Download The jet stream - Atmospheric and Oceanic Sciences

AOS 101 Weather and Climate
Lisha M. Roubert
University of Wisconsin-Madison
Department of Atmospheric & Oceanic Sciences
The Jet Stream
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Jet streams are relatively narrow bands of strong wind
in the upper levels of the atmosphere (troposphere)
that generally blow from west to east at an altitude of
20,000 - 50,000 feet .
Surface temperatures determine where
the jet stream will form. The greater the difference in
temperature, the faster the wind velocity inside
the jet stream. This abrupt change in temperature
causes a large pressure difference, which forces the air
to move. Jet streams can flow up to 200 mph (322
km/h), are 1000's of miles long, 100's of miles wide,
and a few miles thick.
Jet streams follow the boundaries between hot and cold
air. Since these hot and cold air boundaries are most
pronounced in winter, jet streams are the strongest for
both the northern and southern hemisphere winters.
Clouds along a jet stream over
Canada.
Why is it important to know about Jet
Streams?
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Meteorologists use the location of
some of the jet streams as an aid
in weather forecasting.
The main commercial relevance
of the jet streams is in air travel,
as flight time can be dramatically
affected by either flying with the
flow or against the flow of a jet
stream. Clear-air turbulence is
often is found in a jet stream's
vicinity.
Jet streams flow from west to
east in the upper portion of the
troposphere.
What causes Jet Streams?
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Jet streams are caused by a combination of a
planet's rotation on its axis and atmospheric
heating (by solar radiation on Earth and on
other planets by internal heat).
The jet stream results from latitudinally and
vertically large gradients of temperature and
pressure at the intersection of a colder air
mass from the north with a warmer air mass
from the south.
The jet tends to occur at about the height of
the tropopause, the transition from the
troposphere (where temperature decreases
with height) to the stratosphere (where
temperature increases with height). The
combination of these differences in air mass,
plus some help from the coriolis force (coriolis
acceleration) creates the acceleration of winds
into the jet stream.
The Polar and Subtropical Jet Stream
1. Polar Jet Stream
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Cold polar air flowing down from the north
meets the warmer air mass over the United
States causing the polar jet stream to form.
2. Subtropical Jet Stream (stronger in the winter)
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The ascending air in the equator flows
towards the poles after reaching the
tropopause. It is deflected by coriolis force
during it’s path towards the poles and
becomes a westerly wind. To conserve angular
momentum the air blows rapidly from west to
east creating the subtropical jet stream. The
Subtropical Jet stream is formed primarily
because of conservation of angular
momentum.
The Polar and Subtropical Jet Stream
Wind Speeds in the Jet Stream
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Within the jet stream, currents travel at varying
speeds but are greatest at the core.
Jet streaks are areas inside the
jet stream where the velocity is higher than the
rest of the stream. Jet streaks cause air to rise,
which lowers pressure at the surface. When
surface low pressures form the rising air can
cause clouds, precipitation and storms.
The jet stream can also contain wind shear, a
violent and sudden change in wind direction
and speed.
Wind shear can occur outside the jet stream as
well, even at the surface. When vertical winds
blast downward it can cause an airplane that is
in the process of take off to suddenly lose
altitude and potentially crash. For this reason
all commercial planes in the U.S. since 1996
have been equipped with windshear warning
systems.
Jet Streams on Weather Maps
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The color filled regions indicate wind speed in knots.
The shades of blue indicate winds less than 60
knots, while winds greater than 120 knots are given
in shades of red.
Displacement of the Jet Stream
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Areas of high and low pressure act like a moving riverbed, buckling and
snaking the path of the jetstream as it flows to the east.
Winds flow clockwise around
areas of High Pressure
H
Winds flow counter-clockwise
around areas of Low Pressure
L
Displacement of the Jet Stream
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At times, the polar jet stream may dip further south into the U.S., bringing
cold weather with it. At other times it retreats into Canada, leaving milder
weather in the U.S.
Fig. 1. Average jet stream winds (250 mb,
~25,000 feet) during Monsoon Season 2009.
Fig. 2. Average jet stream winds (250 mb, ~25,000
feet) difference from climatology during Monsoon
Season 2009. The jet stream was displaced to the
south from its typical location due to El Niño
development.
Displacement of the Jet Stream
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El Niño and La Niña
cause changes in the
distribution of Low
and High Pressure
regions. For this
reason El Niño and La
Niña cause shifts of
the Jet Stream.
The Pacific/North American Teleconnection
Pattern (PNA) and the Jet Stream
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The Pacific/ North American teleconnection pattern (PNA) is one of the most
prominent modes of low-frequency variability in the Northern Hemisphere
extratropics.
The PNA pattern is associated with strong fluctuations in the strength and
location of the East Asian jet stream.
 Positive phase →associated with an enhanced East Asian jet stream and
with an eastward shift in the jet exit region toward the western United
States.
 Negative phase→associated with a westward retraction of that jet
stream toward eastern Asia, blocking activity over the high latitudes of
the North pacific, and a strong split-flow configuration over the central
North Pacific.
PNA Negative and Positive Phases
Negative Phase
Positive Phase
Higher pressure
Lower pressure
•Stronger high / low pressure systems
•Jet stream and region of storm
formation shift eastward
Winds
•Weaker high / low pressure systems
•Jet stream and region of storm
formation shifts west toward central
Pacific.
Surface Air Pressure and Jet Stream Showing
Blocking During Negative Phase of PNA Pattern
H
L
COLD
H
Negative phase of PNA pattern favors blocking and strong cold-air outbreaks
into western North America.
Homework
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1)
2)
3)
Go to the following website
http://www.nytimes.com/2010/04/16/opinion/16winches
ter.html?_r=1 and read the article : “A Tale of Two
Volcanoes”. Answer the following questions based on the
article:
What impact did the eruption of Krakatoa have in the
field of atmospheric sciences?
Explain how the eruption of Krakatoa contributed to this
discovery.
If Krakatoa had never erupted, do you think we would
have been able to make the discovery of what you
indicated in question 1 eventually? Through which
methods and/or instrumentation do you think we would
have been able to prove it’s existence?