Transcript Ocean Circulation

Ocean Circulation
Current Notes
Key Points
• Warming and expansion of the atmosphere
causes winds which drag the water along
• The effects of temperature and the rotation of
the earth on its’ axis and the positions of the
continents lead to the complex patterns found
on the sea surface.
• Currents have considerable effect on human
activities. From transportation to moderation
of local climates, humans depend on ocean
currents.
Surface Circulation
• Surface currents are a response to the uneven heating
of the earth by the sun.
• Temperature differences cause warmer water near the
equator to swell and move toward the poles.
• Wind energy is converted to water movements called
"currents" by friction between the wind and the water
surface. The surface currents resemble the surface
winds. Once these surface currents are set in motion
they are influenced by three other factors: Coriolis
effect, presence of coasts, and horizontal pressure
gradients.
The Coriolis Effect
• Objects, or ocean currents, moving through the Northern
hemisphere are deflected to the right because of changes in
angular momentum that occur with the changes in latitude.
• Because of the earth's rotation, any freely moving object or
fluid will appear to:
– Turn to the right of its direction of motion in the
Northern Hemisphere and
– Turn to the left of its direction of motion in the Southern
Hemisphere.
• This causes currents to travel clockwise around high
pressure systems in the Northern Hemisphere, and counterclockwise in the Southern Hemisphere.
Wind Patterns
• Surface currents are driven by winds.
• The wind is again influenced by the Earth's rotation and
in the latitudinal band of 30-60 degrees north and south,
the Westerlies move currents towards the northeast (in
the Northern Hemisphere) and southeast in the
Southern Hemisphere.
• Between the equator and 30 degrees north and south,
the trade winds blow toward the southwest in the
Northern Hemisphere and the northwest in the Southern
Hemisphere.
• The air currents moving along the surface of the Earth from
the poles (90 degrees) to 60 degrees north and south of the
equator, flow from east to west and are referred to as the polar
easterlies
Surface Wind Patterns
Gyres
• Winds together with the direct action of the Coriolis
force move tremendous volumes of surface water in a
large circular patterns known as gyres which are
centered around 30 degrees latitude both north and
south. These gyres move clockwise in the Northern
Hemisphere and counter-clockwise in the Southern
Hemisphere.
The Ekman Current
• In 1905, Eckmann determined a theoretical value for this
phenomenon known as the Eckmann spiral. It
contributes to the creation of the gyres.
• Ekman currents always carry water to the right of the
wind in the northern hemisphere. The Ekman currents
push warm water to the center of the ocean. This
surface convergence creates a mound of surface water
that is elevated up to one meter above the equilibrium
sea level.
The Effect of Currents
The currents regulate earth’s climate by
transporting heat from tropical to polar regions
and greatly affect the geographic distribution of
organisms.
Thermohaline Circulation
And the Great Ocean Conveyor
The Three Layer Ocean (A review)
• Surface layer
• Intermediate Layer (\cline)
• Deep or Bottom Layer
Mixing (Overturn)
• Downwelling
– Surface water sinks and
displaces/mixes with deep water
– Temp and Density profile is consistent
and straight down.
Upwelling
• Where offshore surface water movements transport
water away from an area, this area is compensated by
the upward movement of deeper waters. The process of
upwelling brings nutrient rich waters to the surface
allowing large phytoplankton blooms to develop and
feed which in turn support major fisheries.
• This is a common process near coastlines of continents
when the water carried out to the open ocean is
replaced with water coming from depths between 100
and 200m.
The Great Ocean Conveyor
• Global Thermohaline circulation
• Critical in regulating the earth’s climate
• Brings dissolved oxygen to the deep
ocean.
Cold water that flows north in the Atlantic
Ocean forms around Antarctica. Antarctic
Bottom Water forms on the continental shelf
and sinks to spread through the bottom of the
world's oceans. It is the coldest, deepest
water in the ocean. Antarctic Intermediate
Water forms further north and flows at a
shallower depth.
Marine Biology Text Reading
Pages 53-61