1. Chapter 10
Wind: Global Systems

2. General Circulation of the atmosphere
Average air flow around the world
actual winds at any one place vary considerably from the average
But, the average can answer why and how winds blow around the world
Cause is unequal heating of the Earth’s surface
- energy balance not the same for each latitude
-tropics (gain) and poles (lose) energy
Atmosphere transports warm air pole-ward and cool air equator-ward

3. Single-cell Model
Assumptions are made
- Earth’s surface is uniformly covered with water
- sun is always over the equator
-Earth does not rotate (only for PGF)

4. Single-Cell Circulation Model - Hadley Cell
The basis for average air flow around the earth can be examined using a non-rotating, non-tilted, ocean covered earth.
Heating is more intense at the equator, which triggers Hadley cells to redistribute rising heat from the tropical low to the polar highs.

5. Hadley Cell
referred to as a thermally direct cell
excessive heating at equator produces a broad region of surface low pressure
excessive cooling at the poles creates a broad region of high pressure
Horizontal Pressure Gradient forces cool surface air toward the equator while at higher levels air flows towards the poles
air rises near the equator and sinks over the poles
circulation does not actually exist

6. Three cell circulation model
considerably more complex than the single-cell model but there are some similarities
-the tropics still receive an excess of heat
-poles receive a deficit
three cells (instead of one) have the task of energy redistribution
surface high pressure located at the poles
surface low pressure near the equator
for equator to 30° - Hadley cell

7. Three cell circulation model
Over equatorial waters
- air is warm
- horizontal pressure gradients are weak
-winds are light
-region referred to as the doldrums
Warm air rises here and condenses into huge thunderstorms
air reaches the tropopause (top of the troposphere)
air moves laterally toward the poles
coriolis deflects poleward flow toward the right in the NH and toward the left in the SH
result is westerly winds aloft in both hemispheres

8. Three Cell Circulation Model
A rotating earth breaks the single cell into three cells.
The Hadley cell extends to the subtropics, the reverse flow Ferrel cell extends over the mid latitudes, and the Polar cell extends over the poles.
The Coriolis force generates westerlies and NE trade winds, and the polar front redistributes cold air.
Figure 11.2A

9. Subtropical Highs
air moves pole-ward and constantly cools by giving up IR radiation
air begins to converge as it approaches the mid-latitudes
convergence of air aloft increases mass of air above the surface
air pressure at the surface increases
at latitudes near 30°
here we find the major deserts of the world

10. Trade winds
some of the surface air moves back toward the equator
does not flow straight back
coriolis force deflects air causing it to blow from the NE in the Northern Hemisphere and SE in the Southern Hemisphere
NE trades converge with the SE trades along a boundary called the Intertropical Convergence Zone (ITCZ)
Westerlies
not all air moves equatorward
some moves towards the poles and deflects to the East
resulting in westerly flow

11. Observed Winds in January
Observed average global pressure and winds have increased complexity due to continents and the tilted earth.
Differential ocean-land heating creates areas of semi-permanent high and low pressure that guide winds and redistribute heat.

12. Observed Winds in June
Global pressure and wind dynamics shift as the Northern Hemisphere tilts toward the sun, bringing the inter-tropical convergence zone, the Pacific high, and blocking highs in the southern oceans northward.

13. January Winds Aloft
Land-sea temperature differences trigger ridges and troughs in the isobaric surface.

14. June Winds Aloft
Horizontal temperature gradients establish pressure gradients that cause westerly winds in the mid latitudes.

15. Jet Stream
High velocity Polar and subtropical jet stream winds are located in the lower tropopause, and they oscillate along planetary ridges and troughs.

16. Ocean Oscillations
Figure 11.20A
In the southern Pacific Ocean, high pressure in the east pushes surface winds, and waters, toward the low in the west.
This Walker Circulation becomes disrupted during El Nino events, which impacts upwelling and rains.

17. El Nino Ocean Temperature
Satellite data of sea surface temperature (SST) illustrate the difference between a non-El Nino year, with cool easterly waters, and the warmer SST El Nino year.

18. ENSO Index
El Nino Southern Oscillation (ENSO) intensity has been tracked using 6 parameters, including air and sea temperature, sea level pressure, wind speed and direction, and cloudiness.
A graph of the ENSO index shows eastern Pacific warm El Nino and cool La Nina years.

19. Global El Nino Impacts
The El Nino Southern Oscillation (ENSO) is part of a planetary ocean-atmosphere interaction, and can take several years to run its course.
ENSO causes abnormalities around the globe.