Brad de Young

The circulation and hydrography of conception bay, Newfoundland

Abstract The hydrography and circulation of Conception Bay (Newfoundland) are described based on hydrographic, current‐meter and drifter data collected over four years (1988–1991). The seasonal cycles of temperature (‐1.6 to 13–17°C) and salinity (31–32.5) in the bay closely follow those on the adjacent shelf. Exchange of bottom water was observed in April 1989. Deepwater exchange was observed from late fall to early winter of 1989–90. Tidal currents are weak, 1–2 cm s‐1 for the M2 and K1 constituents. Observed Eulerian mean currents (<3 cm s‐1) are smaller than the standard deviation (1–11 cm s‐1); however, there is a persistent outflowing current of 10 to 20 cm s‐1 within 2 km of the shoreline on the eastern side of the outer bay. The Lagrangian correlation length scale is from 4 to 10 km, in agreement with the weak coherence squared (≤0.4) found between the fixed current‐meter sites separated by greater than 4–5 km. The variable currents (up to 20 cm s‐1) tend to be cyclonic. Cyclonic eddies were obser...

Asymmetry in the response of a stratified coastal embayment to wind forcing

We investigate mechanisms that lead to asymmetry in the response of a stratified coastal embayment following the onset of a uniform, steady wind that is blowing both along the axis and out of the bay. We focus on bays on the east coast of Newfoundland where the typical duration of wind events is 5 days and stratification representative of June conditions yields a first baroclinic mode wave speed of 0.51 m s-1. We use several numerical models ranging from a linear, reduced gravity model with a single baroclinic mode, to a nonlinear, prognostic, primitive equation model (CANDIE). We investigate the effect of factors such as continuous stratification, vertical mixing, nonlinearity, and realistic bottom topography. If the linear dynamics of only the first baroclinic mode is considered, the response of the idealized bay to 5 days of steady wind forcing is symmetric about the axis of the bay. Continuous stratification allows for higher-order vertical modes. These slower modes increase the response time of the bay, yielding asymmetry in the circulation pattern after 5 days of constant wind forcing. Model results using realistic geometry demonstrate that realistic bottom topography has little effect on near-surface circulation on the 5 day timescale. Adding nonlinearity allows a significant cross-bay transport of upwelled water and leads to the characteristic along-bay pattern of the surface isotherms evident in observations and can also lead to the separation of the coastal jet from the upwelling favorable shore.

The Local and Nonlocal Response of Conception Bay to Wind Forcing

Abstract In this paper the response of Conception Bay to wind forcing is discussed. Current meter and thermistor chain observations are analysed and compared with output from a reduced-gravity numerical model. The model incorporates realistic coastal geometry and is driven by wind stress calculated from observed winds. Moorings were deployed in the bay during 1989 and 1990. In 1990 the moorings were placed within the coastal waveguide around the head of the bay and show that southwesterly winds generate an upwelling event on the western side that moves around the head of the bay and is suggestive of Kelvin wave propagation. Data analysis shows that the thermocline response is strongly coherent between each mooring at periods of 2–10 days, and winds measured at a nearby station are found to be strongly coherent with the observed temperature fluctuations. Two versions of the reduced-gravity model are applied—one models Conception Bay alone and ignores “upstream” influences and another includes neighboring T...

A Diagnostic Coastal Circulation Model with Application to Conception Bay, Newfoundland

Abstract A diagnostic circulation model is developed for application to coastal regions. The three-dimensional velocity field can be calculated from a specified density field and wind-stress distribution provided transport is given on boundaries where f/H contours enter the model domain (here f is the Coriolis parameter and H is the ocean depth). The model is an extension of that of Mellor. It includes the effect of vertical mixing and bottom friction and avoids explicit calculation of the JEBAR (joint effect of baroclinicity and relief) term, which can be noisy when a realistic density field is combined with realistic topography. The model can also be used in regions of closed f/H contours. An application of the model to Conception Bay, Newfoundland, illustrates the case of calculation and yields comparisons with the more classical technique of dynamic height analysis.

Oceanic responses to Hurricane Igor over the Grand Banks: A modeling study

A three-dimensional (3-D) baroclinic finite-volume ocean model (FVCOM) was developed to examine the oceanic response to Hurricane Igor over the Grand Banks of Newfoundland. Hurricane Igor generated a storm surge of almost 1 m at St. Johns and about 0.8 m at three nearby coastal tide gauge stations (Bonavista, Argentia and St. Lawrence). The surge magnitude from the 3-D baroclinic model agrees approximately with tide-gauge observations at all four stations, slightly better than that from an alternative 3-D barotropic case. The sudden drop of sea surface temperature caused by the storm, approximately 6oC as observed by buoys, is well simulated by the baroclinic model with a k-e turbulence closure. A sensitivity simulation with the Mellor-Yamada turbulence closure significantly underestimates sea surface cooling. It is shown that the sea surface cooling is mainly associated with turbulent mixing, and to a lesser degree with Ekman upwelling. The model solution shows that the largest surge occurred between Bonavista and St. Johns. Further analysis suggests the generation of a free continental shelf wave after the storm made landfall, with the peak surge propagating from St. Johns to St. Lawrence.

A 3‐D data‐assimilative tidal model of the northwest Atlantic

Abstract A three‐dimensional (3‐D) barotropic tidal model for the northwest Atlantic is developed for eight leading semi‐diurnal (M2, S2, N2, K2) and diurnal (K1, O1, P1, Q1) tidal constituents based on the Princeton Ocean Model (POM). Multi‐mission altimetric tidal data are assimilated into the model using a simple nudging scheme. The assimilative model results are validated against independent in situ observations and compared with a non‐assimilative run and previous tidal models. The root‐sum‐square error for the assimilative M2, S2, N2, K1 and O1 tidal elevations is 3.1 cm excluding the Bay of Fundy region and 11.1 cm otherwise. Assimilation improves the accuracy of the model tidal elevation by 40–60% and that of the tidal currents by 20–30%. The semi‐diurnal tidal currents agree better with observations than do the diurnal constituents. The model K1 and O1 tidal currents are intensified on several outer‐shelf areas, qualitatively consistent with shelf‐wave theory and moored measurements, but quantitatively overestimated. Results show that the present assimilative model reproduces the primary tidal constituents better than previous regional and inter‐regional models. In particular, the present model results are as accurate as those of Egbert and Erofeeva (2002) for the northwest Atlantic shelf seas as a whole and better if the Bay of Fundy is excluded, pointing to the importance of the high‐resolution multi‐satellite tides to partially compensate for the simple assimilation technique.

Working towards seafloor and underwater iceberg mapping with a Slocum glider

This paper reports on the integration and evaluation of a Tritech Micron mechanical scanning sonar into a Slocum underwater glider. The intend is to use the Slocum glider with the scanning sonar, to conduct seafloor and iceberg mapping tasks. The mechanical scanning sonar is installed in the extended, free flooded area of nose of the glider. After the successful integration, initial field trials were conducted in order to evaluate the performance in both seafloor surveying, and iceberg mapping modes. To achieve optimal performance, tuning of sonar parameters and vehicle trajectory control becomes significant. The performance of the vehicle and sonar are investigated in the field. Due to the transmission power absorbed by the extended nose cone, backscatter intensity is reduced, and receiver gain had to be increased, when compared to uncovered operations. With the experience gained from the initial field trial, areal surveys and autonomous iceberg mapping missions will be conducted in the future.

Application of a barotropic model to North Atlantic synoptic sea level variability

A barotropic, shallow-water model of the North Atlantic is used to investigate variability in adjusted sea level on time scales of a few days to a few months (by adjusted, we mean that the inverse barometer is removed from both the model-computed sea level and the observations). The model has 1/3° × 0.4° resolution in latitude and longitude, respectively, and is forced using atmospheric pressure and wind stress data derived from European Centre for Medium Range Weather Forecasts (ECMWF, 1994) analyses. The model results are compared with coastal tide gauge data. Along the western boundary, from St. Johns, Newfoundland, to Fernandina Beach, Florida, coherence squared between model and data is greater than 0.5 in the period range 3 to 10 days. South of Cape Hatteras, the model underestimates the amplitude seen in the data, with much better agreement north of the Cape. Model performance on the eastern boundary is generally poor. We suggest this is because on the eastern boundary, the shelf width is much narrower, compared to the internal radius of deformation, than on the western boundary. In addition, the model resolution is insufficient to adequately represent the shelf on the eastern boundary. The poorer agreement south of Cape Hatteras may be due Gulf Stream effects not accounted for by the model dynamics. Finally, we discuss the model-computed variability in the ocean interior.

Synoptic bottom pressure variability on the Labrador and Newfoundland Continental Shelves

A high-resolution, barotropic model of the North Atlantic is used to investigate bottom pressure variability on the Labrador and Newfoundland Shelves. The model has a free surface and is forced by surface atmospheric pressure and wind stress derived from twice-daily analyses from the European Centre for Medium-Range Weather Forecasts. Model output is compared with a yearlong set of bottom pressure data (1985-1986). Coherence squared between model and observations is,significant at the 95% confidence level at almost all stations and exceeds 0.6 over most of the inner shelf, with good agreement in the phase. The model shows a tendency to overestimate the autospectral energy at periods less than 10 days but to underestimate the energy at longer periods. The model does less well at the outer shelf, where coherence squared is only marginally significant. The weaker performance of the model near the shelf break may indicate a role for physical processes trot accounted for in the model (e.g., eddies, stratification) or, alternatively, might be due to the low signal-to-noise ratio in the bottom pressure measurements made at the shelf break. Model/data comparisons show that wind forcing dominates over pressure forciing, except at the northern end of the Labrador Shelf, where both forcings are important. Model experiments run with and without Hudson Bay in the model domain demonstrate that Hudson Bay influences the Labrador Shelf in the 2 to 10-day period range

Reconstructed Wind Fields from Multi-Satellite Observations

We present and validate a method of reconstructing high-resolution sea surface wind fields from multi-sensor satellite data over the Grand Banks of Newfoundland off Atlantic Canada. Six-hourly ocean wind fields from blended products (including multi-satellite measurements) with 0.25° spatial resolution and 226 RADARSAT-2 synthetic aperture radar (SAR) wind fields with 1-km spatial resolution have been used to reconstruct new six-hourly wind fields with a resolution of 10 km for the period from August 2008 to December 2010, except July 2009 to November 2009. The reconstruction process is based on the heapsort bucket method with topdown search and the modified Gauss–Markov theorem. The result shows that the mean difference between the reconstructed wind speed and buoy-estimated wind speed is smaller than 0.6 m/s, and the standard deviation is smaller than 2.5 m/s. The mean difference in wind direction between reconstructed and buoy estimates is 3.7°; the standard deviation is 40.2°. There is fair agreement between the reconstructed wind vectors and buoy-estimated ones.