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MET 10 Lecture 9
Global Winds, Air Masses,
and Fronts
Chapter 8 & 9
Dr. Craig Clements
San Jose State University
Westerly winds and the Jet Stream
Jet streams: rivers of fast-moving air 100’s of miles long,
several hundred miles wide, less than a mile thick.
Upper-level winds above the middle latitudes in both
hemispheres blow in a wavy west-to-east direction.
Aloft, we generally find higher pressure over equatorial
regions 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, sometimes
200 knots.
There are two jet streams in the northern hemisphere:
The polar jet and the subtropical jet.
Westerly winds and the Jet Stream
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.
Polar and subtropical jet streams
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, creating
pressure 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.
Ocean Temperatures
Winds and Upwelling
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.
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 meaning
boy 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?
Why does the ocean become so warm over the
eastern Pacific?
Normally, in the tropical Pacific, the trade winds are persistent
and 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.
Why does the ocean become so warm over the
eastern Pacific?
Toward the end of this warming, the atmospheric pressure over
the 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 or ENSO.
Now if cooling occurs (a cold-water episode) this is termed:
La Nina
(a) Average sea surface temperature
departures from normal as measured
by satellite. During El Niño 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 Niña conditions, strong
trade winds promote upwelling, and
cooler than normal water (dark blue
color) extends over the eastern and
central Pacific. (NOAA/PHEL/TAO)
During El Niño conditions, a
persistent trough of low pressure
forms 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 southern
part of the United States.
During La Niña conditions, a
persistent high-pressure area forms
south 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 moist
air from the ocean into the Pacific
Northwest, producing a wet winter
for that region.
Air Masses and Fronts
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.
Fronts
A front is the transition zone between two air masses of
different densities. Thus, they separate air of different
temperatures and humidities too.
Upward extend of a front is referred to as a frontal surface
or frontal zone.
Stationary front- has essentially no movement.
Cold front- represents a zone where cold, dry, stable polar
air meets warm, moist, unstable subtropical air.
Fronts
Criteria used to locate a front on a surface weather map.
1. Sharp temperature changes over relatively short distance.
2. Changes in the air’s moisture content (indicated by
changes in the dew point).
3. Shifts in wind direction.
4. Pressure and pressure changes
5. Clouds and precipitation patterns.
Surface weather map
Vertical view of clouds, precipitation, winds
across the warm front
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.