Download Understanding Weather and Climate Ch 8

How Does Air Move Around the Globe?
Review of last lecture
• Know 3 Forces that affect wind speed /direction
• Especially work on Coriolis force, as this is the hardest
to understand. Which direction is air deflected to by
Coriolis force?
• What is the geostrophic balance? At which level is it
valid? Difference between upper level and surface
winds
• Troughs, ridges, cyclones and anticyclones. Do they
correspond to high or low surface pressure? Is the air
moving clockwise or counter-clockwise around them?
The most common atmospheric
circulation structure
CE
H
L
Cooling
or No
Heating
Heating
Friction
H
L
CE
Imbalance of heating
 Imbalance of temperature
 Imbalance of pressure
 Wind
Introduction
• Well-defined heating, temperature and pressure patterns exist
across the globe
• These define the general circulation of the planet
• In describing wind motions:
– Zonal winds (east-west): flow parallel to lines of latitude
• Flowing eastward: Westerly wind
• Flowing westward: Easterly wind
– Meridional winds (north-south): flow parallel to lines of
longitude
• Flowing northward: Southerly wind
• Flowing southward: Northerly wind
Annual mean precipitation (heating)
Extratropical stormtrack
Tropical rainfall
Extratropical stormtrack
Primary Highs and Lows
Equatorial low
Subtropical high
Subpolar low
Polar high
Three-cell model
Zonal mean circulation
Each hemisphere is
divided into 3 distinct
cells:
Hadley Cell
Ferrel Cell
Polar Cell
Vertical structure and mechanisms
Polar Cell (thermal):
Driven by heating at
50 degree latitude
and cooling at the
poles
Ferrel Cell (dynamical):
Dynamical response to
Hadley and polar cells
Polar
Hadley
Hadley Cell (thermal):
Heating in tropics  forms
surface low and upper
level high  air converges
equatorward at surface,
rises, and diverges
poleward aloft 
descends in the subtropics
Zonal mean structure of temperature
Two characteristics:
•
Horizontally uniform
in the tropics
•
Steep gradient in
the extratropics
Zonal mean structure of zonal wind
Two characteristics:
•
Westerly winds in
the extratropical
troposphere
•
Jet streams: local
maximum of winds
Westerly winds in the extratropical troposphere
• The existence of the upper level
pressure gradient  air is being pushed
toward poles  Coriolis effect deflects
upper air (to the right)  Westerlies
dominate upper troposphere
• Strongest during winter  thermal
gradient is large
• Explains why storms move eastward,
flight times
The Jet Streams
• Caused by steep temperature
gradients between cold and
warm air masses
• Polar front - marks area of
contact, steep pressure
gradient  polar jet stream
• Low latitudes  subtropical
jet stream
• Stronger in winter, affect daily
weather patterns
A Jet Stream seen from satellite
The subtropical jet is seen as a band of clouds extending
from Mexico on an infrared satellite image
Video: The jet streams
Semipermanent Pressure Cells
• Instead of cohesive pressure belts circling the Earth,
semipermanent cells of high and low pressure exist; fluctuating
in strength and position on a seasonal basis.
• These cells are either dynamically or thermally created.
• Sinking motions associated with the subtropical highs promote
desert conditions across specific latitudes.
• Seasonal fluxes in the pressure belts relate to the migrating Sun
(solar declination).
South Pacific
high
South Atlantic
high
For NH winter:
1. Aleutian and Icelandic lows
2. Siberian and Bermuda-Azores highs
3. South Pacific, Atlantic, Indian highs
South Indian
high
South Pacific
high
South Atlantic
high
For NH summer:
1. Tibetan low
2. Hawaiian and Bermuda-Azores highs
3. South Pacific, Atlantic, Indian highs
South Indian
high
Low pressure: clouds and precipitation
Extratropical stormtrack
Tropical rainfall
Extratropical stormtrack
High pressure: warm surface temperature,
drought and desert
Global distribution of deserts (all near high pressure cells)
General circulation of the oceans
• Ocean surface currents – horizontal water motions
• Transfer energy and influence overlying atmosphere
• Surface currents result from frictional drag caused by
wind - Ekman Spiral
• Water moves at a 45o angle (right)
in N.H. to prevailing wind direction
• Due to influence of Coriolis effect
• Greater angle at depth
Global surface currents
• Surface currents mainly driven by surface winds
• North/ South Equatorial Currents pile water westward, create the Equatorial
Countercurrent
• western ocean basins –warm poleward moving currents (example: Gulf Stream)
• eastern basins –cold currents, directed equatorward
Summary
• Three precipitation (heating) belts. Primary high and
lows
• Three-cell model. Mechanism for each cell
• Two characteristics of zonal mean temperature structure
• Two characteristics of zonal mean wind structure. Why
does westerly winds prevail in the extratropical
troposphere? What cause the jet streams?
• Semipermanent pressure cells. Low pressure is
associated with clouds and precipitation. High pressure
is associated with warm surface temperature, drought,
and desert.
• What drives the ocean surface currents? In the case of
Ekman spiral, what is the direction of surface current
relative to surface wind?
Works cited
Images
•http://pulleysandgears.weebly.com/gears.html
•http://visual.merriamwebster.com/earth/meteorology/meteorological-measuringinstruments/measure-wind-direction.php