Week8 AirMasses Front

7/29/2019 Week8 AirMasses Front

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Global Winds, Air Masses,and Fronts


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Westerly winds and the Jet Stream

Jet streams: rivers of fast-moving air 100s of miles long,several hundred miles wide, less than a mile thick.

Upper-level winds above the middle latitudes in bothhemispheres blow in a wavy west-to-east direction.

Aloft, we generally find higher pressure over equatorialregions and lower pressures over polar regions.

Jet streams are usually found at the tropopause, 10-14 km.

Wind speeds in jet streams are typically 100 knots, sometimes200 knots.

There are two jet streams in the northern hemisphere:The polar jet and the subtropical jet.


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Westerly winds and the Jet Stream


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6/27

During the summer, the Pacific high moves northward. Sinking

air along its eastern margin (over California) produces a strong

subsidence inversion, which causes relatively dry weather to

prevail. Along the western margin of the Bermuda high,southerly winds bring in humid air, which rises, condenses, and

produces abundant rainfall.


7/27

Polar and subtropical jet streams


8/27

Global Wind Patterns and

Ocean Currents

As the wind blows along the ocean, it causes the

surface water to drift along with it.

The moving water gradually piles up, creatingpressure differences within the water itself. This

leads to further motion of the water at greater depths.

Because of the larger frictional drag in water, ocean

currents move more slowly than wind.


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As winds blow parallel to the west coast of North

America, surface water is transported to the right

(out to sea). Cold water moves up from below

(upwells) to replace the surface water.

Winds and Upwelling


12/27

El Nino and the Southern Oscillation

Typically, over the eastern pacific off the Peruvian coast,southerly winds promote upwelling of cold, nutrient rich water.

This is followed by a few weeks of warm water moving south and

replacing the cold water. This reversal is called El Nino meaningboy child(Christ child) because it coincides with Christmas.

At times this warming of the ocean waters off of Peru lasts for

many months. When this occurs, this is considered a major

El Nino event.

Why does the ocean become so warm over the eastern Pacific?


13/27

Why does the ocean become so warm over the

eastern Pacific?

Normally, in the tropical Pacific, the trade winds are persistentand blow westward from a region of higher pressure over the

eastern Pacific toward a region of lower pressure centered near

Indonesia. The easterly trades create upwelling, moving the

surface water to the west. As the water moves westward,

it is heated by the sun.

In the Pacific Ocean, surface water along the equator is usually

cool in the east and warm in the west.

A break down in surface pressure patterns occurs every few

years. The pressure rises over the western Pacific and falls

over the eastern Pacific producing east winds that moves the

warmer waters to the east towards South America.


14/27

Why does the ocean become so warm over the

eastern Pacific?

Toward the end of this warming, the atmospheric pressure overthe eastern Pacific now begins to rise while it lowers in the

western Pacific. This see-saw pattern of reversing surface air

pressure at opposite ends of the Pacific Ocean is called the

Southern Oscillation.

Because the pressure reversals and ocean warming are more or

less simultaneous, scientists call this phenomenon:

The El Nino/Southern Oscillation orENSO.

Now if cooling occurs (a cold-water episode) this is termed:

La Nina


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(a) Average sea surface temperature

departures from normal as measured

by satellite. During El Nio conditions

upwelling is greatly diminished and

warmer than normal water (deep red

color) extends from the coast of

South America westward, across the

Pacific.

(b) During La Nia conditions, strong

trade winds promote upwelling, andcooler than normal water (dark blue

color) extends over the eastern and

central Pacific. (NOAA/PHEL/TAO)


17/27

During El Nio conditions, a

persistent trough of low pressureforms over the north Pacific and,

to the south of the low, the jet

stream (from off the Pacific)

steers wet weather and storms

into California and the southernpart of the United States.

During La Nia conditions, a

persistent high-pressure area formssouth of Alaska forcing the polar jet

stream and accompanying cold air

over much of western North

America. The southern branch of

the polar jet stream directs moistair from the ocean into the Pacific

Northwest, producing a wet winter

for that region.


18/27

Air Masses and Fronts


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21/27

An infrared satellite image that shows maritime tropical air (heavy yellow arrow)

moving into northern California on January 1, 1997. The warm, humid airflow

(sometimes called the pineapple express) produced heavy rain and extensive

flooding in northern and central California.


22/27

A front is the transition zone between two air masses ofdifferent densities. Thus, they separate air of different

temperatures and humidities too.

Upward extend of a front is referred to as a frontal surface

orfrontal zone.

Stationary front- has essentially no movement.

Cold front- represents a zone where cold, dry, stable polarair meets warm, moist, unstable subtropical air.

Fronts


23/27

Criteria used to locate a front on a surface weather map.

1. Sharp temperature changes over relatively short distance.

2. Changes in the airs moisture content (indicated by

changes in the dew point).

3. Shifts in wind direction.

4. Pressure and pressure changes

5. Clouds and precipitation patterns.

Fronts


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Surface weather map


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26/27

Vertical view of clouds, precipitation, winds

across the warm front


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Drylines are not warm fronts or cold fronts, but represent a narrow

boundary where there is a steep horizontal change in moisture as indicated

by a rapid change in dew-point temperature.

A dryline separates warm, moist maritime tropical (mT) on its eastern side

from hot dry continental tropical air (cT) on its western side

Week8 AirMasses Front

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