Winter/Spring 2005

No seminar this week

Thursday, January 6, 2005

Mechanisms and Predictability of North Atlantic-European Climate

Holger Pohlmann

Max Planck Institute for Meteorology
Hamburg, Germany

3:00 p.m., Friday, January 14, 2005

Seminar cancelled due to snowstorm

Thursday, January 20, 2005

Update on MSC data assimilation

Hal Ritchie

Meteorological Research Branch (MSC)
Adjunct Professor at Dalhousie

4:30 p.m., Wednesday, January 26, 2005

Abstract: A key mandate of the Meteorological Service of Canada (MSC) is to protect life and property with respect to extreme weather events. This is primarily achieved through the production and dissemination of forecasts based on numerical models. The accuracy of those forecasts depends on the quality of the numerical models, the data, and the assimilation system that is used to produce the initial analyses from which the forecasts run. Numerical models which predict the evolution of the atmosphere by solving equations describing the physics of fluid flow and other processes have become indispensable tools for modern weather forecasting. Launching these models requires determining initial values for variables such as pressure, temperature, wind speed and direction, and humidity on a three-dimensional grid covering the entire globe. Data assimilation is used to produce a best estimate of the state of the atmosphere by combining information from past observations, carried forward in time by the model, with information from new observations using statistical techniques. The data assimilation component of the system has now become as important as the model itself in terms of its impact on the quality of the forecasts, and provides an excellent framework to meet future challenges for ingesting space remote sensing and other earth observing data for both atmospheric and oceanographic fields.

This presentation will begin with an introduction to the MSC's data assimilation and prediction infrastructure which is the foundation on which this activity is built. It will give an overview of the evolution of data assimilation within MSC and provide an update on recent developments in variational data assimilation. Examples of current and proposed research projects using these methods in collaborative projects at Dalhousie University will also be mentioned.

Hydrodynamic control of the transfer of particulate mattter on a Northwestern Mediterranean margin: from observations to modeling

Maud Guarracino

Department of Oceanography
Dalhousie University

4:30 p.m., Thursday, February 3, 2005

Abstract: A long term experiment of monthly downward particle fluxes and hourly currents and temperature has been initiated in 1993 on the continental slope of the Gulf of Lions. This study aims at describing the spatial variation of the intensity and nature of particle fluxes, discriminating the temporal flux variability, and analysing the role of some forcing factors in the control of particle exchange across the margin. Forcing variables include sources of particulate matter on the shelf (river inputs, atmospheric input and resuspension) and cross- slope exchange mechanism derived from in-situ temperature and current records. A previous statistical analysis of the long term time series has underlined that the transfer of particulate matter to the deep ocean is not forced by the sources of matter, and mass fluxes are likely controlled by the meandering of the Northern Current and by winter dense water formation. Numerical simulations have been carried out to check these hypotheses as well as to define the spatial structure of the water exchanges between the continental shelf and the basin and the major hydrodynamic mechanism which controls the export of matter to deep ocean. This modelling approach scans the impact of local atmospheric forcing (wind stress, heat fluxes, precipitation-evaporation budget) on the variability of the oceanic circulation and of mass fluxes within the canyons. Some results showed an East-West gradient of matter export on the shelf, a positive correlation between matter inputs from the shelf and particle fluxes measured on the slope as well as a positive correlation between anomalies of dense water formation rates and interannual variability of particle fluxes.

How to Interpret Box Models with transition matrices and POPS

Barry Ruddick and Pierluigi Pantalone

Department of Oceanography
Dalhousie University

2:30 p.m., Thursday, February 10, 2005
NOTE special time!

Vortical structures generated by a localized forcing

Vasily Korabel

Ph.D. Candidate
Dept. of Physics and Physical Oceanography
Memorial University of Newfoundland

4:30 p.m., Thursday, February 17, 2005

Abstract: Vortex structures (monopoles, dipoles, quadrupoles) as well as more complex structures (vortex streets) are fundamental elements of geophysical turbulence. Because they can effectively transport momentum, heat, salt and biochemical products, they play an essential role in ocean dynamics. Organized vortex structures are a well known feature of quasi-two-dimensional flows where motion in one direction is suppressed due to one of the following physical mechanisms: background rotation of the system, density stratification or geometrical restrictions such as for the flows in thin layers or soap films.

Vortex dipoles are formed in a viscous fluid when a force is applied locally to some volume of fluid. If the force acts impulsively, a translating vortex dipole is generated. If the force starts at t = 0 and then acts continuously a starting jet with a dipole at its front is generated. Solutions for unsteady viscous flows generated by the action of continuous or impulsive localized forces are obtained in Oseen approximation. The solutions are compared with direct numerical simulations of vortex dipoles as well as with laboratory experiments. The comparison shows good quantitative agreement in both cases.

A physical problem where the localized force acting continuously on fluid is placed in a uniform stream is equivalent to a problem of a fixed body in a uniform stream, while the couple of forces acting in opposite direction are equivalent to the problem of self propelled body moving at constant velocity through a fluid. The solutions for the two dimensional far-field wake are obtained in both cases. At a certain Reynolds number, wakes become unstable and form vortex streets. New series of high-resolution 2D numerical simulations is performed to study the characteristics of the wakes including the shedding frequency for a wide range of control parameters such as translational velocity, magnitude and spatial extent of a localized force. The results of numerical experiments of unsteady wake flow show existence of a great variety of flow regimes and are in good qualitative agreement with laboratory experiments.

Nearshore Oceanography or Mud, Sewage and Aquaculture

Gary Bugden

Fisheries and Oceans
Bedford Institute of Oceanography

4:30 p.m., Thursday, February 24, 2005

The Life of Brine
Thermal and thermohaline convection models for the meridional overturning circulation

Julia Mullarney

Department of Oceanography
Dalhousie University

4:30 p.m., Thursday, March 3, 2005

Why do we need to continue surveying the Labrador Sea?

Igor Yashayaev

Bedford Institute

4:30 p.m., Thursday, March 10, 2005

Abstract: Over the past 15 years, the Bedford Institute of Oceanography (BIO) has conducted annual occupations of a hydrographic section across the Labrador Sea. These observations, combined with earlier hydrographic data, reveal some prominent changes through the whole water column on interannual to decadal time scales. Switching from a regime of intense winter mixing and production of the famous intermediate water mass (the Labrador Sea Water - LSW) to a regime of weaker ventilation has resulted in a rapid change of heat and fresh water content not just in the Labrador Sea, but in the whole subpolar gyre. Combining hydrography and satellite altimetry, we show that a change in steric height (computed from the sea water density distribution) caused by this regime switch is in a very good agreement with the sea surface height.

Freshening imposed on the deep intermediate layers by extreme convection in the early 1990's penetrated to the deep layers. We discuss the nature of the time lag between the intermediate and deep freshening peaks, which can be partially attributed to a spreading of LSW across the subpolar gyre of the North Atlantic. Combining BIO observations in the Labrador Sea with measurements to the east of Greenland (collected by European teams), we obtain a fairly accurate estimate of the LSW spreading time from its source to different sub-polar basins.

The unprecedented warming of the Labrador Sea is partitioned into a change in the water masses and a structural shift in the water mass distribution. The major change is marked by the appearance of a large volume of warm and salty water from the Irminger Sea. This water spread westward from its usual location along the eastern boundary, reaching the midpoint of the Labrador Basin, and filling the whole eastern part of the basin between 100 m and 800 m. Due to this expansion of the Irminger water, the water at 700 m in the eastern part of the Labrador Sea became about 0.6 warmer and 0.05 saltier between 2003 and 2004. Recent observations reveal fresh and cold anomalies that are propagating from the Denmark Strait, entering the deep and bottom layers of the Labrador Sea, and spreading westward. The freshest event was recorded in the early spring of 2004 in the northern Irminger Sea and is expected to arrive to the abyss of the Labrador Sea in 2005. The continuation of Labrador Sea monitoring in 2005 and beyond, combined with observations at the various sources of its deep and bottom waters, will provide the most reliable indication of the operation and regimes of the grand ocean conveyor.

No seminar this week

Thursday, March 17, 2005

No seminar this week

Thursday, March 24, 2005

No seminar this week

Thursday, March 31, 2005

The role of anomalously warm sea surface temperatures on the intensity of Hurricane Juan (2003) during its approach to Nova Scotia

Christopher T. Fogarty, Richard J. Greatbatch and Harold Ritchie

Department of Oceanography
Dalhousie University

4:30 p.m., Thursday, April 7, 2005

Abstract: When Hurricane Juan tracked toward Nova Scotia in September 2003 forecasters were faced with the challenge of predicting the intensity and timing of the hurricane's landfall. There were two competing factors dictating the storm's intensity: 1) the decreasing sea surface temperatures (SSTs) over which the hurricane tracked which was conducive to weakening, and 2) the increased forward motion of the storm which enhanced the surface winds on the right (storm relative) side of the storm. Since Hurricane Juan was moving very quickly (forward speed approximately 15 m s-1) it spent less time over the cooler continental shelf waters between Nova Scotia and the >26oC water of the Gulf Stream than would have been the case for a slower moving storm. However, the cooler shelf waters were warmer than normal during this event, by ~4oC. We demonstrate that these warmer SSTs made a significant contribution (among other factors) to this rare category-two hurricane at landfall in Nova Scotia.

To assess the role of SSTs on the decay rate of Hurricane Juan during its approach to Nova Scotia, we use a mesoscale model of the atmosphere called the Mesoscale Compressible Community (MC2) model. The model consists of a fixed and nested 3 km grid driven by a coarser 12 km grid, and is initiated using a synthetic hurricane vortex constructed from observational information such as storm size and intensity, thus giving a decent representation of the real storm. Multiple experiments were conducted for two SST configurations: 1) observed SST and 2) climatology SST representative of late September. Results from the 3 km simulations initiated approximately 1 day prior to landfall (and others initiated 2 days prior to landfall) indicated that the intensity of Hurricane Juan.s maximum surface winds near landfall were 5-8 m s-1 (~ ± 1.5 m s-1) weaker in the climatology-SST simulations. The destructiveness of these winds was only 50-70% of the observed (warmer than normal) SST runs. This difference suggests that damage from Hurricane Juan could have been much less had SSTs been near normal! Model performance is measured using surface weather data, as well as data collected from a research aircraft that flew into the storm just prior to landfall.

I will also discuss how the model system is being applied to other tropical cyclone events that affected eastern Canada and discuss how the system can be used to improve forecasting of these storms in the future.

High-resolution marine wind retrieval

Rick Danielson

Department of Oceanography
Dalhousie University

4:30 p.m., Thursday, April 14, 2005

Abstract: Synthetic aperture radar (SAR) measurements by Radarsat-1 may be well suited for incorporation into a coastal data assimilation system, but first it is necessary to demonstrate that a surface marine wind analysis improves when SAR observations are employed. The feasibility of combining SAR measurements with high-resolution simulated wind fields is examined for coastal regions of eastern and western North America. A 2D-variational approach is used to retrieve wind fields for a subset of SAR acquisitions. Observations by ships of opportunity and buoys, taken within 90 minutes of these acquisitions, are used to quantify the improvements in the retrieved wind fields. Initial estimates of error bias and spatial covariance are also examined.