HF radar-derived origin and destination of surface waters off Bodega Bay, California

As an integral part of the WEST study of the role of wind-driven transport in shelf productivity, HF radar currents are analyzed to determine typical surface flow patterns off Bodega Bay in northern California. Radar-derived surface trajectories and surface velocity divergences are used to determine the proximal origins and destinations of surface waters in the area. Surface trajectory results show a strong bimodality, with water over the entire shelf originating in the north under upwelling conditions and waters over the inner/mid-shelf originating in the south during relaxation conditions. Outer shelf waters have more variable transport patterns during relaxation conditions, with limited equatorward or onshore movements being most typical. The destinations of surface waters starting at the outer shelf are predominantly offshore, with the majority of particles exiting the radar domain west of Pt Reyes along the shelf edge in less than 2 days. Significant proportions of water from the inner/mid-shelf are exported southward and exit the radar domain inshore or within 20 km of the tip of Pt Reyes, creating possibilities for either nearshore retention in the Bodega region or entrainment of water into the Gulf of Farallons. Approximately 15% of all trajectories remained in the radar domain for 6 days, suggesting that a biologically significant percentage of larvae might be retained in the area for time periods approaching typical larval durations. Calculations of surface divergence indicate where vertical flux may be significant. An extensive area of positive divergence is observed off Bodega during upwelling conditions, while weakly convergent flow is observed where upwelling flows approach Pt Reyes. Positive divergence also is observed during relaxation periods when poleward flow separates from the shore just north of Pt Reyes. Estimates of vertical flux in these divergence zones point to a significant contribution of recently upwelled waters to the observed horizontal fluxes at the surface. Determination of the ultimate source and fate of phytoplankton-rich waters requires further analysis of the detailed time dependence of phytoplankton concentration relative to the time dependence of wind-forced currents.     

Transport and retention in an upwelling region: The role of across-shelf structure

The paradox of upwelling is the relationship between strong wind forcing, nutrient enrichment, and shelf productivity. Here we investigate how across-shelf structure in velocity and hydrography plays a role in the retention (inshore) and export (offshore) of particles such as nutrients, plankton and larvae. We examine the spatial structure of the coastal currents during wind-driven upwelling and relaxation on the northern Californian Shelf. The field work was conducted as part of the Wind Events and Shelf Transport (WEST) project, a 5-year NSF/CoOP-funded study of the role of wind-driven transport in shelf productivity off Bodega Bay (northern California) from 2000 to 2003. We combine shipboard velocity profiles (ADCP) and water properties from hydrographic surveys during the upwelling season to examine the mean across-shelf structure of the hydrography and velocity fields during three contrasting upwelling seasons, and throughout the upwelling-relaxation cycle. We also present results from two winter seasons that serve as contrast to the upwelling seasons.During all three upwelling seasons clear spatial structure is evident in velocity and hydrography across the shelf, exemplified by current reversals inshore and the presence of a persistent upwelling jet at the shelf break. This jet feature changes in structure and distance from the coast under different wind forcing regimes. The jet also changes from the north of our region, where it is a single narrow jet, adjacent to the coast, and to the south of our region, where it broadens and at times two jets become evident. We present observations of the California Under Current, which was observed at the outer edge of our domain during all three upwelling seasons. The observed across-shelf structure could aid both in the retention of plankton inshore during periods of upwelling followed by relaxation and in the export of plankton offshore in the upwelling jet.     

Buoy measured wind, wind stress and wind stress curl over the shelf off Bodega Bay, California

An array of five buoys and three coastal stations is used to characterize the winds, stress, and curl of the wind stress over the shelf off Bodega Bay, California. The wind and wind stress are strong and persistent in the summer and weak in the winter. In the summer, wind and stress decrease strongly across the shelf, toward the coast. Combinations of buoys are used to compute the curl of the wind stress over different portions of the shelf. The mean summer 2001 curl of the wind stress over the array depends upon the area selected, varying between −1.32×10⁻⁶ and +7.80×10⁻⁶ Pa m⁻¹. The winter 2002 wind-stress curl also depends on location, varying from −2.06×10⁻⁶ to +2.78×10⁻⁶ Pa m⁻¹. Mean monthly curl of the wind stress is a maximum in the summer and a minimum near zero in the winter. In both the summer and the winter, the correlation between the wind-stress curl for different portions of the shelf varies between moderate negative, though insignificance, to high positive. A wind measurement at a single point can be poorly related to the measured curl of the wind stress at other locations over the shelf. The measurements show that the use of one wind measurement to characterize the curl of the wind stress over the shelf without further investigation of the local wind-stress curl structure is risky.     

WEST: A northern California study of the role of wind-driven transport in the productivity of coastal plankton communities

The “Wind Events and Shelf Transport” (WEST) program was an interdisciplinary study of coastal upwelling off northern California in 2000–03. WEST was comprised of modeling and field observations. The primary goal of WEST was to better describe and understand the competing influences of wind forcing on planktonic productivity in coastal waters. While increased upwelling-favorable winds lead to increased nutrient supply, they also result in reduced light exposure due to deeper surface mixed layers and increased advective loss of plankton from coastal waters. The key to understanding high levels of productivity, amidst these competing responses to wind forcing, is the temporal and spatial structure of upwelling. Temporal fluctuations and spatial patterns allow strong upwelling that favors nutrient delivery to be juxtaposed with less energetic conditions that favor stratification and plankton blooms. Observations of winds, ocean circulation, nutrients, phytoplankton and zooplankton off Bodega Bay and Point Reyes (38°N) were combined with model studies of winds, circulation and productivity. This overview of the WEST program provides an introduction to the WEST special issue of Deep-Sea Research, including the motivation for WEST, a summary of study components, an integrative synthesis of major research results to-date, and background on conditions during field studies in May–June 2001 (the upwelling period on which this special issue is focused).     

Coastal upwelling in the California current system

Coastal upwelling in the California Current system has been the subject of large scale studies off California and Baja California, and of small scale studies off Oregon. Recent studies of the winds along the entire coast from 25°N to 50°N indicate that there are significant along-shore variations in the strength of coastal upwelling, which are reflected in the observed temperature distribution. Active upwelling appears to be restricted to a narrow coastal band (about 10–25 km wide) along the entire coast, but the region influenced by coastal upwelling may be much wider. Intensive observations of the upwelling zone during summer off Oregon show the presence of a southward coastal jet at the surface, a mean vertical shear, a poleward undercurrent along the bottom, and persistently sloping isopycnals over the continental shelf; most of the upwelling there occurs during relatively short periods (several days long) of upwelling-favorable winds. During the upwelling season off Oregon, the offshore Ekman transport is carried by the surface Ekman layer, and the onshore return flow occurs through a quasi-geostrophic interior. It is not known whether the structure and dynamics observed off Oregon are typical of the upwelling zone along the entire coast, though some of the same features have been observed off Baja California. Current and future research will eventually show whether the Oregon results are also applicable in the region of persistently strong upwelling-favorable winds off northern California, and in the region of complex bathymetry off central and southern California.     

Inter-annual variability in larval supply to populations of three invertebrate taxa in the northern California Current

We investigated sources of inter-annual variability in larval supply to crab and sea urchin populations at Bodega Head and Point Reyes in northern California. During the spring and summer upwelling seasons of the years 1992 through 1997 we monitored the weekly settlement rates of nine species of crabs and two species of sea urchins. As observed in previous studies, daily values of alongshore windstress, temperature and salinity provided evidence for the poleward flow of relatively warm, low salinity water from south of Point Reyes, an apparent retention zone, during upwelling relaxation events. In years dominated by these events (1992, 1993, 1995 and 1996) we observed that alongshore windstress, temperature and salinity were coherent and temperature was significantly correlated with cancrid crab settlement. During these years the magnitude of cancrid crab settlement and the fraction of cancrid crabs relative to other crab species settling were high. Over four years of concurrent sampling there was consistently greater cancrid crab settlement at the Point Reyes site, within the retention zone, than at Bodega Head. Settlement of non-cancrid crabs (porcellanids, grapsids, pagurids and majids) was not as closely linked to intra-annual patterns of upwelling and relaxation, possibly due to the shorter seasonal availability of larvae allowing for the influence of fewer relaxation events. Settlement of this group among years was positively correlated with environmental indicators of strong seasonal upwelling; high salinity, Bakun upwelling index and low temperature. Sea urchin settlement events were observed in June and July of 1992, 1994 and 1997 during warming periods when salinity and temperature were increasing and alongshore windstress was low. Across the six years of the study, we found that cancrid crab larvae had a more even seasonal availability than larvae of non-cancrid species, which settled in greatest numbers during the early portion of the upwelling season. Sea urchins settled in greatest numbers during the later part of the upwelling season. Together these patterns demonstrate the taxon-specific way that inter-annual variability in larval supply is forced by the coincidence of larval availability with favorable physical transport mechanisms.     

Comparison of atmospheric forcing in four sub-arctic seas

A comparative analysis was conducted on climate variability in four sub-arctic seas: the Sea of Okhotsk, the Bering Sea shelf, the Labrador Sea, and the Barents Sea. Based on data from the NCEP/NCAR reanalysis, the focus was on air–sea interactions, which influence ice cover, ocean currents, mixing, and stratification on sub-seasonal to decadal time scales. The seasonal cycles of the area-weighted averages of sea-level pressure (SLP), surface air temperature (SAT) and heat fluxes show remarkable similarity among the four sub-arctic seas. With respect to variation in climate, all four seas experience changes of comparable magnitude on interannual to interdecadal time scales, but with different timing. Since 2000 warm SAT anomalies were found during most of the year in three of the four sub-arctic seas, with the exception of the Sea of Okhotsk. A seesaw (out of phase) pattern in winter SAT anomalies between the Labrador and the Barents Sea in the Atlantic sector is observed during the past 50 years before 2000; a similar type of co-variability between the Sea of Okhotsk and the Bering Sea shelf in the Pacific is only evident since 1970s. Recent positive anomalies of net heat flux are more prominent in winter and spring in the Pacific sectors, and in summer in the Atlantic sectors. There is a reduced magnitude in wind mixing in the Sea of Okhotsk since 1980, in the Barents Sea since 2000, and in early spring/late winter in the Bering Sea shelf since 1995. Reduced sea-ice areas are seen over three out of four (except the Sea of Okhotsk) sub-arctic seas in recent decades, particularly after 2000 based on combined in situ and satellite observations (HadISST). This analysis provides context for the pan-regional synthesis of the linkages between climate and marine ecosystems.     

An Inshore Poleward Current in the NW of the Iberian Peninsula Detected from Satellite Images, and its Relation withG. catenatum and D. acuminata Blooms in the Galician Rias

Toxic Gymnodinium catenatum blooms usually occur in the Galician Rias at the end of the upwelling season, which necessitates a ban on harvesting shellfish extraction, with subsequent economic losses for this sector. One of the possible causes cited in the literature is the advection of populations from outside the area but no evidence was available to substantiate this.Oceanographic conditions at the end of the upwelling season in the NW coast of the Iberian Peninsula (39°–43°N) have been studied for the years 1986, 1990, 1995 and 1998. Sea surface temperature data from satellite images, wind data, drifter tracks and in situ oceanic data from the Galician Rias have been combined to clarify the oceanographic structures in the area at the commencement of the blooms. On the inner shelf, an inshore poleward current advecting warm water has been identified after the cessation of upwelling. On the middle and outer shelf, a tongue of cold water as a remnant of the previous upwelling continued to move southward. On the slope and offshore, the poleward counter current reported by several authors was detected carrying warm oceanic water northwards.It is suggested that the inshore poleward current, not previously reported in the literature, could advect initial populations of dinoflagellates to the Rias from northern Portuguese waters. This would explain why blooms such as G. catenatum have been found usually in Portuguese waters several weeks before the Galician Rias, showing an apparent northward movement, but cells of this toxic dinoflagellate species have not been found in waters of the offshore poleward counter current.     

Zooplankton distribution and cross-shelf transfer of carbon in an area of complex mesoscale circulation in the northern California Current

We conducted a research cruise in late summer (July–August) 2000 to study the effect of mesoscale circulation features on zooplankton distributions in the coastal upwelling ecosystem of the northern California Current. Our study area was in a region of complex coastline and bottom topography between Newport, Oregon (44.7°N), and Crescent City, California (41.9°N). Winds were generally strong and equatorward for >6 weeks prior to the cruise, resulting in the upwelling of cold, nutrient-rich water along the coast and an alongshore upwelling jet. In the northern part of the study area, the jet followed the bottom topography, creating a broad, retentive area nearshore over a submarine shelf bank (Heceta Bank, 44–44.4°N). In the south, a meander of the jet extended seaward off of Cape Blanco (42.8°N), resulting in the displacement of coastal water and the associated coastal taxa to >100 km off the continental shelf. Zooplankton biomass was high both over the submarine bank and offshore in the meander of the upwelling jet. We used velocities and standing stocks of plankton in the upper 100 m to estimate that 1×10⁶ m³ of water, containing an average zooplankton biomass of ～20 mg carbon m^?3, was transported seaward across the 2000-m isobath in the meandering jet each second. That flux equated to offshore transport of >900 metric tons of carbon each day, and 4–5×10⁴ tons over the 6–8 week lifetime of the circulation feature. Thus, mesoscale circulation can create disparate regions in which zooplankton populations are retained over the shelf and biomass can accumulate or, alternatively, in which high biomass is advected offshore to the oligotrophic deep sea.     

Bio-optical signatures and biogeochemistry from intense upwelling and relaxation in coastal California

The NSF-sponsored Coastal Ocean Processes Wind Events and Shelf Transport (WEST) experiment investigates the interplay between wind-driven transport and shelf productivity; while eastern boundary shelves are characterized by high productivity due to upward fluxes of nutrients into the euphotic zone, wind forcing also represents negative physical and biological controls via offshore transport and deep (light-limiting) mixing of primary producers. Although this interaction has been well documented for eastern boundary systems generally and for California specifically, one of the primary goals of WEST was to characterize more fully the interplay between positive and negative effects of wind stress, which result in the consistently elevated biological productivity in these shelf regions. During 3 month-long summer cruises (2000–2002) we observed extremes in upwelling/relaxation, using both in situ instrumentation and remotely sensed data. Relationships between optical and physical properties were examined, with emphasis on biogeochemical implications. During 2000, the WEST region was optically dominated by phytoplankton and covarying constituents. During 2001 and 2002, periods of more intense upwelling favorable winds, we observed a transition to optical properties dominated by detrital and inorganic materials. In all years, the continental shelf break provided a natural boundary between optically distinct shelf and open ocean waters. During 2002, we obtained discrete trace-metal measurements of particulate iron and aluminum; we develop a bio-optical proxy for acetic-acid leachable iron from backscatter and fluorescence, and demonstrate that particulate iron is not well correlated to traditional upwelling proxies such as macronutrients, temperature, and salinity. We conclude that the shelf break between ca. 100 and 200 m water depth serves as a natural break point between coastal and oceanic water masses in this region, and that the elevated biomass and productivity associated with this eastern boundary current regime is dominated by these iron rich, shallow shelf waters.     

Shelf-slope exchanges by frontal variability in a steep submarine canyon

We study the dynamics of a frontal jet and its short-timescale variability generated by the interaction with a submarine canyon using a limited-area fine-resolution three-dimensional coastal ocean model. The focus is on the steep and narrow Palamós Canyon located off the northeast Catalan coast (northwestern Mediterranean) that is characterized by the presence of a permanent along-slope density-driven current. First, we analyse the stationary circulation induced with different jet locations and show a deflection of the flow in the vicinity of the canyon. Significant vertical motions develop as a result of these current adjustments; the general pattern such as downwelling upstream of the canyon and upwelling downstream are always observed. Second, we analyse the circulation and exchanges associated with an onshore displacement of the jet; thus produces a meander propagating with the flow that interacts with the canyon. We find that the resulting three-dimensional patterns present an oscillation characterized by an intense downwelling followed by upwelling. As a result of this interaction, shelf-slope exchanges and vertical motions are enhanced in the area compared with the passing of a meander above a shelf that is not indented by a submarine canyon. The resulting horizontal transports through the Palamós canyon represent up to 10% of the along-shore fluxes on the shelf and appear to be sufficient to exchange the shelf water of the Gulf of Lions and Catalan sea in 2.5 years. Considering the number of canyons existing in the area, we can estimate an exchange of all the shelf waters in less than 3 months.     

Nearshore retention of upwelled waters north and south of Point Reyes (northern California)—Patterns of surface temperature and chlorophyll observed in CoOP WEST

Retentive embayments can be found near capes in upwelling regions, where they stand out as relatively warm features with higher chlorophyll a concentrations than surrounding waters. Within the area of the “Wind Events and Shelf Transport” study site (WEST) from the Gulf of the Farallones to Point Arena (37.5–39°N and 122.5–124°W), we describe two retentive embayments, extending approximately 20 km north of Point Reyes, and 30 km south of Point Reyes in the northern Gulf of the Farallones. These areas are identifiable from chlorophyll a (SeaWiFS) and sea-surface temperature (MODIS) satellite radiometry. Additional data from moorings and drifters were used to further characterize these retentive features. The persistence of these features is on time scales between 2 and 10 days, determined from time-series analysis of mooring data for chlorophyll and temperature. The alongshore wind stress is negatively correlated with chlorophyll and temperature mooring values. The time scale of persistence of these retentive features was similar to both the upwelling-relaxation timescale and the timescale of phytoplankton bloom development. The WEST region is notable in that the spring and summer is subject to the strongest wind stress along the US west coast, yet the coastal waters are highly productive. Based on our observations, we suggest that the apparent persistence of high biomass coastal waters in this strongly advective and wind-dominated system may be partially explained by the presence of retentive features identified in this study.     

Remotely sensed variability of temperature and chlorophyll in the southern Benguela: upwelling frequency and phytoplankton response

High-resolution (1km) satellite data from the NOAA AVHRR (Advanced Very High Resolution Radiometer) and OrbView-2 SeaWiFS (Sea-viewing Wide Field-of-view Sensor) are used to investigate the upper layer dynamics of the southern Benguela ecosystem in more detailed space and time scales than previously undertaken. A consistent time-series of daily sea surface temperature (SST) and chlorophyll a concentration images is generated for the period July 1998–June 2003, and a quantitative analysis undertaken. The variability in SST, upwelling and phytoplankton biomass is explored for selected biogeographic regions, with particular focus on intra-seasonal time scales. The location and emergence of upwelling cells are clearly identified along the length of the southern Benguela, being distinct on the narrow inner and the mid-continental shelves. Most notable is the rapidly pulsating nature of the upwelling, with intense warm/cold events clearly distinguished. The phytoplankton response to this physical forcing is described. Chlorophyll concentration on the inner shelf largely mirrors the pattern of SST variability, similarly dominated by event-scale processes. Over the mid-shelf, higher chlorophyll is observed throughout all seasons, although low biomass occurs during winter. The variability of the offshore extent of SST and chlorophyll is identified at locations of differing shelf width. Cooler upwelled water is confined primarily to the narrow inner-shelf, with event-scale pulses extending considerable distances offshore. Agulhas Current influences are readily observed, even on the Cape Peninsula inner-shelf. Chlorophyll concentrations vary considerably between the locations of differing shelf width. SST, upwelling and phytoplankton indices are derived for selected locations to quantify the intra-seasonal variations. The SST indices show marked temperature changes associated with rapid pulsation on the event scale. No strong seasonal signal is evident. In contrast, the upwelling indices display a strong seasonal signal, with most intense upwelling occurring in spring/summer in the south. The phytoplankton response to the seasonal upwelling index differs between the selected locations. This study concludes that, although low-resolution SST and chlorophyll data may be useful for investigating general patterns over large scales, higher resolution data are necessary to identify finer scale spatial and temporal variability, especially in the inshore coastal zones.     

Nutrient controls on new production in the Bodega Bay, California, coastal upwelling plume

A theoretical framework for the time-dependent processes leading to the high rates of new production in eastern boundary upwelling systems has been assembled from a series of past upwelling studies. As part of the CoOP WEST (Wind Events and Shelf Transport) study, new production in the Bodega Bay upwelling area and it's control by ambient nitrate and ammonium concentrations and the advective wind regime are described. Data and analyses are focused primarily on the WEST 2001 cruise (May–June 2001) when the two legs differed greatly in wind regimes but not nutrient concentrations. Elevated concentrations of ammonium in upwelled water with high nitrate were observed in both legs. Nitrate uptake by phytoplankton as a function of nitrate concentration was linear rather than Michaelis–Menten-like, modulated by inhibitory levels of ammonium, yielding coefficients that enable the specific nitrate uptake element of new production to be estimated from nutrient concentrations. The range of specific nitrate uptake rates for the two legs of WEST 2001 were similar, essentially a physiological response to nutrient conditions. However, the low “realization” of new production i.e. incorporation of biomass as particulate nitrogen that occurred in this system compared to the theoretical maximum possible was determined by the strong advective and turbulent conditions that dominated the second leg of the WEST 2001 study. These data are compared with other upwelling areas using a physiological shift-up model [Dugdale, R.C., Wilkerson, F.P., Morel, A. 1990. Realization of new production in coastal upwelling areas: a means to compare relative performance. Limnology and Oceanography 35, 822–829].     

Surface boundary-layer variability off Northern California, USA, during upwelling

A five-element mooring array is used to study surface boundary-layer transport over the Northern California shelf from May to August 2001. In this region, upwelling favorable winds increase in strength offshore, leading to a strong positive wind stress curl. We examine the cross-shelf variation in surface Ekman transport calculated from the wind stress and the actual surface boundary-layer transport estimated from oceanic observations. The two quantities are highly correlated with a regression slope near one. Both the Ekman transport and surface boundary layer transport imply curl-driven upwelling rates of about 3×10⁻⁴ m s⁻¹ between the 40 and 90 m isobaths (1.5 and 11.0 km from the coast, respectively) and curl-driven upwelling rates about 1.5×10⁻⁴m s⁻¹ between the 90 and 130 m isobaths (11.0 and 28.4 km from the coast, respectively). Thus curl-driven upwelling extends to at least 25 km from the coast. In contrast, upwelling driven by the adjustment to the coastal boundary condition occurs primarily inshore of the 40-m isobath. The upwelling rates implied by the differentiating the 40-m transport observations with the coastal boundary condition are up to 8×10⁻⁴ m s⁻¹. The estimated upwelling rates and the temperature–nitrate relationship imply curl-driven vertical nitrate flux divergences are about half of those driven by coastal boundary upwelling.     

Physical and biological features across an upwelling front in the southern Benguela

The upwelling front of the Cape Columbine upwelling centre was intensively studied, physically and biologically, along a repeated transect during December 1984 following a quiescent phase in the upwelling cycle. Three distinct zones were evident, an inshore zone influenced by upwelling, an offshore warm oligotrophic zone and a transitional frontal zone separating the two. Salinity proved to be a useful indicator of recent water movements. There was evidence of intrusions and mixing of water types within the frontal zone, possibly accounting for the elevated phytoplankton biomass recorded there. Floral and faunal changes occurred between the frontal and offshore zones, corresponding to the thermal front. The predominant flow was alongshore, with strong equatorward jet currents, making the interpretation of cross-shelf gradients difficult in this dynamic area. Aspects of the distributions of organisms and their productivity across the upwelling front are described with respect to the hydrographic parameters and associated flow-field.     

Spatial and temporal variation in surfclam (Spisula solidissima) larval supply and settlement on the New Jersey inner shelf during summer upwelling and downwelling

To examine the relationship between near-bottom larval surfclam concentrations and surfclam settlement at an inner continental shelf site off New Jersey (USA), four consecutive sets of settlement experiments were carried out at three stations at the Long-term Ecosystem Observatory (LEO-15) from 14 to 31 July 1997 during upwelling and downwelling. Two inshore stations were on the landward and seaward sides of Beach Haven Ridge at ∼12 m depth, and a third station was 8 km further offshore at ∼20 m depth. In each experiment, four replicate trays of azoic sand from Beach Haven Ridge were placed flush with the seafloor and exposed for 3–7 days. Larval surfclam concentrations were measured every 4 h at 1 m above the bottom (mab) using Moored, Automated, Serial Zooplankton Pumps at the three stations. At all three stations, larval surfclam concentrations (1 mab) were low during upwelling, and higher during and after downwelling. Pulses of highest larval surfclam concentrations coincided with the initial arrival of downwelled warm water. In addition, larval surfclam concentrations were higher at the two inshore stations than at the offshore station. Larval surfclam settlement in the trays was higher during and following downwelling than during upwelling at one inshore station and at the offshore station. At the other inshore station (landward of Beach Haven Ridge), surfclam settlement did not increase during and following downwelling. Overall, surfclam settlement was higher inshore than offshore. The results indicate that spatial and temporal variation in larval surfclam supply was controlled by upwelling and downwelling circulation and that surfclam settlement was influenced by larval supply. Bottom flows across Beach Haven Ridge during a storm may have reduced larval surfclam settlement on the upcurrent side of the ridge, affecting initial densities on a small (∼1 km) scale.     

Three-dimensional circulation in the NW Africa coastal transition zone

High-resolution data collected southeast of the Canary Islands during late winter 2006 are analyzed to describe the hydrography and three-dimensional circulation in the coastal transition zone off NW Africa. The data are optimally interpolated over a regular grid, the geostrophic velocity field is calculated and the Q-vector formulation of the omega equation is used to compute the quasi-geostrophic (QG) mesoscale vertical velocity. The coastal transition zone is divided into upwelling, frontal and offshore regions with distinct physical and dynamic characteristics. The upwelling region is characterized by cold and weakly stratified waters flowing towards the equator, with a poleward undercurrent of approximately 0.05 m s⁻¹ over the continental slope. The frontal region exhibits a southwestward baroclinic jet associated with cross-shore raising isopycnals; the jet transport is close to 1 Sv, with maximum velocities of 0.18 m s⁻¹ at surface decreasing to 0.05 m s⁻¹ at 300 db. Vertical sections across the frontal region show the presence of deep eddies probably generated by the topographic blocking of the islands to the southward current, as well as much shallower eddies that likely have arisen as instabilities of the baroclinic upwelling jet. The QG mesoscale vertical velocity field is patchy, estimated to range from −18 to 12 m day⁻¹, with the largest absolute values corresponding to an anticyclonic eddy located south of Fuerteventura Island. These values are significantly larger than estimates for other vertical velocities: diapycnal vertical velocities associated with mixing in the frontal region (a few meters per day), and wind-induced vertical velocities (non-linear Ekman pumping arising from the interaction between the wind stress and the background vorticity, maximum values of a few meters per day; linear Ekman pumping due to the divergence of Ekman transport, a fraction of a meter per day; or the coastal constraint in the upwelling region, about 0.7 m day⁻¹). However, the patchiness in both the QG mesoscale vertical velocity and the non-linear Ekman pumping velocity cause their integrated vertical transports to be one order of magnitude smaller than either coastal Ekman transport (0.08 Sv), integrated linear Ekman pumping (−0.05 Sv) or diapycnal transfer (about 0.1–0.2 Sv). The pattern of the near-surface fluorescence field is a good indicator of these different contributions, with large homogeneous values in the coastal upwelling region and a patchy structure associated with the offshore mesoscale structures.     

The phytoplankton bloom response to wind events and upwelled nutrients during the CoOP WEST study

In the coastal waters off northern California, seasonal wind-driven upwelling supplies abundant nutrients to be processed by phytoplankton productivity. As part of the Coastal Ocean Processes: Wind Events and Shelf Transport (CoOP WEST) study, nutrients, CO₂, size-fractionated chlorophyll, and phytoplankton community structure were measured in the upwelling region off Bodega Bay, CA, during May–June 2000, 2001 and 2002. The ability of this ecosystem to assimilate nitrate (NO₃) and silicic acid/silicate (Si(OH)₄) and accumulate particulate material (i.e. phytoplankton) was realized in all 3 years, following short events of upwelling-favorable winds, followed by periods of relaxed winds. This was observed as phytoplankton blooms, dominated by chlorophyll in cells greater than 5 μm in diameter, that reduced the ambient nutrients to zero. These communities were located over the near-shore shelf (<100 m depth) and were dominated by diatoms. An optimal window of 3–7 days of relaxed winds, following an upwelling pulse, was required for chlorophyll accumulation. The large-celled phytoplankton that result are likely important players in coastal new production and carbon cycling.