Winter 1999


Measuring Turbulence in the Ocean Mixed Layer with a Tethered Free-Fall Glider

Blair Greenan

Ocean Sciences Division
Bedford Institute of Oceanography

4:30pm, Thursday, Jan 14, 1999

Abstract:

The EPSONDE-Glider is a tethered free-fall glider designed to provide quasi-horizontal profiles of microstructure and turbulent shear near the ocean surface. A series of tests were performed with EPSONDE-Glider on the CSS Parizeau from June 17-30, 1996. The experiment site was located at Emerald Bank on the Scotian Shelf in a relatively flat area of 100 m deep water. As a complement to glider tests, the following measurements were also performed: 1) EPSONDE vertical profiles of ocean microstructure, 2) air-sea flux measurements with a bow anemometer system, 3) boundary layer meteorological data collected with a minimet buoy, 4) wave spectra from a wave rider buoy, 5) ADCP profiles, 6) CTD profiles, and 7) wave measurements using a ship-mounted radar. Results of recent tests carried out in Bedford Basin in July 1998 will also be discussed.


Stochastic model of edge waves

Christine Pequignet

Department of Oceanography
Dalhousie University

4:30pm, Thursday, Jan 21, 1999

Abstract:

Edge waves are one of the different types of motion which, over a wide range of frequencies, play an important role in nearshore hydrodynamics and sediment transport. In spite of earlier studies, it is still unclear how edge waves are forced.

To test different forcing mechanisms, a linear model has been developed. The forced shallow water equations are solved numerically for edge wave solutions with longshore wavenumber, k. A periodic structure of form exp(iky) is assumed in the longshore, y, direction and this allows also for shear wave solutions. This results in a set of partial differential equations in eta, u, and v (respectively surface elevation, cross-shore and longshore velocities) functions of the cross-shore variable, x, and time, t, and solved using a finite difference scheme. Stochastic forcing terms for the momentum equations are specified at each cross-shore grid points. The equations are linear and therefore solutions for different wavenumbers, k, can be summed to retrieve the full spectrum of solutions.

The model allows for any cross-shore beach profiles h(x) and arbitrary longshore current profiles V(x) as well. The dynamical matrix is used to calculate the spectral transfer function between the forcing and the response. Analysis in the time domain and the frequency is applied to the outputs of the model to understand the edge wave response to different type of forcing mechanisms.


No seminar this week

4:30pm, Thursday, Jan 28, 1999


Cross-shore lunate megaripple migration and bedload sediment transport models

Amani Ngusaru

Dept. of Earth Sciences
Memorial University of Newfoundland

4:30pm, Thursday, Feb. 4, 1999

Abstract : Rotary fan-beam sidescan sonar and rotary pencil-beam sonar images are used to determine lunate megaripple geometrical scales, orientations, cross-shore migration rates and their relation to the hydrodynamic forcing. Megaripples were observed to migrate in both onshore and offshore directions depending on the strength of mean cross-shore flow velocity. Stress-based bedload transport model was successful in relating the migration rates with bedload sediment transport. The observed migration rates were reasonably well modelled using the measured waves and currents in a natural nearshore zone. The orientation of lunate megaripples tends to be controlled by the inshore wave angles of incidence. The measured orientations were consistent with the calculated net bedload sediment transport direction and showed no relationship with the gross bedform-normal sediment transport direction.


Title: TBA

Speaker

Institute

4:30pm, Thursday, Feb 11, 1999


A very easy and useful way of fitting theoretical spectra to observations

Barry Ruddick

Department of Oceanography
Dalhousie University

4:30pm, Thursday, Feb 18, 1999

Abstract: SCAMP measures temperature every millimetre in a vertical profile and miraculously converts this to segment-averaged measures of Chi-theta and Epsilon. It does this by fitting the observed spectra to the theoretical "Batchelor" form, whose cutoff wavenumber depends on Epsilon and thermal diffusivity.

In overcoming some minor difficulties in the SCAMP software, we have developed a simple technique for fitting spectra that should be more broadly used. All one needs is a functional form for the theoretical spectrum, and an estimate for the instrumental noise spectrum. The method has several advantages over other fitting techniques:

1. It is unbiased in comparison with other least-squares or cost function approaches.

2. It is robust, i.e., insensitive to dips and wiggles in the spectrum. This is because the built-in noise model tells the routine to ignore the spectrum as it gets down towards the noise level.

3. Error bars on the fitted parameters. There is a theoretical estimate for the variance of the estimated Batchelor wavenumber, based on how broad or narrow the likelihood function peak is.

4. We calculate statistical quantities that indicate how well the observed spectrum fits the theoretical form. This is extremely useful in automating analysis software, to get the computer to automatically ignore "bad" fits.

The method is demonstrated using SCAMP data, compared to the SCAMP-generated fits and other least-squares techniques, and tested against pseudo-data generated by Monte-Carlo techniques.

A possible application of the method to the EPSONDE data analysis system is described.


Recent Changes to the Slope Waters off the Scotian Shelf and the Gulf of Maine and their Effects

Kenneth Drinkwater

Bedford Institute of Oceanography

4:30pm, Thursday, Feb 25, 1999

Abstract: Warm Slope Waters adjacent to the continental break along the Scotian Shelf and Gulf of Maine were replaced by colder, fresher Labrador-type Slope Water during the autumn of 1997 and winter of 1998. Temperatures dropped by 2-4 deg. C along the slope. This cold slope water was first observed off Banquereau Bank in September 1997 and reached offshore of Emerald Bank by October. By January 1998 it was at the entrance to the NE Channel in the Gulf of Maine and a month later extended all along the southern flank of Georges Bank. These cold, low salinity offshore waters have penetrated onto the Scotian Shelf and into the Gulf of Maine producing hydrographic properties not seen since the 1960s. Possible causes of the change in the offshore Slope Water properties will be discussed.


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4:30pm, Thursday, March 4, 1999


Air-sea interaction near ocean fronts: some mechanisms

Yuri Geshelin

Department of Oceanography
Dalhousie University

4:30pm, Thursday, March 11, 1999

Abstract: The ocean frontal zones are known to play an important role in global air-sea interaction. They are unique in big air-sea temperature differences, fairly sophisticated structure of heat fluxes and anomalous atmospheric response to high sea surface temperature (SST) gradients. In this study, we investigate two mechanisms of that response and a wind-driven process of water drift in the vicinity of ocean fronts. We use field data for the analysis.

First, we take a close look at the relationship between the SST gradient and wind speed. The climatological analysis is confined to the frontal zones of the Newfoundland Basin and Kuroshio areas. Two independent data sets are used in the study: COADS and the data set collected on 83 Russian cruises to the Newfoundland Basin in 1980-1991. Our findings are shown to differ for both regions and different data sets. The inferred component of thermally driven wind is compared to the Hsu, 1984, estimate. In the Newfoundland Basin, the thermal wind circulation is simulated better than in the Kuroshio area. We discuss possible reasons for this.

We also infer some estimates of quite an interesting atmospheric phenomenon, one type of marine fog, known as sea smoke, and describe the conditions in which it occurs. This is done for the Gulf Stream area only. In terms of SST gradients, there is some resemblance between the occurrences of sea smoke and thermally driven winds. The physics of the process is discussed.

Finally, we furnish some estimates of wind-driven drift of the surface water in the Gulf Stream area. The process is fairly weak in the frontal zone and becomes more intensive as one moves away from the main jet of the current.


Hydrostatic and Non-hydrostatic Convection Studies with DieCAST

Dr. David Dietrich

in collaboration with
Charles Lin
Dan Wright
Avichal Mehra

Center for Air-Sea Technology
Mississippi State University

4:30pm, Thursday, March 18, 1999

ABSTRACT. A non-hydrostatic ocean model and a hydrostatic version of the same model are used to simulate convection in a 40 km wide channel with no along-channel variations, but with along-channel flow generated by Coriolis coupling to the cross-channel flow. The channel bathymetry is parabolic, with maximum depth 500 m at center and zero depth at the edges. Model results for four cases are compared: with and without the non-hydrostatic terms, and with 0.1 and 1.0 km resolution. Although significant resolution and non-hydrostatic effects occur in the convective plumes, there is much less effect on the convectively adjusted state (after the plumes "flip" the fluid over). The adjusted state reflects strong contra-diffusive vertical density advection effects that cannot be described by traditional instant convective adjustment and other diffusive parameterizations. The four cases are repeated after applying traditional instant convective adjustment to the initial state, and results are compared.

Although Coriolis terms strongly inhibit convection on scales > 10 km, contra-diffusive advection effects that occur in subgrid scale plume ensembles can be can be emulated using practicable resolution for basin- and global- scale models, by using a vorticity-selective filter (thus reducing the Coriolis constraint on the smallest resolved scales). This strongly favors models having low numerical dissipation, such as the DieCAST model used in these numerical experiments.

Implications for modeling intermediate and deep watermass formation, a major ocean climate modeling issue and a focus of the Dynamo Project, are discussed.


Modelling Organic Aerosols for Climate Studies

Glen Lesins

Atmospheric Science Program
Department of Physics
Dalhousie University

4:30pm, Thursday, March 25, 1999

Organic atmospheric aerosols contribute to much of the total particulate loading. The last decade has seen improvements in the measurements of organic gases and aerosols making it now more feasible to provide them with a better treatment in climate models. The modelling of organic aerosols will be discussed with the goal of assessing their direct and indirect radiative forcing potential.


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4:30pm, Thursday, April 1, 1999


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4:30pm, Thursday, April 8, 1999


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4:30pm, Thursday, April 15, 1999


Seasonal Cycle in the North Pacific and the North Atlantic Oceans.

Igor M. Yashayaev

Ocean Sciences Division
Bedford Institute of Oceanography

4:30pm, Thursday, April 22, 1999

Spatial distributions of seasonal cycle contribution to total variability, amplitudes and phases of its harmonics, annual phase difference and semiannual to annual amplitudes ratio are analyzed in details. Sea surface temperature (SST), air temperature (AT) and sea level pressure (SLP) time series were constructed from merged releases of the Comprehensive Ocean-Atmosphere Data set (COADS). The time series were decomposed into seasonal and non-seasonal (short and long-term) components. The contribution of seasonal cycle to the total variance of SST and AT exceeds 80% in the mid and some high latitudes, and reaches its peak (>95%) in the centers of Subtropical Gyres. In the most cases a combination of annual and semiannual harmonics accounts for more than 95% of seasonal variability.

Amplitudes of SST and AT annual cycles are the highest near the western boundaries of the oceans. SST and AT annual phases increase toward the eastern tropical parts of the oceans, revealing southeastern propagation of annual cycle over the Northern Hemisphere oceans. AT annual cycle leads one of SST by 1 to 3 weeks. The highest phase differences are observed along the western coasts of the North Pacific and the North Atlantic in the regions of western boundary currents. This is consistent with spatial patterns of integral air-sea heat fluxes.

A belt of low amplitudes of SLP annual harmonic stretches along the Equator (0-10N) in both oceans. There are three distinct areas of high annual amplitudes of SLP in the North Pacific ocean: Asian, Aleutian and Californian, but only one in the North Atlantic, centered to the west of Iceland. A remarkable feature in the climate of the North Pacific is a maximum of semiannual SLP amplitudes, centered near 40N and 170W. It is also an absolute maximum in the entire Northern Hemisphere. Analysis of phases of harmonics of SLP seasonal cycle revealed trajectories of propagation of annual and semiannual cycles. Basing on analysis of semianual to annual amplitudes ratio the regions of semiannual cycle dominance are defined.


Dynamics of the Buoyancy-Driven Coastal Jet: The Gaspe Current

Jinyu Sheng

Department of Oceanography
Dalhousie University

4:30pm, Thursday, April 29, 1999

Abstract: A three dimensional primitive equation ocean model is used to study the dynamics of the Gaspe Current and the cyclonic circulation over the northwestern Gulf of St. Lawrence. The model is driven by buoyancy forcing associated with St. Lawrence River discharge and barotropic boundary inflows. The model domain is initially filled with horizontally uniform but vertically stratified waters of summer mean temperature and salinity over this region. After a short-period adjustment, a buoyant plume is developed inside the estuary with an eastward current over the north shore. Due to the Coriolis effect, the north shore current veers anticyclonically and flows southward along the offshore front of the plume. It turns abruptly eastward as it reaches the south shore, forming a surface-intensified Gaspe Current that advects estuarine waters seaward along the right-bounded coastline. The Gaspe Current follows closely the irregular coastline initially but becomes unstable with multiple backward breaking waves developed. With the river discharge as the only driving force, however, the offshore front of the buoyant estuarine plume expands continuously seaward that eventually leads to a large-scale anticyclonic circulation in the northwestern Gulf region. The addition of a barotropic westward jet along the Quebec shore is able

to restrain the seaward expansion of the the offshore front of the estuarine plume, and therefore form a large-scale cyclonic motion over this region.


SEDIMENT SUSPENSION IN THE NEARSHORE ZONE; IN-SITU 2-D OBSERVATIONS AND 1-D NUMERICAL RESULTS. (Do simple 1-D tke models capture suspension characteristics?)

Phillip N. MacAulay, Alex Hay, and A. J. Bowen

Department of Oceanography
Dalhousie University

4:30pm, Thursday, May 6, 1999

ABSTRACT. Nearshore sediment suspension and transport take place in an environment of complex boundary layer dynamics driven by waves, infra-gravity waves and mean currents simultaneously. Both the hydrodynamics and resultant suspension can be highly temporally and spatially variable. One might expect spatial variability for rippled beds, but at times the scales appear much larger than those available through the bed roughness alone and the causes of similar structure for flat bed are even more obscure. To investigate the causes we compare in-situ observations of sediment suspension made with a 2-D acoustic method, the sector scan, with results generated using a standard 1-D ke numerical model which predicts both cross- and long-shore flows and suspension. The objective of this comparison is twofold. Does the model predict the observed dynamics? What does it miss, and why. Secondly, what can the model tell us about the observed processes.

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4:30pm, Thursday, May 13, 1999


Coastal Ocean Predictions System for the East Coast of Canada

Josko Bobanovic

Department of Oceanography
Dalhousie University

4:30pm, Thursday, May 20, 1999
LSC 3655 (classroom on 3rd floor)

PLEASE NOTE special location

Abstract: In recent years, there has been increased pressure to supply accurate predictions of the circulation on Canadian Atlantic shelves. The need for such predictions comes from intensified maritime transportation, search and rescue operations and pollution control. Increased number of offshore structures for hydrocarbon exploration requires careful management of exploration fields and their environmental impact assessment. We report on the development of an operational forecast system for the east coast of Canada. The approach taken is based on nesting. First, we use a large-scale non-linear storm surge model that covers the complete east Canadian shelves and adjacent deep ocean from 38N (Gulf of Maine) to 60N. Forecast fields (sea level and depth averaged currents) are used as a first guess for the open boundary conditions of the nested model.

The nested model covers the Gulf of Saint Lawrence and the Scotian Shelf and parts of the adjacent open ocean (roughly from 40N to 52 N). The ocean model is based on POM (Blumberg and Mellor, 1987), three-dimensional, non-linear, prognostic model that uses sigma-coordinates in vertical. Density fields were obtained from the archives of the Bedford Institute of Oceanography and gridded seasonally. At present the model is run in diagnostic mode, i.e. T and S fields are held fixed in time (season). We use a historic data set to validate the forecast scheme on the Scotian Shelf during winter 1996.


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4:30pm, Thursday, May 27, 1999


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4:30pm, Thursday, June 3, 1999


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4:30pm, Thursday, June 10, 1999


Application of a Direct Inverse Data Assimilation Method to the M2 Tide on the Newfoundland and Southern Labrador Shelves

Zhigang Xu

Bedford Institute of Oceanography

4:30pm, Thursday, June 17, 1999

A study of data assimilative modelling of the M2 tide over the Newfoundland and southern Labrador Shelves and adjacent deep ocean is reported. The dataset includes harmonic tidal constituents from TOPEX/POSEIDON altimetry, coastal tide gauges, bottom pressure gauges, and moored current meters. A linear harmonic finite-element model and a newly-developed direct inverse method for data assimilation are used. Three modelling approaches are taken: a conventional modelling approach with boundary conditions specified from along-boundary observations, a full interior data assimilative approach which provides an optimal domain-wide solution, and a partial data assimilative approach in which the roles of various data subsets are investigated.

The optimal solution from the full assimilation approach has root-mean-square (rms) distance misfits of 3.5 cm and 1.2 cm/s for elevation and current, respectively (in terms of distances on the complex plane), compared to overall rms amplitudes of 30cm and 6cm/s. These misfits are reductions by over 40% and 70% from those in the conventional solution. Formal confidence limits for the optimal solution can be estimated, but depend on assumptions about the spatial covariance of the observational residuals. The partial-assimilative sensitivity cases provide quantitative indications of the importance of the quantity and location of the observational data. In particular, the inclusion of a fraction of the velocity data in the assimilation results in a significant improvement in the model fit to the velocity observations.


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4:30pm, Thursday, June 24, 1999