ch8_general_circulation1

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
• 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
• 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?
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Westerly winds in the extratropical troposphere Jet streams: local maximum of winds
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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
Works cited
Images •/gears.html •/earth/meteorology/meteorological-measuringinstruments/measure-wind-direction.php
• 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 equatoprecipitation (heating) belts. Primary high and lows • Three-cell model. Mechanism for each cell • Surface low pressure is associated with clouds and precipitation. Surface high pressure is associated with warm surface temperature, drought, and desert. • 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? • What drives the ocean surface currents? In the case of Ekman spiral, what is the direction of surface current relative to surface wind?
High pressure: warm surface temperature, drought and desert
Global distribution of deserts (all near high pressure cells)
Zonal mean structure of temperature
Annual mean precipitation (heating)
Extratropical stormtrack
Tropical rainfall
Extratropical stormtrack
Primary Highs and Lows
Equatorial low
Subtropical high Subpolar low
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
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
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
Two characteristics: • • Horizontally uniform in the tropics Steep gradient in the extratropics
Zonal mean structure of zonal wind
Two characteristics:
The most common atmospheric circulation structure
CE
H
L
Cooling or No Heating
Friction
Heating
H L CE Imbalance of heating Imbalance of temperature Imbalance of pressure Wind
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
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
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
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?