Download Document

Chapter 7
Global-Scale Winds
Figure CO: Chapter 7, Global-Scale Winds--Jet stream from space
Image courtesy of the Image Science & Analysis Laboratory, NASA
Johnson Space Center.
Sailors Understood Global
Winds
• Trade winds off the Atlantic coast of North
Africa blow steadily from the northeast
• Farther north along the coast of Europe
the winds typically blow from west to east
• At about 20°S the trade winds blow
steadily from the southeast
• Winds in the horse latitudes (near 30°N
and 30°S) are usually light or calm
Figure 01: Columbus’ route
Figure 02: The route of the HMS Beagle.
What a description of global winds should
explain
• Steady and calm winds observed by
mariners
• Seasonal patterns of precipitation around
the world
• Seasonal patterns of cloudiness around
the world
• The relationships between average wind
patterns and pressure patterns and
upward and downward air motions
6
• The jet streams
Figure 05A: Climatology of precipitation (January).
Source: MATLAB
Figure 05B: Climatology of precipitation (July).
Source: MATLAB
The surface winds over Earth
• Are very complicated because of the
changing seasons, differences between
land and water, and differences in latitude.
• Can be simplified using a conceptual
model.
• Have been described using a 3-cell model
with no land and no seasons. Only
temperature differences from equator to
pole are included.
9
Figure 06: Conceptual model of global-scale winds.
The Conceptual Model
• Begins with cloud band that nearly
encircles the tropics
• These convective clouds are nearly
always present and require upward motion
• Rising air explains cloudiness and
precipitation in the tropics
• Aloft, the rising air spreads out to the north
and south, and the Coriolis force deflects it
towards the east
Figure 07: Subtropical jet stream schematic.
Figure 08: The conservation of angular momentum
Figure 09: Satellite image of subtropical jet stream
Courtesy of CIMSS, University of Wisconsin-Madison clouds.
The Conceptual Model
Continues Aloft
• The air aloft accelerates as it approaches
30° latitude and forms the west-to-east
subtropical jet stream
• Converging air near latitude 30° sinks
toward the surface where there is a region
of high pressure, light winds, and a
minimum of cloudiness and precipitation
• The subsiding air spreads out to the north
and south
The Conceptual Model Continues
at the Surface
• Air flowing towards the equator near the
surface away from the region of high
pressure and light winds is deflected by the
Coriolis Force
• This air becomes the trade winds
• The trade winds of the NH and SH come
together in the Intertropical Convergence
Zone (ITCZ)
• This circulation is called the Hadley cell
• The ITCZ is also known as the doldrums, for
the light winds often observed there
Figure B01A: Full-disk satellite image with labels
Courtesy of SSEC and CIMSS, University of Wisconsin-Madison
Figure B01B: Temperature vs elevation
Figure 10: Satellite image of clouds in equatorial Pacific
Courtesy of Earth Observatory/NASA
The Conceptual Model near the
Poles
• At the poles more energy is lost to space
than gained from the sun
• The air there cools and sinks, warming
adiabatically and creating an inversion that
inhibits cloudiness and precipitation
• At the ground, the sinking air moves outward
from the poles, is deflected by the Coriolis
force, and becomes the polar easterlies
• Eventually the air rises to complete the
circulation of the polar cell
The Conceptual Model in Mid
Latitudes
• On the poleward sides of the descending
branches of the Hadley cells, surface air
moves poleward
• The Coriolis forces deflects this air to form
the midlatitude westerlies
• The midlatitude westerlies encounter the
polar easterlies at about latitude 60°, a clash
of air masses called the polar front
• The temperature gradient causes the
westerly polar front jet
Figure 7.11ab:
Sea-level
pressure maps
for a typical
January and July
Figure 7.11cd
Wind maps for a
typical January
and July
Figure 12: Subtropical and polar jet stream
Figure 7.13: The approximate positions of the polar front jet
stream and the subtropical jet stream over the Northern
Hemisphere during winter.
Modified from S. Lee and H.-K. Kim, J. Atmos. Sci 60 [2003]: 1490–1503.
Upper-Air Midlatitude Westerlies
• The jet streams meander like rivers,
producing a wave-like pattern of troughs and
ridges
• The air flow through these waves results in
storms that move warm air poleward and cold
air toward the equator.
• Each trough-ridge pattern is called a Rossby
wave
• Rossby waves drift slowly eastward, with
rising air near the troughs and sinking air
near the ridges
Figure 14: 500 mb maps for Jan and July
Waves
• Waves are described by their wavelength
(distance between successive troughs or
ridges) and amplitude (north-south extent)
• Amplitude and wavelength determine the
type of weather associated with the waves
Names for Upper-level Wind
Patterns
• When the waves are small, and the ridges
and troughs are weak, the pattern is called
zonal, or high index, meaning roughly
west to east at constant latitude.
• When the waves have greater amplitude
(north-south dimension), the pattern is
called meridional, or low index, meaning
that there is a lot of north/south motion.
Figure 15: Zonal, meridional, and split flows
More about upper-level patterns
• Sometimes there is zonal flow at high
latitudes and meridional flow at low
latitudes. This is a split-flow pattern.
• Sometimes persistent closed highs and
lows form in a split-flow pattern when the
meridional pattern is extremely meridional.
This is called a blocking pattern,
because it can be extremely persistent.
Figure 7.16: Normal (a) and “blocking” (b) wind patterns
above North America in summer
Modified from “Written in the Winds: The Great Drought of ’88.” J. Namias,
Weatherwise, Jan. 4 1989, vol. 42, pp. 85–87. Reprinted by permission of the
publisher [Taylor & Francis Group, http://www.informaworld.com]
Implications of Upper-level
Winds
• Blocking highs can lead to drought
conditions and prolonged heat waves.
• Meridional flow accomplishes poleward
energy transport that helps balance the
energy balance of the Earth and
atmosphere.
Figure 17a: (a) Anomalies in 500-mb heights in the
vicinity of North America during December 2009 to
February 2010.
Courtesy of ESRL Physical Science Division/NOAA.
Figure 17b: Actual 500-mb heights in the vicinity of North
America during December 2009 to February 2010.
Courtesy of ESRL Physical Science Division/NOAA.
More about Waves in the
Westerlies Aloft
• Shorter waves move eastward faster than
the longer Rossby waves.
• Waves of different length can add and
subtract to/from one another’s amplitude.
• Forecasting waves in the westerlies aloft is
vital for everyday weather forecasting.
• El Niño/La Niña affect the westerlies aloft.
Figure 18: Short waves
The Poleward Transport of
Energy
• The poleward transport of energy is a
feature of the conceptual model
• Without this transport of energy the poles
would be much colder and the tropics
much warmer
• Poleward energy transport is
accomplished by both the atmosphere and
the oceans
• The region of maximum energy transport
lies between latitudes 30° and 60°
Figure 19: Heat transport by atmosphere and ocean
Seasonal Shifts and Monsoons
• The ITCZ, the subtropical highs, and the
polar front all shift southward in NH winter
and northward in NH summer.
• Seasonal shifts are most intense over
Asia, which has the largest continental air
mass.
• The summer monsoon is wet, with low
pressure over land; the winter monsoon is
dry, with high pressure over land.
Figure 20: Location of ITCZ in January and July
More Seasonal Shifts
• The polar jet stream is displaced further
poleward in summer than in winter
• During summer the positions of the
subtropical highs shift poleward
• The polar jet stream is weaker in summer
• The subtropical jet stream is weaker in
summer
• Lows associated with the ITCZ shift
seasonally as do the lows associated with
the polar front
Thermal Lows
• Lows associated with deserts are called
heat lows or thermal lows
• They develop because of intense surface
heating
• They develop at about the same latitude
as the subtropical highs
• They occur in the southwest U.S. in
summer, and the vicinity of Iraq
Monsoons
• Monsoons are weather features driven by
seasonal differences in the heating of land
and ocean along with seasonal shifts in
global-scale circulations.
– Monsoons are not present in the three-cell model
– Indian summer monsoon has cooler air over
water, heated air over land, upslope onshore
wind and generation of clouds and precipitation—
wet season
– Indian summer monsoon has return flow aloft
from land to water and sinking air over the
Arabian Sea and the Bay of Bengal
Figure 21: Summer Indian monsoon circulation
schematic.
Figure 22: Monsoon winds in summer and winter.
The Winter Monsoon
• During autumn and winter, air above land
cools faster than over the water,
establishing a PGF from land to water.
• The winds are reversed from the summer
monsoon flow—at the surface from land to
sea
• Sinking air above the land suppresses
cloud development and precipitation.
• Winter monsoon is a dry season
Unsolved Problems for
Research
• What determines the location of the ITCZ?
– The ITCZ has been moving northward at a rate of
about 1.4 km per year.
• What controls the poleward extent of the Hadley
cell?
• What causes the summertime subtropical highs in
the Northern Hemisphere?
– Why is it that the subtropical highs of the Northern
Hemisphere are strongest when the sinking branch of
the Hadley cell is at its weakest?
• What controls the locations of the jet streams?
Figure 23: Computer simulations of the location of the
polar and subtropical jet streams for varying amounts of
Modified from S. Lee and H.-K. Kim, J. Atmos.
Sci. 60
tropical
convection.
[2003]: 1490–1503.
Figure B02: Air over a mountain