SCIENCE REVIEW 1987 - Bedford Institute of Oceanography

Research
Fig. 5 The CASP oceanographers used current meters (A) to record currents and water properties
every half hour for four months. Surface waves were measured by instrumented buoys (B). Sea level
gauges (C) were installed along the Nova Scotia coast to record sea level fluctuations. Research vessels
(D) made surveys of the temperature and salinity of the water overlying the shelf: Anemometer towers
(E) recorded wind speed and direction every ten minutes.
equipment, and to the researchers themselves.
Nevertheless, as a result of careful
planning, and a run of good luck, the
CASP winter field program was very
successful. Despite the expected loss of some
data, the CASP atmospheric and oceanographic
observations contain a wealth of
information about winter storms that will
take several years to analyse.
Results of the program.
The oceanic and atmospheric observations
collected during the CASP field program
can be used to test our understanding of
many aspects of the ocean’s response to
storms. Most oceanographic studies of
wind-driven currents and waves do not
have the benefit of simultaneous measurements
from large meteorological networks,
making it more difficult to correctly
interpret their results. By working with
AES, the BIO oceanographers obtained an
especially detailed description of the storm
winds that affected the Scotian Shelf during
the CASP field program. For example,
detailed wind measurements made on the
Nova Scotia mainland during CASP gave
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BIO oceanographers good estimates of the
movements of warm and cold fronts, an
important element in explaining the generation
of the observed wind-driven currents.
Ocean currents
Under the influence of the earth’s rotation,
large-scale patterns of storm-driven currents
and sea level fluctuations, known as
“shelf waves,” are established on continental
shelves and migrate slowly along the
coast over periods of several days. The
currents and sea level fluctuations associated
with these waves are generally
strongest adjacent to the coast, but unlike
ocean surface waves, shelf wave motions
are not visible to the eye. Studies of the
CASP sea level data have shown a direct
relationship between shelf waves detected
on the Scotian Shelf and the wind over the
shelf. They also showed that shelf waves
generated by winds over the Grand Banks
of Newfoundland travel to the Scotian
Shelf, arriving one or two days after the
passage of a storm. The significance of
these findings is that in order to predict
storm-driven currents and sea levels in a
given coastal area, it is necessary to take
into account not only the local wind, but
the effects of storms in remote areas as
well.
The most immediate oceanic response to
storms is the generation of horizontal
currents known as “inertial currents.” The
CASP current and wind measurements
clearly showed the generation of inertial
currents by the rapid increase in wind
speed at the onset of storms, and by the
abrupt changes in wind direction that
accompany the passage of storm fronts.
Inertial currents are stronger than the
currents associated with shelf waves, and
reached speeds of up to 1 km/hr during the
largest storms in CASP They are weak
near the coast, however, where the effects
of shelf waves tend to be strongest.
Therefore, ocean forecasting models will
have to accurately predict both the shelf
wave and inertial motions. The models will
also require information about the passage
of storm fronts in order to correctly
forecast the inertial currents.
Surface waves
The CASP surface wave measurements are
one of the largest and most complete sets of
wave data ever gathered. The wave buoys
were anchored in water of varying depth
near the coast, and therefore are useful in
describing the alteration of wind-driven
surface waves as they approach the coast
and enter shallow water. Such information
is required to test future wave forecasting
systems that attempt to predict wave
heights near coasts. The data also provide
information about the rate of growth of
waves due to the wind in the early stages of
their development, for example shortly
after the onset of a storm.
Conclusions.
The Canadian Atlantic Storms Program is
contributing to a more complete understanding
of winter storms on Canada’s east
coast and of the response of the ocean to
storms. Cooperation between oceanographers
and meteorologists has produced an
especially useful set of storm data for the
eastern Canadian coastal region. Oceanographic
measurements made in CASP are
guiding the development of ocean forecasting
models, and can be used to test the skill
of the resulting models.