Kenneth H. Brink

The Coastal Transition Zone program

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.

The Nantucket Shoals Flux Experiment (NSFE79). Part I: A Basic Description of the Current and Temperature Variability

Abstract The Nantucket Shoals Flux Experiment (NSFE79) was conducted across the continental shelf and upper slope south of Nantucket from March 1979 to April 1980 to study the flow of shelf water from the Georges Bank/Gulf of Maine region into the Middle Atlantic Bight. The experiment included a moored array of current meters and bottom instrumentation deployed at six locations across the shelf and upper slope spanning a depth range from 46 to 810 m, and supporting hydrographic observations. A basic description of the moored current and temperature data is given here with an emphasis on the low-frequency variability. In the summer period (April–August) when the local vertical stratification reached a maximum due to increased surface heating and reduced wind mixing, the mean flow over the shelf at all instruments was primarily along 1ocal isobaths towards the west. The subtidal current fluctuations were coherent both horizontally and vertically over the shelf, but not with current fluctuations observed ove...

The near-surface dynamics of coastal upwelling

Abstract An attempt is made to examine some observational and theoretical aspects of upper ocean dynamics in regions of strong coastal upwelling. “Upper ocean” is roughly defined as about the upper 10–30 m of the water column for most systems. First, the basic surface Ekman and mixed layers are discussed, including some of the modifications due to upwelling. Next, coastal upwelling fronts and their associated circulation are treated. Finally, areas of strongly three-dimensional upwelling are classified and discussed. Horizontal advection of heat and momentum appear to be generally important for the near-surface dynamics of coastal upwelling, and these phenomena make realistic theoretical treatments especially difficult.

A Comparison of Long Coastal Trapped Wave Theory with Observations off Peru

Abstract The agreement between coastal trapped wave theory and observation is studied for the case of observations made off Peru during the 1977 CUEA JOINT-II experiment. Wave properties are calculated using a numerical model with realistic, horizontally uniform stratification and realistic bottom topography. These properties are then explored as a function of the ratio of the first internal Rossby radius of deformation to the shelf-slope width. The agreement of observed and calculated first-mode, free wave phase speeds (230 cm s−1) is excellent, while modal structures agree more poorly. A forced wave calculation, using observed winds and currents as input, is used to hindcast alongshore currents and sea level in the frequency band where Smith (1978) observed free coastal trapped waves during 1977. The model suggests that most of the observed sea level and alongshore velocity fluctuations in the 5–10 day period band are due to free waves originating equatorward of 5°s, while winds between 5°s and ...

The Response of Stratified, Frictional Flow of Shelf and Slope Waters to Fluctuating Large-Scale, Low-Frequency Wind Forcing

Abstract Numerical and analytical models are developed to discuss large-scale, low-frequency, wind-driven fluctuations over the continental shelf and slope in the presence of realistic density stratification and bottom friction. The models employ the “long-wave” assumption that frequencies are small relative to the inertial frequency and that alongshore scales are large relative to cross cross-shelf scales. An attempt is made to make the effect of bottom friction on the flow as realistic as possible by taking into account the influence of surface gravity waves on bottom stress. The results were obtained for sinusoidal wind forcing having frequency ω and alongshore wavenumber l and are described below. A general criterion for barotropic shelf water response for wind forcing frequencies ω in the “weather” band (2π/few weeks to 2π/few days) can be derived. For the purpose of rapid calculation, this criterion reduces, approximately, tohere Ns2 is a shelf-averaged value of the buoyancy frequency squared, α is ...

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.

Variability of the near-surface eddy kinetic energy in the California Current based on altimetric, drifter, and moored current data

Low-pass-filtered velocities obtained from World Ocean Circulation Experiment (WOCE) surface drifters deployed in the California Current off northern California during 1993-1995 have been compared with surface geostrophic velocity estimates made along subtracks of the TOPEX/POSEIDON altimeter and with moored acoustic Doppler current profiler (ADCP) data. To obtain absolute geostrophic velocities, a mean sea surface height (SSH) field was estimated using the mean drifter velocities and historical hydrographic data and was added to the altimetric SSH anomalies. The correlation between collocated drifter and altimetric velocities is 0.73, significant at the 95% level. The component of the drifter velocity which was uncorrelated with the altimetric velocity was correlated with the wind in the Ekman transport sense. Monthly averages of eddy kinetic energy (EKE), estimated using all drifter and altimeter data within the domain (124°-132°W, 33°-40.5°N), show energy levels for the drifters that are about 13% greater than those for the altimeter. Drifter, altimeter, and ADCP measurements all exhibit similar seasonal cycles in EKE, with the altimeter data reaching maximum values of about 0.03 m 2 s -2 in late summer/fall. Wavenumber spectra of the altimeter velocity indicate that the velocity fluctuations were dominated by features with wavelengths of 240-370 km, while the ADCP data suggest that the temporal scales of these fluctuations are at least several months. Between 36° and 40.5°N, the region of monthly maximum EKE migrates westward to about 128°W on a seasonal timescale. This region of maximum EKE coincides with the maximum zonal SSH gradient, with increased EKE associated with increased southward flow. A simple model shows that much of the seasonal cycle of the SSH anomalies can be produced by linear processes forced by the curl of the wind stress, although the model cannot explain the offshore movement of the front.

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.

Tidal and lower frequency currents above Fieberling Guyot

An intensive, yearlong deployment of current meter moorings was made over and around Fieberling Guyot (32.5°N, 127.75°W) in the North Pacific. The measurements are used to investigate tidal currents, the mean flow, and subinertial current fluctuations. The K1 and O1 diurnal tides were greatly amplified over the seamount, apparently through dynamics resembling nonlinear seamount-trapped waves. The diurnal tides interacted nonlinearly to induce constituents at the sum frequency (M2) and difference frequency (fortnightly) above the summit. Diurnal amplification was strongest at about 450 m, and it extended from the seamount center out over the flanks. The S2 semidiurnal tide was not substantially amplified over the seamount. The mean flow was primarily along isobaths and was strongest about 50 m above the seamount rim. It was driven by tidal rectification, so that mean along-isobath velocity at a given location correlated well with the amplitude of the local fortnightly tide at that location. Accompanying the mean flow was a “cold dome” over the seamount that was presumably maintained by a balance of mean downward advection and eddy (tidal) radial heat transport. Lower frequency current fluctuations were enhanced near the seamount and were apparently driven by fluctuations in the ambient flow.

Mean Currents Driven by Topographic Drag over the Continental Shelf and Slope

Abstract A sequence of numerical simulations is described of wind-driven flow over irregular continental shelf topography. The model is barotropic, nonlinear, and forced by a periodic, spatially uniform alongshelf wind stress. The objective of the study is to determine whether topographic drag, known to be asymmetric for barotropic flow over the shelf, can generate substantial time-averaged alongshore currents in the presence of a fluctuating zero-mean wind stress. With realistic parameters, mean maximum alongshore currents of 0.05 to 7.0 cm s−1 are realized with flow in the direction of freely propagating shelf waves. The residual current strength is a strong function of wind stress period and bottom bump wavelength: larger forcing periods and shorter bump wavelengths enhance the time-mean circulation. Particle paths are generally observed to be chaotic, in contrast to the nearly cyclic behavior of the Eulerian velocity field. However, cross-shore particle dispersion is well correlated with the mean alon...