Charles G. Hannah

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,...

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.

Decadal-scale hydrographic and circulation variability in the Scotia–Maine regionSUM ☆ ☆☆

Abstract Historical data, geostrophic computations and numerical circulation models are used to examine decadal-scale hydrographic “regime shifts” and associated circulation changes in the Scotian Shelf and Gulf of Maine region. Ocean temperature and salinity data indicate multi-year periods with cooler (by a few °C) and fresher (by a few 0.1 psu ) conditions over the shelf and slope around 1940 and 1960, apparently associated with increased Labrador Current transport as suggested previously. Three-dimensional seasonal hydrographic fields for the cold 1960s period show the detailed structure of the hydrographic anomalies, including largest magnitudes in winter at depth along the shelf edge and extending into shelf basins, and at the surface over the continental slope. Model estimates of the associated circulation changes in winter and spring, obtained through prognostic model refinement of diagnostic fields, suggest that the predominant circulation feature of southwestward shelf-edge flow was increased by 1– 2 Sv during the cold 1960s compared to the warm 1970s — an amount comparable to its climatological mean value. In contrast, the model solutions indicate limited decadal-scale variability in the major shelf circulation features away from the shelf edge. The relation of the Scotia–Maine decadal-scale variability to larger-scale variability in the northwestern Atlantic and its potential implications for ecosystems are discussed.

Seasonal variation of the three-dimensional mean circulation over the Scotian Shelf

The seasonal-mean circulation over the Scotian Shelf is studied numerically by computing mean and tidal current fields for winter, spring, and summer using a three-dimensional nonlinear diagnostic model. The mean current fields are forced by seasonal-mean baroclinic pressure gradients, tidal rectification, uniform wind stresses, and associated barotropic pressure gradients. A historical hydrographic database is used to determine the climatological mean baroclinic forcing. Upstream open boundary conditions are estimated from the density fields to give no normal geostrophic bottom flow and are specified as either along-boundary elevation gradients or depth-integrated normal velocities. The numerical solutions for nominal bimonthly periods (January–February, April–May, and July–August) reveal the dominant southwestward nearshore and shelf-break flows of relatively cool and fresh shelf water from the Gulf of St. Lawrence and Newfoundland Shelf, with speeds up to about 20 cm/s. The seasonal intensification of the southwestward flows is reproduced by the model, with the transport increasing from 0.3 Sv in summer to 0.9 Sv in winter on the inner Halifax section. There are also pronounced topographic-scale influences of submarine banks, basins, and cross-shelf channels on the circulation, such as anticyclonic gyres over banks and cyclonic gyres over basins. Baroclinicity is the dominant forcing throughout the domain, but tidal rectification is comparable on the southwestern Scotian Shelf (e.g., about 0.2 Sv recirculating transport around Browns Bank for all the periods). The mean wind stress generates offshore surface drift in winter. The solutions are in approximate agreement with observed currents and transports over the Scotian Shelf, although there are local discrepancies.

On the geographic and seasonal patterns of the near-surface circulation on Georges Bank — from real and simulated drifters

Abstract We examine the near-surface circulation on Georges Bank by comparing data collected from satellite-tracked drifters in the real ocean and data computed for simulated drifters in a virtual ocean. The observed trajectory data set was obtained from drifters drogued at 10 m during the 1995–1997 US GLOBEC Northwest Atlantic/Georges Bank field program. The simulated drifter trajectories were computed based upon the seasonal circulation as predicted by the Dartmouth Circulation Model, using bimonthly climatological forcing. The observed and simulated drifter patterns indicate well-organized anticyclonic around-bank flow on the northern flank, Northeast Peak, and southern flank throughout the year. The key to recirculation around the Bank is the seasonality of the northward flow in the Great South Channel. Winter months are characterized by little northward flow, while there is significant northward flow in the Great South Channel in summer. In late summer, both observed and numerical drifters indicate a minimum recirculation time on Georges Bank of roughly 40 days. The simulated drifter trajectories generally predict a seasonal climate consistent with the observed drifters, though the effects of weather events on the observed drifters are not captured by the numerical simulations.

Modelling the sea level of the upper Bay of Fundy

Abstract A high resolution model of the upper Bay of Fundy was developed to simulate the tides and sea level. The model includes the wetting and drying (inundation) of the extensive tidal flats in Minas Basin. The model reproduces the dominant M2 tidal harmonic with an error on the order of 0.30 m, and the total water level in Minas Basin with an r.m.s. error of 0.30–0.50 m. Overall the system is capable of a sea level simulation with a relative error of ∼10%. The motivation for the model development was the simulation of the land/water interface (instantaneous coastline) to aid in the validation of coastline retrieval algorithms from remotely sensed observations. Comparison of observed and simulated coastlines showed that a high quality representation of the local topography/bathymetry is as important as the sea level simulation in the calculation of the coastline. For example, long narrow features such as dykes are difficult to resolve in a dynamical model but are important for the inundation of low lying areas.

Inverse Model for Limited-Area Hindcasts on the Continental Shelf*

Abstract The authors have constructed an inverse model for interpreting subtidal velocity observations in a limited-area hindcasting context. The intended use is following removal of a best prior circulation estimate that accounts for tide, local baroclinicity, and the direct response to local wind forcing. The remaining, subtidal velocity signal is inverted to provide far-field subtidal pressure forcing. The forward portion of the model is a linearization of a full-physics 3D simulator (QUODDY). Its inversion is achieved by gradient descent, using an exact algebraic adjoint and strong dynamical constraints. The cost function is a weighted least squares blend of velocity mismatch plus boundary condition size, slope, and tendency. The control parameters are barotropic open-water boundary conditions. Solution is achieved in the time domain, as a complement to the frequency-domain “detiding” inversion presented earlier. A simple test case is introduced to demonstrate features of the inversion: precision, acc...

Wind‐driven depth‐averaged circulation in Queen Charlotte Sound and Hecate Strait

Abstract We develop a wind‐driven depth‐averaged model of the circulation on the continental shelf around the Queen Charlotte Islands. The model captures a major feature of the winter current‐meter observations: a flow in Moresby Trough against the direction of the prevailing winds. Moresby Trough is a steep submarine canyon cutting across the shelf from the Pacific Ocean to the mainland. The flow patterns revealed by simulated drifters lead to four generalizations about the depth‐averaged, wind‐driven flow: (1) the flow is subject to strong topographic steering, (2) the exchange between Queen Charlotte Sound and the Pacific Ocean is limited to small regions near Cape St James and Cape Scott, (3) the exchange between Queen Charlotte Sound and Hecate Strait is controlled by Moresby Trough, and (4) the observed outflows past Cape St James are not explained by the dynamics of this model.

Winter transport and sea level fluctuations in Hecate Strait, British Columbia

Observed winter transport and adjusted sea level fluctuations in Hecate Strait were investigated using empirical orthogonal function and coherence analyses. The responses to large-scale and local wind forcings were identified by the distinctive spatial patterns of their adjusted sea level responses. At the resolved periods of 2 to 48 days, the large-scale and the local wind forcings were of roughly equal importance in driving transport fluctuations. Comparison of observations with a conceptual model indicates that the adjusted sea level in the northeast corner of the strait gives a good measure of response to both the local and large-scale wind forcings. This provides a physical explanation for the observed high correlation between the adjusted sea level at Prince Rupert and the winter transport fluctuations in Hecate Strait. We also show that the transport fluctuations are associated with a particular spatial pattern of the velocity field, which represents roughly 1/4 of the energy in the observed winter velocity fluctuations measured at an array of current meters. This has implications for the use of surrogate transport series to hindcast oceanographic conditions in Hecate Strait.

Flocculation and the Fate of Drill Mud Discharges

Flocculation of the fine particulate portion of drilling mud during discharge into the ocean provides a pathway for an impact on the benthic environment in that the higher settling velocity of flocculated particles leads to higher concentrations near the sea floor. The benthic boundary layer transport model, bblt , has been developed for predicting the dispersion and transport of suspended sediment in the benthic boundary layer on the continental shelf. Past bblt applications aimed at impact zone assessment of drill mud discharges on the Atlantic Canadian shelf have used a constant settling velocity to describe particle settling. However, recent observations on the shelf suggest that natural flocs break up when the turbulent shear stress exceeds 0.1 Pa. We incorporate a variable settling velocity into the bblt model to investigate how stress-dependant flocculation affects the magnitude and frequency of high drill mud concentrations near the sea floor. Applications to the Hibernia drill site on the Grand Banks and the Cohasset site near Sable Island are considered.