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Atmospheric Pressure & Wind: Part 4
Synoptic scale winds
Mesoscale winds
Microscale winds
Air-sea interactions
Lecture 13
March 24, 2010, Wednesday
The largest synoptic scale wind.
Monsoon: Arabic for “season”.
Often incorrectly referred to only
as “heavy rainy summer season”.
Actually refer to the climatic
pattern of seasonal reversal of
winds due to seasonal thermal
differences between large
continents and large water bodies
(high and low pressure cells), and
heavy precipitation alternates with
dry conditions on an annual basis.
Best exemplified by the Asian
monsoon which is characterized by
alternating offshore (onshore) flow
with dry (wet) conditions during
cool (warm) months
Monsoons
Asian MonsoonsWinter: northeast winds from
Himalayas are compressed,
warmed and dry, these offshore
winds are maintained by the
subsidence due to convergence
of subtropical jet streams in the
upper air.
Summer: heating of continent leads
to convergence at the surface and
onshore winds which draw moist air
northward, with strong orographic
lifting, heavy rain falls over south of
the continent.
Monsoon Low
(Depression)
Arizona (Southwest, Mexican) MonsoonLittle like the Asian Monsoon: much less rainfall amount due
to scattered thunderstorms (dessert climate), no west-east
tall mountain barrier, smaller continent of North America
Chinook (Rockies), Foehn (Alps), and Santa Ana
(California) winds: Synoptic Scale Winds
Leeward side
Sinking air is heated
by compression
Foehn: mid-latitude cyclones pass the Alps from southwest
Chinooks: similar winds on the eastern side of the Rocky
Mountains, form when low pressure systems occur east of the
mountains
Both Foehn and Chinook winds are most common in winter
Santa Ana winds: occur during transitional seasons (most
common in the Fall, but also in spring), when high pressure
develops over the Rockies, descending air down the western
slopes, often spread wildfires
As a consequence of the different adiabatic lapse rates of moist and dry air, the air on the leeward slopes becomes warmer than equivalent elevations on the windward slopes. Föhn winds can raise temperatures by as much as 30°C (54°F) in just a matter of hours. Central Europe enjoys a warmer climate due to the Föhn.
A foehn wind or föhn wind is a type of dry down slope wind which occurs in the lee of a mountain range. It is a rain shadow wind which results from the subsequent adiabatic warming of air which has dropped most of its moisture on windward slopes.
Adiabatic warming of downward moving air produces the warm Chinook wind
The Santa Ana winds in Southern California sweep down across the deserts and across the Los Angeles Basin pushing dust and smoke from wildfires far out into the Pacific Ocean.
(Synoptic to Mesoscale)
Locally chilled cold dense air in highland areas (such as Antarctic and
Greenland ice sheets) “falls” to the lower elevations
Katabatic wind in Antarctica Katabatic wind spilling off an ice shelf
Coastal polynyas are produced in the Antarctic by katabatic winds
Sea and Land Breezes
Daily scale temperature gradient
Sea breeze
land warms: thermal low
Land breeze
land cools: thermal high
In principle, lake breeze is similar to sea breeze
A sea-breeze front is a weather front created by a sea-breeze, as a convergence zone. The cold air from the sea meets the warmer air from the land and creates a boundary like a shallow cold front. When powerful this front creates cumulus clouds, and if the air is humid and unstable, cumulonimbus clouds, the front can sometimes trigger thunderstorms.
Sea breezes
moderate
coastal
temperatures
Sea breezes
begin around
noon and reach
10-20 kph in
mid-afternoon
Sea breezes
have the largest
influence in the
tropics near
cold ocean
currents
Sea and Land Breezes
Daytime: air is heated more over mountain slopes than valley
floor (solar angle), air glides up mountain slope: valley breeze
Mountain and Valley Breezes (Mesoscale)
Nighttime: air is cooled more on
the slopes than over valley floor,
dense cold air drains down the
slope: mountain breeze
Country Breeze (Mesoscale)
country country
Heat Island
Cool Cool
Major Wind Systems: Microscales
Dust Devils
A dust devil is a strong, well-formed, and relatively long-lived whirlwind, ranging from small (half a meter wide and a few meters tall) to large (over 10 meters wide and over 1000 meters tall). The primary vertical motion is upward. Dust devils are usually harmless, but rare ones can grow large enough to threaten both people and property.
They are comparable to tornadoes in that both are a weather phenomenon of a vertically oriented rotating column of air. Most tornadoes are associated with a larger parent circulation, the mesocyclone on the back of a supercell thunderstorm. Dust devils form as a swirling updraft under sunny conditions during fair weather, rarely coming close to the intensity of a tornado.
Microscale Winds
Turbulence, dust devils, gusts
Dust devils form on clear days
Tornadoes form in thunderstorms
Dust devils develop from ground up
Tornadoes develop from cloud down
Dust devils rarely reach wind speeds
of 60 mph, and most are short-lived
(few minutes) and reach heights of
only 100 meters
Tornadoes generate winds as high as
280 mph
Dust Devil vs. Tornado (Mesoscale)
Near-Surface
Ocean
Circulation
Gyers
Gulf
stream
Ekman
spiral
Upwelling
Air-Sea
Interactions
El Nino
La Nina
ENSO
U.S.
SST: Sea Surface Temperature
During 1997-1998 El Nino
The Gulf Stream
Satellite Image
Reds-oranges:
25-29 oC
Yellows-greens:
17-24 oC
Blues:
10-16 oC
Purples:
2-9 oC
Gyre
Gyre
Near-Surface Ocean Currents
cold currents - west coasts of continents
warm currents - east coasts of continents
Ekman spiral
Surface ocean currents driven by wind stress (drag force) which is
deflected by Coriolis force, surface ocean water moves at a 45o angle to
the right (left) in the northern (southern) hemisphere of the surface wind
Current speed decreases & direction turns increasingly towards right
(left) with depth in N.H. (S.H.), 180o at ~ 100 m depth, current dies out.
Northern hemisphere
Southern
hemisphere
100 m
South America
coldwarm
thermocline
Upwelling
Strong winds drag surface waters away from coastal locations.
Colder, nutrient rich waters from the deep ocean rise, or
upwell, to replace these waters.
Most pronounced off western coast of South America as cold
water upwelling ensures the driest desert on Earth, the Atacama.
Southern California, hot & dry Santa Ana winds bring people
to the beach but cold water from upwelling keeps people out of
water.
Vertical profile of ocean water temperature
Mixing zone: also called
surface layer, driven by
winds, warmest (by the Sun)
Thermocline transition
zone: thermocline layer,
friction & viscosity dampens
mixing effect temperature
rapidly decreases with
increasing depth greatest
vertical temperature
gradient
Deep cold zone: or deep
water, dark and high
pressure make it hard to
study it, freezes at ~ – 2 oC
Upper Air Circulation at the EquatorEasterly Westerly
Pacific Ocean
El Nino
Spanish for boy child, it tends to occur near Christmas season, so named in reference to the Christ child.
In normal condition, trade winds move warm ocean surface waters near the equator westward, as does the Walker Circulation at equator.
western Pacific: higher temperature and sea level, lower surface pressure, increase in convective precipitation
eastern Pacific: upwelling of cold ocean water, cooler air temperature, higher surface pressure, dry condition.
El Nino develops when trade winds weaken or even reverse direction to flow eastward, and warm water moves eastward.
unusual warm ocean surface water in eastern equatorial Pacific Ocean.
El Nino
Air-Sea Interactions
Normal
El Nino
Walker Circulation
El Nino
Walker Circulation
La Nina Conditions
Sea surface skin temperature anomalies in November 2007 showing La Niña conditions
Southern Oscillation:
pressure pattern seesaw in tropical Pacific
winds shift from easterly to westerly
irregular occurrence 2-5 years
El Nino & Southern Oscillation are linked ENSO
La Nina (Spanish for girl child): opposite to El Nino,
strong easterly trade wind, unusually cold ocean
surface water in eastern Pacific along the coast of
South America (very dry winter).
Southern Oscillation, ENSO, La Nina
Air-Sea Interactions
El Nino
Across North America during La Niña, increased precipitation is diverted into the Pacific Northwest due to a more northerly storm track and jet stream. The storm track shifts far enough northward to bring wetter than normal conditions (in the form of increased snowfall) to the Midwestern states, as well as hot and dry summers. Across the North Atlantic, the jet stream is stronger than normal, which directs stronger systems with increased precipitation towards Europe.
During El Niño, increased precipitation in California due to a more southerly, zonal, storm track. Increased precipitation falls along the Gulf coast and Southeast due to a stronger than normal, and more southerly, polar jet stream. The subtropical jet stream across the deep tropics of the Northern Hemisphere is enhanced due to increased convection in the equatorial Pacific, which decreases tropical cyclogenesis within the Atlantic tropics below what is normal, and increases tropical cyclone activity across the eastern Pacific.