General Circulation of the Atmosphere
and Oceans
Before we start …
• Assignment IV: 7 full scores, 16 9s, 11 8s
• Assignment V: 28 full scores, 2 9s, 1 8s.
• Midterm II:
41-51: 9 (one 51 and one 50!)
31-40: 17 (try to do better next time!)
21-30: 7 (try to do better next time!)
Introduction
• Well-defined pressure patterns exist across the globe
• These define the general circulation of the planet
• In describing wind motions:
– Zonal winds: flow parallel to lines of latitude
– Meridional winds: flow parallel to lines of longitude
• The general circulation of the atmosphere may be
examined through a three-cell model
Primary Highs and Lows
Equatorial low
Subtropical high
Subpolar low
Polar high
Annual mean precipitation (heating)
Tropical rainfall
Extratropical stormtrack
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
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
Vertical structure of temperature
Two characteristics:
•
Horizontally uniform
in the tropics
•
Steep gradient in
the extratropics
Vertical 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
Annual mean 200 mb winds
Subtropical
jet stream
Polar jet
stream
A Jet Stream seen from satellite
The subtropical jet is seen as a band of clouds extending
from Mexico on an infrared satellite image
Semi-permanent Surface Pressure Cells
• Instead of cohesive pressure
belts circling the Earth, semipermanent cells exist
January
• Low: Aleutian, Icelandic, Tibetan
• High: Siberian, Hawaiian,
Bermuda-Azores
• Cells are either dynamically or
thermally produced, and
fluctuate in strength and
position on a seasonal basis
• Subtropical highs are often
associated with deserts
July
Annual mean 850mb winds
Horse latitudes
General circulation of the oceans
• Ocean currents – horizontal water motions
• Transfer energy and influence overlying atmosphere
• 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 Ocean Circulation
• 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
Ocean upwelling
• is an oceanographic phenomenon that
involves wind-driven motion of dense,
cooler, and usually nutrient-rich water
towards the ocean surface, replacing
the warmer, usually nutrient-depleted
surface water.
• Coastal upwelling: Due to Coriolis
effect
• Equatorial upwelling: Due to Coriolis
effect
Deep Water Circulation
Figure 6.23
Christopherson, 2003
Summary
•
•
•
•
Primary high and lows
Three-cell model. Mechanism for each cell
Two characteristics of temperature structure
Two characteristics of wind structure. Why does westerly
winds prevail in the extratropical troposphere? What
cause the jet streams?
• What drives the ocean currents? In the case of Ekman
spiral, what is the direction of surface current relative to
surface wind? Two types of ocean upwelling