California Current System Off Research Articles

The California current system off Baja California Sur

Based on hydrographic data from two cruises (June 2010 and May 2012) off Baja California Sur, Mexico, historical in situ measurements from the World Ocean Database 2018, California Cooperative Oceanic Fisheries Investigations, and Investigaciones Mexicanas de la Corriente de California cruise programs, as well as satellite images and data from the Global Reanalysis, this study describes the California Current System off Baja California Sur. The California Current System is characterized by the interaction of four near-surface currents. Far from the coast, the California Current flows within the North Pacific Subtropical Gyre but does not reach the Pacific Tropical-Subtropical Convergence off Mexico. From December to June, an equatorward flow known as the Tropical Branch of California Current emerges along the coast. This flow intensifies from March to June and is closely related to the mass flux induced by coastal upwelling. In July, a new branch of the Tropical Branch of the California Current turns poleward along the Baja California Sur coast. This flow is often referred to as the California Surface Countercurrent because it flows in the opposite direction to the California Current. Additionally, the California Undercurrent is detected, influencing the water column from 100 to 900 m, with a maximum poleward flow between 200 m and 300 m, decreasing toward the surface. The California Undercurrent persists throughout the year over the continental slope, displaying a significant semiannual component around the Gulf of California entrance. The California Current System off Baja California Sur plays a crucial role in the formation of Transitional Waters within the Pacific Tropical-Subtropical Convergence off Mexico. The importance of equatorward flows by the Tropical Branch of California Current in ventilating the Oxygen Minimum Zone waters in the Central Pacific off Mexico is emphasized.

Diversity and toxicity of Pseudo-nitzschia species in Monterey Bay: Perspectives from targeted and adaptive sampling

Monterey Bay, California experiences near-annual blooms of Pseudo-nitzschia that can affect marine animal health and the economy, including impacts to tourism and commercial/recreational fisheries. One species in particular, P. australis, has been implicated in the most toxic of events, however other species within the genus can contribute to widespread variability in community structure and associated toxicity across years. Current monitoring methods are limited in their spatial coverage as well as their ability to capture the full suite of species present, thereby hindering understanding of HAB events and limiting predictive accuracy. An integrated deployment of multiple in situ platforms, some with autonomous adaptive sampling capabilities, occurred during two divergent bloom years in the bay, and uncovered detailed aspects of population and toxicity dynamics. A bloom in 2013 was characterized by spatial differences in Pseudo-nitzschia populations, with the low-toxin producer P. fraudulenta dominating the inshore community and toxic P. australis dominating the offshore community. An exceptionally toxic bloom in 2015 developed as a diverse Pseudo-nitzschia community abruptly transitioned into a bloom of highly toxic P. australis within the time frame of a week. Increases in cell density and proliferation coincided with strong upwelling of nutrients. High toxicity was driven by silicate limitation of the dense bloom. This temporal shift in species composition mirrored the shift observed further north in the California Current System off Oregon and Washington. The broad scope of sampling and unique platform capabilities employed during these studies revealed important patterns in bloom formation and persistence for Pseudo-nitzschia. Results underscore the benefit of expanded biological observing capabilities and targeted sampling methods to capture more comprehensive spatial and temporal scales for studying and predicting future events.

The iron limitation mosaic in the California Current System: Factors governing Fe availability in the shelf/near‐shelf region

AbstractThe California Current System is a productive eastern boundary region off the coasts of Washington, Oregon, and California. There is strong seasonality to the region, with high levels of rainfall and river input to the coastal ocean during the winter season, and coastal and Ekman upwelling during the spring and summer. Iron (Fe) input to the coastal ocean during the winter months can be stored in the continental shelf mud belts and then be delivered to the surface ocean by upwelling in the spring and summer. There have been a number of studies providing strong evidence of Fe‐limitation of diatom growth occurring in regions of the California Current System off of California, and the occurrence of Fe‐limitation has been linked with narrow continental shelf mud belt width and low river input. We provide evidence for potential Fe‐limitation of diatoms off the southern coast of Oregon in July 2014, just off the shelf break near Cape Blanco in a region with moderate shelf width and river input. Since eastern boundary regions account for a disproportionally large amount of global primary production, this observation of potential Fe‐limitation in an unexpected near‐shore region of the California Current System has implications for global models of primary productivity. In order to re‐evaluate the factors impacting Fe availability, we utilize satellite imagery to compare with historical datasets, and show that unexpected levels of Fe can often be explained by eddies, plumes of upwelled water moving offshore, or lack of recent upwelling.

Diurnal frequency internal waves in the southern part of the California Current ecosystem as a nutrient source

Although the semidiurnal internal tide dominates internal motions at all sites studied to date off Mexico, a diurnal signal was detected on the shelf of the California Current System off Baja California Sur, even though the barotropic tide is exponentially stronger in the semidiurnal frequency along this coastline. In this study, diurnal thermocline shoaling events were measured in kelp beds in late summer and fall of 2004, very near their southern limit in the Northern Hemisphere. Sub-thermocline water bathed the base of the kelp beds at a diurnal frequency. Nitrate concentrations up to 1.4 µmol·L–1 were found in this sub-thermocline water of almost 20 ºC entering the kelp beds, which is a much greater concentration of nitrate at this temperature than is typical for the California Current. Nitrate-rich pulses lasted from 35 min to 19 h and reduced bottom temperatures up to 5 ºC in 1 h. The periodicity of temperature variability at the moorings showed significant coherence with the diurnal variability in zonal winds (>95% confidence interval) and was not coherent with the semidiurnal tide. We propose that windforced, diurnal internal motions may provide an important nutrient source for primary producers during reduced upwelling periods and merit further study as a fertilization source in the southern part of the California Current System.

Effects of ocean climate on life cycles and distribution of small pelagic fishes in the California Current System off Baja California

Due to their sensitivity to climate forcing that may alter their abundance and distribution, small pelagic fish are important ecological indicators of the state of the California Current System. They are schooling, planktivorous fish that provide forage for higher trophic levels. We describe the life cycle histories and patterns of distribution of sardine, anchovy, and mackerel species occupying the waters along the west coast of the Baja California Peninsula (Mexico). The links between the physical and biological forcing and the structure and condition of their habitats, their patterns of movement, productivity, and stock structure (where information is available) are described in relation to the fisheries. We have used the catches as a proxy for the history of the different stocks, while recognizing that fisheries-derived information is biased by the operational scale and only covers areas where fish are available. We have relied mainly on studies of sardines to evaluate the validity of the principal paradigms in fisheries oceanography. We describe how the environment can structure a population by relating an example given by the spatiotemporal variability of the northern and southern stocks of Pacific sardine (Sardinops sagax) and Pacific mackerel (Scomber japonicus) from 2000 to 2014. During this period, the northern stocks showed a marked tendency to contract southwards, as far south as Magdalena Bay, in response to the regional cooling that began in the previous decade. It appears that the combined effects of fishing and climate change may sufficiently alter habitat characteristics so that both the distribution and productivity of a population are shifted.

Seasonality of the transitional region of the California Current System off Baja California

AbstractHydrographic data collected over the period 1997–2013 are analyzed to investigate the seasonality of hydrographic features and associated geostrophic flows off the Baja California peninsula. The upper ocean in the region was found to be homogeneous in winter and spring but subdivided into two regions in the summer and autumn. In the first case, the system typically shows relatively low‐temperature and salinity waters, which give it a subarctic character. In the second, only the region north of Punta Eugenia (28°N) maintains subarctic characteristics, while the southern region receives an inflow of tropical and subtropical waters that results from the weakening of northwesterly winds, which allows the poleward advection of surface waters. Also during this period, a positive wind stress curl promotes the zonal advection of North Pacific's eastern edge waters into the coast and to the north as a surface coastal flow. Average seasonal patterns of geostrophic flow at 200 dbar revealed that the differentiation into provinces is also evident at that depth, with two clearly defined cyclonic structures in summer and autumn, both separated at the latitude of Punta Eugenia. The analyses conducted also showed a clear continuity of the California undercurrent along the shelf break, with more diffuse currents in the winter. Poleward flows were observed throughout the water column, especially in summer and autumn, although the origin of the surface flow does not necessarily involve a surfacing of the California Undercurrent.

Phytoplankton in the California Current system off southern California: Changes in a changing environment

Patterns of phytoplankton biomass, abundance and composition are examined between 1990 and 2009 over three major scales of environmental variability in the California Current system: seasonal, El Niño-Southern Oscillation (ENSO) and interannual. These patterns are compared with patterns of zooplankton biomass for commonalities and for links that might explain the decrease of zooplankton biomass since the 1970s, and the seemingly paradoxical increase in phytoplankton since 1984.Phytoplankton showed a slow, but significant increase in biomass and a change in species composition. The change appears to be quasi-continuous, with no sign of a more abrupt change around the year 2000. The responses of phytoplankton to ENSO events were weak. Biomass was depressed by El Niño conditions, but an effect of La Niña was not evident. There was no consistent response of species composition to either extreme. The seasonal signal of phytoplankton was strong, evident in biomass, abundance and species composition. The timing and magnitude of the annual maximum changed during the study. Prior to 1998, the annual peak of phytoplankton occurred consistently during the spring months, after which the peak shifted to summer. The shift in annual phytoplankton peak from spring to summer was accompanied by a change in composition, including a 75% decrease in the abundance of Hyalochaete species in the annual peak. The decrease in spring phytoplankton has been offset by an increase in the summer, which suggests a change in the source and timing of the nutrient supply.No evidence was found of an immediate link between phytoplankton abundance or composition and zooplankton biomass. It is postulated that the decline of zooplankton biomass seen since the 1970s is due, not to a decrease in annual primary production, but to shifts in the timing and composition of the annual phytoplankton peak which has destabilized the zooplankton cycle.

The California Current system off Monterey, California: physical and biological coupling

Repeated hydrobiological surveys over the period 1988–2002 perpendicular to the central California coast indicate strong coupling between physical circulation and biological production. An equatorward-flowing jet about 100–200 km from shore marked the inshore edge of the California Current (CC). This ‘‘CC Jet’’ had its highest velocities during late winter and spring. The jet divided inshore, biologicallyproductive waters from offshore, low-production waters. Mean flow in the inshore waters is poleward. However, this flow is interrupted in late spring and summer bya surfaceenhanced, equatorward-flowing, coastal upwelling jet. The upwelling jet coincides with maxima of nutrients, chlorophyll-a and primaryproduction. Annual variabilityin the inshore zone is related to (1) vertical py cnocline movements associated with geostrophic adjustments to accelerations of the California Current system, and (2) coastal upwelling. In offshore waters, the annual cycle accounted for a small fraction of the variability, indicating the dominance of eddies and meanders in this zone (J. Geophys. Res. 92 (1987) 12 947). The offshore regime was mesotrophic to oligotrophic, with a subsurface chlorophyll-a maximum above the nutricline. Considerable subduction mayoccur under the California Current jet and be an important process in the export of biogenic material to the deep sea. Published byElsevier Ltd.

Floral patterns in the California Current System off southern California: 1990-1996

In 1990, CalCOFI cruises began routine collection of samples for the enumeration of phytoplank-ton species. From each quarterly cruise, nearsurface samples from 34 stations are pooled into four regional samples prior to counting. This paper summarizes the first 6.5 years of the program in order to identify major large-scale patterns of species composition and fluctuations. A total of 312 species were recognized during this study. Recurrent group analysis defines two major floral clusters. The first is composed of diatoms characteristic of enriched regions. This cluster is most abundant in the northeastern region and often attains maximum abundances in the spring. Seventy-two percent of the variability of chlorophyll is accounted for by the variability of these species. The second cluster is composed of species common in the offshore central North Pacific. These species have relatively low spatial and temporal variability in the study region. There is no detectable seasonality. In neither cluster can interannual variability be detected above seasonal variability, spatial variability and error. These patterns differ from the geographic patterns of zooplankton species in the region, which are often dominated by fauna from the subarctic North Pacific and transition zone. The apparent absence of a similar subarctic flora is briefly discussed.

Drifter observations of a cold filament off Point Arena, California, in July 1988

We use the position fixes and temperature readings from 56 Tristar‐II mixed‐layer drifters, in conjunction with advanced very high resolution radiometer images, to provide a description of the mesoscale variability of the flows associated with cold‐water filaments in the California Current system off northern California in July 1988. We find that the northern, offshore temperature front of the filament is convergent and is closely associated with the core of a high‐speed (>100 cm/s) jet that has a broader spatial scale than the cold‐water filament. Mesoscale features are observed to be related to all developments of the cold‐water filament on a variety of time scales (1–30 days). In one well‐observed feature, estimates of vertical velocity (w) based on a vorticity budget are −20 m/d over an area of 150 km2 and are associated with a gain of vorticity. Estimates of w based on a local heat budget following a drifter are about 3 times larger over an area of 60 km2. These intense downwelling features are also places where horizontal stirring occurs.

Mesoscale variability in current meter measurements in the California Current System off northern California

Current meter data from a triad of moorings located on the continental rise offshore of Point Reyes‐Point Arena and from a mooring site located about 250 km offshore and to the northwest of the array are analyzed. The continental rise moorings were deployed from October 1984 to July 1985, and each had five meters at depths from about 150 to about 3560 m. The offshore mooring was deployed from September 1982 to August 1983 with six meters positioned from about 150 to 3800 m. Velocity components were highly vertically coherent in the upper 600 m and were in phase for periods longer than about 10 days; temperature tended to be less coherent. There was some horizontal coherence in the continental rise array. Coherence at 9–13 days between the deep temperature (T at about 3200 m) measurements at M1 and M2 (located about 120 km offshore and separated by about 100 km) coincided with very high, in phase, coherence in the northward velocity component v. Although there was no coherence between the first baroclinic modal components at M1 and M2, the eastward barotropic velocity component was coherent at periods of 9–11 days, and the northward barotropic velocity component was coherent at 8–15 days and 4.5–6 days. The triad provided a complete data set only at 350 m: the highest coherence was between out of phase v components (and also T) at M2 and M3 (about 100 km seaward of M2) at periods of 45–90‐days, with the dominant time domain empirical orthogonal function (EOF) representing strong horizontal shear between M2 and M3. At periods of 9–13 days, there was bottom and surface intensification in the nearshore kinetic energy spectra, suggestive of a superposition of a barotropic mode with the first two baroclinic modes and partially consistent with the presence of topographic Rossby waves. The vertical structure at M2 differed from that at W10 (the offshore mooring), where the deep (about 3000 m) flow tended to be stronger than that near the continental slope and rotary behavior was apparent at 3000 m. The barotropic modal energy was higher at W10 than at M2, but the first baroclinic modal energy was comparable at the two sites. At M2, when they were coherent, the barotropic and first baroclinic modes enhanced each other in the upper water column (and led to reduced flow at deeper levels); at W10, these modes were mainly out of phase (and so enhanced the flow at deeper levels). These differences, which may be spatial rather than temporal, increase the complications of initialization of predictive models from mass field data alone. There are some indications of current and temperature variability due to fluctuating winds; for example, the northward components of wind stress and barotropic current were coherent at all locations at periods of 8–9 days. At W10 there was also coherence between the eastward components of barotropic current and wind stress at 16–19 days. On the basis of frequency domain EOFs, some of the current variability was forced by large‐scale (i.e., about 1000 km) wind stress at periods of between 5 and 10 days; however, less than half the current variability was demonstrably related to the wind stress. At M2 and M3 at 150 m, large‐amplitude (about 1°C) temperature variations, which are sustained for several weeks, are influenced by alongshore flow convergence and offshore jets. Coherence between T at 350 m and wind stress curl is suggestive of Ekman pumping at M3, but the temperature was also related to alongshore convergence and offshore flow.

Satellite Observations and Modeling of Meanders in the California Current System off Oregon and Northern California

Abstract Infrared satellite images from the west coasts of Oregon and northern California are used to identify meander patterns in sea surface temperature which appear as large cold tongues extending offshore. Two relatively long series of images from 1982 and a few examples from 1980 and 1983 demonstrate the evolution of the cold tongues from an initial variety of scales (60–200 km), to the fastest growing waves (110–130 km) and then finally to tongues with longer wavelengths (400 km). This is observed to occur over periods of about three months in summer and fall when the coastal circulation is composed of a southward surface current over a northward undercurrent. The initial shorter scale features are believed to be excited by the interaction between the mean current and the coastal topography. Baroclinic instability associated with the vertical shear between the surface current and the undercurrent is found to be responsible for the growth of the features observed in the satellite imagery. A nonlinear...

Seasonal variability in meanders of the California Current System off Vancouver Island

Satellite infrared images taken over the past few years reveal seasonal variations in the meanders of the California Current System (CCS) off Vancouver Island. The CCS exhibits meanders with wavelength between 120 and 150 km in both winter and spring, when the upper ocean current all flows northwestward or southeastward, respectively. In summer, the CCS includes the California Undercurrent, which flows northwestward beneath the southeastward surface current and has shorter wavelength (75–80 km) meanders. Around the end of August, these shorter scale meanders are engulfed by the longer 150 km meanders. Finally, in the fall, eddies are detached from the offshore extensions of the longer meanders. A linear stability theory using a four‐layer model of the CCS successfully predicts the wavelengths in the winter and spring, but not in the summer and fall. The meander evolution observed in summer and fall is instead simulated by using a nonlinear numerical model in a domain with topographic obstacles selected to match the alongshore variations in the continental slope. The topographic features with a spatial periodicity of 75 km act as a trigger to initiate the short wavelength (75 km) meanders; following this, these 75 km meanders grow because of baroclinic instability. The 75‐km meanders, which have grown to large amplitudes, are replaced by longer (150 km) meanders because of nonlinear interactions between these two scales of meanders. Cyclonic eddies are finally shed from the longer meanders.

Observation and Modeling of Satellite-Sensed Meanders and Eddies off Vancouver Island

A long time series of satellite infrared images spanning the period 21 July–8 October 1980 reveals the evolution of long (160 km) and short (80 km) wavelength meanders of the California Current System off Vancouver Island. Midway in the series, strong interactions occurred between the two meander scales. Although the shorter meanders were initially more energetic, they were eventually dominated by the longer meanders. Near the end of the time series, detached mesoscale eddies were formed from the longer meanders. Current meter data from the period before meander growth clearly indicates the presence of a strong vertical shear near 150 m, which consists of a southeastward surface current and a northwestward undercurrent. It is argued in this paper that the observed shear flow is the energy source for the meanders and eddies seen in the satellite images. A four-layer, quasi-geostrophic model is used to represent the California Current System off Vancouver Island. The two upper layers represent the surface current and undercurrent respectively, and the two lower layers describe the deep ocean. Linear stability theory suggests that the vertical shear between the two currents maintains meander growth through the mechanism of baroclinic instability. Nonlinear numerical calculations simulate the engulfment of the shorter meander by the longer meanders and also the formation of the detached eddies. An energy analysis indicates two mechanisms at work during engulfment of the shorter meanders. One involves a stronger stabilization of the shorter meanders resulting from an inverse Reynolds stress, and the other consists of a direct nonlinear interaction which transfers energy from the shorter meanders to the longer meanders.