Transcript pressure/winds

Winds and the Global Circulation
System
Objectives:
•Describe the measurement and variation of atmospheric
pressure
•Explain the processes that impact upon wind direction and
weather systems
•Describe the pressure and circulation patterns of the
atmosphere
•Evaluate the role of upper atmospheric circulation
•Describe the circulation patterns of the oceans
Atmospheric Pressure
As the atmosphere is
held down by gravity, it
exerts a force upon every
surface (pressure = force
per unit area)
At sea level the force is
the weight of 1 kg of air
that lies above each
square centimeter of the
surface
(around 15 lbs per inch)
-Atmospheric
pressure
decreases rapidly
with altitude near
the surface
-Therefore a
small change in
elevation will
often produce a
significant change
in air pressure
Differences in air pressure = a pressure gradient
The pressure gradient forces acts at right
angles to the isobars (90 degrees)
820
820
830
830
840
840
850
860
850
870
880
860
weak pressure
gradient
890
strong pressure
gradient
The planet Earth also rotates
• in the northern hemisphere air appears to be
deflected to the right
• in the southern hemisphere, deflected to the
left
• this deflective force = Coriolis force
• because the wind is deflected it now flows
parallel to the isobars = geostrophic wind
Imagine a turntable
• when not turning, a ball traces
straight line
• when moving, ball traces a
curved line
low pressure
pressure
geostrophic
gradient force
winds
992
996
1000
1004
1008
1012
1016
1020
high pressure
Friction forces
• As wind flows over the surface friction reduces
the speed
• Friction also changes the direction of the
geostrophic wind
• The pressure gradient force over powers the
Coriolis effect
• As a result wind flow across the isobars
• In Northern
hemisphere –
cyclones spiral
counter clockwise
• Anticyclones spiral
clockwise
• In Southern
hemisphere –
opposite
• Cyclones spiral
clockwise
• Anticyclones spiral
counter clockwise
Land and Sea breezes
During the day, air over land heats up and the
sea is relatively cool (sea breeze)
land = low pressure and sea = high pressure
At night air over land cools and the sea is
relatively warm (land breeze)
land = high pressure and sea = low pressure
High pressure (anticyclone)
Side View
From above
H
L
surrounding air is
relatively low
L
H
air descends
L
Low pressure (depressions, cyclone)
Side View
From above
L
H
surrounding air is
relatively high
H
L
air ascends
H
General Circulation of the Earth’s
Atmosphere
Deflection is
least at the
equator and
greatest at the
poles
• In Northern
hemisphere –
cyclones spiral
counter clockwise
• Anticyclones spiral
clockwise
• In Southern
hemisphere –
opposite
• Cyclones spiral
clockwise
• Anticyclones spiral
counter clockwise
Cold
High
Pressure
EARTH
But heat is
transported from
the Equator to the
Poles - how?
Warm
Low Pressure
SUN
Warm air rises and flows polewards
LOW
PRESSURE
L
So, around the equator
= (ITCZ) Inter-Tropical
Convergence Zone
= low pressure
because solar heating
Doldrums
• Intense heating
causes air to rise,
leading to little
horizontal motion
at times
• Very calm
conditions
Samuel Taylor Coleridge
“The Rime of the Ancient
Mariner”
“Down dropt the breeze,
the sails dropt down,
'Twas sad as sad could be”
“Day after day, day after
day,
We stuck, nor breath nor
motion ;
As idle as a painted ship
Upon a painted ocean.”
Cooled air sinks at 30 degrees N&S
H
HIGH
PRESSURE
Subtropical High
• Trade winds are
predictable
• North East in
Northern hemisphere
• South East in
Southern hemisphere
• At subtropical high
conditions are calm
• known as the
“horse latitudes”
Cooled air sinks at 30 degrees N&S
Air flows
northwards
towards the
Poles
Some air
flows back
towards the
Equator
HIGH
PRESSURE
Warm air meets cold air and rises
Warmer
air rises
L
Cold air sinks
Subpolar low pressure
because the warmer air
of the mid-latitudes rises
as it meets cold polar air
H
At the Poles the air is
cold and dense
• this produces an area
of high pressure
From the subtropical high
to the subpolar low =
“westerlies”
• Includes variable low
and high pressure
systems
• At the Polar Front storms form
From the Polar High to
the subpolar low are the
Polar easterlies - variable
Hadley
Cells
Monsoons
-January high
pressure over the
land produces dry
winds.
-Air flows towards
the ITCZ.
Monsoons
July - position of
the ITCZ moves
North
• Low pressure
over the land
causes winds to
flow off the
ocean.
• This brings
heavy rainfall.
Permanent
area of high
pressure over
Antarctica
Seasonal
pressure
changes over
the Arctic
Upper
Atmosphere
• At 5-7 kms above
the surface
• Influenced only by
pressure gradient
force and Coriolis
force
• Geostrophic winds
that flow parallel to
isobars
Rossby Waves
-Smooth westward
flow of upper air
westerlies
-Develop at the polar
front, and form
convoluted waves
eventually pinch off
-Primary mechanism
for poleward heat
transfer
-Pools of cool air
create areas of low
pressure
Jet Streams
• Narrow bands of
high velocity
• Form along the
polar front and
above the Hadley
cell in the
subtropics
Circulation patterns (CURRENTS)
produced by:
•
•
•
•
•
Winds
Density differences in sea water
Coriolis force
Shape of ocean basins
Astronomical factors (TIDES)
Ocean Temperatures
• Surface warms – especially in the summer
• Layer of rapid temperature drop thermocline
Ocean Currents
• Driven mostly by
wind blowing over
the surface
• However, currents
move slowly
• Lag behind wind
speed so often
called drifts
wind
Ocean currents
• Large continuously moving loops (gyres)
• Produced by winds, Coriolis force and land
masses
Gyres
• Large circular
currents
• Subtropical
gyre
corresponds to
the subtropical
high pressure
• N. and S.
Equatorial
currents
corresponds to
the trade winds
• Equatorial
countercurrent
corresponds to
the ITCZ
• West winds
around
Antarctica
create
circumpolar
gyre
• Portions
branch off
toward the
equator
Deep-sea currents
• Driven by differences
in temperature and
salinity
• Much slower than
surface currents
• Thermohaline
circulation
• Depends on conditions
in the North Atlantic
El Nino/Southern Oscillation (ENSO)
• Every year warming occurs off the coast of
Peru (~2˚/C)
• Suppresses upwelling
• But every 4 or 5 years it is much more
pronounced = El Nino
• Because there is no upwelling fish die
• May have far wider effects
In normal year, low pressure dominates in
Malaysia and northern Australia.
In an El Niño year, low pressure moves east
to the central part of the western Pacific