Winter/Spring 2003

Transport of suspended sediment on a natural beach

Stephen Henderson

College of Oceanic and Atmospheric Sciences
Oregon State University

4:45 p.m., Thursday, January 9, 2003
Note Special Time

Abstract: Transport of suspended sediment under waves is predicted by coupling an eddy-diffusive numerical model for sediment suspension with a one-dimensional numerical wave boundary layer model. Predictions of beach erosion and accretion, calculated from the divergence (or convergence) of the simulated depth-integrated suspended sediment flux, are compared with the observed erosion and accretion of a natural beach. The effects of gravity and bedload transport on beach change are not simulated. A single free parameter, which determines the rate of sediment pick-up from the seabed, is adjusted to calibrate the model. The calibrated model predicts much of the observed beach profile change, including both the seaward sandbar migration which occurred during a storm, and the shoreward sandbar migration which occurred between two storms. A simplified, analytic model for transport of suspended sediment under waves is derived. Predictions of transport obtained using this simplified model are compared with numerical predictions, and with the observed beach change.

No seminar this week

Thursday, January 16, 2003

Measurement of fluvial bedload transport using an acoustic Doppler current profiler

Colin Rennie

Department of Oceanography
Dalhousie University

4:30 p.m., Thursday, January 23, 2003


Observations from a moored profiled ADV on the Scotian Shelf

David Ciochetto

Department of Oceanography
Dalhousie University

4:30 p.m., Thursday, January 30, 2003

Abstract: A Nortek Vector ADV was incorporated with a SeaHorse Moored Profiler and the Epsmapper turbulence probe. The instrument package was tested in the Bedford Basin during the summer of 2002 and deployed on the shelf during October 2002. Problems with the data and instrument performance were hinted at in the data. A presentation of the work on this data will be presented and discussed. This is still a work in progress so comments and discussion are greatly appreciated.

Naval applications of the acoustical ocean

Dan Hutt

Ocean Sensing and Modelling Group
Defence R&D Canada - Atlantic

4:30 p.m., Thursday, February 6, 2003

Abstract: Modern navies rely on underwater acoustics for communication, reconnaissance and detection but these capabilities are limited by the ocean environment. One of the main constraints is imposed by the thermal structure of the ocean which controls acoustical propagation. Also, there are fundamental trade-offs between range, spatial resolution and temporal resolution. Finally, the state of the ocean surface affects underwater acoustics through the contribution of ambient noise and the effect of interface scattering. The issues affecting naval applications of underwater acoustics will be reviewed and relevant research at DRDC Atlantic will be presented.

No seminar this week

Thursday, February 13, 2003

No seminar this week

Thursday, February 20, 2003

No seminar this week

Thursday, February 27, 2003

Wave-Current Bedforms on Sable Island Bank

Carolyn Smyth

Bedford Institute of Oceanography

4:30 p.m., Thursday, March 6, 2003


Southern Ocean Iron Fertilization Experiment (SOFeX)

Ken Johnson

Monterey Bay Aquarium Research Institute (MBARI)

4:00 p.m., Thursday, March 13, 2003
Location: LSC4263 (Psychology Wing)


A method for adiabatically adjusting ocean models

Richard Greatbatch

Dalhousie University

4:30 p.m., Thursday, March 20, 2003

Abstract: A new and exciting technique, known as the semi-prognostic method, is described. The method can be used to correct models for systematic error, e,g, poor Gulf Stream separation and poor representation of the Northwest Corner, and uses hydrographic data as input. The method is adiabatic, meaning that it does not rely on introducing diabatic sources and sinks. Rather, the idea is to use hydrographic data to introduce forcing terms into the model momentum equations. The method is easy to implement, computationally very cheap, and has application beyond that to be described here. In particular, it can be used a method for transferring information between the different subcomponents of a nested modelling system. The method is illustrated using a regional model of the Northwest Atlantic as well as an eddy-permitting model of the entire North Atlantic. Some comparisons are shown with a 1/12 deg eddy-resolving model of the North Atlantic.

Tracing Arctic Ocean Near-Surface Waters

Peter Jones

Bedford Institute of Oceanography

4:30 p.m., Thursday, March 27, 2003


A Numerical Study of Tidal circulation in Lunenburg Bay

Jinyu Sheng and Liang Wang

Dalhousie University

4:30 p.m., Thursday, April 3, 2003


On the role of air-sea fluxes in extra-tropical hurricanes

Will Perrie

Bedford Institute of Oceanography

4:30 p.m., Thursday, April 10, 2003


Calculating the Flow Around Ships

David Hally

DRDC Atlantic

4:30 p.m., Thursday, April 17, 2003

Abstract: The first attempts to calculate the flow of the water around a ship were undertaken well over 100 years ago. However, it is only in the last decade that accurate flow predictions have been possible. This seminar will describe the features typical of ship flows, why DRDC Atlantic is interested in calculating them, the methods we use to calculate them, and topics that we are still researching.

Interannual variability of meridional transports in the Atlantic: a model intercomparison

Jens-Olaf Beismann, C.W. Boening, D. Stammer (and the FLAME group)

Institut fuer Meereskunde an ber Universitaet Kiel

4:30 p.m., Thursday, April 24, 2003

Abstract: Three different medium-resolution ocean models (based on MOM, OPA, and the MIT codes) are used to study the response of the meridional overturning circulation to interannual to decadal atmospheric variability associated with the NAO. The emphasis is on the identification of robust elements in the simulations as well as on the differences caused by different numerical representations of physical processes. The simulations are analyzed with respect to spatial patterns and time scales of oceanic transport fluctuations. Using sensitivity experiments with a higher resolution (1/3 degree) version of one of the models we investigate the relation between low-frequency variations of meridional transports in the North Atlantic and transport indices derived from hydrographic time series. Furthermore, we study the pathways and mechanisms by which dynamical and passive tracer anomalies originating from fluctuations in deep water formation propagate southward.

No seminar this week Thursday, May 1, 2003

Breakup of the stratospheric polar vortices

Pingping Rong

Earth & Planetary Sci., Johns Hopkins University

4:30 p.m., Thursday, June 26, 2003

Abstract: The temperature in the Earth's stratosphere (between 12 to 50 km) increases with altitude due to ultraviolet heating of ozone. A strong circumpolar flow, called the stratospheric polar vortex, forms during polar winter when ultraviolet heating ceases in this region. These polar vortices last until spring and then break up as ultraviolet heating returns. The structure and evolution of the polar vortices have significant implications on ozone depletion and on global and regional climates.

40 years of meteorological analyses are used to examine the evolution of, and stirring around, the Arctic vortex, with focus on the breakup stage. Two extreme regimes of vortex breakup are found, late and early breakups, with very different decay characteristics. The occurrence of these different breakups is closely related to the interannual variability of the forcing due to the upward propagating planetary waves.

A shallow water model is used to examine the basic processes responsible for the different observed regimes. This model includes topographic forcing, to represent the upward propagating waves, and relaxation to an equilibrium state, to represent radiative processes. The roles of, and competition between, topographic wave forcing and radiative relaxation are examined by systematically varying the different parameters. Both steady and vacillation states are produced, which may be related to cold and warm polar stratospheres, respectively. Calculations with an annual cycle for the radiative equilibrium state and different timing and seasonality of the bottom forcing are also performed to produce the observed breakup regimes.

A new hybrid numerical method which includes diabatic forcing and can resolve steep gradients is also tested and used to further examine the competition between the external forcing and radiative relaxation. Calculations of idealized planar flows show that the relaxation damps out the oscillations in the elongation as well as orientation of a forced vortex, but steep vorticity gradients are preserved for at least 10 vortex overturning periods.

Volcanic Eruptions and Climate: Winter Warming and Summer Cooling

Alan Robock

Department of Environmental Sciences
Rutgers University

11:00 a.m., Tuesday, July 29, 2003

Abstract: Large volcanic eruptions inject sulfur gases into the stratosphere, which convert to sulfate aerosols with an e-folding residence time of about one year. Large ash particles fall out much quicker. The radiative and chemical effects of this aerosol cloud produce responses in the climate system. Using examples from major eruptions of the past and results from experiments with numerical models of the climate system, this talk illustrates the major impacts. One of the most interesting is the "winter warming" of Northern Hemisphere continents following major tropical eruptions. During the winter in the Northern Hemisphere following every large tropical eruption of the past century, surface air temperatures over North America, Europe, and East Asia were warmer than normal, while they were colder over Greenland and the Middle East. This pattern and the coincident atmospheric circulation correspond to the positive phase of the Arctic Oscillation. Using the Max Planck Institute ECHAM4 and the Geophysical Fluid Dynamics Laboratory SKYHI GCMs, we have successfully simulated this response following the 1991 Mount Pinatubo eruption. In spite of the decrease in surface solar heating, surface air temperature increases in high and midlatitudes of the Northern Hemisphere in the winter because of changes in tropospheric circulation caused by stratosphere-troposphere dynamical coupling and volcanically-induced ozone depletion. The phase of the Quasi-Biennial Oscillation also affects the dynamical response.

The June 15, 1991 Mount Pinatubo eruption was a large but relatively short-lived shock to the climate system. It thus provided an excellent opportunity to study its workings, to test climate models, and to examine the impacts of climate change on life. When forced with observed aerosols, the Geophysical Fluid Dynamics Laboratory (GFDL) R30 climate model reproduces the observed cooling and drying of the atmosphere for several years after the Pinatubo eruption. By comparing model simulations with and without water vapor feedback, we demonstrate the importance of the atmospheric drying in amplifying the temperature change, and show that without the strong positive feedback from water vapor the model is unable to reproduce the observed cooling. These results provide quantitative evidence of the reliability of water vapor feedback in current climate models. This confirmation of climate model physics and sensitivity, combined with our other work on climate system dynamics and connections between stratospheric and tropospheric circulation, give strong validation to the models used for attribution and projection of anthropogenic effects on climate.

Recent developments in shallow sea/shelf edge modelling

Alan Davies

Proudman Oceanographic Laboratory, UK

11:00 a.m., Tuesday, August 19, 2003

Abstract: A brief overview of the development of a three dimensional baroclinic shelf edge model will be presented with its subsequent application to the study of internal tides. The nature of wind forced flow in the region and the importance of the resonant non-linear coupling between near inertial internal waves and the internal tide producing energy at the M2 frequency will be examined. The importance of internal wave-wave interaction upon mixing at the shelf edge and its implications for ocean circulation models will be briefly discussed. Trappping of wind induced near-inertial internal waves by along shelf flows and frontal jets, and the implications for enhanced local turbulent mixing will be illustrated.

The role of internal tides, stratification and wind waves in determining sediment resuspension at the shelf edge will be presented.