Adriana Huyer

Coastal Upwelling in the California Current System

Abstract 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.

Upwelling-driven nearshore hypoxia signals ecosystem and oceanographic changes in the northeast Pacific.

Seasonal development of dissolved-oxygen deficits (hypoxia) represents an acute system-level perturbation to ecological dynamics and fishery sustainability in coastal ecosystems around the globe. Whereas anthropogenic nutrient loading has increased the frequency and severity of hypoxia in estuaries and semi-enclosed seas, the occurrence of hypoxia in open-coast upwelling systems reflects ocean conditions that control the delivery of oxygen-poor and nutrient-rich deep water onto continental shelves. Upwelling systems support a large proportion of the worlds fisheries, therefore understanding the links between changes in ocean climate, upwelling-driven hypoxia and ecological perturbations is critical. Here we report on the unprecedented development of severe inner-shelf (<70 m) hypoxia and resultant mass die-offs of fish and invertebrates within the California Current System. In 2002, cross-shelf transects revealed the development of abnormally low dissolved-oxygen levels as a response to anomalously strong flow of subarctic water into the California Current System. Our findings highlight the sensitivity of inner-shelf ecosystems to variation in ocean conditions, and the potential impacts of climate change on marine communities.

The nature of the cold filaments in the California current system

Data from the Coastal Transition Zone (CTZ) experiment axe used to describe the velocity fields and water properties associated with cold filaments in the California Current. Combined with previous field surveys and satellite imagery, these show seasonal vaxiability with maximum dynamic height ranges and velocities in summer and minimum values in late winter and early spring. North of Point Arena (between 39oN and 42oN) in spring-summer the flow field on the outer edge of the cold water has the chaxacter of a meandering jet, carrying fresh, nutrient-poor water from farther north on its offshore side and cold, salty, nutrient-rich water on its inshore side. At Point Arena in midsummer, the jet often flows offshore and continues south without meandering back onshore as strongly as it does faxther north. The flow field south of Point Arena in summer takes on more of the chaxacter of a field of mesoscale eddies, although the meandering jet from the north continues to be identifiable. The conceptual model for the May-July period between 36 oN and 42oN is thus of a surface jet that meanders through and interacts with a field of eddies; the eddies are more dominant south of 39oN, where the jet broadens and where multiple jets and filaments axe often present. At the surface, the jet often separates biological communities and may appeax as a barrier to cross-jet transport, especially north of Point Arena early in the season (March-May). However, phytoplankton pigment and nutrients are carried on the inshore flank of the jet, and pigment maxima axe sometimes found in the core of the jet. The biological effect of the jet is to define a convoluted, 100 to 400-km-wide region next to the coast, within which much of the richer water is contained, and also to carry some of that richer water offshore in meanders along the outer edge of that region.

The Leeuwin Current off Western Australia, 1986–1987

Abstract The Leeuwin Current in the Indian Ocean off Western Australia differs from the other major eastern boundary currents, e.g., California Current, since it flows rapidly poleward against the prevailing equatorward wind. The first large-scale study of the Leeuwin Current was conducted between North West Cape (22°S) and the south-western corner of Australia (35°S) from September 1986 to August 1987. As part of this Leeuwin Current Interdisciplinary Experiment (LUCIE), current meters were deployed along the shelf-edge (from 22° to 35°S) and across the shelf and upper slope (at 29.5° and 34°S), and CTD surveys were made out from the shelf at several latitudes. Except for about one month (January) the flow between the surface and about 250 m was strongly poleward within 100 km of the shelf-edge, with a poleward transport of about 5 Sv (Sv ≡ 6 m3 s−1). The 325-day mean currents at the shelf-edge were poleward at about 10 cm s−1, opposing a mean equatorward wind stress of 0.3 dyn cm−2. The monthly mean cur...

Horizontal transport and the distribution of nutrients in the Coastal Transition Zone off northern California: Effects on primary production, phytoplankton biomass and species composition

Conductivity-temperature-depth surveys during 1988 encountered strong baroclinic jets that were evident in acoustic Doppler current profiler and hydrographic data. During June and July 1988 a filament with high surface nitrate, high chlorophyll, abundant populations of neritic centric diatoms, and higher rates of primary production was evident perpendicular to the coast between Point Arena and Point Reyes. However, the high-nutrient and phytoplankton regions were not in the baroclinic jets but were south and inshore of them. Surface water transported offshore by the strong baroclinic jets was found to have relatively low nutrient content, suggesting that the jets themselves do not carry significant levels of coastally upwelled, high-nutrient water to the ocean interior. The low nutrient and salinity content of the jet suggests that the water originated several hundred kilometers upstream. Although the jets themselves do not appear to transport significant levels of nutrients directly from the coastal regime to the oceanic regime, dynamic processes associated with a meandering jet are likely responsible for high surface nutrients found several hundred kilometers offshore. Processes such as upwelling along the southern edge of the seaward jet result in significant enrichment of the coastal transition zone and in large blooms of neritic diatoms. During 1988 the high-nutrient, high-phytoplankton filament was present when the survey sequence began but then decayed after a month. The surface and subsurface nitrate fields were coherent with the dynamic topography field throughout the survey sequence; however, the surface and integrated chlorophyll fields were coherent only through the first two surveys. A decrease in phytoplankton biomass began during the third survey coincident with physical changes which occurred in that time frame: (1) an intensification of the undercurrent and (2) changes in the surface circulation from predominantly offshore to predominantly longshore. Understanding the processes responsible for the uncoupling between biology and physics is paramount for realistic biological models of this region.

The structure of the transition zone between coastal waters and the open ocean off northern California, winter and spring 1987

Physical and biological fields in the coastal transition zone off northern California were measured during February, March, May and June 1987 in an extended alongshore region between 60 km and 150 km offshore. The spring transition, as seen in coastal sea level and winds, occurred in mid-March. Surface variability during the two spring cruises was stronger and of larger scale than that seen during the two winter cruises. An equatorward-tending current, flowing along the boundary between low steric sea level inshore and high steric sea level offshore, dominated both the directly-measured (acoustic Doppler current profiler) and geostrophic current fields during spring. Current jets of comparable strength directed both offshore and onshore were seen off Cape Mendocino and Point Arena; these evolved significantly in the 3 weeks between cruises. Inshore of the current, properties associated with upwelled water were found near the surface, including low temperature and high salinity, nutrients and chlorophyll; offshore of the current, waters were warmer, less saline, lower in nutrients and more oligotrophic. Geostrophic and directly measured volume transports in the current were about 2–3 Sv. Isopycnals inshore of the spring upwelling front were displaced vertically by O(40–80 m) from their depths during the winter survey; these displacements extended deep into the water column and were largely independent of depth between 100 and 400 m. Surface mixed layers tended to be deep in winter and shallower inshore of the upwelling front in spring. A connection between the equatorward-tending frontal jet off northern California and the more well-studied California Current further south is suggested by the similarity of their transports and of their dynamic height values.

The physical environment of the Peruvian upwelling system

Abstract Knowledge of the characteristics of the physical environment of the Peru coastal upwelling zone has increased greatly during the past decade. Observations made during 1976 and 1977, many made as part of the Coastal Upwelling Ecosystem Analysis JOINT-II expedition, provide the basis for an interpretative description of the physical processes and phenomena occurring in Peru coastal waters. Four important phenomena are discussed: wind-driven upwelling, the poleward undercurrent, surface mixed layer deepening, and low-frequency coastal-trapped waves. The coastal winds were invariably favorable for coastal upwelling, even during the 1976 El Nino. The agreement between the offshore transport in the relatively shallow (30 m) surface layer and the Ekman transport, deduced from the local wind, was good for both the mean and variable state. The agreement with the deeper onshore transport was less good, consistent with the marked three-dimensionally and spatial variability of the upwelling. The poleward undercurrent was apparently continuous from 5°S to at least 15°S, flowing just below the surface layer over the continental shelf and slope, and supplying the upwelling water. The variability in the alongshore velocity field was dominated by baroclinic free coastal-trapped waves with periods of 5–20 days. Although there is appreciable temporal and spatial variability in the Peru coastal upwelling system, the upwelling, the undercurrent, and the low-frequency coastal-trapped waves were ubiquitous.

Currents and water masses of the Coastal Transition Zone off northern California, June to August 1988

In summer 1988, we made repeated mesoscale surveys of a grid extending 200 km offshore between 37°N and 39°N in the coastal transition zone off northern California, obtaining continuous acoustic Doppler current profiler data and conductivity-temperature-depth data at standard stations 25 km apart on alongshore sections 40 km apart. All surveys showed a baroclinic equatorward jet, with core velocities of >50 cm s−1 at the surface decreasing to about 10 cm s−1 at 200 m, a width of 50–75 km, and a baroclinic transport of about 4 Sv. The core of the jet lay between the 8.6 and 9.4 m2 s−2 contours of geopotential anomaly (relative to 500 dbar). Three current meter moorings, deployed at 25-km separation across the jet at the beginning of the survey sequence, provided time-series of the velocity; throughout the 37-day deployment, at least one mooring was within the core defined by the 8.6 and 9.4 m2 s−2 contours. The jet flowed southwestward across the grid from late June until mid-July 1988, when the jet axis moved offshore in the north and onshore in the southern portion of the grid. Temperature-salinity analysis shows that jet waters can be distinguished from both the freshly upwelled coastal waters and the offshore waters. Isopycnal maps indicate alongshore advection of relatively fresh, cool water from farther north, as well as small-scale patchiness not resolved by our survey grid. The baroclinic jet observed here may be continuous with the core of the California Current off central California. The later surveys clearly showed a poleward-flowing undercurrent adjacent to the continental slope, with core velocities up to 20 cm s−1 at depths of 150–250 m. Its baroclinic transport (relative to 500 dbar) increased from 1.0 Sv between late June and early August 1988. Within the survey grid, there was a definite onshore gradient in the characteristics of North Pacific Intermediate Water. The subsurface waters adjacent to the continental margin were warmer and more saline than those offshore, indicating net northward advection by the California Undercurrent over the inshore 100 km and equatorward advection farther from shore.

Cold halocline, increased nutrients and higher chlorophyll off Oregon in 2002

Received 24 March 2003; accepted 18 June 2003; published 31 July 2003. [1] Observed changes in the nutrient levels in the halocline of the California Current during 2002 indicated a natural eutrophication that was accompanied by increased chlorophyll and oxygen in surface water. Decreased oxygen in the lower water column over the shelf indicated that much of the phytoplankton production was respired rather than passedontohighertrophiclevels.In2002thehaloclinewater was >1� C colder than usual and 0.5� C colder than any previous observation. Four transect lines off the coast of Oregonshowa50%increaseinnitrate,phosphateandsilicate at 33 psu in 2002 compared to 1998–2001. The increase in nutrients resulted in a 2-fold increase in chlorophyll standing stocks during the summer of 2002 compared with the preceding four years. A significant portion of the increased production was subsequently respired resulting in low oxygen water over the shelf. INDEX TERMS: 4215 Oceanography: General: Climate and interannual variability (3309); 4845 Oceanography: Biological and Chemical: Nutrients and nutrient cycling; 4283 Oceanography: General: Water masses. Citation: Wheeler, P. A., A. Huyer, and J. Fleischbein, Cold halocline, increased nutrients and higher chlorophyll off Oregon in 2002, Geophys. Res. Lett., 30(15), 8021, doi:10.1029/ 2003GL017395, 2003.

Statistical properties of near‐surface flow in the California Coastal Transition Zone

During the summers of 1987 and 1988, 77 near-surface satellite-tracked drifters were deployed in or near cold filaments near Point Arena, California (39°N), and tracked for up to 6 months as part of the Coastal Transition Zone (CTZ) program. The drifters had large drogues centered at 15 m, and the resulting drifter trajectory data set has been analyzed in terms of its Eulerian and Lagrangian statistics. The CTZ drifter results show that the California Current can be characterized in summer and fall as a meandering coherent jet which on average flows southward to at least 30°N, the southern end of the study domain. From 39°N south to about 33°N, the typical core velocities are of O(50 cm s−1) and the current meanders have alongshore wavelengths of O (300 km) and onshore-offshore amplitude of O(100–200 km). The lateral movement of this jet leads to large eddy kinetic energies and large eddy diffusivities, especially north of 36°N. The initial onshore-offshore component of diffusivity is always greater than the alongshore component in the study domain, but at the southern end, the eddy diffusivity is more isotropic, with scalar single particle diffusivity (Kxx + Kyy) of O(8 × 107 cm2 s−1). The eddy diffusivity increases with increasing eddy energy. Finally, a simple volume budget for the 1988 filament observed near 37°N off Point Arena suggests that subduction can occur in a filament at an average rate of O (10 m d−1) some 200 km offshore, thus allowing the cold water initially in the filament core to sink below the warmer ambient water by the time the surface velocity core has turned back onshore. This process explains why satellite temperature and color imagery tend to “see” only flow proceeding offshore.