward of the continental margin (Kosro and Huyer 1986).
The observational component of the Coastal Jet Separation Experiment consisted of three hydrographic and velocity surveys during 1994 and 1995 using the Research Vessel (RV) Wecoma, tracking surface drifters and analyzing satellite SST imagery. The hydrographic data were collected using a towed, undulating vehicle, SeaSoar (Pollard 1986), which cycles rapidly from the surface to depth while towed at 8 knots (4 m/s) behind the RV Wecoma. The majority of the conductivity-temperature-depth data was collected by towing the SeaSoar on a bare cable cycling the vehicle between 0 and 120 m every 4 minutes. The result is hydrographic data with very high spatial resolution (1 km between along-track surface points) obtained rapidly (cross-shelf sections in 2-3 hours and large-area maps in 1-2 days) so that a detailed "snapshot" of the system can be studied. Over the shallow continental shelf, the SeaSoar was flown from 0 to 55 m cycling every 1.5 minutes, resulting in along-track profile spacing as little as one-third of a kilometer. The SeaSoar was also towed occasionally on a cable equipped with aerodynamic fairing to reduce drag; this allows the vehicle to cycle between the surface and in excess of 300 m every 8 minutes.
The seasonal cycle in the wind stress is apparent in the 1994 winds as measured at Newport, Oregon (Fig. 1, top). July was a month of typical, strong upwelling-favorable winds; however, August was a period of anomalously weak equatorward winds. An SST image from 18 August 1994 (Fig. 1, bottom) reveals a band of cold upwelled water near the coast, confined to inshore from the continental shelf break north of Cape Blanco (the wide cold feature near lat. 44°N is the influence of Heceta Bank—note the bottom topography in Fig. 3) and extending much farther seaward south of the Cape. The equatorward upwelling jet, associated with the temperature front between cold upwelled water inshore and warmer water offshore, meanders westward at Cape Blanco but returns toward the coast downstream in a counter-clockwise bend before meandering over 300 km offshore near lat. 41°N. A ship survey of the region on 25 to 30 August 1994 (Fig. 2) confirmed the existence of the strong alongshore coastal jet north of the Cape as exemplified by the strong gradient in dynamic height found there. Geostrophic currents flow along contours of dynamic height, and their speed is inversely proportional to the distance between the contours. The dynamic height map shows continuity of the southward jet with a counter-clockwise (cyclonic) eddy roughly 80 km in diameter offshore from the Cape. A second dynamic height map reveals the pinching off of the eddy and a reconnection of the equatorward jet on the inshore side, as denoted by the 0.9-m dynamic height contour. This creation of a large counter-clockwise eddy is termed "cyclogenesis" and is a result of a flow-topography interaction between the coastal upwelling jet and the Cape. The eddy contains water of coastal origin, and this process is an important mechanism for injecting biologically important material into the deep ocean.
Three satellite-tracked surface drifters were released across the coastal jet to the north of Cape Blanco (Fig. 3) at the beginning of the survey. All three drifters were initially swept offshore by the separating jet, but the most inshore drifter (long dashed curve) split from the other two to follow an inshore pathway to the south (compare with the 0.8-m dynamic height contour in Fig. 2). The other two drifters moved together around the northern edge of the cyclonic eddy at speeds of up to 0.6 m/s, before one drifter (dashed curve) moved off to the northwest while the other drifter made one complete revolution of the eddy before exiting to the north. Both drifters then made counter-clockwise revolutions around a cold eddy (near lat. 43°N, long. 126.5°W in the SST image) formed earlier (July) in the upwelling season. One drifter (dashed curve) again left the cyclonic feature to the north and spent the next 5 weeks in the low velocity region well offshore from central Oregon, while the other drifter remained trapped in the counter-clockwise eddy. In mid-October, the winds became south-southwesterly through the fall transition (Fig. 1) and the circulation responded as shown by drifter tracks in the right-hand panel of Figure 3. The drifter in the stagnant offshore area was swept back onto the continental shelf, returning very close to its deployment point after spending 2.5 months offshore. This demonstrated a Lagrangian pathway for passive particles to leave the coastal ocean, but to then return through the influence of seasonally and spatially varying circulation. The other drifter executed additional revolutions around the eddy before also being swept onto the central Oregon shelf. Since the cold offshore eddy was formed in July, this indicated a minimum eddy lifetime of 4-5 months. Finally, both drifters were swept rapidly poleward by the Davidson Current (Jones 1918) transiting to north of Vancouver Island.
In 1995, winds became upwelling-favorable for an extended period beginning in mid-May (Fig. 4, top). A satellite SST image from 18 May 1995 (Fig. 4, bottom) showed a relatively narrow band of cold, upwelled water near the coast during this early part of the upwelling season. The southward-upwelling jet and front meandered only slightly near Cape Blanco (compare the SST image in Fig. 1). A ship survey of the Cape Blanco region (not shown) confirmed the existence of the nearly straight southward jet near the continental shelf break. Five satellite-tracked surface drifters were released across the continental margin north of Cape Blanco on 21 May 1995 (Fig. 5, left). The four inshore drifters transited rapidly to the south at speeds up to 0.6 m/s, again demonstrating the relatively straight alongshore flow (compare with Fig. 3, left). The drifter released farthest west was placed in the surface salinity minimum associated with the Columbia River influence offshore from the coastal upwelling jet and front, and transited slowly (0.05-0.25 m/s) to the southwest. Throughout the remainder of the upwelling season, the drifters were swept equatorward in the eastern boundary current region by a meandering jet (Fig. 5, right). Evidence for eddies, meanders, swift jets, and more sluggish flow far offshore (e.g., near lat. 37-40°N, long. 129-132°W) exists in the drifter tracks. These Lagrangian trajectories confirm the hypothesis that the separating coastal upwelling jet off Oregon contributes significantly to the meandering jet now accepted as making up a large fraction of the equatorward flow in the California Current System (Huyer et al. 1991). None of the drifters released in May returned to the coast, nor did they remain or return to their release latitude. This demonstrates that material carried by the coastal upwelling jet off Oregon early in the upwelling season can be carried far from its point of origin by the swift, relatively linear southward flow.
A ship survey and drifter release was conducted near the end of August 1995 after a period of typical strong upwelling-favorable wind stress (Fig. 4, top). A map of dynamic height (Fig. 6, left) shows a strong southward jet centered on the continental shelf break north of Cape Blanco which then meandered offshore near the Cape and separated from the coast to become an oceanic jet. The jet gained in strength downstream from the Cape, in part from a flow contribution that joined the separating jet from the northwest (lat. 43.25°N, long. 125.5°W) and was associated with flow around Heceta Bank upstream. Five satellite-tracked drifters were released on the continental shelf north of Cape Blanco (Fig. 6, right) and initially all were carried to the south in the upwelling jet. While the most inshore drifter grounded south of Cape Blanco, the other four drifters were swept swiftly (speeds in excess of 1 m/s) offshore in the separating jet. The drifters executed both clockwise (anticyclonic) and cyclonic loops associated with the strong, unstable meandering jet. Drifters released on the shelf transit over 400 km from the coast before turning southward and delineating the equatorward eastern boundary current jet with its core located between long. 127°W and 128°W near lat. 40°N. As the winds become south-southwesterly in the fall, the circulation responds, and all four drifters were swept shoreward between lat. 37°N and 39°N. By 7 February 1996, one drifter (thin dashed curve) had come ashore south of Cape Mendocino, another drifter (thick solid curve) was transiting poleward over the continental slope, one drifter (thin solid curve) had exited from a cyclonic eddy near lat. 38°N, long. 127°W and was transiting shoreward to the northwest, and the fourth drifter (thick dashed curve) stopped transmitting over the continental slope just north of Point Arena. In summary, as a result of the jet-topography interaction which leads to a meandering jet with a substantial east-west component, passive material released on the shelf in the coastal upwelling jet north of Cape Blanco, Oregon can return to the coast within approximately 5 degrees of latitude to the south under the influence of the seasonal change in wind stress and the resulting ocean circulation.
The results presented here demonstrate the existence of strong spatial variability in the upwelling circulation off the Oregon and northern California coasts. Understanding the processes which lead to and control this spatial variability is critical in assessing the influence of ocean circulation on biological productivity and the transport of biogeochemical and anthropogenic material onto and off of the continental shelf. The existence of separated coastal upwelling jets, recirculating eddies, meandering equatorward jets, and return flows to the coast has been documented. There is a distinct difference in the behavior of the system early (May) versus later (August) in the upwelling season. In May, the coastal jet remains relatively straight as it transits around Cape Blanco, and drifters released on the shelf are flushed far to the south in the eastern boundary current region. This differs from the behavior later in the upwelling season (August) when the upwelling circulation is more fully developed and the jet-topography interaction creates a spatially complex flow pattern that can retain drifters released on the shelf near their release latitude, enabling their return to the continental margin after the seasonal winds change direction.
Finally, interannual changes in the strength and timing of upwelling occur and are important to levels of biological productivity. While this paper has not concentrated on interannual variability, marked differences exist between the two August realizations presented here. In August 1994, upwelling-favorable winds were anomalously weak and the jet-cape interaction resulted in the generation of cyclonic eddies. In August 1995, after a month of typical strong upwelling-favorable winds, the coastal jet was fully separated from the coast near Cape Blanco. To better understand interannual variability, both further understanding of dynamic processes (which lead to strong spatial variability) and long time series of oceanographic and atmospheric conditions are needed.
We are indebted to Jane Huyer (OSU) for her encouragement during this research and for her insights on coastal and eastern boundary current circulation. Thanks also to the OSU Marine Technicians, Marc Willis, Tim Holt, Linda Fayler, and Mike Hill, who were responsible for the highly successful SeaSoar operations. The officers and crew of the RV Wecoma performed superbly—only occasionally would they comment on the fact that we insisted on towing the SeaSoar in an east-west direction, invariably in the trough of the prevailing northwest summer swell. Ted Strub and Corinne James (OSU) generously provided the satellite imagery. This work was funded by National Science Foundation Grant OCE-9314370.
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