OCEAN CURRENTS AND
CLIMATE
Ocean Currents and Climate
There are two types of Ocean Currents:
1. Surface Currents are driven by surface
wind circulation. These waters make up
about 10% of all the water in the ocean.
These waters are the upper 200 meters of the
ocean.
2. Deep Water Currents are driven by
Thermohaline Circulation• Makes up the other 90% of the ocean.
• Moves around the ocean basins by
density-driven forces and gravity.
The density difference is a function of
different temperature and salinity.
•These deep waters sink into the deep ocean
basins at high latitudes where the
temperatures are cold enough to cause the
density to increase.
Ocean Currents are influenced
by two types of forces:
Primary Forces start the water
moving
a. solar heat
b. winds
c. gravity
d. Coriolis Effect
Primary Force: solar heat
Causes thermal expansion of water about 8
centimeters higher at the equator
Atmospheric
heating sets
the winds in
motion to the
poles and the
Coriolis Force
adds a “twist”
to its motion.
Primary Force: Winds
Primary Force: gravity
Elevated water at the equator begins
to “flow poleward” due to gravity.
These large mounds of water and the flow
around them are called Gyres. They produce
large circular currents in all the ocean
basins.
Primary Force: Coriolis
The curvature of
motion due to
Earth’s rotation.
Curvature is to
right in the
Northern
Hemisphere, to
left in the Southern
Hemisphere.
Atlantic Gyre– composed
of the:
A. Gulf Stream
B. North Atlantic Drift
C. Canary Current
D. North Equatorial current
Why doesn't the water spin
towards the center of the ocean?
Why does it flow around the hill
in this circular motion?
Answer: Ekman Transport
Ekman Transport –
The friction from underlying
water slows down the upper
current and the Coriolis
Forces gives it a twist (to the
right in the Northern
Hemisphere).
A wind blowing for 10 hours across the ocean
will cause the surface waters to flow at about 2%
of the wind speed.
•Water will pile–up in the direction the wind is
blowing.
•Gravity will tend to pull the water down the
"hill" or pile of water against the pressure
gradient.
•But the Coriolos Force intervenes and cause
the water to move to the right (in the northern
hemisphere) around the mound of water.
The net
movement of
water (averaged
over the entire
upper 330
meters of the
ocean) is 90º to
the right of the
wind direction
(in the northern
hemisphere).
When the water is pushed to the right it forms
the hill we described above. So, when water is
pushed along by the wind it wants to be
turned to the right by the Coriolis force
(in the northern hemisphere) but it must fight
against gravity (trying to move up the hill of
water formed by Ekman transport).
A balance is met between the Coriolis and
the gravity (pressure gradient force).
This balance produces a balanced flow
called a Geostrophic
current.
Geostrophic
Current
Eastern and Western Boundary Currents
Boundary Currents are the major
geostrophic currents around the gyre.
The Gulf Stream is
an example of a
Western Boundary
Current.
The Canary Current
is an example of an
Eastern Boundary
Current.
Major Gyres of the world
Note the location of the Eastern and Western
Boundary Currents.
Western Boundary Currents originate in low
latitudes and move to high latitudes.
They are warm, shallow, fast-moving currents
Eastern Boundary Currents originate in high
latitudes and move to low latitudes.
They are cold, deep, slow-moving currents.
Upwelling along the coast
-caused by Ekman transport of waters
(waters move to the right of the wind).
The waters moved offshore are replaced
by waters from below.
This brings cold, nutrient-rich waters to
the surface. It has a fertilizing effect.
Most of the world’s great fisheries are
found in areas of upwelling.
Upwelling demonstration
http://www.youtube.com/watch?v=XV90dy0ns1U&NR
=1
Downwelling –
caused by Ekman transport onshore
(movement of water to the right of the wind
direction).
It pushes
water to
shore and
then it
sinks.
Global Conveyor System
Global Conveyor System
Thermohaline circulation distributes
heat from tropics to poles
Deep ocean cold currents upwell,
bringing nutrients to the surface.
Speed of flow is thought to have major
impacts on global climate systems