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
Abstract:
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
Abstract:
Title: TBA
Speaker
Institute
4:30 p.m., Thursday, March 21, 2002
Abstract:
Title: TBA
Speaker
Institute
4:30 p.m., Thursday, March 28, 2002
Abstract:
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
Abstract:
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
Dalhousie
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.