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.