how does air move around the globe?. review of last lecture know 3 forces that affect wind speed...
TRANSCRIPT
Review of last lectureReview 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 The most common atmospheric circulation structurecirculation structure
L
H
H
L
HeatingCoolingor No Heating
Imbalance of heating Imbalance of temperature Imbalance of pressure Wind
CE
CE
Friction
IntroductionIntroduction• 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)Annual mean precipitation (heating)
Extratropical stormtrack
Tropical rainfall
Extratropical stormtrack
Primary Highs and LowsPrimary Highs and Lows
Equatorial low
Subtropical high
Subpolar low
Polar high
Zonal mean circulation
Each hemisphere is divided into 3 distinct cells:
Hadley Cell
Ferrel Cell
Polar Cell
Three-cell modelThree-cell model
Vertical structure and mechanismsVertical structure and mechanisms
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
Ferrel Cell (dynamical): Dynamical response to Hadley and polar cells
Polar Cell (thermal): Driven by heating at 50 degree latitude and cooling at the poles
HadleyPolar
Zonal mean structure of temperatureZonal mean structure of temperature
Two characteristics:
• Horizontally uniform in the tropics
• Steep gradient in the extratropics
Zonal mean structure of zonal windZonal mean structure of zonal wind
Two characteristics:
• Westerly winds in the extratropical troposphere
• Jet streams: local maximum of winds
• 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
Westerly winds in the extratropical troposphereWesterly winds in the extratropical troposphere
• 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
The Jet StreamsThe Jet Streams
The subtropical jet is seen as a band of clouds extending from Mexico on an infrared satellite image
A Jet Stream seen from satelliteA Jet Stream seen from satellite
Semipermanent Pressure CellsSemipermanent 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).
For NH winter:
1. Aleutian and Icelandic lows
2. Siberian and Bermuda-Azores highs
3. South Pacific, Atlantic, Indian highs
South Pacific high
South Atlantic high
South Indian high
For NH summer:
1. Tibetan low
2. Hawaiian and Bermuda-Azores highs
3. South Pacific, Atlantic, Indian highs
South Pacific high
South Atlantic high
South Indian high
Low pressure: clouds and precipitationLow pressure: clouds and precipitation
Extratropical stormtrack
Tropical rainfall
Extratropical stormtrack
High pressure: warm surface temperature, High pressure: warm surface temperature, drought and desertdrought and desert
Global distribution of deserts (all near high pressure cells)
• Ocean surface currents – horizontal water motions• Transfer energy and influence overlying atmosphere• Surface currents result from frictional drag caused by
wind - Ekman Spiral
General circulation of the oceansGeneral circulation of the oceans
• Water moves at a 45o angle (right) in N.H. to prevailing wind direction
• Due to influence of Coriolis effect
• Greater angle at depth
• 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
Global surface currentsGlobal surface currents
SummarySummary• 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?