part 3. distribution and movement of air chapter 8 atmospheric circulation and pressure...
TRANSCRIPT
Part 3. Distribution and Movement of Air
Chapter 8
Atmospheric Circulation and Pressure Distributions
Planetary WindsWell-defined pressure patterns exist across the Earth that induce the global wind patterns on the planet
Idealized Single-Cell Convection Model for a Planet
The ITCZ is a band of clouds across the tropics
The Three-Cell Model
Intertropical convergence zone (ITCZ) -- surface low pressure with clouds and rain
Subtropical high -- Air subsides (dry climate)
Hadley cell -- tropical convection cell
Ferrel cell -- southwesterly winds at surface
Polar cell -- northeasterly winds at surface
The three-celled model vs. reality:
• Hadley cells are close approximations of real world equatorial winds
• Ferrel and polar cells do not approximate the real world winds very well at all
• Model is unrepresentative of westerly flow aloft
• Continents and topographic irregularities cause significant variations in real world wind patterns compared to the model
Semi-Permanent Pressure Cells are large areas of higher or lower atmospheric pressure than the surface average
They may be thermally induced (rising warm air or subsiding cold air) or they may be caused dynamically by converging or diverging wind patterns)
They fluctuate seasonally
Northern hemisphere semi-permanent cellsThe Aleutian, Icelandic, and Tibetan lows
Siberian, Hawaiian, and Bermuda-Azores highs
ITCZ (low)
Average atmospheric air pressure and wind patterns in January
Average atmospheric air pressure and wind patterns in July
The Sahelreflects seasonal migrationof the ITCZ
The Sahel is rainy during northern hemisphere summer and dry during northern hemisphere winter as the ITCZ shifts north and south through the year. It is the reason for annual flooding of the Nile River.
Shifts in the ITCZ affect the Sahel
Mid-latitude winds in the middle and upper troposphere are controlled b the pressure gradient force and the Coriolis force, giving the westerly winds
Stronger pressure gradients in winter give stronger westerlies
Westerly winds in the upper atmosphere• Air motions directed towards poles • Redirected by Coriolis deflection
– Westerly winds aloft result
The polar front and jet streams• Fast stream of air in upper troposphere
– Above polar front– Stronger in winter
Polar jet profile
The fastest middle and upper troposphere mid-latitude winds (the jet stream) are at polar front. There is a sharp temperature contrast and horizontal pgf to the south and north of the polar front.
300 mb
500 mb
800 mb
Strongest pgf here
The subtropical jet transports low latitude moisture and energy
Subtropical front and jet
Ridges and trough profileRidge Ridge
Trough
A trough over the mid-continent in (b)
Rossby waves are the wave-like pattern of ridges and troughs in the upper troposphere winds. Ridges and troughs (Rossby waves) will migrate either east or west with time
Sequence of Rossby wave migration -- the dashed line shows the migration and evolution of a trough with time across the country
Ocean currents are driven by wind stresses and are deflected by the Coriolis force. Thus, the water moves at about a 45° angle to the winds. The diection of movement of the ocean changes with depth following a pattern called the Ekman
spiral.
• Western basins usually experience warm ocean currents
• Eastern basins usually experience cold ocean currents
Upwelling in along-shore winds• Cold waters rise due to Ekman transport
Global Ocean Circulation
Infrared Satellite Image of the Gulf Stream
Major Wind Systems of the EarthMonsoons• Thermal induced seasonal wind patterns
associated with shifts of the ITCZ
• Monsoons are characterized by dry offshore winter flow and wet onshore summer flow
• The monsoon in East Asia experiences orographic enhancement
Winter monsoon
Summermonsoon
Topography enhances monsoonal effects
Foehn winds are strong, downslope winds that adiabiatically compress, raising the air temperature. Foehn winds are associated with hot, dry, clear weather
Chinook winds are foehn winds along the east slope of the Rockies (“snow eaters”)
Santa Ana winds are foehn winds that blow from the deserts and over the mountains into the valleys of southern and central California
Katabatic winds are cold, dense winds that flow down mountain slope. They warm as they descend, but they are still colder than the surrounding air.
• Boras and mistral winds are forms of katabatic winds in Europe
Southern California Santa Ana induced fires
Sea and land breezes form due to temperature differences over land and sea. Sea breezes form during the day, and land breezes form at night.
Valley and mountain breezes form due to heating and cooling on mountain sides. Valley breezes form during the day, and mountain breezes form at night (similar to katabatic winds)
Sea breezedevelopment
Sea breeze-initiated clouds over Hawaii
Valley and mountain breeze development
Air-Sea Interactions in the Equatorial PacificEl Niño, La Niña, and the Walker circulation• El Niño events
– Unusually warm water in the eastern equatorial Pacific Ocean
– Linked to global weather anomalies – 2 to 5 year recurrence
• La Niña events -- wind and temperature patterns reversed of El Niño patterns
• Walker circulation – Vertical and horizontal tropospheric flow in
the equatorial Pacific that controls areas of heavy rainfall
The “Normal” Walker Circulation (no El Niño conditions)
ENSO (El Niño/Southern Oscillation) events -- oscillations between El Niño and La Niña conditions• ENSO results in global teleconnection
patterns (weather effects far from the equatorial Pacific)
El Niño SST Anomalies
La Niña SST Anomalies
The Southern Oscillation Index
The PDO Index
Warmer waters in western tropical Pacific
Cooler waters in western tropical Pacific