Transcript Air/Ocean Circulation

Chapter 6
Atmospheric and
Oceanic
Circulations
Robert W. Christopherson
Charlie Thomsen
Wind is a vector variable
Temperature is a scalar variable.
Atmospheric and Oceanic
Circulations
Wind Essentials
Driving Forces within the Atmosphere
Atmospheric Patterns of Motion
Oceanic Currents
Wind Essentials
Air Pressure and Its Measurement
Mercury barometer
Aneroid barometer
Wind: Description and Measurement
Wind
Anemometer
Wind vane
Global Winds
Barometers
Figure 6.2
Air Pressure Readings
1013.25mb=101.325kpa=14.66lb/in2
Figure 6.3
Atmospheric Pressure and Elevation
Which point (p1 vs. p2) has higher air
pressure? Why?
How are pressure change with elevation?
(1) Uniformly decrease with elevation
(2) decreases faster with higher close to
see level than at high elevations.
p2
p1
Wind Vane and
Anemometer
Wind: horizontal movement of air across
Earth surface.
Vector: Speed measured by Anemometer
Direction measured by wind vane
wind direction is defined as the
direction from which it originates.
Standard measurement of wind
is 10 m above ground.
Old weather forecast refer wind speed in
scales, a commonly used one is Beaufort
Wind Scale.
Figure 6.4
Driving Forces within the
Atmosphere
Pressure Gradient Force
Coriolis Force
Friction Force
Gravity
Pressure Gradient Force
Pressure difference is
primarily caused by uneven
heating of Earth surface
Without pressure gradient
force, the air will not move,
then there will be no
Coriolis force, no friction
force.
Figure 6.7
Coriolis Force
It deflects anything that flies or flows across Earth surface:
wind, airplane, ocean currents etc.
Coriolis Force only changes the direction of movement, not
the speed. It is always perpendicular to the direction of
movement, to the right hand side on Northern Hemisphere.
F=2vΩsin(φ), where v=wind speed, Ω=angular velocity of
earth rotation 7.29x105 radians per second, φ=latitude
The stronger the wind, the stronger Coriolis force.
Figure 6.9
Pressure + Coriolis + Friction
Pressure Gradient Force only
Pressure Gradient+Coriolis+Friction Forces
Pressure Gradient +Coriolis Forces
Friction force: Always in the opposite
direction of wind.
Strength: depending on wind speed, surface
condition (topography, vegetation, …)
Figure 6.8
Scales of Atmospheric Movement
The movement of atmosphere around the globe is a composite
of multiple scale motion, like a meandering river contains
larege eddies composed o f smaller eddies containing still
smaller eddies.
Macroscale: Large Planetary wide movement of
atmosphere, e.g. trade winds, monsoon, hurricanes, which
can blow for weeks or longer.
Mesoscale: lasts for several minutes or hours, usually less
than 100 km across, e.g. thunderstorms, tornadoes
Microscale: smallest scale of air motion, lasts for seconds
at most for minutes, e.g. wind gust, dust devils
The Westerlies
tropopause
sun
Earth
500mb
600mb
700mb
800mb
900mb
Pressure gradiant
1000mb
Equator
N. Pole
Single Cell Model (Hadley 1735)
sun
Figure 6.12
Three-Cell Model (1920s)
Polar Cell
Ferrel Cell
Hadley Cell
Figure 6.12
General Atmospheric Circulation and pressure zones
Figure 6.12
General Atmospheric Circulation
Figure 6.12
Primary High-Pressure and
Low-Pressure Areas
Equatorial low-pressure trough: thermal
Polar high-pressure cells: thermal
Subtropical high-pressure cells: dynamic
Subpolar low-pressure cells: dynamic
Equatorial Low-Pressure Trough
Intertropical convergence zone (ITCZ)
Clouds and rain
Trade winds: The trade winds were named
during the era of sail ships that carried trade
across the seas.
Global Barometric Pressure
Icelandic Low
Siberian High
Aleutian Low
Hawaiian High
Azores High
The subtropical high pressure zone broke into three high pressure centers: Hawaiian, Azores, Siberian Highs
The subpolar low pressure zone broke into two low pressure centers: Aleutian and Icelandic LowsFigure 6.10
High Pressure Center
East side drier and more
stable, feature cooler
ocean currents than west
side. Earth major deserts
extend to the west coast
of each continent.
Global Barometric Pressure
Pacific high
Bermuda high
The high pressure centers are pushed northern. As a result, the subpolar low pressure centers are weakened
significantly.
Figure 6.10
Global Patterns of Pressure
Westerlies
Wind Portrait of
the Pacific Ocean
Pacific High
Trade wind
Wind pattern derived from
a radar scatterometer aboard
Seasat on a day in September.
Note: compare wind pattern
and the visible earth below:
Figure 6.6
June–July ITCZ
Figure 6.11
Monsoonal Winds
Larger than
average
northward
migration of
ITCZ.
Regional wind systems seasonally changes direction and
intensity associated with changes temperature and
precipitation.
Winter: cold dry wind blow off the continents
Summer: warm moist-laden wind blow from sea toward land
Figure 6.20
Upper Atmospheric Circulation
Jet stream: a fast flowing narrow air
currents in the upper atmosphere
Rossby waves
Jet Streams
An concentrated band
of wind occurring in
the westerly flow aloft.
Flat in vertical
direction
Speed up to 190 mph
Stronger in winter
It is caused by the large
pressure gradient
caused by the large
horizontal temperature
difference over short
distance.
30-70oN
Influence surface
weather systems.
20-50oN
Figure 6.17
Rossby Waves
Note: in the upper atmosphere, artic area
has low pressure, thus the air circles
counterclockwise parallel to the pressure
gradient (Why?). But this circle is not
perfect. Instead, it follows a wavy path.
Discovered by Carl G. Rossby in 1938. It
refers to the waving undulations of
geostrophic winds of the arctic front.
Figure 6.16
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
transfere
Pools of cool air
create areas of low
pressure
Figure 6.16
Local Winds
Land-sea breezes
Mountain-valley breezes
Katabatic winds
Land-Sea
Breezes
Figure 6.18
Mountain-Valley
Breezes
Katabatic Wind: A regional
scale gravity driven wind,
usually needs a high plateau to
cool the air, and become dense
and flow downslope.
Figure 6.19
Oceanic Currents
Function: Mixing sea water
Surface warm water with deep cold water
CO2 absorption
Climate
Biogeochemical processes: phytoplankton growth
Driving force: the frictional drag of winds
Thus we have an Atmosphere-Sea are coupled system. Once the
current starts to move, the Coriolis force will kick in. Then there is
“friction” between upper and lower water, the shear stress.
Major Ocean Surface Currents
Surface ocean currents are driven by air circulation around subtropical high pressure cells.
Figure 6.21
Equatorial Currents/Western Intesificaitn
Corresponding to trade winds on both sides of the
Equator, these winds drives the surface current
westward along the equator, called equatorial
currents.
The equatorial currents push water piles up against
the eastern shores of the continent. This is called
western intensification. The piled up water will go
either up north or down south . The Gulf Stream is
one caused by western intensification.
Upwelling/Downwelling Currents
Upwelling Currents: When surface water is swept away
from a coast, an upwelling current occurs. This cool water
generally is nutrient rich, e.g. Pacific Coast of North and
South America
Downwelling Currents: Accumulation of surface water
(e.g. western end of equatorial current) can gravitates
downward to generate a downwelling current.
Figure 6.22