摘要:

Filaments are cold surface features often found in eastern boundary current regions. A typical filament originates near the shelf and extends as a narrow (100 km or less) tongue extending hundreds of kilometers offshore. These features represent the main focus of the the Coastal Transition Zone program, which took place in 1987 and 1988 off the northern coast of California. The historical background for the program is discussed, as well as the questions that motivated it. The general approach of the program is described, followed by an overview of the initial findings, representing a summary of our current understanding of these features and their relation with eastern boundary current dynamics.

ISSN:0148-0227    

DOI:10.1029/91JC01206

年代:1991

数据来源: WILEY

摘要:

We use a 4‐year time series of high‐resolution coastal zone color scanner imagery to study mesoscale variability in phytoplankton pigment (as a surrogate for biomass) distributions off central California during the spring‐summer upwelling season. We use empirical orthogonal functions to decompose the time series of spatial images into its dominant modes of variability. Similarly, we analyze wind fields derived from the Fleet Numerical Oceanography Center pressure fields. The results from these analyses are used to investigate the coupling between wind forcing of the upper ocean and phytoplankton distributions on mesoscales. Prior to the spring transition, wind stress is dominated by strong northward conditions, and pigment is low and relatively uniform throughout the California Current domain. The first strong southward wind event soon results in a bloom of phytoplankton that is also uniformly distributed offshore. Nearshore values fluctuate relatively little in response to variations in wind forcing. After the spring transition, the wind becomes more steady (southward) and the fluctuations are dominated by changes in the strength of the curl. During southward events, the curl tends to become more positive. Depending on the fluctuations and strength of these curl episodes, filaments begin to form at particular locations. The overall pigment concentration in the California Current drops as a larger fraction of the pigment is distributed within the filaments. The filaments also tend to become shorter during this period, which starts soon after the spring transition and usually reaches its maximum intensity in early summer. Winds (and curl) become much weaker in the mid‐July to August period, and the filaments become much less prominent in terms of concentration and their length increases. Isolated wind events occasionally reinvigorate the filaments. A final bloom in the fall occasionally appears with spatial structure similar to the spring bloom. We conclude that wind forcing, in particular the curl of the wind stress, plays an important role in the distribution of phytoplankton pigment in the California Current. Although the underlying dynamics, especially of the filaments, may be dominated by processes other than forcing by wind stress curl (such as eddies or topographically induced instabilities), it appears that curl may force the variability of the filaments and hence the pigment p

ISSN:0148-0227    

DOI:10.1029/91JC01207

年代:1991

数据来源: WILEY

摘要:

A formalism is presented for quantifying the sampling error of an arbitrary linear estimate of a time‐averaged quantity constructed from a time series of irregularly spaced observations at a fixed location. The method is applicable to any irregularly sampled time series; it is applied here to satellite observations of chlorophyll from the coastal zone color scanner (CZCS). The two specific linear estimates considered here are the composite average formed from the simple average of all observations within the averaging period and the optimal estimate formed by minimizing the mean squared error of the temporal average based on all of the observations in the time series. The formalism requires a priori knowledge of the variances and correlation functions of the chlorophyll signal and CZCS measurement error. In the usual absence of the necessary detailed information on these parameters, values obtained here from in situ measurements of chlorophyll and fluorescence off the coast of southern California can be used. The resulting estimates are referred to here as “suboptimal estimates,” which are optimal only if the assumed values for the parameters are correct. Suboptimal estimates are shown to be much more accurate than composite averages. Moreover, suboptimal estimates are also shown to be nearly as accurate as optimal estimates obtained using the correct signal and measurement error variances and correlation functions for realistic ranges of these parameters. Suboptimal estimation is thus a very useful and practical alternative to the composite average method generally used at pr

ISSN:0148-0227    

DOI:10.1029/91JC00704

年代:1991

数据来源: WILEY

摘要:

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 ofO(50 cm s−1) and the current meanders have alongshore wavelengths ofO(300 km) and onshore‐offshore amplitude ofO(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) ofO(8 × 107cm2s−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 ofO(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” onl

ISSN:0148-0227    

DOI:10.1029/91JC01072

年代:1991

数据来源: WILEY

摘要:

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 byO(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

ISSN:0148-0227    

DOI:10.1029/91JC01210

年代:1991

数据来源: WILEY

摘要:

Satellite‐tracked drifters deployed in the Iberian coastal transition zone were gradually carried northward in a poleward flowing continental slope current between September 1986 and March 1987 in convoluted trajectories which revealed the ubiquitous presence of mesoscale eddies. We estimate the rate of dispersion from the Lagrangian statistics of the drifting buoys and present a comparison of the results with those obtained from other oceanic regions. The dispersion of the drifters is well modelled by a simple description of eddy diffusion assuming homogeneous isotropic turbulence (Taylor's theory). The assumption of homogeneity and stationarity facilitated the derivation of stable Lagrangian statistics. After correction for non stationarity, the diffusivities obtained werek11= 3.4 × 106cm2s−1in the zonal direction andK22= 2.5 × 106cm2s−1in the meridional direction. The Lagrangian integral time scales were respectivelyT11= 1.25 days andT22= 1.9 days. Our results and those of other authors, in environments ranging from truly oceanic to eastern boundaries, have all indicated that Taylor's theory fits the observations well. As a first order approximation the assumption of a homogeneous and stationary field would appear quite

ISSN:0148-0227    

DOI:10.1029/91JC00907

年代:1991

数据来源: WILEY

摘要:

Data from the Coastal Transition Zone (CTZ) experiment are 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 variability with maximum dynamic height ranges and velocities in summer and minimum values in late winter and early spring. North of Point Arena (between 39°N and 42°N) in spring‐summer the flow field on the outer edge of the cold water has the character 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 farther north. The flow field south of Point Arena in summer takes on more of the character 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°N and 42°N is thus of a surface jet that meanders through and interacts with a field of eddies; the eddies are more dominant south of 39°N, where the jet broadens and where multiple jets and filaments are often present. At the surface, the jet often separates biological communities and may appear 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 are 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

ISSN:0148-0227    

DOI:10.1029/91JC01024

年代:1991

数据来源: WILEY

摘要:

In June 1987 the geostrophic flow in the coastal transition zone off northern California (between 50 and 150 km off the coast from Point Reyes to just north of Cape Mendocino) was dominated by a well‐defined, southward‐meandering current. Three vertical sections are presented that show the hydrographic structure of the current down to 100 m and its relationship to the distribution of phytoplankton biomass. The sections show that the geostrophic adjustment brought cold, saline, deep water up to the surface on the low steric height, or cold side of the flow, and that this upwelled water supported a relatively large diatom biomass (chlorophyll‐aconcentrations between 1 and 10 mg m−3). We present particle size spectra and photomicrographs of the phytoplankton that show that the diatom biomass was dominated by chain‐forming species (e.g.,Skeletonema costatum, Chaetocerosspp.,Thalassiosiraspp., andRhizosolenia alata, but also a single‐celledActinocyclussp.). Photosynthetic light response measurements reveal that these diatom communities were capable of high photosynthetic rates (Pmaxbetween 5 and 25 mg C mg Chla−1h−1). Although most of the diatoms were located in cold, slow‐moving water on the low side of the current, some were being carried downstream. High chlorophyll concentrations were observed at depths>75 m in and along the cold edge of the flow in all of our sections; we show evidence that in two out of three cases this was the result of wate

ISSN:0148-0227    

DOI:10.1029/91JC01208

年代:1991

数据来源: WILEY

摘要:

In July 1988 we sampled upper layer zooplankton distributions in and around a major cold filament located off Point Arena, California. Average zooplankton biomass levels declined inshore to offshore, but relatively high levels extended seaward along the cool side of the filament jet. A series of transverse station lines shows strong shifts in community composition across the axis of the filament. The cross‐filament compositional sequence was recognizably similar along each line, giving an impression (supported by cluster analysis and along‐flow versus cross‐flow spatial autocorrelation) of banding parallel to the filament axis. The “standard” sequence was characterized by local maxima ofDolioletta gegenbauriin nonfilament waters to the south and east,Euphausia pacificaalong the southeast margin of the filament,Eucalanus californicusand euphausiid larvae within the cool core of the filament and extending partway across the strong seaward jet on the north and west margin of the filament, and heteropod larvae, chaetognaths,Dolioletta, and a mixture of small copepods (including several with southern and offshore faunal affinities) along the warm side of the jet and extending into nonfilament waters to the north and west. A “core” group of samples characterized by high abundance ofEucalanusmatched the trajectories of drifters released at the upstream end of the filament. Both crossed geopotential anomaly contours (to larger Ф) as they moved out the filament, suggesting a cross‐jet com

ISSN:0148-0227    

DOI:10.1029/91JC01037

年代:1991

数据来源: WILEY

摘要:

Despite the intermixing that planktonic populations might be expected to experience in the ocean's fluid regime, zooplankton species may be subdivided across their range into genetically distinct populations. This subdivision, or population genetic structure, may be generated by the interplay of biological processes (reproduction, dispersal, differential mortality) and physical forces governing planktonic distributions. Significant population genetic structure in the planktonic copepodMetridia pacificaoccurred during a period of upwelling in the coastal transition zone off the west coast of the United States during April and May 1987. During this period a coastal eddy of saline, recently upwelled water was bordered by a southward flowing current stream; offshore waters were nutrient poor and slower flowing.Metridia pacificawas the most abundant copepod in zooplankton samples collected in this domain; the species was sufficiently abundant in 16 samples to allow genetic analysis. Individual copepods were assayed for allozymic variability by polyacrylamide gel electrophoresis. The samples were genetically heterogeneous (by a contingency chi‐square analysis of allozymic frequencies). The genetic differentiation of the samples was characteristic of geographically separated conspecific populations. Cladistic (tree building) analyses were used to visualize the genetic relatedness of the 16 samples, based on the similarity of allozymic frequencies. This analysis resulted in two heterogeneous groups (of five and six samples each) and five anomalous samples that neither constituted a third group nor belonged to the two groups. Overlay of these groupings on the dynamic height topographies showed a concordance between the population genetic and oceanographic structures. Five of the six offshore samples belonged to one group; the five samples of the other group were found either in the eddy (two of three central eddy samples) or in the current jet. The remaining five samples were scattered in nearshore regions or in the frontal region between the eddy and offshore. The division of the samples into eddy and offshore groups may result from redistribution by currents of planktonic populations of different geographic origin and distinct genetic character. Thus the eddy may have entrained copepod populations originating from different source regions than the populations in offshore waters. Further genetic analysis on appropriate time and space scales will be required to determine the mechanisms generating structure in oceanic zooplankto

ISSN:0148-0227    

DOI:10.1029/91JC01209

年代:1991

数据来源: WILEY