Current Topics

1
Current Topics
2
Put in notes
Surface ocean currents driven by wind and deep
currents driven by density differences in water
masses combine to transport 1/3 of the total heat
transferred from the equator to the poles.
3
now for the details
How fast can you swim?
An Olympic class swimmer can sprint the 100 m
freestyle in 40-50 seconds 100 m/40 sec
2.5 m/s
YOU are much slower! A good amateur swimmer can
swim 2- 2.5 mph 1 m/s
Keep this in mind as we discuss currents today.
4
Surface currents driven by wind affect only
about 10 of the oceans water

Thermohaline currents affect the other 90 of the
water.
5
A geostrophic gyre is an oceanwide
current bounded by the continents, westerly
winds, and the trade winds.
How to remember? Gulf Stream goes north, cold
water on CA coast comes south.
Southern hemisphere is opposite.
The gyres circulate CW in the northern hemisphere
and CCW in the southern hemisphere.
6
Geostrophic gyres result from a balance of forces
1. Wind friction (in direction of wind)
2. Coriolis force
3. Pressure gradient in water from water piling
up downwind.
7
Result currents move at a 45 angle from the
wind direction. To the right in
northern hemisphere and to the left in the
southern one.
now, lets look at WHY.
8
Wind friction and Coriolis force
motion gets smaller with depth because of
frictional losses.
and so on down the water column.
9
The average movement for the whole column of
water is to 90 the right of the
wind direction.
10
The average flow is to the right all around a
gyre, so water piles up towards the middle.
This creates a pressure difference between the
center and the outside that partially
counteracts the movement of this water towards
the middle.
11
Pressure gradient lt Coriolis effect, so curvature
still occurs.
12
The net result is current flow at 45 to the wind
direction.
13
Max height 2 m
The hill is shifted to the western edge because
the western currents as these currents flow
north, they experience increasing Coriolis
forces.
14
In the northern hemisphere, the western edge of
the gyre is a northbound current
transporting warm water northward.
Western boundary currents are fastest and deepest
and are narrow (lt 100 km).
The Gulf Stream moves 2 m/s, extends to a depth
of 450 m, and is 70 km wide.
15
Eastern boundary currents carry cold water
southward.
They are slower, wider, and shallower.
The Canary Current moves 0.5 m/s and is 1000 km
wide.
16
but the energy for the gyre comes from the
transverse currents
Steadily blowing westerlies and trade
winds continually provide energy for the
currents.
17
The biggest current Antarctic Circumpolar Current
Why do you think this current is so large?
(No roadblocks continents!)
18
There are also vertical currents induced by the
wind at continental edges
coastal upwelling (CA nutrient-rich water and
cooling at coast)
coastal downwelling
19
BIG upwelling at equator!
downwelling
upwelling
brings nutrient-rich water up for abundant life
there.
20
The gyres modulate climate on the continents
cold water moving south tends to
produce cool,foggy summer weather
warm water makes hot, humid summer weather
21
but probably the most noticeable effect is on
the United Kingdom and northern Europe
Which do you think is more important for
warming Europe currents or wind?
22
And THAT is why you can find tropical plants on
islands just off the coast of England
23
Interplay between ocean currents and atmospheric
circulation is responsible for El Niño and La
Niña events
24
NORMAL YEAR
upwelling brings nutrients for fish --gt happy
fishermen!
25
EL NIÑO YEAR
downwelling shuts off nutrients for fish --gt sad
fishermen!
26
NORMAL Warm water in SW Pacific
27
EL NIÑO Trade winds slow and warm water starts
moving.
28
EL NIÑO CONSEQUENCES
Warm water ? Increased evaporation ? Increased
precipitation ? Flooding and all sorts of other
nasty stuff
29
LA NIÑA rapid return to normal conditions
30
This is often referred to as ENSO El
Niño-Southern Oscillation
31
So far, we have talked about wind driven currents
and the consequences of these currents.
There are also currents driven by density
differences between water masses.
32
Surface currents driven by wind affect only
about 10 of the oceans water

Thermohaline currents affect the other 90 of the
water.
33
Denser water is usually colder and saltier.
34
Two ways to form denser water
Freeze seawater (Arctic, Antarctic)
Ice is pure water, so this leaves behind saltier
water
Evaporate seawater (Mediterranean)
35
Distinctive water masses form under ice and may
keep their identities for as long as 1600 years.
36
Salty water created by evaporation in
the Mediterranean Sea sinks to its bottom, but
some of it flows out over the sea bottom ridge
at the Straits of Gibraltar.
37
Sinking is often concentrated in a narrow zone,
while return flow is much more diffuse.
38
Water masses formed in these ways keep their
characters (temperature, salinity, and density)
for a long time
Antarctic Bottom Water -- 1600 years
Other deep water -- 200-300 years
Time to complete a circuit of the North Atlantic
gyre 1 year
39
(No Transcript)
40
Speculate What would happen if there was no
friction between the wind and the water?
41
Speculate What do you think would happen if the
surface warmed to the point where all of the ice
melted and no more could form because the
poles were too warm?