Fall 2004

Title: TBA



4:30 p.m., Thursday, October 7, 2004


Application of surface-following coordinate systems to the interaction of waves, surface currents, turbulence, etc.

Alastair D. Jenkins

Bjerknes Centre for Climate Research
Geophysical Institute
Bergen, Norway

10:00 a.m., Tuesday, October 12, 2004

Abstract: A dynamically consistent framework for modelling atmosphere--ocean interaction must take account of surface waves, either implicitly or explicitly. In order to calculate the effect of wind forcing on waves and currents, and vice versa, it is necessary to employ a consistent formulation of the energy and momentum balance within the airflow, wave field, and water column, taking into account the Earth's rotation. It is also advisable to use a coordinate system which can represent vertical variations at scales smaller than the wave height, for example, a surface-following coordinate system. We may account for the waves explicitly by employing a numerical spectral wave model, and applying a suitable theory of wave-mean flow interaction. A closed system of equations may be obtained to second order in wave slope by applying wave action conservation equations in the propagation of the spectral wave components. Other effects which should be taken account are the effect of turbulence and depth-varying currents on wave propagation and dissipation, the presence of surface films and sea ice, the effect of waves on the mean water level, and the generation of Langmuir circulations.

Sources of Eddy Variability in the Labrador Sea

Bernard Barnier

Directeur de Recherche CNRS
Equipe Modelisation des Ecoulements Oceaniques de Moyenne et grande echelle (MEOM)
Grenoble, France

4:30 p.m., Thursday, October 14, 2004

Abstract: Sources of eddy variability in the Labrador Sea are investigated from high resolution numerical model simulations. First, the dynamical processes involved in open ocean convection and re-stratification are reviewed. Then, realistic simulations of the ocean circulation in the Labrador Sea are used to investigate the role of mesoscale eddies on the convection/restratification cycle. Several 10 year long realistic simulations have been carried out with an eddy-resolving (4 km resolution) regional model of the Labrador Sea, embedded in an eddy-permitting (20 km resolution) model of the North Atlantic. Two different sources of eddy variability are identified. The first one is an instability of the Irminger Current at a particular location of the west coast of Greenland where a strong convergence of the isobaths is found. Eddies generated there ("Irminger Eddies") appear as coherent structures, do not mix with surrounding waters and live long (a few years). Many drift from east to west in the Labrador Sea along the 2000 m isobath and re-enter the Irminger Current on its Canadian side. They are found to contribute very little to the seasonal re-stratification of the Labrador Sea, but do limit convection in the northern part of the Labrador Sea. The second source of eddy variability is the baroclinic instability which arise after winter convection in the Labrador Sea. These "Labrador Sea Eddies" are significantly less intense than the "Irminger eddies", and have a much shorter life. They are found to efficiently mix waters horizontally, and to be the main driver of the seasonal re-stratification of the interior Labrador Sea.

No Seminar -
Open House prep

4:30 p.m., Thursday, October 21, 2004

Title: TBA



4:30 p.m., Thursday, October 28, 2004


No seminar today -
Riley Lecture

4:30 p.m., Thursday, November 4, 2004

Stratified Flow Past Topography

Dr. David Farmer

University of Rhode Island

3:00 p.m., Friday, November 5, 2004
Room 5263 (psychology building)

Abstract: The mathematical framework for stratified flow over topography will be presented in a simplified way, starting with steady inviscid layered flow to show the crucial role of nonlinearity in modifying the response. Ways of extending this representation to include friction and entrainment will be discussed, together with model calculations of internal wave generation which can be compared with observations.

Remembrance Day - no seminar

Thursday, November 11, 2004

Interannual changes in heat and salt in the Labrador Sea

Ross Hendry

Fisheries and Oceans Canada
Bedford Institute of Oceanography

4:30 p.m., Thursday, November 18, 2004

Abstract: Annual spring and early-summer surveys of the Labrador Sea since 1990 by Ocean Sciences Division, DFO Maritimes Region have now provided a 15-year time series. These observations include a period of severe winters in the early 1990's that resulted in intense deep convection and the formation of large amounts of cold, fresh Labrador Sea Water. Recent winters have been milder than normal. During the period 1996-1999 following the years of deep convection, the upper 1000m of the west-central Labrador Sea restratified as warm, saline Irminger Water replaced the remnants of convection. Since 2000 winters have been milder still. The heat and buoyancy content of the upper 1500m have continued to rise. However, recent surveys show evidence of a resumption of wintertime convection. This has produced increasing amounts of a warm and saline water mass in the upper 1500m with potential density much less than the potential density of the Labrador Sea Water formed in the early 1990's. Interannual changes in upper layer heat content during the decade-long cycle of deep convection and restratification seem closely linked to changes in local air-sea heat flux. The balance of terms controlling the upper-layer salt content is much less certain.

Stratified Tidal Flow Over a Bump

Richard Dewey

School of Earth and Ocean Sciences
University of Victoria

3:00 p.m., Wednesday, November 24, 2004
Note special day and time

Abstract: The interaction of a stratified flow and an isolated topographic feature can introduce numerous disturbances into the flow, including turbulent wakes, internal waves, and eddies. Measurements made near a "bump" east of Race Rocks, Vancouver Island, reveal a wide range of phenomena associated with the variable flow speeds and directions introduced by the local tides. Up-stream and down-stream flows were observed by placing acoustic Doppler current profilers (ADCP) on one flank of the bump. Simultaneous ship-board ADCP surveys corroborated some of the more striking features. Froude number conditions varied from subcritical, to critical, to supercritical as the tidal velocities varied from weak (0.2 ms-1), to peak speeds of 1.5 ms-1. During the strong ebb, when the moored ADCPs were located on the lee side, a persistent full water-depth lee wave was detected in one of the moored ADCPs and the ship-board ADCP, as the flow became supercritical and the lee wave became separated from the bump. ADCP beam velocities suggest enhanced turbulence during the strong flow periods, and the internal form drag associated with the development of the lee wave is examined. Many of the three-dimensional flow characteristics are in good agreement with the laboratory studies of Hunt and Snyder, 1980, while some, such as the bottom boundary shear, are not.

Recent Progress on Understanding the Caribbean/Loop Current System

Lie-Yauw Oey

Princeton University

4:30 p.m., Thursday, December 2, 2004

Abstract: The Caribbean/Loop Current system is an important component of the subtropical gyre circulation of the North Atlantic Ocean. I will give a summary of recent progress in understanding the regional circulation, especially with regard to the variability of the Loop Current eddy-shedding and its connection with remote forcing from the Atlantic Ocean.

Questions of Freshwater Transport and Variability in the Labrador Sea

Paul Myers

University of Alberta
Department of Earth and Atmospheric Sciences

4:30 p.m., Thursday, December 9, 2004

Abstract: The climate of the North Atlantic and the Arctic are linked in a number of ways. Decadal variability in ocean properties, winds, precipitation, etc. have been linked to both the North Atlantic and Arctic oscillations and to each other through feedback loops. A key feature of all these loops is the role of freshwater. In this talk, issues of freshwater in the Labrador Sea are considered from modelling studies, atmospheric reanalyses and historic oceanic data.

The transport of freshwater is analyzed in an eddy-permitting regional model of the sub-polar North Atlantic, focusing on the export of freshwater (in liquid form) through Davis Strait. The results show that, in these model simulations at least, convection in the Labrador Sea is not sensitive to enhanced export of freshwater from the Canadian Arctic Archipelago, as the enhanced freshwater does not escape from the Labrador Current into the gyre. Analysis of net precipitation minus evaporation over the Labrador Sea since 1950, from NCEP and ERA40 reanalyses are also considered. Results from both datasets are comparable (and consistent with satellite based measurements over the more recent part of the study period) and suggest a significant increase in precitation over the Labrador Sea since the mid 1970s, mainly in spring and summer. Potential linkages between changes in the atmospheric precipitation and historical sea surface salinity changes, and the great salinity anomalies is considered.

No seminar this week

Thursday, December 16, 2004

No seminar this week

Thursday, December 23, 2004

University closed for Holidays - no seminar

Thursday, December 30, 2004