Francisco E. Werner

Diagnostic model for baroclinic, wind-driven and tidal circulation in shallow seas

Abstract A three-dimensional diagnostic model for continental shelf circulation studies is presented. The model is based on the linearized hydrodynamic equations subject to surface stress, density gradient, and remote (boundary) forcing. Finite elements are used to resolve real topography. Solutions are obtained in the frequency domain, including the limit of zero frequency. A test case based on analytic solutions for tidal front circulation demonstrates the successful representation of sensitive baroclinic circulation. Representative applications to the Gulf of Maine region, including the Bay of Fundy, Georges Bank, and a portion of the Scotian Shelf, are shown for wind, along-shelf transport, and tidal front circulation on Georges Bank.

Drift of sea scallop larvae Placopecten magellanicus on Georges Bank: a model study of the roles of mean advection, larval behavior and larval origin

Abstract The drift and exchange of sea scallop larvae ( Placopecten magellanicus ) on Georges Bank is investigated by tracking particles in three-dimensional flow fields consisting of the semidiurnal tidal current and autumn mean circulation on realistic topography. Three composite flow fields are considered, each forced by non-linear tidal current interactions, seasonal-mean density gradients and seasonal-mean wind stress. The around-bank flow rates are in approximate agreement with the observed residual gyre, while the cross-isobath currents in the flow fields are consistent with observations only in being generally weak. In most cases it is unclear whether the discrepancies arise from observational uncertainties or from model approximations. In the simulations the particles are given the behavior and planktonic period expected of sea scallop larvae. Particle starting positions correspond to the three major scallop aggregations: the Northeast Peak (NEP), the Southern Flank (SF), and the Great South Channel (GSC). Simulations are run to examine the sensitivity of the particle trajectories and settlement numbers to aspects of larval biology (vertical distribution, ascent and descent rates, search times, growth and mortality rates), and to various flow field components. The pattern and extent of larval exchange and settlement are most sensitive to the duration and depth of planktonic drift, gyre strength, weak cross-isobath flow, and mortality rate. The simulations indicate significant larval exchange among the three aggregations, with self-seeding possible for the GSC and NEP, and unlikely for the SF. Given the high retention of particles on Georges Bank as a whole (10–73% before mortality), Georges Bank scallops should be considered self-sustaining.

Modelling some effects of behavior on larval settlement in a turbulent boundary layer

Abstract A one-dimensional model of larval concentration and settlement flux in a turbulent boundary layer was used to consider how some aspects of larval behavior in the plankton or on the bottom might affect settlement rates. The focus was on behavioral modifications of two terms in the model—the vertical (fall) velocity ( w f ), a composite measure of swimming and gravitational sinking, and the probability of settlement per unit time ( p ) for larvae interacting with the bottom. Depth-independent changes in vertical speed can increase the settlement rate by up to an order of magnitude. Such changes might be produced by negative phototactic responses that induce passive sinking of larvae throughout the water column during daylight, by photonegative swimming responses that may occur in relatively shallow or clear water (where swimming responses are fairly uniform throughout the water column), or by photonegative swimming responses that are of similar magnitude over a broad range of light intensities. In addition to increasing the long-term average settlement rate, negative phototactic responses should increase temporal variability in settlement at time scales of ≦24 h, as long as larvae are responding to changes in light intensity at depth. This prediction, coupled with results from Gross et al . [(1992) Journal of Marine Research , 50 , 611–642], suggests that for larvae that respond to light there may be two sources of substantial (order-of-magnitude) temporal variability in settlement that operate at time scales of 24 h or less— diel periodicity in light intensity and semi-diurnal or diurnal tidal periodicity in boundary shear stress and turbulence intensity. The additional effect of phase shift between tidal and light cycles introduces substantially less variability to the average (>24 h) settlement rate than do individual effects of the two forcings. Behavioral responses that produce a depth dependence in vertical speed can affect the settlement rate only if w f varies roughly by a factor of 2 or more very close to the bottom (i.e. within a few percent of total boundary layer thickness for tidal boundary layers). Phototactic or barokinetic responses of larvae are not likely to produce depth gradients in vertical speed strong enough to affect settlement rates. However, such gradients might be produced by a recent contact with a potential settlement site, by effects of velocity gradients near the bottom on larval orientation, or by larval responses to chemical cues associated with the bottom. Behavioral responses to fluid forces exerted on larvae on the bottom can alter the settlement rate by an order of magnitude or more. However, the settlement rate is predicted to be most sensitive to effects of weak, rather than strong, flow on the settlement probability. Thus, understanding larval responses to weak flows may be especially important to predicting settlement rates.

Harmonic structure of English Channel/Southern Bight tides from a wave equation simulation

Abstract The harmonic structure of a long-term time-stepping simulation is obtained by least-squares decomposition. The structure of 11 major constituents used to force the model is displayed, reproducing the major tidal features of this system. The problem of incomplete field data is discussed and analyzed, in terms of these and 40 additional constituents which were generated internally. At least two semi-diurnal constituents (2 MS 2 and 2 MN 2 ) which had been previously neglected are shown to be contributors to apparent errors in time-domain comparisons with field data. By excluding these and all other unforced constituents, improved RMS error estimates are obtained in the time domain.

An exploratory numerical study of the currents off the southern coast of Mallorca including the Cabrera Island complex

Abstract Exploratory results obtained from a 3-D numerical model of the continental shelf circulation off the southern coast of the Island of Mallorca are described. The responses to idealized wind forcings (sea breezes and storms) and prescribed inflow through the open boundaries are considered. The model results identify circulation patterns consistent with features inferred from existing biological and geological studies and suggest specific future experimental studies.

Residual Circulations Due to Bottom Roughness Variability under Tidal Flows

Abstract Tidal flows over irregular bathymetry are known to produce residual circulation flows due to nonlinear interaction with gradients of depth. Using the depth-averaged vorticity equations, the generation of residual vorticity and residual flows due to variation of the frictional coefficient are examined. The authors find that the contribution due to bottom roughness variations can be as large as that arising from gradients of depth and velocity. Specific cases are considered on the northern California shelf, Georges Bank, and the U.S. South Atlantic Bight. The generation of residual vorticity is a strong function of the length scales at which roughness or depth vary. Length scales of bottom roughness variation are commonly within the range of greatest effect (e.g., sand patchiness, cobbly outcrops, etc.). The site-specific cases show that the bottom roughness variability can generate as much residual circulation as that expected from depth variability. The implication for numerical modeling studies ...