Winter/Spring 2002

Hindcasting the NAO using diabatic forcing of a simple AGCM

Drew Peterson

Department of Oceanography
Dalhousie University

4:30 p.m., Thursday, January 24, 2002

Abstract: A primitive equation, dry atmospheric model with a rudimentary representation of the model physics is used to hindcast the NAO using diabatic forcing diagnosed from NCAR/NCEP reanalysis data in the period 1949-1999. The correlation of the 30 member ensemble mean with the observed NAO index is 0.79.

By further prescribing time dependent forcing only in the tropics (30oS-30oN), we show that the recent upward trend in the NAO is related to tropical forcing. Further specification of the tropical forcing into sub-sectors leads us to believe this upward trend is connected with the effect of diabatic forcing in both the Eastern Pacific and Indian Oceans. High frequency, interannual variability of the NAO, however, seems to be more closely related to extra-tropical forcing.

The model, with the full global diabatic forcing, is also able to reproduce the recent eastward shift of the sea level pressure pattern associated with the NAO. By investigating the behavior of the model using forcing that has been linearly regressed against the NAO index we show that this eastward shift is associated with non-linear dynamical processes.

North Atlantic Crossroads: Coordinated Changes in the Deep and Intermediate Waters of the Sub-Polar Gyre

Igor Yashayaev

Bedford Institute of Oceanography

4:30 p.m., Thursday, January 31, 2002


I.) We demonstrate the dramatic changes in the deep and intermediate water masses of the northwest Atlantic since the late 1950s by compiling time series of water mass properties for the various waters at different locations along the deep western boundary current of the sub-polar gyre. That analysis demonstrated the sustained freshening of the water masses over the past 4 decades. We grouped the data within the time frames representing extreme conditions and mapped volume, temperature and salinity of each water mass of the study and show the sources of this freshening and how it is carried around the sub-polar gyre by the deep western boundary current and then into the interior by the North Atlantic Current.

II.) Labrador Sea Water (LSW) scored more citation than any other water mass formation. However, the published estimates of its spreading time across the North Atlantic were biased by the desire of authors to see LSW there ASAP. Applying a wide range of tools to the most complete collection of the hydrographic data across the North Atlantic, we identify LSW in different basins of the North Atlantic, quantify its volume and property and show that the spreading time of LSW is, at least, twice slower than reported by Sy at el (1997). It takes 1 to 2 years for the bulk of LSW to arrive to the Irminger Sea (0.5 in Sy at el.) and 5 years to the Iceland Basin (2 in Sy at el.). We also suggest that the transport of LSW to the Iceland Basin is mostly associated with the North Atlantic Current and oppose the existence of "shortcut" by which LSW reaches the Iceland Basin directly from the Labrador Sea.

III.) The changes in the stratification of the deep water are of great dynamical consequences. The thickness of the density surfaces associated with the Labrador Sea waters have increased significantly between the 60s and the 90s over the entire sub-polar gyre. These means that the waters in the upper 1.5 km have become denser while those deeper than 2 km have become less dense. These changes will impact the strength and deep structure of the North Atlantic current.

The North Water Polynya, Baffin Bay: A Modeling Study Tom Yao, Bedford Institute of Oceanography

Tom Yao

Bedford Institute of Oceanography

4:30 p.m., Thursday, February 7, 2002

Abstract: We use a multi-category sea ice model coupled to the Princeton ocean model to study mechanisms for the formation and maintenance of the North Water, Baffin Bay in winter. Monthly climatological, atmospheric data from the NCEP/NCAR reanalysis provides the forcing. A recent objectively analysed climatology provides the initial ocean temperature and salinity. Wind drives the ice in a cyclonic gyre around northern Baffin Bay. The interruption of the gyre by the coastline of northwest Greenland is the cause of the polynya. Ice builds up on the windward coast (Melville Bay) but thins on the leeward coast (the North Water). The North Water is a region of enhanced ice growth and ocean heat flux which increase towards the Greenland coast. We attribute the heat flux to buoyancy flux from ice growth.

No seminar this week

Thursday, February 14, 2002

The Changing Relationship Between the NAO and Northern Hemisphere Climate Variability

Jian Lu

Department of Oceanography
Dalhousie University

4:30 p.m., Thursday, February 21, 2002

Abstract: A secular change in the link between the North Atlantic Oscillation and the ice export through Fram Strait is unraveled and related to an eastward shift in the SLP pattern associated with the NAO in recent studies. Here, a proxy for Fram Strait ice export, that is, SLP difference across the Strait is used to verify this change. Two additional variables, Siberian winter temperature, and an index of North Atlantic storm activity, are also found to switch from being uncorrelated with the ice export proxy in the 1950's and 60's to being strongly correlated in the 1980's and 90's, suggesting the emergence of a new climate regime associated with the NAO. We argue that the establishment of this new climate regime is related to an upward trend throughout the whole of the 20th century in the cross-correlation between the NAO index and Rogers' first storm activity mode for the North Atlantic.

No seminar this week

Thursday, February 28, 2002

No seminar this week

Thursday, March 7, 2002

No seminar this week

Thursday, March 14, 2002


Title: TBA



4:30 p.m., Thursday, March 21, 2002


Title: TBA



4:30 p.m., Thursday, March 28, 2002


The role of salinity in interannual variablity of the upper ocean at high latitude

Charles Tang

Bedford Institute of Oceanography

4:30 p.m., Thursday, April 4, 2002

Abstract: The role of salinity in the upper-ocean variability of the Labrador Sea is investigated using data from OWS Bravo in two representative periods: 1964-1965 and 1969-1971 and two mixed-layer models (bulk model and turbulence closure model). Salinity itself cannot be simulated with one-dimensional models because errors in precipitation rate are possibly large and the inter-annual variation is controlled mainly by large-scale circulation. By specifying salinity in the models according to data, good simulations of sea surface temperature (SST) are obtained. A comparison with simulations using salinity climatology shows that SST and mixed-layer depth are sensitive to errors in salinity during the cooling phase (winter and spring). This is in contrast to mixed-layer simulations for mid-latitude Pacific in which the results are not very sensitive to salinity. The main reason for this is that density depends more strongly on salinity in high-latitude Atlantic than it does in mid-latitude Pacific. During the heating phase (summer and fall), the mixed-layer properties are mainly controlled by surface heat flux and water optical type because the mixed layer is shallow and short-wave radiation dominates the heat flux. A comparison of the two periods reveals that in 1964-1965, a period of normal salinity, the maximum mixed-layer depth is 300 m and the minimum SST is 3o C, and in 1969-1971, a period marked by abnormally low salinity (referred to as the Great Salinity Anomaly) and a relatively warm winter, the same quantities are about 100 m shallower and 10 C lower. These differences raise the question as to which factor is more important in controlling the mixed-layer properties - surface heat flux or stratification. Results of sensitivity runs indicate that the mixed-layer depth is mainly controlled by stratification, and SST depends both on stratification and surface heat flux. The influence of surface heat flux, however, is weak for a deep mixed layer.

Towards Interdisciplinary Modelling of the NW Atlantic: Development of a Plankton Model

Vézina, A., Bedford Institute of Oceanography
Casault, B., Bedford Institute of Oceanography
Pahlow, M., Dalhousie University

4:30 p.m., Thursday, April 11, 2002

Abstract: Monitoring of the NW Atlantic shelf since 1998 has documented strong interannual variability in physical conditions and in the timing, magnitude and spatial distribution of plankton production events (e.g. spring bloom). To better understand the processes responsible for this variability requires a physical-biological modelling system that can assimilate and interpret monitoring data. We report here on the development of a plankton model that would be suitable for such a system. We focus initially on the Scotian shelf off Halifax, Nova Scotia. Different PZND (phytoplankton zooplankton nutrients and detritus) model versions are compared against climatologies of chlorophyll and nutrients and against higher resolution time series at monitoring stations. For development purposes, the models are run in 0D mode, with mixed-layers prescribed from climatologies or from higher resolution observations. Inverse methods applied to simplified versions of the PZND models are used to estimate poorly known parameters and to improve the functional representation of key processes (e.g. grazing, sinking). The capability of 0D and 1D PZND models to reproduce the seasonal production cycle in different areas of the Scotian shelf will be discussed.

The meaning of "heat flux" in the ocean and the parameterization of mesoscale eddies in ocean models

Trevor McDougall

CSIRO Marine Research
Hobart, Tasmania, Australia

4:30 p.m., Thursday, April 18, 2002

Abstract: This work answers the question "what is heat content in the ocean"? Mixing processes in the ocean conserve enthalpy and mostly destroy potential temperature in a similar fashion (and at a similar rate) to how entropy is universally produced by mixing processes. Potential enthalpy - the enthalpy that a water parcel would have if raised adiabatically and without exchange of salt to the sea surface - is shown to be more conservative than potential temperature by more than two orders of magnitude. This work proves that potential enthalpy is the quantity whose advection and diffusion encapsulates the physical meaning of the First Law of Thermodynamics in the ocean. A new temperature variable called "conservative temperature" is advanced which is simply proportional to potential enthalpy. It is shown that present ocean models contain typical errors of 0.1 degrees C in their temperature due to the neglect of the nonconservative production of potential temperature.

Mesoscale eddies in the ocean mix fluid parcels in a way that is highly constrained by the stratified nature of the fluid:- so much so that much of our intuition about ocean mixing comes from thinking in density coordinates. Temporal-residual-mean (TRM) theory provides the link between the different views that are apparent from averaging turbulent flow in height coordinates and in density coordinates. The TRM theory reduces the parameterization problem from three dimensions to two dimensions and it shows how the divergent part of the relevant eddy density flux is skew-symmetric in height coordinates and that the total advection velocity can be adiabatic. The Gent-McWilliams scheme is best interpreted as a scheme for implementing the temporal residual mean. The TRM theory has very specific things to say about how the tracers in coarse-resolution models should be interpreted and how the skew diffusion streamfunction should approach zero at boundaries. It is emphasized that the extra advection in the Gent-McWilliams scheme the TRM theory is not the bolus transport because the extra TRM advection is non-divergent and diapycnal in character while the bolus velocity is divergent and adiabatic.

No seminar this week because of Riley lecture.

Thursday, April 25, 2002

A Three-Dimensional Ocean Circulation Model of the Western Caribbean Sea

Jinyu Sheng and Liqun Tang

Department of Oceanography
Dalhousie University

4:30 p.m., Thursday, May 2, 2002

Abstract: A three-dimensional ocean circulation model is used to study circulation, water mass distributions and their seasonal and mesoscale variabilities in the western Caribbean Sea. The model is forced by monthly mean climatologies of temperature and salinity, COADS surface wind stress and heat flux, and volume transports across the model open boundaries. The western Caribbean Sea model is initialized with the January mean temperature and salinity and integrated for two years. The model results reproduce many well-known circulation features in the region, including the persistent throughflow known as the Caribbean Current, the cyclonic Panama-Colombian Gyre, and strong seasonal variabilities of temperature and salinity in the surface mixed layer. The model calculated annual mean transport across the Yucatan Strait is about 27 Sv, which is comparable with previous estimates. Vertical distributions of the model currents across the Strait also agree reasonably well with the current observations made by Pillsbury (1890) and most recently by Sheinbaum et al. (2002). In addition, the annual mean near-surface currents produced by the model are in agreement with the decadal mean near-surface currents inferred from trajectories of the satellite-tracked drifters by Fratantoni (2001).

No seminar this week

Thursday, May 9, 2002


Development of Guidelines for Ocean Model Data Assimilation: examples from the NW Atlantic

H. Jean Thiébaux

Recently retired from the U.S. NCEP
Visiting Dalhousie University

4:30 p.m., Thursday, May 16, 2002

Abstract: To date, few comparisons of analyzed, and nowcast and forecast fields, with in situ ocean observations have been made, to evaluate the impacts of assumptions on which present ocean data assimilation schemes have been constructed and to provide guidance for future developments.

This seminar will bring together the results of a program of evaluation of ocean temperature analyses, nowcasts, and hindcasts, with the object of establishing definite guidelines for the next generation of ocean model data assimilation. We begin with a review of the theoretical framework for ocean model data assimilation; and describe three ocean models and one analysis system, providing different applications of the principles. Comparative evaluations of surface and subsurface temperature fields produced by the models and analyses, using observations from moored buoys and bathy profiles, are presented for the Northwest Atlantic.

The evaluations of comparative outputs of model and analysis systems have been made in the following ways. Time series plots of sea surface temperature records from the moored buoys in the Atlantic, are compared with time series plots for collocated, model and analysis output SSTS. Temperature sounding records from bathythermographs deployed in the region by commercial, research, and Coast Guard and Navy ships, are compared with collocated "virtual soundings" created from the multi-level model and analysis output files.

Together, the results of the comparisons show that both the construction of analysis algorithms and the timeliness of the data have significant impacts on product evaluations; and lead to a set of clear guidelines for the next generation of ocean model data assimilation. Primary among these are the importance of using nearest real?time data in a multivariate algorithm based upon covariance structures that represent the spatial relationships of analysis increments as closely as possible.

Influence of Turbulence and Scatterer Size Distribution on Acoustic Backscatter Statistics: Comparison of Measurements and Theory

Keath Borg

Department of Oceanography

4:30 p.m., Thursday, May 23, 2002

Abstract: The large variations in sand concentration found near the seabed requires a receiver with a wide dynamic range. The instrument which motivated this work achieves this dynamic range by measuring log amplified amplitudes. This measurement is used as a surrogate for intensity (amplitude squared), which is what backscattering theory is based on. Most acoustic backscatter profilers assume incoherent scattering in which intensities add. This instrument also measures spatially and temporally coincident velocities using coherent pulse pairs. The influence of the coherence between consecutive acoustic pings and the log amplifier is examined for a turbulent jet using a numerical simulation. The use of the log amplifier biases the mean amplitudes to smaller values, as well as modifies the statistics of the amplitude and intensity distributions. Introducing turbulence to the simulation reproduces many of the characteristics in the correlation histograms generated from measurements made in the lab. Advection further improves the results since this factor leads to a dependence of the log amplitude distribution on the scatterer concentration. Amplitude noise, due to a distribution in the scatterer sizes, introduces skew; however, these results are inconsistent with the measurements. Electronic noise is also considered, but found to be insignificant in magnitude.

Short-Term Physical, Chemical and Biological Variability on the Scotian Shelf

Blair Greenan

Fisheries and Oceans Canada
Bedford Institute of Oceanography

4:30 p.m., Thursday, May 30, 2002

Abstract: Daily to weekly timescale variability on the Scotian Shelf is studied using a combination of historical data sets and new field data collected in the Fall 2000. The intent of this study is to improve the understanding of the relationship between the physical forcing mechanisms and the response of the chemical and biological fields on these short time scales. This is accomplished through the use of a newly developed mooring platform (SeaHorse) that uses surface wave energy to enable the instrument to climb down the mooring wire and then float upwards while sampling the water column. This provides bi-hourly profiles of CTD and chlorophyll at one location over month-long periods. Results from the Fall 2000 deployment indicate a subsurface chlorophyll maximum below the pycnocline during the first part of the time series. An event occurred in mid-October during which the temperature, salinity and density iso-surfaces rise approximately 25 m in the water column. During the peak of this event, a small bloom begins as nutrients are brought into the upper part of the water column. SeaWiFS ocean color satellite images prove to be valuable in providing a spatial context for chlorophyll concentrations, however, the lack of temporal resolution due to poor quality images means that this data set provides limited information for short-term chlorophyll variability. Using SeaHorse CTD data and ADCP current measurements, a trend of decreasing Richardson number in the ocean mixed layer with increasing surface wind stress has been demonstrated.