Transcript Ocean Circulation - stjohns

Ocean Circulation
Ch. 3
Objectives
Demonstrate an understanding of:
1.
2.
3.
4.
Ocean circulation
Coreolis effect
Global wind patterns
Great ocean conveyor
Firing a missile
• Fig 3.16
• Aim the missile at Charleston --> 2 1/2 hours later
it hits San Diego. Why?
• Coriolis clip (about halfway)
• Landing a jet (Explore--Coriolis Effect)
The earth spins!
• Causes objects to follow a curved path.
What’s this called?
• Coriolis Effect:
– Deflects large-scale motions
(winds/currents) to the right in the northern
hemi and to the left in the southern hemi
• Show with globe
• Demo: spinning paper/record on board
• Why don’t we notice it when we are
walking or driving?
• Detailed explanation
• How does this affect the wind
patterns?
• Let’s look at how convection works
first . . .
Convection Demo
• Convection-wind demo
• What is convection? After seeing this demo, converse
with your neighbor and come up with a definition and
explanation of what’s going on
– movement of heat in a fluid (air is a fluid) caused by
differences in temp/density
• What does this have to do with wind?
Wind Patterns
• Flashlight/earth demo--what’s
happening?
• Sun heats the surface of the
earth at the equator
• What happens to the warm
air?
• Warm air rises
• Cooler, denser air blows in to
replace it
• Area of low pressure and not
much wind
– Doldrums
• Why not much wind?
This doesn’t just happen at the
equator . . .
The Doldrums
• Area of low pressure where
the trade winds meet along
the equator
• Calm, light winds
• Warm air rises steadily
• Cool air moves in and
warms rapidly before it
moves too far  no wind
Wind Patterns
• What happens to the warm,
rising air after it releases it’s
moisture?
• It becomes dry and cold and
descends towards earth N & S
of equator.
– Where does this happen (in terms
of latitude)?
• 30° N and S.
• What type of environment do
we usually see here?
Global Vegitation.
Where are the
deserts
located?
Horse Latitudes
• 30° N and S are also called
the Horse Latitudes
• Winds are variable --> trade
winds and westerlies are
moving apart
• Sailors, after being “stuck”
for days, and short of
drinking water and food,
used to throw dying horses
overboard -- it also
lightened the load --> easier
to sail
Wind Patterns
• What do you notice at the
60° parallel?
– Rising warm, moist air
• At the 90 °?
– Descending, cool, dry air
• What types of pressure
zones exist at each of these
parallels?
• Is there a relationship
between these pressure
zones and whether the air
sinks or rises?
Low vs. High Pressure
• What’s the relationship?
• Low pressure:
– Hot, moist, rising air
– Clouds and rain
• High Pressure:
– Cool, dry, sinking air
– Sunny
• Notice the flow from high to
low pressure---This is
convection!!!
• Explanation (Fig. 7o-3)
Wind Patterns
• There are six cells
• Which directions does
the wind flow in each
cell?
• Always from high to
low pressure!
• Why doesn’t the
air/wind flow in
straight lines?
• Because the earth
spins causing the
Flying a plane?
Coriolis effect
Play merrry-go-round clip
Nike and Currents?
• Let’s do an activity
• hand out current maps
• Keep the maps, you
will be quizzed on the
major currents
• Read introduction
Wind and Currents
• What does the wind have to do with the currents?
• Do you see a relationship between wind and major surface
currents?
• Check with a neighbor and discuss it
Wind and Currents
• Wind pushes the water
• Water moves at 45° angle
due to Coriolis effect.
• Top layer of water pushes
on layer below it
• Coriolis effect causes this
second layer to move
slightly to right and
slower.
• And so on, and so on
until?
Ekman Spiral
• The water is moving in
an opposite direction
and eventually no
current at all!
• Ekman Spiral:
– The spiral change of
movement of water in
the water column
• What do you notice
about the net movement
of water?
Ekman Transport
• Ekman transport:
– The net movement of ocean
water at right angles caused
by the wind
• Is this diagram of the
northern or southern
hemisphere?
• Northern. What direction
would the net transport of
water be if this was the
southern hemi?
Currents and Winds
With your knowledge of the Ekman transport, explain why the currents flow in the
direction they do. (Hint: what do you notice about the wind direction?)
Can you identify the trade winds and the westerlies? Chat with a neighbor.
Gyres
• Currents combine into huge gyres:
– Large circular systems of surface currents
• remember---you will be quizzed on these currents!!!
• Adopt a Drifter program website ---- let's look at some drifting
buoys
Thermostat?
•
•
•
•
Giant thermostat -- why?
Bringing cold water to the tropics and warm water to the poles
Regulates climate
Remember though--- these are average patterns over a large
distance over a ling time---currents shift with the season, bottom
contour, coastline and tides--also, these are surface currents.
The ocean is layered
• 3 main layers:
– Surface/mixed layer
– Intermediate layer
– Deep layer
• What do you notice?
– (a) Salinity varies-precipitation, evaporation, river
runoff
– (a) Temp and density are mirror
images--temp controls density
– (b) Surface temp varies with
latitude--deep water temp much
more uniform
– (c) Seasonal thermoclines
The Great Ocean Conveyor
• Where does it “begin?”
• Unequal distribution of solar radiation---equator heats up
• Water moves as current from tropics towards higher latitudes
– i.e., the Gulf stream
– cools, transfers heat to atmosphere --> N.W. Europe is warmer than other
regions at same latitude
The Great Ocean Conveyor
• Water moving northward cools
• What happens to cooler water
–
–
–
–
It is denser and it sinks
What else makes water more dense?
Salinity. Why would salinity in the water increase at the poles?
As ice forms, salt ions are left behind
The Great Ocean Conveyor
• As the colder, saltier water sinks, warm water takes it
place, and then too begins to cool
• This sinking cold water is called the North Atlantic
Deep Water (NADW)
The Great Ocean Conveyor
• NADW joins the Antarctic bottom water then splits
– Circles Antarctica, heads south of OZ
– Up east coast of Africa, warms, rises (causes monsoons), and
then begins the 1000 year journey all over again.
10 minute animation on currents
• Ocean Currents
animation
The Day After Tomorrow?
• The conveyor has a major effect on global climates
– Who saw this movie? What caused the freeze?
– Global warming --> more freshwater runoff --> less salty, dense water
sinking --> slows down conveyor
Local Currents: Longshore
• Longshore current:
– Currents of water flowing parallel to the shore
– Waves break, water piles up during a set and travels along
shore
Longshores at OB
Sloat, 2003
Rip Current
• Rip current:
– Fast moving belt of
water traveling offshore
• Breaking waves over
bars --> increase of
water level near shore
• Converging of
longshore currents
• Water rushes back out
through break in bar
• What if you get stuck in
one?
Swim parallel to shore!