Circulation
of the Atmosphere
I Scales of motion
II local winds
·
Land
/ sea breezes
·
Mountain
/ valley breezes
·
Chinook
(foehn) winds
·
Katabatic
winds
III global circulation
·
Single-cell
and three-cell models
·
Observed
distribution of pressure / winds
·
Monsoons
·
The
westerlies and jet stream
·
Ocean
currents
IV El Nino / La Nina
V global precipitation patterns
I Scales of motion
In
the atmosphere, motion occurs at all spatial scales simultaneously.
|
Scale |
Time |
Distance |
Example |
|
Planetary |
Weeks,
or longer |
1000
to 40,000 km |
Westerlies,
trade winds |
|
Synoptic |
Days
to weeks |
100
to 5000 km |
Cyclones |
|
Mesoscale |
Minutes
to hours |
1
to 100
km |
Tornadoe,
T-storm |
|
Microscale |
Seconds
to minutes |
<
1 km |
Turbulence,
wind gusts |
II local winds
Wind
systems at smaller spatial scale result from local heating differences between
different surfaces, which result in local pressure gradients and winds; or,
topographical features.
Land / sea breezes convection cell caused by differential heating of land / ocean
surfaces.
Valley / mountain breezes
·
Daytime:
upslope, or valley breeze, summer thunderstorms near the tops of peaks
·
Nighttime:
cold air drainage into valley (why does it “drain”?)
Katabatic winds large pool of cold air builds up at higher elevations, then
rapidly drains downslope. If channeled through narrow valleys, can cause very
fast, strong winds. Greenland, Antarctica, Alps, Rockies.
Chinook (foehn) winds related to “rainshadow” effect, wind blows upslope, cools as its
rising causing condensation and precipitation; when it descends down the “lee”
side of the mountain, it warms again, and is very dry. Thus, the Chinook wind is the warm, dry wind
descending down the lee side. Rockies,
Santa Ana winds in California.
III General Circulation of the
Atmosphere
Because
the atmosphere is a fluid on a rotating sphere, with differential heating near
the surface, the air is constantly in motion (winds). These occur on many different scales: there are large, or global,
scale features, and small or local scale features. The general circulation
refers to the large scale features.
Single-cell circulation
model
·
Uniform
surface, no rotation
·
One
convection cell in each hemisphere
Three-cell circulation model
·
Uniform
surface, with rotation
·
Three
convection cells in each hemisphere
Which features of the
earth’s real circulation are captured by each of these? Which are not? Can you
explain why?
Observed distribution of
pressure and winds
Seven (in each hemisphere)
primary features of the general circulation:
Polar
highs
Polar
easterlies
Subpolar
lows
mid-latitude
westerlies
subtropical
highs
trade
winds
intertropical
convergence zone (equatorial low)
subtropical highs: high pressure areas centered around latitudes 30 North and
South. Anticyclonic circulation,
associated with subsidence, clear skies, dry and sunny weather. Winds
equator-ward of them are easterly, winds pole-ward of them are westerly
trade winds: easterly winds found in the tropics between latitudes 25N and
25S. Very reliable, and pick up a lot of moisture from evaporation over the
oceans. Northeasterly in northern hemisphere, southeasterly in southern
hemisphere.
Intertropical convergence
zone (ITCZ): “belt” around the equatorial region, where
the SH and NH trade winds meet, or converge. Between latitudes 10N and 10S,
location depends on season. Low pressure associated with convergence, ascending
air, lots of tall cumulonimbus clouds and thunderstorms.
Polar high pressure system: located over the cold polar regions, associated with subsiding,
cold, anticyclonic circulation. Extend from poles close to latitude 60 N and
60S.
Polar easterlies: just equator-ward of the polar highs, easterly winds predominate.
Located at latitude around 60 N and 60S.
Sub-polar low pressure
systems: areas of cyclonic flow, convergence, rising
air, clouds, precipitation. Located just south of the polar easterlies, between
latitudes around 50-60 N and S. Icelandic
low and Aleutian low.
Seasonal variations in the
large scale circulation:
latitudinal shifts in “primary features”
discussed previously
shifts in important
features:
·
Siberian
High (winter)
·
Aleutian
Low (winter)
·
Icelandic
Low (winter)
·
Bermuda
(winter) / Azores (summer) High
Monsoons: seasonal shift of winds from offshore
(land-to-ocean, dry) during winter to onshore (ocean-to-land, wet) during
summer.
South Asian, or Indian,
monsoon: Heavy rains in June and July associated with
onshore winds in summer and interaction with the Himalayan Mountains.
North American monsoon: summertime temperatures over southwestern US (centered over
Arizona) cause a thermal low pressure system, which draws moist winds from the
Pacific Ocean as well as the Gulf of Mexico. Summer rains begin early July in
northwestern Mexico, Arizona, New Mexico, and partially felt in southern
California, Utah, and Colorado.
Other
monsoon areas:
·
East
Asian, or China
·
Australia
·
West
Africa.
mid-latitude westerlies: Between latitudes 30-60 North and South. Winds meander in a wave
like pattern, with the crests and troughs associated with cyclonic and
anticyclonic systems moving generally west to east. Upper tropospheric flow is close to geostrophic, thus the winds
follow the isobars and the winds are smoother than close to the surface.
Jet Stream: fast moving core of the upper tropospheric westerlies.
·
Polar front jet stream: located near the “front”, where the cold polar air meets the
warmer mid-latitude air. Wave-like
pattern called Rossby waves. Very important for determining the strength and
locations of storm systems in the mid latitudes.
·
Subtropical jet stream: located just poleward of the subtropical high pressure systems.
Meridional
flow-
stormy weather, mixing of warm and cold air
Zonal flow – calm weather, little
mixing
Ocean Currents and Climate
Ocean
currents and winds are closely related, in the sense that winds affect the
direction of currents and currents affect winds. Along with the global
atmospheric circulation, the ocean currents transfer energy from tropics to
polar regions.
Ocean
currents affect regional climate:
·
The
Gulf Stream / North Atlantic Drift
·
Peruvian
and Benguela currents
·
California
current
On
longer time scales it is thought that major shifts in ocean currents affect
global climate, such as during the ice ages.
Ocean currents and upwelling
Upwelling: water from deeper layers
of ocean rise to the surface.
Typically
along eastern shores of oceans (western shores of continents) brings cold,
nutrient rich water up to the surface. Affects temperatures, stability, as well
as fish populations.
V El Nino / Southern Oscillation (ENSO)
What
is El Nino?
·
Originally
got the name El Nino for the
shifting current in December off the west coast of South America near Ecuador
and Peru. This is a local event that has
been known for many years.
·
Normally
the cold Peruvian current flows equatorward along the coast, and is associated
with upwelling of cold nutrient-rich
waters that feed the fishing industries (anchovies).
·
In
December, a warm current dominates for a few weeks.
·
Sometimes,
the warm current lasts for months, or even years. These are called El Nino periods.
How
does this warm current affect the fishing industry? How does it affect weather
patterns near the coast?
The Southern Oscillation
El
Nino is a local feature of a large scale circulation shift called the Southern
Oscillation.
Normal situation:
·
Usually,
the pressure over the western equatorial Pacific Ocean is lower than over the
eastern equatorial Pacific Ocean.
·
So,
what does this mean for winds? Ocean currents? Precipitation? Sea level?
During El Nino events:
·
A
shift occurs in the locations of the high and low pressure centers. The low
pressure center moves eastward, sometimes all the way to the eastern part of
the basin.
·
So,
what does this mean for winds? Ocean currents? Precipitation? Sea level?
ENSO teleconnections
Teleconnection a relationship between circulation patterns in one region and
weather in another region that is in a different part of the world
Why
do you think teleconnections exist?
ENSO
& North America: warm in northwest, wet along the central coast of
California, wet in southeast, dry in the northeast. Fewer Hurricanes in Atlantic Ocean.
What
does this have to do with the jet stream?
Other
teleconnections:
·
wet
over parts of Indian Ocean and eastern equatorial Africa
·
dry
over India, southeastern Africa, and northeastern South America
Two strongest El Ninos of
this century:
1982-83, and 1997-98.
Global precipitation
patterns
Annual precipitation
Why
are some regions relatively wet?
·
Tropics,
especially eastern portion of continents.
·
Western
coast in mid latitudes on the windward side of mountain ranges
Why
are some regions are dry?
Variability