John W. Loder

Predicted positions of tidal fronts in the Gulf of Maine region

Abstract The distribution of Simpson and Hunters (1974, Nature , 250, 404–406) tidal mixing parameter in the Gulf of Maine region, calculated from Greenbergs (1983, Journal of Physical Oceanography , 13, 886–904) numerical model of the M 2 tide, is presented and used in a discussion of the summertime positions of tidal fronts and their variability. The role of wind mixing is assessed using a version of Simpson and Hunters criterion reformulated to include a fixed fraction of the climatological wind-derived energy in the Gulf. Two alternative criteria for the extent of tidally well-mixed areas, one based on a near-surface energy partition and the other on the bottom Ekman layer thickness, are also examined. It is suggested that all of these factors should be given further consideration in criteria for the extent of vertically well-mixed areas on continental shelves.

Seasonal Circulation on the Western and Central Scotian Shelf

Abstract A realistic representation of 3D seasonal circulation and hydrography on the western and central Scotian Shelf has been obtained from historical observations and a combination of diagnostic and prognostic numerical models with forcing by tides, wind stress, and baroclinic and barotropic pressure gradients. The major current features—the southwestward Nova Scotian and shelf-edge currents, and partial gyres around Browns and Sable Island Banks—are found to persist year-round but with significant seasonal changes. Comparison with current meter observations shows good agreement for the Browns Bank, southwest Nova Scotia, and inner-shelf regions, and poorer agreement in the Sable Island Bank and shelf-edge regions where current and density observations are sparser and tidal influences weaker. There is significant spatial structure in the seasonal circulation and hydrography, and in the underlying dynamical processes. On the shelf scale there are substantial changes in stratification, potential energy,...

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.

Seasonal variation of the three-dimensional residual circulation on Georges Bank

The seasonal variation of the low-frequency circulation in the Georges Bank region is studied numerically by computing six bimonthly circulation fields subject to forcing from the barotropic M2 tide, mean baroclinic pressure gradients, and mean wind stresses. The model is three dimensional, diagnostic, and nonlinear, with quadratic vertical eddy viscosity that is stratification-dependent. Tidal forcing is imposed at the boundary of the domain, and an extensive set of density and wind data is used to determine climatological mean forcings. The magnitude of the M2 tidally rectified around-bank velocities and transports is sensitive to stratification influences on the eddy viscosity, with up to a 50% intensification relative to computations which assume no influences. The dependence occurs through both the local advective tidal stress terms and the large-scale barotropic pressure field. The six bimonthly solutions (January–February, March–April, May–June, July–August, September–October, November–December) indicate important contributions from tidal rectification, baroclinic pressure gradients, and mean wind stress to the seasonal intensification of the Georges Bank gyre. Tidal rectification and baroclinicity are the dominant forcings, while wind stress generates opposing around-bank flow and cross-bank surface drift in winter. Overall, the solutions are in approximate agreement with observed Eulerian around-bank currents and transports, although there are both local and bank-wide discrepancies. The seasonal intensification of recirculation around Georges Bank is well produced by the model, with estimates of the recirculating transport increasing from under 0.02 Sv (January–February) to over 0.13 Sv (September–October).

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.

Tidal rectification and frontal circulation on the sides of Georges Bank

Using Wright and Loders (l985a,b) depth-dependent tidal rectification model and Garrett and Loders (1981) diagnostic frontal circulation model, predictions of the residual circulation associated with the topographic rectification of tidal currents and the summertime density field on the northwestern and open ocean sides of Georges Bank are made and compared with observations. In general, the estimates of both wintertime and summertime along-isobath currents are in qualitative agreement with observations, but the agreement between predicted and observed cross-isobath currents is poor. The circulation associated with tidal rectification is primarily along isobaths in an anticyclonic sense around the Bank at all depths. The cross-isobath circulation is much weaker and, in the Eulerian specification, is dominated by two cells with opposing current directions. However, a significant Stokes velocity is predicted such that the along-isobath Lagrangian current is generally less than its Eulerian counterpart, and the cross-isobath Lagrangian current is sometimes in the opposite direction to its Eulerian counterpart. Both the along-isobath and cross-isobath currents associated with tidal rectification are predicted to be significantly stronger in summer than in winter due to a reduction in the strength of friction as a result of reduced wind stress and increased density stratification. An additional contribution to the anticyclonic circulation around Georges Bank is associated directly with the summertime tidal front around the Bank. This flow component is predicted to form a second intense along-isobath jet on the northwestern side, slightly off-bank of that due to tidal rectification, and a broader flow on the open ocean side. The associated cross-isobath circulation is predicted to be much weaker than the along-isobath circulation, with a general on-bank bottom flowon both sides of the Bank.

Turbulence dissipation rates and nitrate supply in the upper water column on Georges Bank

Abstract Measurements of velocity microstructure in the upper water column on Georges Bank are used to contrast the summertime structure of turbulent kinetic energy dissipation rate between the central mixed area and the tidal-mixing front, and to compare vertical nitrate fluxes with frontal-zone primary production demands. In the mixed area during weak winds, the dissipation rate varies strongly over the semidiurnal tidal period in close relation to the tidal current strength, varies with the monthly/fortnightly tidal modulation, and generally increases with distance below the sea surface. Collectively, these features provide strong support for the elevated vertical mixing rates on Georges Bank being primarily due to the tides, although wind forcing also contributes significantly. In the frontal zone on northern Georges Bank, the upper-ocean dissipation rates are about an order of magnitude weaker than in the mixed area, have a more complex temporal variation during the tidal period, and also vary with the monthly/fortnightly tidal modulation. The vertical eddy flux of nitrate into the frontal euphotic zone varies over the tidal period and with the tidal modulation. Averaged over the tidal period, the estimated fluxes are about one-third of the nitrogen demand estimated from concurrent primary production measurements, supportive of an important contribution from turbulent mixing to new production in the frontal zone, but also pointing to additional processes and/ or inadequate data coverage of this complex zone. The measured dissipation rates at both the mixed and frontal sites are in approximate agreement with the turbulence levels in two 3-D numerical models for summertime tidal and mean circulation on the Bank, one with and eddy-viscosity and the other an advanced turbulence closure. The latter model has more realistic vertical turbulence distributions and indicates strong sensitivity of the turbulence levels to horizontal position in the frontal zone.

Detailed structure of currents and hydrography on the northern side of Georges Bank

A suite of observations from July 2–3, 1988, is used to describe the spatial structure and temporal evolution over the tidal period of currents and hydrography across the northern side of Georges Bank in summer under light winds. The data set includes moored current and hydrographic observations at four cross-bank positions, fast response thermistor chain observations at two of the sites, a conductivity-temperature-depth section, 10 repeated sections over the tidal period using a towed Batfish and ship-mounted acoustic Doppler current profiler, and two surface drifter trajectories. The observations provide a detailed description of previously identified features such as the strong semidiurnal tidal currents, an internal tide, a tidal front, and a residual current jet and also reveal a hierarchy of energetic smaller-scale structures. These include an internal hydraulic jump during off-bank tidal flow and subsequent internal waves propagating onto the bank and also a surface convergence in the frontal zone. The physical oceanographic regime on the northern side of Georges Bank during spring-fall can be conceptualized as a hybrid of a stratified shelf break with strong tidal advection and a tidal(mixing) front. Key factors to the regime are the strong tidal currents and abrupt topographic variation over the banks side. The result is a nonlinear and baroclinic tide-topography interaction at the bank edge and a frontal zone with strong variability over the tidal period associated with tidal advection and large-amplitude internal waves. The along-bank transport in the residual jet is estimated to be 0.91 Sv, indicating that it may be the largest summertime transport feature on the northwestern Atlantic shelf between Cape Hatteras and the Grand Bank.

UPPER-OCEAN TRANSPORT MECHANISMS FROM THE GULF OF MAINE TO GEORGES BANK, WITH IMPLICATIONS FOR CALANUS SUPPLY

Abstract Potential upper-ocean pathways for the supply of biota from the Gulf of Maine to Georges Bank are investigated by numerically tracking particles in realistic 3-d seasonal-mean and tidal flow fields. The flow fields, obtained from a prognostic model forced by observed M2 tides and seasonal-mean wind stress and density fields, include the major known observational features of the circulation regime in winter, spring and summer — a wind-driven surface layer (in winter and early spring) overlying seasonally-evolving baroclinic and tidally-rectified topographic gyres. The surface layer in winter and early spring, with generally southward drift for typical northwesterly wind stress, can act as a conveyor belt for the transport of biota to Georges Bank, provided that the biota can spend a substantial fraction of time in the surface Ekman layer. The numerical experiments indicate that the upper-ocean drift pathways for biota in the southern Gulf of Maine are strongly sensitive to biological and/or physical processes affecting vertical position in relation to the surface Ekman layer and horizontal position in relation to topographic gyres. The seasonality and location of the identified pathways are generally consistent with observed distributional patterns of Calanus finmarchicus based on the 11-year MARMAP surveys.

Seasonal-Mean Hydrography and Circulation in the Gulf of St. Lawrence and on the Eastern Scotian and Southern Newfoundland Shelves

Abstract The climatological seasonal-mean hydrography and circulation in the Gulf of St. Lawrence and on the eastern Scotian and southern Newfoundland shelves are studied by reconstructing high-resolution temperature, salinity, and density fields for four seasons and numerically computing the associated circulation fields. The current fields are obtained from a three-dimensional diagnostic model, forced by baroclinic pressure gradients, seasonal wind stresses, and additional barotropic inflows across the Strait of Belle Isle and southern Newfoundland shelf upstream boundaries. The hydrographic fields suggest strong gulf–shelf interconnections, including outflow of relatively fresh surface water from the gulf to the eastern Scotian shelf, penetration of slope water at depth onto the shelves and into the gulf, and flow into the gulf through the Strait of Belle Isle. The circulation is generally cyclonic in the gulf, reinforced by inflows of Labrador and Newfoundland shelf water through the Strait of Belle I...