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
Abstract:
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.
Title
Speaker
Institute
4:30pm, Thursday, Oct. 7, 1999
Abstract:
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
Institute
4:30pm, Thursday, Oct 28, 1999
Abstract:
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
LSC238
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.