SCIENCE REVIEW 1987 - Bedford Institute of Oceanography

same all year but moves close to then away
from our instrument bringing faster then
slower regions of the current to the
instrument. We know this is probably not
the reason because of an interesting fact.
Sea level changes from one side of an
ocean current to the other. This fact is
similar to the phenomenon reported daily
in the weather forecasts which tell us how
the atmospheric pressure is changing as the
currents of air pass over our heads. The sea
level change associated with the pressure
change across a current of water is small
but measurable. Across the strongest
currents in the ocean such as the Gulf
Stream the sea level changes by about one
metre. Across the Labrador Current the
change is only about 15 centimetres, but
that is enough to be calculated using sea
level observations at the shore and temperature
and salinity measurements from the
other side of the current in the middle of
the Labrador Sea. Temperature affects sea
level in the middle of the ocean because,
like most other substances, it expands with
heating. The salinity of the water also
affects the height of sea level. Higher
salinity like cooling makes the water denser
which reduces the sea level.
By doing these calculations we find that
the sea level goes up and down with the
seasons at the shore and in the middle of
the Labrador Sea but that it goes up and
down more at the shore than in the middle
of the sea. From this we conclude that the
sea level, or pressure, drop across the
Current varies throughout the year. This
indicates that the speed of the current must
be changing with the seasons just as the
current meter observations in Figure 4
indicated. We therefore conclude that the
strength of the current really does change
through the year but what makes it
change?
One reason is found in our observation
that the sea level goes up and down more
near the shore than in the open ocean. Our
measurements of temperature and salinity
from these areas show that the sea level
goes up in summer partly because the
water is warmer but this effect is about the
same on both sides of the current. Sea level
also goes up in summer because of the
increased amount of fresh water being
mixed into the seawater. The fresh water is
from the spring melting on land and sea
and is carried to the Labrador Current by
the rivers emptying along the the Labrador
coast and into Hudson Bay, Foxe Basin,
Baffin Bay. Much of the fresh water comes
from the Arctic Ocean via the currents
around Greenland but this flow probably
takes more than a year. Unlike the temperature
effect the sea level rise due to the
fresh water is much greater over the
continental shelf than over the open ocean
because it tends to be confined there by the
current. This uneven rise and fall between
the waters over the shelf and open ocean
gives rise to the annual variation in sea
level across the current. We tentatively
conclude, therefore, that the annual variation
in the speed of the current is mainly
due to the annual cycle in fresh water flow.
The fact that the maximum flow occurs
late in the year is because the fresh water
has to come a long way and only moves at
about 20 cm s -l .
Another thing we would like to know
about the annual cycle is how constant it is
difference between the minimum and the
maximum current over the year, is about
the same from year to year. The timing of
the winter decrease, however, changes
dramatically from year to year.
Every year, for example, the speed goes
from its highest values to its lowest values
in winter, but in one year the drop occurs
around Christmas and in another it occurs
three months later at the end of March.
This seems like a large change from one
year to the next but so far we don’t know
what causes it or what the consequences
are. Maybe the drop in the current speed in
the winter has something to do with freezeup
in the north and the shift from one year
to the next occurs because freeze-up comes
at different times. This, of course, is just
speculation but continued study in the
years ahead will help us to understand how
these changes are connected with climatic
changes in other parts of the world and
with the changes in the local fishery.
Figure 5 Monthly temperature at 200m depth west of Hamilton Bank, showing that from late 1982
through 1985 temperature was about 1°C colder than during the rest of the record
from year to year. Does the current, for
example, always have the same maximum
and is it always reached on the same date?
The curves in Figure 4 show the speed of
the current in summer to vary somewhat
but to be more or less the same from year
to year. In winter the current is seldom the
same. In February for instance the curves
show the smoothed current as low as 5
centimetres per second (cm s -l ) and as high
as 25 cm s -l , but in July the range is only
about 5 cm s -1 . This difference from
summer to winter in the year to year
variation on a particular date has more to
do with the phase or timing of the cycle
than with its amplitude. The amplitude, or
Temperature Changes
Over the years we have also been lucky to
observe a large shift in the temperature of
the water over the Labrador shelf. This was
measured close to the bottom in 200
metres of water on the western side of
Hamilton Bank. These data are shown in
Figure 5 as a series of monthly anomalies.
The point for January 1980 is the monthly
mean for that month subtracted from the
mean of all the Januarys in the record. This
presentation emphasizes long term departures
from the average. The temperature
clearly was much higher, relative to the
average, during the first two years than in
the period between the autumns of 1982
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