51% of
short-wave solar radiation is converted to heat (longer-wave radiation)
upon striking the Earth's surface
Why is there less heating of the
poles than
the tropics even at the spring and autumn equinox
when all areas on Earth are experiencing the same number of hours of
sunlight?
Reflection by ice and lower light in winters
further contribute to cooler polar temperatures.
As a result, there
is
a net surplus of heat near to equatorial regions
and a net loss nearer to poles.
So why don't the equatorial oceans
boil away and the polar oceans freeze solid?
Considering only unequal heating across the Earth's surface,
how might air move
over the global ?
Complications to this circulation
pattern:
How
will the rotation of the Earth affect the path (trajectory) of wind as
it moves from the poles toward the equator (what will happen to a
cannonball shot southward)?
So air
rising from the equator is deflected, but it also cools and looses
moisture as it moves towards the polar, sinking at about 30° N and
30° S (Hadley
cell).
Most of this sinking air as it reaches the Earth's surface moves back
toward the equator, but some is deflected toward the poles (Ferrel cell).
This air mass encounters air that has been cooled over the pools and is
warming as in moves toward the equator forcing air upward at
50°-60° latitude (polar cell).
Gobal patterns in air circulation are further complicated by seasonal
differences in heating and the irregular distribution of continents and
oceans (
Do land and water have the
same specific heat?)
Surface Currents -
horizontal circulation
What explains the pattern of
circulation in surface currents shown in the figure below for the
Atlantic Ocean?
Ocean water circulates in
currents caused mainly by
wind
friction at the surface (10% of the world ocean. i.e. mostly
above the pycnocline).
The
Coriolis effect and
obstruction to
flow by continental margins
result in surface
currents organized
into huge circuits known as
gyres.
The Coriolis effect causes
surface waters under ideal conditions to
move ~45° to the right (in the northern hemisphere) of the
direction
of wind. Each successive layer of water underneath is
effected further by the Coriolis effect (but the foward movement of
water is less) creating an Ekman spiral (image the path of a cannonball
fired from a small cannon that sits upon a moving
cannonball). The net (sum)
movement of this water column is
theoretically
~90° to the movement of the wind
(Ekman transport).
However,
the movement of water in a
gryre due to the Coriolis effect is opposed
by a gravitation pressure gradient resulting from a build-up of water
(higher elevation) toward the center of the gyre. As a result
Ekman transport is not greater than ~45°. Hence, gyres
are referred to a
geostrophic
("Earth turning") currents.
Some
surface currents are rapid and riverlike, with well-defined boundaries,
especially along the western gyre currents. Western
intensification is due to the Coreolis effect which is stronger closer
to the poles.
Eastern
boundary
currents are braod,
shallow and less defined, carrying cooler
water toward the equator.
An example of a
western
boundary
current is the Gulf
Stream along the western North
Atlantic
(55 million m3/s, 300X the flow of the Amazon,
160
km/d).
Why does
England experience such a
temperate climate given its high latitude?
Major currents meander
significantly over periods of weeks (analogous
to rivers meandering over geological time) resulting in formation of
eddies. Warm-core eddies
rotate clockwise and cold-core counter clockwise.
What
might be the
ecological significance of
these eddies?
Wind-induced
vertical circulation
Upwelling and
downwelling
describe the vertical movements of water masses. Upwelling is often due
to the divergence of surface currents.
How might upwellings affect biological
productivity of that area? Downwelling is often caused by
surface current convergence.
El Niño, an anomaly
in surface circulation, occurs when the trade winds falter, allowing
warm water to move eastward across the Pacific at the
equator.
Why should
currents reverse direction under these conditions?
Smaller scale, closer to home:
The
Georgia Bight
Observation of cooler waters along the
shores of Dayton Beach than at
Savannah or Miami led to the discovery of
summer upwellings from greater
depths (cooler) waters of the
Gulf Stream.
Three conditions are necessary:
- passage of a Gulf Stream eddy
- sustained northward winds
- onshore position of the Gulf Stream
The intruded patch of water can be over
100 km wide and persist for
several weeks. The water is nutrient-rich and when it reachs the
photic zone induces phytoplanton growth on the continental shelf which
in turn causes increases in zooplankton population sizes and species
composition.
