California Undercurrent Research Articles

Looping RAFOS floats in the California Current System

Looping motions of RAFOS floats deployed off the Central California coast between 1992 and 2010 are described. Most floats were deployed in the California Undercurrent. Floats at 300m were observed to loop 26% of the time, with anticyclonic rotation observed about twice as often as cyclonic rotation. Characteristics of anticyclonic rotation at 300m included median swirl speeds of 14cm/s for diameters of 59km and 14-day periods. Long-lived (>70 days and at least 8 consecutive loops) anticyclonic loopers are identified as California Undercurrent eddies (or “cuddies”). One cuddy was observed to move southwestward 1650km over a period of 520days before observations ended. Kinematically, cuddies are similar to Mediterranean Water eddies (meddies) in the Iberian Basin, except that meddies are somewhat stronger, larger and deeper.Over the slope, most cuddies were formed between either Pt. Sur and Pt. Reyes or Cape Mendocino and Cape Blanco. The region north of Cape Mendocino is estimated to generate six cuddies each year. These eddies form from waters on the inshore side of the California Undercurrent as they move past Cape Mendocino. Topographic drag is the most likely formation mechanism.Two of eight deep floats were observed to loop, one anticyclonically (1545m) for 279 days and the other cyclonically (1167m) for 192 days. Looping for these floats ended near Taney Seamounts.

Spatial distribution and offshore export of total organic carbon along the eastern boundary of the Subtropical North Pacific

Data collected in February, 2003, along the upper continental slope of western North America between Monterey Bay, California (37°N, 122°W), and Cabo San Lucas, Mexico (23°N, 118°W), document, for the first time, the alongshore distribution of total organic carbon (TOC). Highest TOC concentrations (>70 µM) were observed for waters above the nitracline and associated with both California Current and southern surface waters. The northward advection of tropical waters in the California Undercurrent did not have any discernible impact on TOC distributions. An estimate of the average rate at which TOC in surface waters was exported offshore by Ekman transport in February 2003 was 1.73 × 103 kg C yr−1 for each meter of coastline. The offshore flux estimate is thought to be conservative with respect to the annual mean offshore flux because the offshore Ekman transport and primary production increase in late spring and early summer and the contribution of upwelling filaments has not been considered. Analysis of TOC contributions to pelagic respiration suggested that TOC accounted for 45% of the oxygen decrease in southern oxic waters. In California Current and oxygen minimum zone waters, TOC did not contribute to pelagic respiration.

Amplification of hypoxic and acidic events by La Niña conditions on the continental shelf off California

[1] Low-oxygen and low-pH events are an increasing concern and threat in the Eastern Pacific coastal waters, and can be lethal for benthic and demersal organisms on the continental shelf. The normal seasonal cycle includes uplifting of isopycnals during upwelling in spring, which brings low-oxygen and low-pH water onto the shelf. Five years of continuous observations of subsurface dissolved oxygen off Southern California, reveal large additional oxygen deficiencies relative to the seasonal cycle during the latest La Niña event. While some changes in oxygen related to the isopycnal depression/uplifting during El Niño/La Niña are not unexpected, the observed oxygen changes are 2–3 times larger than what can be explained by cross-shore exchanges. In late summer 2010, oxygen levels at mid-depth of the water column reached values of 2.5 ml/L, which is much lower than normal oxygen levels at this time of the seasons, 4–5 ml/L. The extra uplifting of isopycnals related to the La Niña event can explain oxygen reductions only to roughly 3.5 ml/L. We find that the additional oxygen decrease beyond that is strongly correlated with decreased subsurface primary production and strengthened poleward flows by the California Undercurrent. The combined actions of these three processes created a La Niña-caused oxygen decrease as large and as long as the normal seasonal minimum during upwelling period in spring, but later in the year. With a different timing of a La Niña, the seasonal oxygen minimum and the La Niña anomaly could overlap to potentially create hypoxic events of previously not observed magnitudes.

Eddy properties in the California Current System

[1] Eddy detection and tracking algorithms are applied to both satellite altimetry and a high-resolution (dx = 5 km) climatological model solution of the U.S. West Coast to study the properties of surface and undercurrent eddies in the California Current System. Eddy properties show remarkable similarity in space and time, and even somewhat in polarity. Summer and fall are the most active seasons for undercurrent eddy generation, while there is less seasonal variation at surface. Most of the eddies have radii in the range of 25–100 km, sea level anomaly amplitudes of 1–4 cm, and vorticity normalized by f amplitudes of 0.025–0.2. Many of the eddies formed near the coast travel considerable distance westward with speeds about 2 km/day, consistent with the β effect. Anticyclones and cyclones show equatorward and poleward displacements, respectively. Long-lived surface eddies show a cyclonic dominance. The subsurface California Undercurrent generates more long-lived anticyclones than cyclones through instabilities and topographic/coastline effects. In contrast, surface eddies and subsurface cyclones have much more widely distributed birth sites. The majority of the identified eddies have lifetimes less than a season. Eddies extend to 800–1500 m depth and have distinctive vertical structures for cyclones and anticyclones. Eddies show high nonlinearity (rotation speed higher than propagation speed) and hence can be efficient in transporting materials offshore.

The California Current System: A multiscale overview and the development of a feature-oriented regional modeling system (FORMS)

Over the past decade, the feature-oriented regional modeling methodology has been developed and applied in several ocean domains, including the western North Atlantic and tropical North Atlantic. This methodology is model-independent and can be utilized with or without satellite and/or in situ observations. Here we develop new feature-oriented models for the eastern North Pacific from 36° to 48°N – essentially, most of the regional eastern boundary current. This is the first time feature-modeling has been applied to a complex eastern boundary current system. As a prerequisite to feature modeling, prevalent features that comprise the multiscale and complex circulation in the California Current system (CCS) are first overviewed. This description is based on contemporary understanding of the features and their dominant space and time scales of variability. A synergistic configuration of circulation features interacting with one another on multiple and sometimes overlapping space and time scales as a meander-eddy-upwelling system is presented. The second step is to define the feature-oriented regional modeling system (FORMS). The major multiscale circulation features include the mean flow and southeastward meandering jet(s) of the California Current (CC), the poleward flowing California Undercurrent (CUC), and six upwelling regions along the coastline. Next, the typical synoptic width, location, vertical extent, and core characteristics of these features and their dominant scales of variability are identified from past observational, theoretical and modeling studies. The parameterized features are then melded with the climatology, in situ and remotely sensed data, as available. The methodology is exemplified here for initialization of primitive-equation models. Dynamical simulations are run as nowcasts and short-term (4–6 weeks) forecasts using these feature models (FM) as initial fields and the Princeton Ocean Model (POM) for dynamics. The set of simulations over a 40-day period illustrate the applicability of FORMS to a transient eastern boundary current region such as the CCS. Comparisons are made with simulations initialized from climatology only. The FORMS approach increases skill in several factors, including the: (i) maintenance of the low-salinity pool in the core of the CC; (ii) representation of eddy activity inshore of the coastal transition zone; (iii) realistic eddy kinetic energy evolution; (iv) subsurface (intermediate depth) mesoscale feature evolution; and (v) deep poleward flow evolution.

Poleward reach of the California Undercurrent extension

The California Undercurrent is known to transport relatively warm, high‐salinity, nutrient‐rich water from the equatorial Pacific to Vancouver Island along the western continental slope of North America. This transport helps maintain the high productivity of the eastern boundary California Current system. In this study, we use several decades of water property survey data for the coasts of Oregon, Washington, British Columbia, and Alaska to show that equatorial Pacific water carried poleward by the undercurrent can eventually reach the Aleutian Islands, roughly 11,000 km from the source region. Long‐term current meter records confirm the undercurrent as far north as Vancouver Island, where the current is found to be weakest in spring but then to strengthen through the summer and fall before merging with the wind‐forced, poleward flowing Davidson Current in winter. The core depth of the equatorial water increases from 150 m ± 25 m off northwest Washington (near the northern end of the western North America coastal upwelling domain) to 225 m ± 25 m off southeast Alaska (near the southern end of the Gulf of Alaska coastal downwelling domain).

Temporal variability of the physical and chemical water characteristics at a coastal monitoring observatory: Station ENSENADA

The temporal variability of the physical and chemical conditions of coastal waters off Ensenada, Baja California (Mexico) was characterized. A historical analysis was made based on 11 years (1998–2008) of temperature and salinity data records measured quarterly by IMECOCAL, along a transect perpendicular to the coast (CalCOFI line 100). Moreover, the physical and chemical conditions at a coastal monitoring observatory called station ENSENADA were described using a 2-year data series (October 2006–November 2008) obtained with improved temporal resolution. The historical analysis of line 100 showed marked seasonal variability in the thermohaline conditions associated with fluctuations in the flow of the equatorward California Current and the poleward California Undercurrent, as well as with coastal upwelling events whose magnitude and frequency increase towards spring–summer. Interannual variability was also observed, related to warm and/or cold ENSO phases that modify the characteristics of the water column in this coastal region. The most striking characteristics of the interannual variability at station ENSENADA were La Niña conditions recorded from summer 2007 to mid 2008. During this cold ENSO phase, temperature, salinity, dissolved oxygen, density, and dissolved inorganic carbon data revealed the anomalous presence of subsurface water at the surface layers in spring 2008. Results suggest that the coastal observatory is sensitive to the temporal variability of hydrographic conditions on shelf coastal waters (<50 km) off Ensenada in the northern BC region. Consequently, station ENSENADA would be a good location to high-frequency monitors the oceanographic conditions of the transitional region between tropical/subtropical and subarctic systems of the California Current System.

Dynamics of wind‐driven upwelling and relaxation between Monterey Bay and Point Arena: Local‐, regional‐, and gyre‐scale controls

In north and central California, equatorward winds drive equatorward flows and the upwelling of cold dense water over the shelf during the midspring and summer upwelling season. When the winds temporarily weaken, the upwelling flows between Point Reyes and Point Arena “relax,” becoming strongly poleward over the shelf. Analytical and numerical models are used to describe the effect of alongshore variability of winds, bathymetry, and basin‐scale pressure gradients on the strength of upwelling and its relaxation. Alongshore winds weaken to the south of Point Reyes, and the shelf becomes narrower from Point Reyes to Monterey Bay. Both of these lead to reduced upwelling at and to the north of Point Reyes, causing an alongshore gradient of temperature and density on the shelf. These alongshore gradients lead to an along‐isobath pressure gradient over the shelf that drive the relaxation flows. A simple analytical model is used to explain the dynamics, magnitude, and structure of the relaxation flows. The modeling also suggests that the depth of origin of the upwelled waters, and thus their temperature, is controlled by the along‐isobath pressure gradient that exists over the continental slope. This along‐slope pressure gradient is also responsible for the California undercurrent in this region. This pressure gradient is not generated in a model of the Californian coast extending from 32°N to 42°N and integrated for several months, suggesting it is caused by dynamics whose spatial or temporal scales are larger than the Californian coast and/or longer than several months.

Mean structure and seasonal variability of the poleward undercurrent off southern California

The magnitude, location, and extent of the California Undercurrent (CUC) off southern California is investigated using cruises of the California Cooperative Oceanic Fisheries Investigations (CalCOFI) from 1993 to 2003 which provide hydrographic, biochemical, and acoustic Doppler current profiler (ADCP) velocity data on a quarterly basis. This study is the first use of the decade‐long ADCP time series; it improves on prior geostrophic calculations by providing an absolute velocity reference for estimates of currents and transport. The long‐term mean reveals two undercurrent cores in the region south of Point Conception and north of Baja, California: one in the region of the continental slope within the Southern California Bight (SCB) and a second off the Santa Rosa Ridge (SRR). A single core is observed off Point Conception. Spiciness is found to be a good indicator of the presence of the CUC; however, direct velocity observations or a deep reference level are required to resolve the full strength of the CUC cores. In particular, the core off the SRR would be almost entirely missed by geostrophic calculations relative to 500 m, the maximum depth sampled by CalCOFI. The undercurrent transport off Point Conception is estimated to be about 1.7 ± 0.1 Sv, slightly less than the sum of that estimated for the SCB (1.0 ± 0.1 Sv) and that estimated off the SRR (1.1 ± 0.1 Sv) and consistent with some of the flow turning offshore at Point Conception. The CUC in the SCB is strongest in summer, while that off the SRR is strongest in fall. The CUC off Point Conception is strong in both summer and fall, reflecting the confluence of these two branches. Interannual variability is also present, and the velocity and spiciness of the CUC appear to peak during El Niño periods.

Late Quaternary paleoproductivity history on the Vancouver Island margin, western Canada: a multiproxy geochemical studyThis article is one of a series of papers published in this Special Issue on the theme Polar Climate Stability Network.

Multiproxy analysis of a 38 m long sediment core recovered from the Vancouver Island margin (48.97°N, 127.04°W, water depth 1243 m) has yielded a millennial-scale history of upwelling and export production over the last ∼50 ka. Higher concentrations of marine organic carbon, opal, and trace Mo suggest that production was higher, and sedimentary pore waters more anoxic, during the warm Holocene, Bølling–Allerød, and interstadial events between 31 and 44 ka BP. Relatively lower production and higher inputs of terrigenous organic matter occurred during the last glacial (14.7–31 ka BP; Cordilleran ice sheet proximal to coring site at ∼19.5 ka BP) and from 44–50.4 ka BP. Enrichments in sedimentary δ15N during interstadial events are interpreted to reflect episodic delivery and upwelling of isotopically heavy nitrate to the surface waters and subsequent vectoring to the seafloor via settling planktonic detritus. Similar patterns are seen in southern California and other areas along the western margin of North America, implying that heavier nitrate generated by denitrification in the Eastern Tropical North Pacific has in the past been carried northward in the California Undercurrent at least as far as central Vancouver Island. This inference is consistent with modern hydrographic observations in the region. Comparison of the coherent Vancouver Island, Oregon, California, and northwest Mexico margin records with late Pleistocene climate history in Greenland reinforces the conclusion that a tight physical and biogeochemical coupling has existed for at least 50 ka between the North Atlantic and North American margin waters, including those off Vancouver Island.

Development, implementation and evaluation of a data-assimilative ocean forecasting system off the central California coast

Abstract The development and implementation of a real-time ocean forecast system based on the Regional Ocean Modeling System (ROMS) off the coast of central California are described. The ROMS configuration consists of three nested modeling domains with increasing spatial resolutions: the US West coastal ocean at 15-km resolution, the central California coastal ocean at 5 km, and the Monterey Bay region at 1.5 km. All three nested models have 32 vertical sigma (or terrain-following) layers and were integrated in conjunction with a three-dimensional variational data assimilation algorithm (3DVAR) to produce snapshots of the ocean state every 6 h (the reanalysis) and 48-h forecasts once a day. This ROMS forecast system was operated in real time during the field experiment known as the Autonomous Ocean Sampling Network (AOSN-II) in August 2003. After the field experiment, a number of improvements were made to the ROMS forecast system: more data were added in the reanalysis with more careful quality control procedures, improvements were made in the data assimilation scheme, as well as model surface and side boundary conditions. The results from the ROMS reanalysis are presented here. The ROMS reanalysis is first compared with the assimilated data as a consistency check. An evaluation of the ROMS reanalysis against the independent measurements that are not assimilated into the model is then presented. This evaluation shows the mean differences in temperature and salinity between reanalysis and observations to be less than 1 °C and 0.2 psu (practical salinity unit), respectively, with root-mean-square (RMS) differences of less than 1.5 °C and 0.25 psu. Qualitative agreement is found between independent current measurements and the ROMS reanalysis. The agreement is particularly good for the vertically integrated current along the offshore glider tracks: the ROMS reanalysis can realistically reproduce the poleward California Undercurrent. Reasonably good agreement is found in the spatial patterns of the surface current as measured by high-frequency (HF) radars. Preliminary results concerning the ROMS forecast skill and predictability are also presented. Future plans to improve the ROMS forecast system with a particular focus on assimilation of HF radar current measurements are discussed.

Influences of the Juan de Fuca Eddy on circulation, nutrients, and phytoplankton production in the northern California Current System

A diagnostic circulation model and water mass analyses are used to examine variability in the structure and circulation of the Juan de Fuca Eddy, a highly productive region at the northern end of the California Current. Results from three years of field studies demonstrate that the eddy increases in spatial extent from early to late summer as the vertically averaged contribution of California Undercurrent source water grows from ∼60% in June to ∼80% in September. Typical near‐surface eddy radii range from ∼15 km in the early summer to ∼30 km in September and increase with depth. Below 100 m, eddy radii are ∼40 km. Fresher water, associated with the estuarine outflow from the Juan de Fuca Strait, is advected around the eddy margin. During southward wind conditions, the combination of cyclonic geostrophic flow and wind‐driven currents in the surface Ekman layer cause the eddy to be “leaky” on its southern perimeter. Eddy surface circulation becomes more retentive (up to ∼32 d observed) during periods of weak winds or frequent northward reversals. The presence of the eddy facilitates large inputs of dissolved inorganic nutrients into the region through two mechanisms: doming of California Undercurrent water within the eddy and enhanced cross‐shelf advection of Juan de Fuca Strait outflow. The combination of these sources results in a persistent, broad (100 km offshore) region of elevated macronutrients. The retentive circulation patterns combined with persistent nutrient supply may favor the development of toxigenic diatom blooms of Pseudo‐nitzschia species in this region.

Modeling the generation of the Juan de Fuca Eddy

Numerical simulations with the Regional Ocean Modeling System are used to study the generation of the cyclonic Juan de Fuca Eddy located off the entrance of Juan de Fuca Strait in summer. An initial simulation forced with average summer upwelling favorable winds, tides, and buoyancy boundary conditions that maintain an estuarine flow in Juan de Fuca Strait produces an eddy and currents that are in reasonable agreement with observations. Sensitivity studies are then carried out to explore the importance of these three forcing mechanisms. The relative proximity of dense water in the bottom estuarine flow entering the strait is shown to lead to enhanced upwelling off Cape Flattery when either wind or tidal forcing is applied. This upwelled water then mixes with the estuarine outflow and is advected offshore. The tidal upwelling arises through three mechanisms: M2 vertical excursions of nearly 20 m at 50 m depth west of the cape on the flood tide; strong tidally rectified vertical velocities west of the cape; and the spilling of denser bottom water over the western wall of Juan de Fuca Canyon on the ebb tide. The cyclonic eddy is a consequence of geostrophic adjustment to the doming isopycnals that arise from the upwelling. These model simulations refute an earlier hypothesis that the eddy is generated when California Undercurrent water is drawn up Tully Canyon and onto the Vancouver Island shelf, suggesting instead that these canyon dynamics play only a secondary role in maintaining the eddy once it is formed.

Nonhydrostatic simulations of the regional circulation in the Monterey Bay area

The regional circulation in the vicinity of Monterey Bay is complex and highly variable. We use a one‐way coupled, nonhydrostatic version of the Dietrich Center for Air Sea Technology (DieCAST) ocean model to simulate the regional circulation. Seasonally varying local wind stress, topographic irregularities, coastal upwelling, and forcing from the open ocean are all important in this region. Satellite imagery often shows a cyclonic eddy inside the bay and an anticyclonic eddy outside the bay. The offshore anticyclonic eddy is also associated with a year‐round anticyclonic eddy over the Monterey Submarine Canyon (MSC). The offshore eddy is better organized during winter. It is found that the California Undercurrent (200–400 m) does not enter the bay itself but is diverted offshore past the entrance of the bay, presumably to reform farther north along the coast. The main branch flows northward contributing to the deep anticyclonic eddy located approximately 50 km offshore of Monterey Bay. The simulations show that vertical motion is greater during summer than winter, as expected. During spring upwelling, the deep waters often upwell along the walls of the canyon and then spread and mix with surrounding waters. The deep circulation enhances mixing significantly due to the topography. We further investigate the regional circulation by comparing it with the cases where the deep canyon was filled gradually. Vortex stretching over the canyon just beyond the entrance to Monterey Bay and along the adjacent continental slopes contributes to cyclonic circulation at deeper levels. Vertical sections of velocity along the axis of MSC indicate horizontal and vertical patterns of flow that are generally consistent with past observations on the circulation of Monterey Bay.

Foraminiferal Evidence of Younger Dryas Age Cooling on the British Columbia Shelf

Cluster analysis of foraminifera from a ~12,000-9000 radiocarbon year old piston core from Goose Island Trough, Queen Charlotte Sound, indicates that a cold interval correlative with the Younger Dryas stadial occurred during a shallow water phase. The reduction in depth was caused by the passage across the area, between 11,500 and 10,000 years BP, of a glacial forebulge associated with the retreat of the Late Wisconsinian ice sheets. Published sedimentological evidence indicate that water depths decreased to ~75-90 m, placing the site above the permanent North Pacific pycnocline (100 m). Low salinity-near glacial conditions, at these depths, between -11,100 and 10,000 years BP were recognized by abundant populations of Cassidulina reniforme and lslandiella helenae. This cold interval has also been recognized in cores from elsewhere in Queen Charlotte Sound. The depressed salinity and temperature may have resulted from a modification of regional weather patterns. Decreased mean continental summer temperatures could have reduced the seasonal influence of the North Pacific High and lengthened that of the Aleutian Low. This would have resulted in a near continuous onshore surface Ekman transport and enhanced coastal runoff, effectively blocking the movement onto the shelf of deep, saline, warm water of the California Undercurrent. The resultant isolated inshore basin comprised of present-day Hecate Strait and Queen Charlotte Sound is tentatively named the "Hecate Sea". By ~10,000 years BP, weather and ocean circulation had returned to near modern patterns as indicated by the disappearance of lslandiella helenae and by the development of an Epistominella vitrea - dominated biofacies.

The habitat of mesopelagic scyphomedusae in Monterey Bay, California

In the mesopelagic zone, at depths of 200–1000 m in the Monterey Submarine Canyon, CA, medusae in three genera of scyphozoa, Atolla, Periphylla and Poralia, were observed, videotaped and collected over a 9-year period (1990–1998). Environmental data were obtained simultaneously using a remotely operated vehicle (ROV) with sensors for depth, temperature, salinity and dissolved oxygen. Shipboard measurements of these same properties at two reference stations in the region defined the local water masses and helped identify species niches using the metric of spiciness and oxygen levels of the waters in which medusae were visually “captured”. The most abundant genus of mesopelagic scyphomedusae was Atolla, found associated most strongly with the spicy (warm, salty) waters of the California Undercurrent, usually above the core of the oxygen minimum zone (OMZ; O 2>0.5 ml/l). The least abundant mesopelagic scyphomedusa was Periphylla, which occurred in more variable waters, including those with a greater contribution of fresher, colder (less spicy), subarctic water and, hence, most like those at the offshore California Current station in the most depleted oxygen zone (averaging O 2 <0.3 ml/l). Poralia was mostly confined to the densest, coldest water, with peak abundance at the lower boundary of the OMZ (i.e., 0.3< O 2<0.5 ml/l). These spiciness measures on local isopycnal surfaces within the mesopelagic zone, supported by data on dissolved oxygen concentrations, indicate highly significant but fine-scale habitat differences in species habitats in Central California waters. This in situ investigation appears to be one of only a few studies to document fine-scale, water mass affinities of mesopelagic zooplankton.

A subsurface warm‐eddy off northern Baja California in July 2004

Upper‐ocean eddies are commonly observed from remote sensing, but submerged eddies are more difficult to detect. During July 2004, a 21‐day hydrographic survey in the southern region of the California Current was carried out to investigate the mesoscale variability. We observed for the first time a subsurface anticyclonic eddy off northern Baja California with the same water mass characteristics as the California Undercurrent. The core of the eddy was quasi‐circular with radii of 35 km and thickness of 250 m. The maximum swirl velocity was ∼3 cms−1. The water mass of the core of the eddy was characterized by potential temperature of 11°C, salinity of 34.5, and dissolved oxygen of 1.4 mll−1. The eddy propagated westward. The subsurface warm‐eddy could transport relatively saline water into the North Pacific subtropical gyre.

Intermediate level Lagrangian subsurface measurements in the northeast Pacific: Isobaric RAFOS float data

Isobaric RAFOS floats have been used to track the California Undercurrent and to investigate its continuity since 1992. The data include 61 quasi‐Lagrangian subsurface trajectories sampled for the most part between 150 and 600 m. The data set allows estimates of Eulerian and Lagrangian statistics for the region, studies of mesoscale eddy activity, and analysis of seasonal variability of circulation patterns off Central California. A browsable Web‐based inventory and a Graphical User Interface have been developed to provide access to this data set, including interactive manipulation of the data.

Hydrographic conditions near the coast of northwestern Baja California: 1997–2004

The effects of the 1997–1998 and 2002–2004 El Niño on the upper waters in the continental shelf and slope regions off northwestern Baja California are explored with data from eight cruises taken in late spring from 1998 to 2004 and the summers of 1997 and 1998. Geostrophic velocities were calculated referenced to a specific volume anomaly surface separating the generally southward flowing California Current waters from the waters advected to the north by the California Undercurrent. The resulting fields show equatorward flow near the surface except in the summer of 1997, when a poleward jet was found in the upper 40 dbar. This shallow jet advected anomalously warm and salty waters characteristic of the 1997–1998 El Niño, with its core found within 20–30 km from the coast. By spring of 1998, the waters brought into the region by the jet had mixed across the pycnoline with the salty California Undercurrent waters below, resulting in high salinity levels on the density surfaces corresponding to the otherwise fresh California Current waters (25–26 σ t ). By contrast, the 2002–2004 El Niño stands out for the very fresh and cold waters found on the same density surfaces in late spring of 2003 and 2004, marking a pronounced presence of subarctic waters. The fresh conditions found on the latter years represent a nearshore expression of the anomalous intrusion of subarctic waters observed 50–150 km from the coast of Southern California and Punta Eugenia, reported from July 2002 until April 2003. Our results suggest that the presence of this intrusion has continued to influence the region at least until May 2004.

Cross-shelf subtidal variability in San Pedro Bay during summer, 2001

A total of 16 moorings were deployed across the San Pedro shelf, one of the two wider embayments in the Southern California Bight, from near the surfzone to the upper-slope. On the middle and outer shelf in the summer of 2001, the currents flowed strongly equatorward at the surface and had large vertical shears through the well-stratified water column. This equatorward flow differs from predominantly poleward flow found in previous studies of the coastal margin further west. In deeper water, near the shelf break, the shears were such that near-bottom flows were poleward and incorporated into the upper parts of the Southern California Undercurrent over the slope. Mid-shelf current fluctuations, with periods of 10–25 days, along with upwelling over the shelf, were not related to local winds, but were significantly correlated with the large-scale alongshore pressure gradient. Shorter period (∼7–10 days) inner shelf alongshore currents, however, were significantly correlated with the alongshore wind at the shelf break. A CEOF analysis gives two significant modes, with the first mode dominant over the outer and middle shelf. The wind-forced second mode connects the inner shelf to the poleward undercurrent over the slope such that increases in the poleward flow over the slope are correlated with increases in the equatorward current inshore of the 15 m isobath.