2 Atmospheric Circulation Patterns and Global Climate

gases in the atmosphere move around in response to two primary driving forces

2.1 Convection

(a) Warm air is less dense and rises -- causes low pressure

(b) Cool air is more dense and sinks -- causes high pressure

(c) On large scale, this causes circulation from the equator to the poles

-- on a small scale, winds blow from highs to lows

2.2. Coriolis Effect

(a) Spinning Earth causes lateral deflection of air currents and makes large convection cells unstable

- right deflection in north, left deflection in south

(b) Causes generation of several smaller convection cells called Hadley Cells, each about 30° wide

2.3 Hadley Circulation Cells and Deserts

- amount of moisture air can hold is directly proportional to atmospheric pressure

- amount of moisture air can hold is directly proportional to atmospheric temperature

- remember warm air rises and cold air sinks

(a) Air gets heated and rises at equator

- rising hot air carries a lot of water

(b) As air cools and flows laterally to north and south, it can hold less water

- condensation and rainfall adjacent to equator - rain forests

(c) Continues to cool and flow laterally until about (+ or - ) 30° latitude where Coriolis effects and increased density make it sink

- descending, warming air pulls moisture into it making deserts
- deserts associated with aridity, not temperature

2.4 El Niņo

a regional climate pattern that results from a change in oceanic circulation patterns off the coast of Peru

(a) Atmospheric and Oceanic Components of El Nino

(1) distribution, movement and temperature of surface waters (< 200 m)

(2) distribution, movement and temperature of deeper waters (> 700 m)

(3) strength of westward-moving tradewinds

- affects the movement of surface water and upwelling

(b) The Process

(1) "Normal" Conditions

- warmer surface water is pushed westward by tradewinds and replaced by upwelling, cold, deep nutrient-rich water

- anchovies and other fish thrive in this nutrient-rich water

- storms over western Pacific

(2) El Niņo Conditions

- weakening of westward tradewinds allows eastward flow of warmer surface waters, shutting down upwelling of deep, cold water

- warming waters pump heat and moisture into eastern Pacific atmosphere causing low pressure systems and associated storms that repeatedly pound western South America

(3) La Niņa Conditions

- very strong tradewinds push warmer surface waters far into the western Pacific, cooling the eastern Pacific and causing intense storms in the western Pacific

the 1997 El Niņo event


back to lecture outline | on to the next heading