The University of Alberta, the University of Tromso and the University of the Arctic invite you to explore this four week course that examines the environment and climate of the circumpolar North. This course is the result of an international collaboration and provides you with an insight into our planet's North. Following an overview of regional geography, we will focus on the cryosphere (ice), as well as the atmosphere and ocean of the region. We will learn why the Arctic is cold and ice covered, and how that impacts its climate and ecosystems. We will also consider how the Arctic is connected to the rest of the world. Finally, we will examine present day climate change, the processes driving it, and evidence for it in the Arctic, before looking at the implications in the rapidly evolving North. Watch a preview of the course here: https://uofa.ualberta.ca/courses/arctic-climate
2.2B Ocean Circulation and the Arctic Ocean
Loading...
Из курса от партнера University of Alberta
Introduction to the Arctic: Climate
169 оценки
Из урока
Climate Systems
In this lesson we’ll learn about the climate of the Arctic, and the way the climate interacts with the rest of the Earth system. In Lesson 1, we saw how the Earth receives energy from the sun and how heat is trapped on our planet through the natural greenhouse effect. We will now discuss how this energy is distributed throughout the globe and how it is transferred from the tropics to the poles via the large-scale circulation systems of the atmosphere and the ocean. We’ll learn the difference between the natural and the human-induced greenhouse effect - driven by the input of greenhouse gases created by burning fossil fuels, such as carbon dioxide and methane - and discuss what the future holds for the Arctic’s climate. As you will see, the global climate system is tightly linked and climate change in the Arctic also strongly affects temperate latitudes.
Познакомьтесь с преподавателями
Paul Myers, Ph.DProfessor
Department of Earth and Atmospheric Sciences
Only a few narrow gateways to the Atlantic and Pacific Oceans exist.
These gateways limit the exchange of water and contribute to the lower salinity.
As we saw in lesson one, the largest connection between the Arctic Ocean and
the Atlantic Ocean, Fram Strait between Greenland and
Svalbard, is 350 kilometers wide and 2,700 meters deep.
There are smaller gateways through Nares Strait,
between Ellesmere island and northwest Greenland, and
via the Canadian Arctic Archipelago and Baffin Bay.
The only connection to the Pacific Ocean is via the 80 kilometer wide and
less than 50 meter deep Bering Strait, which separates Siberia from Alaska.
In spite of being a narrow gateway, the Bering Strait is the only connection
between the Pacific and Atlantic oceans in the Northern Hemisphere.
It transfers about 800,000 cubic meters of water
every second from the Pacific to the Arctic Ocean.
In a common unit of volume transport in oceanography, that is 0.8 sverdrups.
A sverdrup is a measure to indicate a flow rate of 1 million
cubic meters of water per second through some region of the ocean.
To put this in context,
the Bering Strait has four times the flux of the Amazon River.
Which drains the entire Amazon rain forest of South America.
The Bering Strait flow is essentially a one-way downhill
flow into the Arctic Ocean,
because the Pacific Ocean surface elevation is higher than the Arctic Ocean.
This elevation difference is because the Pacific ocean is fresher.
We'll talk more about how the salinity effects elevation
when we talk about sea level.
The Bering Strait volume transport fluctuates seasonally,
decreasing with temperature in the winter while increasing in salinity.
In summer, it increases in volume transport and
temperature, but decreases in salinity.
Pacific water is very important to the Arctic ecosystem,
as it is rich in nutrients and fuels biological productivity.
Particularly on the wide continental shelves.
It is an important source of relatively warm water
affecting sea ice in the western Arctic.
It is more acidic than Atlantic Ocean water
which means it can increase ocean acidification in the Arctic Ocean.
Arctic Ocean water is only exported to the Atlantic Ocean, and
not to the Pacific, due to the sea surface elevations.
Atlantic Ocean water also flows into the Arctic Ocean.
The Atlantic Ocean inflow of about ten sverdrups is ten
times larger than the Pacific Ocean inflow and is both warmer and saltier.
The primary conduit of Atlantic water input is eastern Fram Strait.
But lesser amounts also cross the Barents Sea shelf.
Arctic rivers, along with glacial runoff and icebergs, and
the net difference between precipitation and evaporation,
make up the other water inputs to the Arctic Ocean.
Rivers, glaciers, icebergs, and
precipitation are the only pure fresh water inputs.
Given the details of the topographic structure of Fram Strait,
and the passages of the Canadian Arctic Archipelago,
which do you think exports more water from the Arctic Ocean to the Atlantic Ocean?
A, the Canadian Arctic Archipelago exports more water than Fram Strait.
B, the Canadian Arctic Archipelago and
Fram Strait export about the same amount of water.
C, the Canadian Arctic Archipelago exports less water than Fram Strait.
Or D, water flows into the Arctic through the Canadian Arctic Archipelago.
C is correct.
Outflows from the Arctic Ocean to the Atlantic Ocean are also primarily carried
through Fram Strait with about ten sverdrups.
To the west of Greenland, about one to two sverdrups flow through
the marine channel system of the Canadian Arctic Archipelago into Baffin Bay.
Since the sea surface is higher in the Arctic Ocean than Baffin Bay,
the flow through the Canadian Arctic Archipelago is
effectively unidirectional towards Baffin Bay, and then the Atlantic Ocean.
Arctic Ocean circulation is driven primarily by winds and
thermohaline processes based on salinity and temperature.
Connection of the Arctic Ocean to the other world oceans,
though restricted, mean that water masses found here
have various sources with distinctive temperature and salinity characteristics.
This results in a strong layering of water masses called stratification.
The Arctic Ocean has a cold surface layer that has low salinity and
is therefore less dense.
It is normally 5 to 10 meters thick, but where sea ice is absent,
it is up to 20 meters thick.
This layer forms from the freshwater input
along with the relatively low salinity ocean water from the Pacific.
The low density of this capping water, primarily arriving from rivers such as
the Mackenzie, Ob, Yenisei, and Lena, means that it is more buoyant.
And floats above the saltier and denser underlying ocean water.
The surface mixed layer is still salty due to mixing, allowing its temperature
to decrease to minus 1.8 degrees Celsius, just above its freezing temperature.
Sea ice forms from this surface layer, especially on the shallow continental
shelves when atmospheric temperatures are sufficiently cold to form ice.
Between the surface mixed layer and the underlying ocean,
is a sharp change in salinity and temperature known as the halocline.
In the Eurasian Basin, the halocline varies between 20 and 100 meters in depth.
It is sustained by cold salty water from the wide Eurasian continental
shelves associated with brine expelled during sea ice formation.
When saltwater freezes into sea ice,
salt is expelled, creating very cold salty water in the process.
This cold salty brine sinks due to its higher density, accumulating at
the depth where its density is equivalent to that of the underlying water.
Due to its density and
cold temperature, the water of the halocline effectively insulates
the surface mixed layer from the warmer water below by acting as its heat sink.
In concert with this, the mixing of surface water with the warmer, saltier,
deeper water is further limited by the capping sea ice
that prevents wind driven stirring.
The near surface stratification in the Amerasian Basin is
slightly more complex than in the Eurasian Basin.
The Amerasian halocline is primarily composed of Pacific water
entering through Bering Strait, which forms a thicker band known as a halostad.
Salinity gradually increases between the surface mixed layer and 200 meters depth.
The upper halostad is made up of warmer Pacific summer water,
whereas the lower halostad is made up of colder Pacific winter water.
As a result, the Amerasian Basin's surface mixed layer and
sea ice is less insulated from warmer deep water than it is in the Eurasian Basin.
This results in larger annual summer sea ice reductions.
However, this Amerasian halocline
is sufficient to maintain the surface stratification of the water column and
does limit heat transfer from the deep ocean water.
Look at this section of Arctic Ocean stratigraphy.
Based on what you have learned so far about the waters of the Arctic Ocean,
which water mass is the Atlantic water?
Is it A, B, C, or D?
Water mass C is correct.
As the Atlantic water is relatively warm and salty,
consistent with the previous thermohaline circulation discussion.
Beneath the respective haloclines, throughout the Arctic Ocean,
lies Atlantic water.
It can be found at depths of up to about 1,200 meters.
But in its warmest waters,
the temperature maximum is between 200 meters and 400 meters.
Atlantic water flows in through Fram Strait between Greenland and
Svalbard, originating from the warm and saline West Spitsbergen Current.
It can be considered a northern extension of the North Atlantic current and
the famous Gulf Stream.
The warm salty water cools as it enters the Arctic Ocean.
Becoming increasingly dense and finally sinking beneath the halocline.
The sheer volume of Atlantic water entering the Arctic Ocean
causes circulation within the Atlantic water layer.
It flows counterclockwise around the Arctic Ocean,
at a rather sluggish 0.02 meters per second.
Maintaining the Arctic Ocean boundary current.
The deepest Arctic Ocean water mass is the cold, salty,
and dense Arctic bottom water, found below about 1200 meters depth.
This water is primarily of mixed Atlantic and Arctic Ocean origin.
Where sea ice formation occurs on continental shelves,
the resulting brine is the coldest, saltiest, and densest water
in the Arctic Ocean, as it cannot be heavily diluted while on the shelf.
When such Arctic waters reach the continental slope, they descend, and
bring Atlantic water with them.
There is some stratification within the Arctic bottom water.
This can vary from location of the deep water formation, and
on the temperature and salinity levels of the originating water.
The deepest water on the floor of the Arctic Ocean
is slightly warmed by geothermal heating.
Resulting in slow vertical circulation within the deep water.
Two major ocean circulation systems operate in the Arctic Ocean
these are the Transpolar Drift and the Beaufort Gyre.
These systems influence the large scale circulation
in the Arctic Ocean as a whole.
The Beaufort Gyre is located in the Canada basin sector of the Arctic Ocean.
It is driven by wind, moving sea ice, and
water, in a clockwise motion around the North Pole.
The Beaufort Gyre rotates in a clockwise direction.
Why might that lead to the Beaufort Gyre being able to store
large amounts of low salinity water?
Is it because A, surface divergence transports water out of the gyre.
B, the Arctic Boundary Current carries sea ice in this direction.
C, that the coriolis force acts to the right in the Northern Hemisphere.
Or D, Pacific water entering via Bering Strait is
forced to turn south when it reaches the Lomonosov Ridge.
Only answer C is true.
The Arctic Ocean boundary current rotates counter clockwise.
The Lomonosov Ridge is too deep to affect the Beaufort Gyre.
Although topographic features can constrain surface ocean circulation.
Coriolis deflections to the right mean inwards, with a clockwise circulation,
causing low salinity water to converge and be trapped in the gyre.
Thus, the Beaufort Gyre also holds a considerable amount of
fresh water within it.
The Transpolar Drift is the other major current system
that operates in the Arctic Ocean.
It flows from the Amerasian Basin towards Fram Strait, and
exports sea ice from the Russian Arctic Ocean through the East Siberian and
Laptev seas, and towards the North Atlantic.
It takes about two to four years for
sea ice formed in these seas to reach the open North Atlantic.
Sea ice formation and its seasonal growth and decay play a crucial
role in the oceanography and water mass formation of the Arctic Ocean.
Once formed, sea ice is move by prevailing winds and ocean currents.
This shows us that the atmosphere and oceans are tightly connected.
These atmospheric conditions are marked by pressure highs and lows.
The Beaufort high is interesting because it drives the clockwise movement
of the Beaufort Gyre.
This pressure system varies through the seasons over the year.
In the winter it is located over the Central Canada Basin.
And during the summer, it moves closer to Siberia and North America.
Though sea ice is a major characteristic of the Arctic Ocean,
it is not the only ice found drifting around in this sea.
There are also chunks that break off from terrestrial glaciers and
ice sheets that make their way into the Arctic Ocean.
Most come from Greenland ice sheet, but some are also from Ellesmere Island
at the North Eastern edges of the Canadian Arctic archipelago.
These chunks of ice, or icebergs, are made up of freshwater ice
that melts while floating to more southerly latitudes.
Icebergs can be enormous.
And, as implied by the expression the tip of the iceberg,
have most of their volume underneath the ocean surface.
This is a function of the density of freshwater ice
being about 90% of that of seawater.
The largest icebergs in the world are usually found in the Southern Ocean around
Antarctica.
Still, huge pieces of drifting ice can also be found at the Circumpolar North.
These are usually broken off, or calved, from the Greenland ice sheet.
One of the biggest arctic icebergs ever recorded
broke off the Petermann Glacier in northwestern Greenland in 2010.
It measured 260 square kilometers,
about three times the area of Copenhagen, Denmark.
Because of their large size, and the fact that most of their mass is below
the sea surface, icebergs pose a serious hazard to shipping.
For example, the famous Titanic collided with an iceberg that
measure 1.6 kilometers in length.
Most icebergs, including the Titanic iceberg,
travel south from western Greenland and Baffin Bay and the Labrador sea.
Drifting with the Labrador current along the east coast of Canada.
Icebergs can also pose a hazard to underwater cables and structures
because they can plow through the sea bed, sometimes to several meters depth.
By now, you know all about the Arctic atmosphere and ocean.
And how the Arctic and Antarctic receive their heat, that is,
heat trapped by the natural greenhouse effect
is redistributed across the Earth's surface by the atmosphere and oceans.
Do you remember from lesson one what the major greenhouse gases
with the greatest heat retention effect are?
Select all of the answers you think are correct.
A, carbon dioxide.
B, methane.
C, ozone.
D, water vapor.
All answers but C are correct.
Carbon dioxide, methane, and water vapor, are all greenhouse gases.
Whereas concentrations of ozone are too low for
it to have a significant radiative effect.
Ознакомьтесь с нашим каталогом
Присоединяйтесь бесплатно и получайте персонализированные рекомендации, обновления и предложения.
Coursera делает лучшее в мире образование доступным каждому, предлагая онлайн-курсы от ведущих университетов и организаций.
© Coursera Inc., 2019 Все права защищены.
