Download G The Global Circulation

The Global Circulation
G
The Global Circulation
Aims
To understand the concept of scale
To describe and explain the major wind systems and their seasonal changes
To understand the global circulation
Objectives
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To locate on a map the principal prevailing wind regimes associated with the planetary-scale circulation.
To locate on a map the semi-permanent surface pressure systems or "centers of
action" that influence the planetary-scale circulation.
To relate the named prevailing wind fields to the global air pressure distribution.
To sketch and label a cross-section diagram showing the idealized three - cell model
of global atmospheric circulation.
To describe the seasonal changes in the major features of the planetary-scale circulation regime.
To discuss the influence of ocean - continent distribution on global pressure distribution.
To explain why the midlatitude flow aloft is predominantly westerly, with upper tropospheric jet streams, located over the near surface polar front.
.30-17.30, Wednesday 13.30-14.30 and by appointment
Tel.: (0116) 252 3848
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The Global Circulation
Outline
Introduction
• Scales of atmospheric motions
• Observations of surface winds and winds aloft
• Uneven Heating and Resulting Pressure Differences as causes for
winds
Observed distribution of pressure and winds
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Vertical, meridional and zonal winds
Seasonal changes
Changes in wind speed with height
Distribution of pressure zones and systems
The global circulation
• Relationships between heating and major pressure patterns
• Relationships between prevailing winds and major pressure patterns
• Impact of Coriolis effect
The Westerlies
• Geostrophic balance
• Pressure gradient dependence
• Increase in wind speed with height
Jet streams
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Fast flows at height
The polar or midlatitude jet at about 10km above the polar front
Seasonal changes in speed and location
Other jets
The Global Circulation
Bullets
Introduction
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Scales of atmospheric motions
Categories according to observations of different phenomena
Planetary scale (mainly horizontal), synoptic/weather map scale, mesoscale - shows
large vertical flow, mircoscale - chaotic irregular
Example: Averaging across a hurricane: lots of local winds, variation in winds speed,
various rain events, but on large scale a slowly westward moving system
Surface Winds are mostly zonal or around centers.
Aloft the wind is nearly only zonal, except in the tropics. In the high northern latitudes
more meandering than in the high southern latitudes.
Winds are labeled by the direction from which they blow
All winds result from pressure gradients that arise due to unequal heating
Observed distribution of pressure and winds
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Vertical winds indicate a three cell circulation on each hemisphere, rising over the
tropics, sinking over the subtropics, rising over the midlatitudes and sinking over the
poles.
Meridional winds are towards the equator on the surface and away from the equator
aloft between the subtropics and the tropics, away from the subtropics on the surface
and towards the subtropics between the subtropics and the mid-latitudes and towards
the mid-latitudes on the surface and away from the mid-latitudes between the mid-latitudes and the poles.
Zonal winds are easterly over the tropics and westerly over the mid-latitudes. They
are much faster aloft than near the surface.
Zone of tropical low pressure (intertropical convergence zone, ITCZ), zone of subtropical high pressure systems - most dominantly over the oceans, zone with low
pressure systems in the mid-latitudes and polar highs. These zones are not continuous due to land-sea contrasts and move seasonally North-South.
The global circulation
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Pressure difference between the tropics and the poles due to uneven heating.
Pressure gradient force from equator to poles.
Moist warm air in the tropics rises. Remains fairly warm due to condensation. Can
rise to very high heights.
Continually rising air in the tropics leads to divergence aloft in the tropics as the air
has to move away.
As the air moves away from the equator radiation cooling leads to denser air.
Northward air movement due to PGF is deflected to the right on the Northern Hemisphere because of the Coriolis force restricting poleward flow
Thus convergence aloft around 25o. Cooling and convergence lead to subsidence.
Subsiding dry air - deserts. Surface winds are weak in this zone
Air flows on the surface back to the low in the tropics - Trades, due to Coriolis forces,
or to the North - Westerlies
The tropical convection cell (convective rising driving a large vertical and South-North
circulation) is called Hadley cell after George Hadley who first suggested that the global circulation on a hemisphere would be similar to a land-sea breeze because of the
temperature gradients between the tropics and the poles.
Seasonal shifts of the winds due to shifts in heating and consequently pressure differences
The Global Circulation
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Seasonally winds tend to be much stronger than in the annual mean - this results
from averaging over the whole year for the annual mean.
The Westerlies
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Westerlies - geostrophic winds result from the geostrophic balance, i.e between PGF
and Coriolis effect acting on poleward moving air.
As a parcel of air is accelerated due to pressure gradient force the Coriolis effect
increases.
Coriolis force becomes larger and larger as long as the parcel accelerates until finally
the pressure gradient and Coriolis force balance. Then the flow is parallel to the isobars because otherwise there would be a force towards the lower pressure, thus
acceleration and a further increase in the Coriolis force.
The winds generated in this way are called geostrophic winds. They flow in a straight
path parallel to the isobars at a constant speed.
The closer the isobars - the larger the pressure gradient - the higher the wind speed
Idealization!
Pressure gradient increases with altitude - thus wind speed increases as well, up to
the tropopause.
Jet streams
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Fast flows at height due to large pressure differences
Most prominent and consistent - the polar or midlatitude jet between 30o and 70o
North depending on time of year and synoptic situation
Polar jets at about 10km above the polar front in lower layers
Polar front is the region where warm air from the midlatitude Westerlies meets the
cool air form the polar Easterlies. Thus there is a large South-North temperature gradient near the surface and as consequence a large South-North pressure gradient at
height.
Since the pressure gradient increases with height the PGF increases with height and
therefore the wind speed.
Large changes in speed of the polar jet seasonally due to much larger temperature
and thus pressure gradients in Winter than in Summer - in Winter on average about
125km/h and about half of that in Summer (compare temperature map and latitudinal
heat budget)
Seasonal changes in the location of the polar jets due to the South-North movement
of the region with the largest temperature and thus pressure gradients as the incident
solar radiation varies.
Other jets exist but are less studied and are usually seasonally absent.
Links
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http://www.geog.le.ac.uk/staff/jk61/teaching/gy1003/lectures/lecture16.html
http://cwx.prenhall.com/bookbind/pubbooks/aguado2/chapter8/deluxe.html
The Global Circulation
Introduction
Hurricane Floyd on September 9, 1999
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The Global Circulation
Table 1: Time and space scales for atmospheric motions
SCALE
TEMPORAL
SCALE
Macroscale
Planetary
Weeks to Years
SPATIAL
SCALE
EXAMPLES
1000-40,000 km Westerlies and
trade winds
(Anti)Cyclones,
Synoptic
Days to weeks
100-5000 km
Hurricanes
Land-sea breeze,
Mesoscale Minutes to days
1-100 km
thunderstorms
Microscale Seconds to minutes <1 km
Turbulence,
dust devils
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The Global Circulation
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The Global Circulation
Based on sea breeze
analog could explain
a circulation like this:
PGF
Observed circular and
zonal winds due to:
Coriolis force
Friction
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The Global Circulation
Observed distribution of pressure and winds
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The Global Circulation
Faster
than at
surface
More
zonal
Opposite
direction in
tropics
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The Global Circulation
Sea level pressure
and surface winds
Northern hemisphere winter
(Jan)
Northern hemisphere summer
(July)
Movement of
wind zones
pressure regions
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The Global Circulation
North South flow
Aloft:
•away from the
equator
•towards
equator in midlatitudes,
•towards the
poles
90S
60S
30S
EQ
30N
Positive: northward, Negative: southward
Surface:
• towards the equator
• towards the poles in midlatitudes
• away from poles
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60N 90N
The Global Circulation
Idealized global circulation on rotating Earth
Pressure Zones
Polar
Subpolar
high
low
•Polar High
Polar cell
Polar easterlies
•Subpolar Low
60
•Subtropical High
Ferrel cell
•Tropical Low, ITCZ
Polar front
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30o
Su
Hadley
cell
btr
op
ic a
lh
Westerlies
ig h
Horse lati
tudes
Hadley cell
NE
trade winds
0o
E qu
ator
ia l l o
w
Doldrums
Hadley
cell
SE
trade winds
Hadley cell
Circulation cells
•Polar Cell
•Ferrel Cell
Source:
Lutgens, F.K. and •Hadley Cell
E.J. Tarbuck, 1998.
The Atmosphere
GY1003 - Principles of Physical Geography, Lecture 16, Jörg Kaduk
Wind systems
•Polar easterlies
•Westerlies
•Trades (NE, SE)
The Global Circulation
Idealized and observed pressure zones
Source: Lutgens, F.K. and E.J. Tarbuck, 1998. The Atmosphere
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The Global Circulation
Characteristics of subtropical high-pressure systems
East side
close to continent
subsiding dry air
West side
air from high pressure
system blows over warm
ocean
becomes moist and
unstable
Source: Lutgens, F.K. and E.J. Tarbuck, 1998. The Atmosphere
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The Global Circulation
Jet streams
Polar jet
Subtropical jet
Jets
Narrow, very fast air
currents
Typically > 100 km hr-1
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Polar
front
The Global Circulation
Jets
Geostrophic
winds
Faster in
winter
hemisphere
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Pressure decreases
The Global Circulation
cold air,
high density,
isobars vertically close
700 mb
pressure
surface
warm air,
low density,
isobars spread out vertically
Source: Lutgens, F.K. and E.J. Tarbuck, 1998. The Atmosphere
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The Global Circulation
Meridonal profiles of the zonal mean pressure
NH
SH
Pressure difference
between high latitudes and tropics
110
10000m
NH
winter
280
NH
summer
550
Eq.
850
1020
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larger in winter
because temperature
difference is larger
and thus density difference
The higher in the
atmosphere the
stronger this effect
The Global Circulation
Upper air flow in the midlatitudes
Rarely straight
Mostly wave like
pattern
Rossby waves
Source: Lutgens, F.K. and E.J. Tarbuck, 1998.
The Atmosphere
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The Global Circulation
The index
cycle
Varying waves
in upper airflow
Gently undulating upper airflow Meanders form in jet stream
Very important for heat
exchange
3-8 weeks
•Orography
•Land-Sea
•Rotation
Strong waves in upper airflow
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Return to a period of flatter flow aloft
Source: Lutgens, F.K. and
E.J. Tarbuck, 1998. The
Atmosphere
The Global Circulation
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The Global Circulation
Upper level convergence and divergence
Subsidence
H
Source: Lutgens, F.K. and E.J. Tarbuck, 1998. The Atmosphere
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Uplifting
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The Global Circulation
Summary
• Three cell model of global atmospheric circulation
• Tropical Hadley cells are directly driven convection cells
• Resulting pressure pattern is:
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low in the tropics (warm moist air is rising)
high in the subtropics (dry air descending from above)
low in the mid-latitudes (rising air, moist, flowing over cold air out flow from poles)
high over the poles (cold dry sinking air)
Wind pattern: Trades, Westerlies, Polar Easterlies
Pressure zones broken up due to differential heating
Semi permanent high and low pressure systems
Wind speeds higher in winter due to larger pressure differences
Wind speeds higher at altitude due to larger pressure differences
High altitude jets
Polar/mid-latitude jet meanders, Rossby waves, index-cycle
Role in heat exchange
Convergence/divergence aloft creating high/low pressure below
GY1003