Fall 1999

Numerical Simulation of Internal Kelvin Waves and Coastal Upwelling Fronts in Large Lakes

William P. O'Connor

Department of Oceanography
Dalhousie University

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


Large lakes, with horizontal dimensions of several hundred km and depths of several hundred meters, have their own interesting dynamics. In winter they are isothermal, but by late summer a strong thermocline has developed. The uppermost 5 m may be isothermal at 20 deg C, the depths below 15 m may be isothermal at 5 deg C, and there can be a strong thermocline with a change of 15 C degrees over 10 m. This is a stronger thermoclines than is usually found in the ocean, and it is a significant challenge for an ocean model to represent the dynamics.

Two numerical ocean models, the Princeton Ocean Model (POM) and the DIECAST ocean model are applied to the model the wind forced upwelling dynamics in large lakes with this thermocline. The models are first applied to a circular basin of diameter 100 km and of maximum depth 100 m. Both constant depth and parabolic bathymetries are used. After the wind stops, for light upwelling, and internal Kelvin wave progresses around the basin. For strong upwelling when the thermocline breakes the surface, the upwelling front progression resembles an internal bore. Finally, the models are applied to a realistic case of strong upwelling in Lake Michigan.




4:30pm, Thursday, Oct. 7, 1999


Ocean Wave Extraction from RADARSAT Synthetic Aperture Radar Inter-Look Image Cross-Spectra

Michael Dowd

St. Andrew's Biological Station
St. Andrew's, New Brunswick

4:30pm, Thursday, Oct. 14, 1999

Abstract: This presentation covers some theoretical and practical aspects of extracting directional ocean wave spectra from synthetic aperture radar (SAR) image spectra. A background review of SAR imaging of ocean waves is first undertaken. A set of 6 RADARSAT SAR images co-located with a directional wave buoy off the east coast of Canada are then examined. These SAR data were processed into two looks at the same ocean scene, separated by approximately 0.4 seconds. The associated inter-look image cross-spectra show the expected reduction in speckle noise. Phase spectra correctly identify wave propagation direction for all but one case, but the mean phase values obtained for the wave groups are quite variable. Converting these SAR spectra into ocean wave information requires inversion of the wave-SAR transform. This is examined from the perspective of a general statistical estimation problem in order to address some outstanding issues in the inversion procedure. Next, extraction of spectral wave information from observed RADARSAT SAR image cross-spectra is undertaken, with an emphasis on quantifying the wave information content of SAR. A modified wave-SAR transform is proposed to better account for case-specific geophysical and imaging effects. Analysis of the residual error of simulated and observed SAR spectra lead to a canonical form for the SAR observation error covariance. Wave estimates extracted from the SAR spectra take into account these error covariances, as well as spectral null spaces where the SAR contains no wave information. Finally, this band-limited SAR wave information is combined with prior (buoy) spectral wave estimates, focusing on parameterization of the wave spectral shape and the role of regularization.

Effects of Ocean Currents on Pacific Salmon Migration Inferred From a Fine Resolution Numerical Model

Marie-Claude Bourque

Ph.D. graduate
Department of Oceanography
University of British Columbia

4:30pm, Thursday, Oct. 21, 1999

New Algebraic Turbulence Modeling Parameters Derived from Experimental Data

David S. Ciochetto


4:30pm, Thursday, Oct 28, 1999


Existing two dimensional turbulent boundary layer modeling parameters fail to adequately model the physics of complex three dimensional turbulent boundary layer flow. This failure is illustrated through several lags in turbulence parameters as the flow develops. The turbulent shear stress angle lags the flow gradient angle and the behavior of the Reynolds shear stresses lag that of the mean flow. A minimum set of equations to account for the three dimensionality of turbulent flows of engineering interest is proposed. Eleven sets of experimental data for complex three dimensional turbulent boundary layer flows of engineering interest were examined. From these data, new algebraic turbulence modeling parameters were developed to support the six equations mentioned above. The resulting parameters display a good degree of universality for the variety of experimental geometries that were examined.

No seminar this week

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

No Seminar This Week
It's Remembrance Day

4:30pm, Thursday, Nov. 11, 1999

Observations and simulations of Long-term Temperature Trend in The Upper Pacific Ocean above 400m

Boyin Huang, Zhengyu Liu

Department of Atmospheric and Oceanic Sciences
University of Wisconsin - Madison

4:30pm, Thursday, Nov. 18, 1999

Abstract: The long-term temperature trend in the past 40 years is studied using the observations of the expendable bathythermography (XBT) and the simulations of the ocean General Circulation Model (GCM) in the upper Pacific above 400m. The observations and simulations consistently show that the tropical temperature warms in the upper eastern cold tongue and cools in the lower western warm pool in the tropical Pacific Ocean, although the strengths of the warming and the cooling are stronger in the simulations than in the observations. The characteristics of a warming near the surface and a cooling in the thermocline, and a warming in the east and a cooling in the west are also manifested in the North Pacific extra-tropics.

The simulations show that the tropical warming and the cooling are controlled dynamically by the ocean currents in the Pacific. The warming in the cold tongue is caused by the reduction of cold advection off the equator in the surface ocean, and by the reduction of upwelling cold advection in the thermocline. The warming in the surface warm pool is caused by the reduction of the westward cold advection from the cold tongue. The cooling in the thermocline warm pool is caused by the reduction of the equatorward warm advection. These oceanic advections are mainly controlled by the reduction of the meridional divergent (convergent) current off (into) the equator, the westward equatorial current, and upwelling current near the Peru coast. These currents, in turn, are controlled dynamically by the reduction of atmospheric trade winds. Therefore, the tropical temperature trend is controlled dynamically by the reduction of trade winds in the Pacific. The thermodynamic effect through the surface net heat flux between the ocean-atmosphere interface seems not to be a critical factor to affect the tropical ocean temperature trend. In the North Pacific mid-latitudes, however, the thermodynamic effect seems more important to the surface western and central ocean. But for the eastern and lower ocean, the dynamic effect on the oceanic temperature trend is still dominant.

Special Departmental Seminar

Sedimentation processes on the continental margin of western Europe, 40o-50oN

Professor Nick McCave

Department of Earth Sciences
University of Cambridge

2:00pm, Thursday, Nov. 25, 1999
Please note special time and location.

Abstract: Recent studies of sedimentation processes on the continental margin of N.W. Europe have revealed a variety of transport mechanisms. These include detached bottom boundary layers (BBL) forming intermediate nepheloid layers, strong downslope transport of large aggregates in the BBL, vertical settling, alongslope transport in the Slope Current and rare turbidity currents. In the mid-depth waters of the deeper margin a seasonal signal is found in particulate concentration - higher in summer - suggesting a low seasonal pump of POC to the deep ocean interior.

A Study of the extratropical re-intensification of ex-hurricane Earl using the Canadian Meteorological Center analysis data and ensemble forecast system

Suhong Ma, John Gyakum, Hal Ritchie, Jim Abraham, Chris Fogarty

Atmospheric Environment Service

4:30pm, Thursday, Dec. 2, 1999

Abstract: Ex-hurricane Earl re-intensified as it traveled across Canadian waters south of Nova Scotia between 00 UTC 05 September and 12 UTC 06 September 1998. The central sea level pressure decreased 36 hPa, to 964 hPa, in 36 hours and caused strong wind and heavy rainfall at Sydney, Nova Scotia and Saint JohnUs, Newfoundland. Based on the Canadian Meteorological Center (CMC) regional analysis, three important factors in the re-intensification of ex-Earl were observed:

1. The beginning of re-intensification occurred when the upper level potential vorticity (PV) anomaly intensified and moved southward approaching the lower level PV anomaly.

2. Ex-Earl moved into an area of favorable extratropical cyclogenesis downstream of an approaching mid-level trough(corresponding to the upper level PV anomaly).

3. The system approached a pre-existing region of low level PV anomaly

Ensemble forecasts from 17 members (9 spectral models including control run, 8 grid model ) were used to analyze the performance of the CMC ensemble forecast members. Approximately half of the 17 ensemble forecast members (initialized at 00 UTC 03 September and 00 UTC 04 September) successfully traced the decay of ex-Earl and the subsequent re-intensification after 00 UTC 05 September. Members, which failed to track the remains of Earl because they couldnUt sustain the lower level PV anomaly associated with ex-Earl, developed a new (extratropical) cyclone north of the analyzed track. Many members, among those which had "good" tracks, did not deepen the low significantly (below 980 hPa). This owes to underestimation of the warm core thermal anomaly ,the upper level PV anomaly, the cold-air intrusion after 00 UTC 05 September. Re-intensification was best traced by members which successfully captured the above features.

Vertical distribution of turbulence, larval fish and prey in a stratified water column

Christian Reiss, Ayal Anis, John Dower, and Barry Ruddick

Departments of Oceanography and Biology, Dalhousie University, Halifax, NS, B3H 4J1
Department of Oceanography, Dalhousie University, Halifax, NS, B3H 4J1
Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, B. C.

4:30pm, Thursday, Dec. 9, 1999

Abstract: Water column turbulence has been hypothesized to affect the feeding and thus the survival of fish larvae in food limited environments by increasing the encounter rate between predators and prey (Rothschild and Osborn 1988). They further made two specific predictions 1) that predators and prey would adjust their positions in the water column to to take advantage of permanent, semi-permanent or periodic, localized turbulent motion to capture prey or avoid predation; 2) that through the effcst of turbulence on contact rates, there may be the possibility for linking large cale oceanographic phenomena with microscale events. We test the hypothesis that larval fish (Merluccius bilinearis and Mallotus villosus), a potential predator (Ctenophora) and prey of three size classes early (c1 to c2), late (c3 to c5) and adult stages of copepods would exhibit strong associations with turbulence. Water column turbulence was estimated in three different ways: 1) from wind speed using the empirical model of Mackenzie and Leggett (1991), calculation of the Richardson number (a measure of the ratio of stratifying effect of buoyancy to the mixing effects of vertical current shear), and 3) from velocity fluctuation profiles obtained from the Epsonde profiler. No diel migrations were obvious in any of the species or stages examined. Mallotus villosus and Ctenophora were both found below the pycnocline, where Richardson number was less than 0.25, and turbulence level were >10-6 W/kg. Merluccius bilinearis, was found predominately in waters of high Richardson number (>0.25), in an area of maximal stratification, and minimal turbulence (10-6 to 10-8 W/kg). All three prey developmental stages exhibited a similar relationship with turbulence and Richardson number as Merluccius bilinearis. These results show that in stratified waters, turbulence has little predictive value, indicating for this short survey period, both species-specific and ontogenetic variations in vertical distribution complicate any generalizations of the positive effect of turbulence on trophic interactions between fish and their zooplankton prey.

Recent Changes in the Ocean Climate of the Scotian Shelf and Gulf of Maine

Peter C. Smith, Brian Petrie, Ken Drinkwater and Alex Herman

Coastal Ocean Science, OSD
Bedford Institute of Oceanography

4:30pm, Thursday, Dec. 16, 1999

Abstract: Analysis of hydrographic, chemical and moored current, temperature and salinity measurements from the Scotian Shelf/Gulf of Maine reveals that the 1990's has been a decade of intense interannual variability. During the period 1993-97, flux measurements from the major inflow currents to the Gulf of Maine indicates an exceptional increase of freshwater input in the surface layers, which is reflected throughout the Gulf and on Georges Bank. At the same time, the deep inflow of Warm Slope Water (WSW) in the Northeast Channel diminished markedly. Then, in the autumn of 1997, a significant increase in the transport of the Labrador Slope Water (LSW) into the region along the outer shelf edge (50-300m) led to flushing and mixing of the deep basins of the shelf with a much colder, fresher water mass. The leading edge of this pulse is tracked through the region at rates of 4-10 km d-1. More recently in 1999, the LSW seems to be disappearing from the Northeast Channel, to be replaced by WSW and/or Gulf Stream ring water. Furthermore, an exceptional number of "cross-overs" of Scotian Shelf water to Georges Bank has occurred this year. Attempts will be made to relate this variability to various physical mechanisms and climate indices, such as the North Atlantic Oscillation (NAO) index.

The physical variations associated with these alternating periods of dominance by the two slope water components have been accompanied by systematic variations of dissolved oxygen and nutrients: Labrador Slope Water leads to low nitrate and high oxygen concentrations relative to the times when WSW prevailed. The significant changes of the dissolved oxygen and nitrate concentrations are coherent with those in temperature and salinity, in many ways similar to an episode that occurred in the mid 1960s. The relationship between the hydrographic properties, the nutrients and the dissolved oxygen for Scotian Shelf waters will be discussed, and the role of the Labrador Current in these changes will be examined.