Fall 2003

No seminar this week

Thursday, September 11, 2003

No POMSS seminar this week, because of the

Riley Memorial Lecture

Taking the Hype Out of Hyperspectral, and Putting the System into Ecosystem

Curtis D. Mobley

Vice President and Senior Scientist
Sequoia Scientific, Inc.

CSP Lecture Room 202
New Academic Building, King's College
4:00 p.m., Thursday, September 18, 2003

Reception to follow in the 5th floor lounge.

On the relationship between the water masses pathways and mesoscale variability in the Western Mediterranean Sea

E. Demirov
Department of Oceanography
Dalhousie University

N. Pinardi
Department of Environmental Sciences, Bologna University
Ravenna, Italy

4:30 p.m., Thursday, September 25, 2003

Abstract: The role of the mesoscale variability on the formation and spreading of water masses in the Western Mediterranean Sea (WMED) is studied with an ocean general circulation model (OGCM). The model is forced with interannual variable surface forcing for years from 1979 to 1999. The simulations are compared with some recent observational studies of the mesoscale processes in the WMED. It is found that the model reproduces the major features of the observed mesoscale variability during the preconditioning, and violent mixing phases of the deep convection in the Gulf of Lions and also the large mesoscale eddies evolution in the Algerian basin.

The evolution of the mixed patch formed after a deep convection event is studied for the winters of three years: 1987, 1992, and 1999. It is demonstrated that the mesoscale may play a major role for the spreading of the Western Mediterranean water masses out of the Gulf of Lions and towards the Gibraltar. Equivalently we may argue that the intermediate and deep waters conveyor belt of the WMED is eddy driven.

Because of Hurricane Juan,
there will be no seminar this week

Thursday, October 2, 2003

Circulation in the Santa Maria Basin and Santa Barbara Channel

Kyoko Ohashi

SUNY, Stony Brook

4:30 p.m., Thursday, October 9, 2003

Abstract: The Santa Maria Basin (SMB) and Santa Barbara Channel (SBC) are located to the northwest and southeast, respectively, of a bend in the southern California coastline. Moored observations and numerical model results are used to describe circulation in this area during 1998.

Winds in this area are equatorward throughout the year, and weak in SBC due to sheltering by a coastal mountain range. In spring, the near-surface currents are equatorward everywhere except along the northern shelf of SBC. In summer this poleward flow grows stronger, while the flow at SBCs eastern mouth turns poleward too. SMB currents are only intermittently poleward. SVD analysis of currents and wind suggests that large-scale changes in winds and currents are related, as are poleward flow along the northern shelf of SBC and a sharp wind stress gradient across SBC.

A three-dimensional numerical ocean circulation model is used to simulate the 1998 flow. The model is forced by a daily wind stress pattern derived from buoy winds. The model is run with and without assimilation of observed temperatures. The no-assimilation case tends to have colder water temperatures, which implies that surface heat flux (which is not included in the model, but whose effects can be incorporated into the model by temperature assimilation) is important. The surface heat flux estimated from the model is most similar to the observation-derived values of Winant and Dorman [1997] in summer, and least similar in winter, possibly due to the 1997-1998 ENSO event. They are more similar in SMB than in SBC, which suggests that the model transport is more realistic in the former. In EOF analysis of the Coriolis and pressure gradient terms for modeled near-surface currents, the first mode suggests that the non-geostrophic part of the flow is driven by wind stress. In the second mode, the non-geostrophic term has a relatively large magnitude at the northern shelf of SBC, which is not balanced by wind stress.

The modeled and observed currents are similar in direction, but the former tends to have smaller annual mean and variance values. SVD analysis identifies modes of variability between the two that have similar spatial patterns and temporal evolutions. Thus, in addition to the annual means and variances, the modeled and observed flows are also comparable in terms of their variabilities.

Breakups of the Stratospheric Polar Vortices

Pingping Rong

Department of Oceanography
Dalhousie University

4:30 p.m., Thursday, October 16, 2003


On the tropical forcing of the trend in the Northern Hemispheric tropospheric circulation

Jian Lu

Department of Oceanography
Dalhousie University

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

Abstract: Two aspects of the Northern Hemisphere climate change are mainly concerned in my thesis research: (a) the changing spatial pattern of the interannual variability of the North Atlantic Oscillation(NAO) since the late 1970's; and (b) the forcing mechanisms of the trend in the Northern Hemisphere tropospheric circulation during the last 50 years or so.

In my previous POMSS presentation, I talked about the nature of the eastward shift in the spatial pattern of the interannual variability of the NAO, as unraveled by using an atmospheric model with dry dynamics driven by diabatic forcing diagnosed from observations. The model results reveal the nonlinear dependence of the spatial pattern of the NAO on the NAO index, the pattern being shifted to the east (west) for high (low) NAO index. General agreement is found between the model and observations. Therefore, it can be argued that the recent eastward shift in the NAO pattern is a consequence of the trend towards higher NAO index during the last several decades of the 20th century.

This time, I am going to be focusing on the upward trend of the NAO, as well as the trend in the troposphere over the whole Northern Hemisphere. The hemispheric trend is associated with a deepening of both the Aleutian and Icelandic lows, a pattern that bears close resemblence to the "Cold Ocean Warm Land" pattern with a positive projection on the NAO (PNA) over the Atlantic (Pacific) sector. Experimentation using the same Hall model show that the observed simultaneous deepening trend in both (Aleutian and Icelandic) lows can be largely attributed to the diabatic forcing from the tropics. While the contribution from the extratropical forcing is rather trivial. The mechanism by which the tropical forcing drives the extratropical circulation trend is a planetary wave train emanating from the Indo-Pacific region of the tropical oceans. In the model, the extra-tropical storm tracks also play an important role in amplifying the original wave signal over the North Atlantic sector, resulting a resonant NAO-like dipolar response there. These model results underscore the importance of studying the tropical diabatic processes and the associated teleconnection in understanding the recent climate change during the last half century.

Explicit Filtering in Large-Eddy Simulations and the Approximate Deconvolution Model

Jason Hicken


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

Abstract: Large-eddy simulation (LES) of turbulence is a popular and successful approach to modelling turbulent flows at moderate Reynolds numbers. In LES, a filter is applied to the equations of motion and the resulting filtered equations are solved. Recent research efforts have focused on LES methods which use explicit filtering. One such method is the approximate deconvolution model (ADM) of Stolz and Adams. The ADM aims to reconstruct the unfiltered velocity field, up to the filter cutoff, using information contained in the filtered velocity field.

I will discuss implicit and explicit filtering in LES models, but the focus of the talk will be the ADM and how I hope to extend this model. The original ADM was implemented using a spectral solver while Gullbrand has recently adapted the ADM for staggered, finite difference grids. I will highlight my efforts to extend the ADM to a collocated, finite volume formulation.

The ADM solver I am developing uses an unstructured Cartesian grid with anisotropic adaptation developed by Ham et al. An adaptive grid is very attractive when solving turbulent flows; adaptive grids refine only where necessary, significantly reducing the number of cells for a given accuracy.

Near-bed turbulence, bottom stress, and the wave friction factor under energetic irregular waves

John Newgard

Department of Oceanography
Dalhousie University

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

The two Stommel models for the ocean's meridional overturning circulation

Richard Greatbatch

Department of Oceangraphy
Dalhousie University

4:30 p.m., Thursday, November 13, 2003

Abstract: In the Stommel box model, the strength of the overturning circulation is parameterized in terms of the density (and hence the pressure) difference between the two boxes. Straub has pointed out that this parameterization is not consistent with the Stommel-Arons model for the abyssal circulation. In particular, the zonally-averaged density field implied by the Stommel-Arons model is unrelated to both the strength and the direction of the meridional overturning. Here, we investigate the inconsistency using the abyssal circulation model of Kawase and a variant to include the effect of southern hemisphere wind forcing. The model results have relevance to the closure of zonally-averaged models and provides a simple illustration of how southern hemisphere winds can drive northern overturning.

Meso-Scale Weather Systems and Model Simulations

Yang Fuquan

University of Waterloo

10:30 a.m., Friday, November 14, 2003


Numerical Study of Barotropic Coastal Circulation in Lunenburg Bay, Nova Scotia

Liang Wang and Jinyu Sheng

Department of Oceanography
Dalhousie University

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

Abstract: A high-resolution coastal circulation model is used to study the three-dimensional barotropic circulation in Lunenburg Bay and adjacent areas, Nova Scotia. Two numerical experiments are presented. In the first experiment, the coastal circulation model is forced by the M2 tide at the model open boundaries. The simulated surface elevations of the semi-diurnal constituent in Lunenburg Bay and Upper South Cove agree well with the observations. The model results also demonstrate that the nonlinear tidal dynamics play a very important role in Upper and Lower South Coves, Corkum's Channel and northwest Lunenburg Bay. In the second experiment, the model is forced by tides, local wind and storm surge during Hurricane Gustav in September, 2002. The model results show that surface elevations in Lunenburg Bay during Gustav were forced mainly by tides and storm surge and currents were forced mainly by local wind and tides.

Title: TBA



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


The upper ocean response to a hurricane undergoing extratropical transition

Chris Fogarty, Richard Greatbatch and Alex Medvedev

Department of Oceanography
Dalhousie University

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

Abstract: A coupled atmosphere-ocean modelling system is being developed at Dalhousie University to study the extratropical transition (ET) of hurricanes and tropical storms in the Atlantic Ocean. ET is defined as the transformation of a tropical cyclone (TC) into a baroclinic mid-latitude cyclone characterized by the expansion of the TC wind field, decrease of maximum winds, increasing asymmetries in wind and precipitation and the development of fronts. These transformed disturbances often move into the mid-latitudes as dangerous storms bringing heavy rains and high winds over large areas of the Atlantic Ocean and neighbouring landmasses.

Early stages of the project have involved testing the FLAME (Family of Linked Atlantic Model Experiments) model using prescribed windstress forcing that mimics the lifecycle of hurricanes undergoing ET. The FLAME model is a 1/3 deg x 1/3 deg (lat/lon) z-level, eddy-permitting model of the North Atlantic Ocean. The windstress specification is that of a Rankine vortex. Two simulations have been run for the cases of hurricanes Fabian and Juan of the 2003 hurricane season. The vortex is fit to data from the National Hurricane Center and is moved along the final storm track.

Preliminary results for ET-like wind forcing using an ocean - which has been spun-up by climatological windstress forcing - show the typical right-of-track cooling in SST in the wake of a moving hurricane. The magnitude and breadth of the cooling in the hurricane Fabian simulation are much too large in the original run owing to unrealistically shallow mixed layers in the model. While work was being done to solve the mixed layer representation, we ran a simulation of Hurricane Juan using late-September climatology for the upper ocean over the Scotian Shelf - the area traversed by Hurricane Juan. The location and magnitude of ocean cooling agreed well in overall structure when compared with actual SST data. This talk will highlight data and results from this event, which lend confidence to the model.s mixing scheme.

Special Seminar

2-D Complex Spatial Eigenfunctions of Beach Video Data

Karin Bryan

University of Waikato
New Zealand

4:00 p.m., Monday, December 8, 2003
Note special day and time


Strong cyclones and SST anomalies of the western North Pacific - Inferences based on preliminary seasonal comparisons

Rick Danielson

Department of Atmospheric and Oceanic Sciences
McGill University

4:30 p.m., Thursday, December 11, 2003

Abstract: Studies have long emphasized the importance of western ocean boundary currents and their strong sea surface temperature (SST) gradients to the development of midlatitude cyclones (especially strong ones). However, they have also emphasized that the role of surface heat fluxes in cyclones is not well understood. Prompted by an unexplained atmospheric phenomenon over the North Pacific Ocean, called the midwinter storm track suppression, the hypothesis of a seasonal variation the role of surface heat and moisture fluxes in small groups of strong western North Pacific cyclones is examined. Their net effect is examined using SST anomalies as a proxy. Composite SST anomalies are constructed for each cyclone group, where groups are defined only by the occurrence of events during midwinter or during the early and late cold season. Systematic differences in sea surface temperature anomalies beneath these two groups are interpreted as differences in preconditioning by the upper-oceanic mixed layer. Some supporting evidence of an upward influence is available in terms of submonthly sea level pressure variations and a comparison for two groups of western North Atlantic cyclones. It is suggested that the role of preconditioning heat fluxes in cyclones varies because of large-scale seasonal changes in baroclinicity (or more specifically, static stability) and in the role of the ocean as a local source of water vapour. Similar arguments are being considered in the literature to explain the storm track suppression.

Title: TBA



4:30 p.m., Thursday, December 18, 2003


Merry Christmas!

Thursday, December 25, 2003

Happy New Year!

Thursday, January 1, 2003