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How does ice cover vary in the Bering Sea from year to year?
Dr. Lyn McNutt
Geophysical Institute
University of Alaska
903 Koyukuk Drive
Fairbanks, AK 99775-7230
Sea ice is one of the
most important physical elements of the Bering Sea continental
shelf. Sea ice helps define the ecosystem, provides habitat
for microorganisms, birds and marine mammals, and affects the
migration routes of Arctic marine mammals, such as whales and
seabirds. Changes in sea ice are a direct result of the atmosphere
interacting with the ocean, especially through changes in wind
speed and direction and air temperature. Since the sea ice floats
on the surface of the ocean, it moves around in the wind as storms
pass by. In the Bering Sea storms usually occur every three to
five days. The ice also changes every year due to the paths of
the storms. The storm paths, called storm tracks, relate to long-term
atmospheric patterns, called teleconnections. These teleconnection
patterns range from interannual (several years) to decadal (tens
of years). These patterns, in turn, relate to global climate
patterns. By studying these local and global connections, scientists
can use sea ice in the Bering Sea as an indicator of climate
change and its effects on the marine ecosystem.
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| The Northern
Bering Sea in May 1981 |
Sea ice begins forming in the northern Bering
Sea as late as November, as the ocean reaches a temperature of
-1.7°C, the freezing point for saltwater in this area, and
ice may remain into June of the following year. Most of the sea
ice forms in the northern portions of the shelf and is then blown
southward due to the prevailing north-northeasterly winds. Ice
melts at the edge when it moves into an area of water which is
warmer than the freezing point. In the Bering Sea, sea ice affects
water temperature, salinity and ocean currents. The formation,
motion and melting of the ice at the edge play important roles
in controlling the heat exchanged between the ocean and the atmosphere,
and the amount of salt in the water on the Bering Sea continental
shelf. The growth of sea ice also creates cold, salty water, while
ice melt makes freshwater. These processes are critical to the
physical conditions that influence the way the Bering Sea ecosystem
works. In the northern Bering Sea, water enters the Arctic Ocean
through Bering Strait, making the Bering Sea the only connection
between the Arctic Ocean and the Pacific Ocean.
There are four conditions
important to understanding the role of sea ice in the Bering
Sea: brine rejection, polynya development, formation of
the "cold pool" and variation in ice melt. When the
ice forms, not all the salt in the sea water (brine) can be incorporated
into the ice as it freezes, and is instead returned to the ocean
beneath the ice. This is called brine rejection. By losing this
brine, the sea ice becomes less salty, while the underlying water
becomes more salty. This changes the way the water currents move
under the ice as this cold salty water sinks to the bottom of
the sea. As the floating sea ice is blown southward by the wind,
it reaches an area of water which is above freezing, where the
ice melts rapidly. As the ice melts, it creates a layer of fresher
(less salty) water on the surface of the ocean on top of a layer
of salty water. The boundary between these two layers is called
the halocline. The fresher water on the surface is full of nutrients
that are essential to the health and productivity of the ecosystem,
especially to the microorganisms called phytoplankton.
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| St. Lawrence
Island Polynya. St. Lawrence Island is grey, thicker ice is
shown in green, and warmer ice with open water is yellow. |
Brine rejection also occurs in areas called
polynyas. Polynyas are areas of open water in the sea ice, which
form in the lee of islands and coasts. They are most often created
during northerly winds, but may also occur during southerly wind
events, and they usually occur in the same places every year. The
St. Lawrence Island Polynya is a very large, important polynya
that covers hundreds of kilometers. Polynyas work like conveyor
belts for ice creation. When a polynya appears, it exposes a large
area of water to the cold wind. This water quickly cools and ice
forms on the surface. The wind then blows this ice away from the
coast, and more water appears so that new ice is constantly being
formed and moved around by the wind. All this ice formation creates
cold, salty, more dense water through brine rejection, as described
above. This dense salty water sets up ocean currents that transport
water, and possibly organic matter, to the south and then west
of St. Lawrence Island, providing important conditions for creating
healthy life on the bottom of the ocean, called the benthos.
Polynyas are also very important feeding areas
for marine mammals who live in the Bering Sea all year. Many of
these mammals, especially seals and walrus, stay on the sea ice,
close to polynyas, where they can feed in the winter. The open
water in the polynyas allows them to use the ice as a platform
to dive under the water and feed, yet leaves them areas for breathing
when they come to the surface.
By forming in the same area every year, and
creating cold, dense salty water, the St. Lawrence Island Polynya
helps maintain a an area of cold water on the northern Bering Sea
shelf called a "cold pool". This colder water then remains
throughout the summer, and is often associated with nutrient rich
conditions, leaving a "footprint" of productivity under
a "cold pool" region. Nutrients are necessary for the
benthos, but are also important for development of microorganisms
and fish, who live in what is called the pelagic zone, and for
predators who feed on benthic and pelagic resources, such as birds,
seals, walrus, migrating whales, and man. A second "cold pool" occurs
in the middle shelf of the southeastern Bering Sea as a result
of thicker sea ice which often remains in this area in late spring.
This southern "cold pool" impacts distributions of species
of fish such as pollock. These fish are an especially important
food source for marine mammals, birds, and man. In fact, pollock
from the Bering Sea are often the fish in fish sticks and fish
filets that you find in your local supermarket.
When the sea ice melts at the edge in the spring,
the fresh water and nutrients in the ice are released onto the
surface of the ocean on the Bering Shelf. This water is critical
to the development of microorganisms which form the key to the
entire ecosystem of the Bering Sea. The measure of the success
of these organisms and their contribution to the food chain is
called primary productivity. The important relationships of sea
ice to primary productivity are: how far south the ice has gone,
and when and where does it melt? This brings us back to understanding
how sea ice responds to changes in the atmosphere, teleconnections
and to global climate patterns.
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| Maps of sea ice
in the Bering Sea seen in different types of atmospheric conditions. |
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| Four distinct
meltback patterns |
There have been three
different types of Spring melt in the sea ice cover in the eastern
Bering Sea during the last thirty years. This means that
three different sea ice conditions have affected the Bering Sea
shelf and the ecosystem: 1972-1976 (cold), 1977-1988 (warm) and
1989-2001 (cool). During the cold period, ice extended south
to St. Paul Island near the shelf break, and stayed there for
a month or more. The climate changed dramatically from cold to
warm in the Bering Sea in 1977, and this created changes in the
amount of sea ice, air and ocean temperatures, sea level air
pressure and surface winds. In the warm period, from 1977-1988,
the ice did not reach as far south, and stayed in the southern
area 2-4 weeks less than it did in the colder period. In the
cool period from 1989 to 2001, sea ice again moved to the south,
but it came and went very quickly, often melting before there
was enough sunlight for the microorganisms to use the nutrients
to create a phytoplankton bloom. Since the cold period ended
in 1976, there has not been a return to the extensive sea ice
conditions of the early 1970s. If we look at spring melt in May
in the more recent years, we see that sea ice ranges from almost
none, to heavy ice in the middle of the shelf, to melting in
the western shelf and heavy ice in the east. These changes affect
the primary productivity, especially phytoplankton blooms, which
then affects the entire ecosystem.
Phytoplankton are microscopic plants, also
called microflora, that use sunlight and nutrients from the melted
sea ice to synthesize carbon through photosynthesis, just like
plants on land. Different species of phytoplankton exist in oceans
throughout the world. They all play an important role in the exchange
of carbon dioxide between the atmosphere and the ocean, and hence,
climate change. Zooplankton are microscopic animals, also called
microfauna, that feed on phytoplankton. There are several hundred
different species of both phytoplankton and zooplankton in the
Bering Sea. In the Bering Sea, the phytoplankton that are not consumed
directly by the zooplankton sink through the pelagic zone to the
benthos at the bottom of the ocean. There they are either eaten
by other organisms, or deposited onto the ocean floor.
Phytoplankton blooms in the Bering Sea occur
in two different ways: they can start due to ice melt (early bloom),
or they can happen later in the season as sunlight increases, even
if the ice has melted early (late bloom). The ideal situation is
for the bloom to happen when the ice is melting and there is enough
sunlight for the phytoplankton to perform photosynthesis. This
is not always the case, however, especially due to the changes
to the sea ice that have taken place since 1976. If there is an
early phytoplankton bloom in cold melt-water, when there are not
as many zooplankton, most of the energy from the primary production
goes to the benthos. Late blooms happen in warmer water when there
is more sunlight, and there are zooplankton present, but there
are less phytoplankton due to less nutrients, so most of the energy
remains in the water column, the pelagic region. So the timing
of the sea ice melt creates early versus late blooms, and this
may form two different types of ecosystems in the Bering Sea: benthic
versus pelagic.
In the northern Bering Sea ice still remains
late in the spring, so most spring phytoplankton blooms are associated
with ice melt. However, in the southeastern Bering Sea, the type
of bloom depends on the timing of ice retreat, and we know that
the sea ice in this area has changed. The amount and type of energy
(benthic versus pelagic) created in the Bering Sea ecosystem as
a result of the timing and location of the melting sea ice, has
a cascading effect on all the fish, mammals and birds in the region.
Many scientists agree that the effects of climate
change may first be seen in areas such as the Bering Sea. This
makes the Bering Sea, its sea ice, and the ecosystem it supports,
very important to understanding how global climate may affect a
specific region. The ecosystem in the Bering Sea supports one of
the world's richest and most productive fisheries, and is important
to the economy of Alaska and the United States. It also has been
used by native cultures for centuries. Changes in the sea ice affect
the entire ecosystem, but we do not yet know what all the consequences
will be. This is especially true if the sea ice conditions observed
in the last three decades continue to change as rapidly as seen
in recent years.
References:
Books:
Krupnik, I. and D. Jolly (eds), The Earth is
Faster Now: Indigenous observations of Arctic Environmental change,
ArCUS and Arctic Studies Center, Smithsonian Institution, 356 p.
2002.
Loughlin, T. and K. Ohtani (eds), Dynamics
of the Bering Sea, North Pacific Marine Science Organization (PICES),
University of Alaska Sea Grant Press, Fairbanks, Alaska, 824 p.,
1999.
Articles:
Grebmeier, J.M., and L.W. Cooper (1995). Influence
of the St. Lawrence Island Polynya on the Bering Sea benthos. J.
Geophys. Res. 100:4439-4460.
Hunt, G.L., P. Stabeno, G. Walters, E. Sinclair,
R.D. Brodeur, J.M. Napp, and N.A. Bond. 2002. Climate change and
control of the southeastern Bering Sea pelagic ecosystem. Deep-Sea
Res. II 49:5821-5853.
Macklin, S.A., Saitoh, S.I., Radchenko, V.I.,
Napp, J.M., Stabeno, P.J., and McKinnell, S.M. (Editors). 2002.
Variability in the Bering Sea Ecosystem. Progress
in Oceanography 55(1-2).
Niebauer, H.J., 1998. Variability in Bering
Sea ice cover as affected by a regime shift in the north Pacific
in the period 1947-96. J. Geophys. Res.,
103, 27,717-27,737.
Stabeno, P. J. , N. A. Bond, N. B. Kachel,
S. A. Salo, and J. D. Schumacher, On the temporal variability of
the physical environment of the south-eastern Being Sea, Fish.
Oceanogr., 10:81-98 2001.
Additional Information:
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