Chapter 7: Atmospheric Circulations By the end of this chapter you should: Understand the scales of motion Be able to give an example of an eddy Have an understanding of the general circulation of the atmosphere

Scales of Atmospheric Motions Scales of motion – different sizes of circulation Microscale – smallest scale of motion. 2 meters or less and last seconds to minutes Mesoscale – circulation around a city (20 km). Can last minutes to hours Synoptic scale – Circulations around highs and lows (2,000 km). Can last many days Fig 7.1 Planetary scale – Circulations around the entire Earth.

Scales of Atmospheric Motions Fig 7.1

Scales of Atmospheric Motions Fig 7.1

Eddies - Big and Small Eddy – a circulation formed downwind from an object (examples?) Rotor – rotation formed downwind from a mountain wave Wind shear – change of wind speed or direction with height

Thermal Circulations Thermal circulations Circulations brought on by changes in air Temperature.

Sea and Land Breezes Types of thermal circulations Sea breeze (scale of motion?) Land breeze Sea breeze front – leading edge of the sea breeze Florida sea breezes can have huge walls of clouds. Make for great gliding due to vertical movement

Sea and Land Breezes

Seasonally Changing Winds - the Monsoon Monsoon wind system – “monsoon” means seasonal Asian monsoon – in winter the land is much colder than ocean. In summer, opposite is true

Seasonally Changing Winds - the Monsoon

Mountain and Valley Breezes Valley breeze – explain this Mountain breeze – explain this

Katabatic Winds Katabatic winds – in general, any wind that flows downhill Perfect scenario is elevated plateau with mountains around Bora – katabatic wind in the Adriatic Sea

Chinook (Foehn) Winds Chinook winds – fierce wind that flows on the eastern slope of the Rockies. Compressional heating – main source of the heat from the Chinook. Air descends very rapidly and warms Chinook wall cloud – a bank of clouds forming over the mountains

Santa Ana Winds Santa Ana wind Warm, dry wind that originates from the Desert. Usually needs a high in the Great Basin Compressional heating Fans huge wildfires

Desert Winds Dust storms – formed by surface winds that form from surface heating. Wind picks up dust Dust devils – Formed from combination of surface heating, unstable atmosphere, and an obstacle in the way

General Circulation of the Atmosphere General circulation only represents the average circulation Why are the prevailing winds in Hawaii northeasterly and in san Jose northwesterly? Cause: unequal heating of the earth’s surface Effect: atmospheric heat transport

Single-cell Model Basic assumptions Covered with water Sun is always over equator Earth does not rotate Hadley cell Heat at the equator creates a low that brings heat to the poles aloft

Single-cell Model Why is the single-cell model wrong? Earth has the Coriolis force. In this model, all winds would be easterly. We know that is wrong So let’s spin the planet.

Three-cell Model Model for a rotating earth - keep first two assumptions Hadley cell is still apparent Doldrums – equatorial region where winds are light. Warm air rises, condenses and moves laterally towards the poles Subtropical highs – convergence of air aloft creates highs at 30 degrees. Major deserts and horse latitudes. Trade winds – some of the surface air from the Subtropical highs move back to the equator and deflect due to the Coriolis force. Intertropical convergence zone – converge of the trade winds from both hemisphere to get rising air and thunderstorms

Three-cell Model Westerlies – some of the air from the Subtropical highs move towards the poles and deflect towards the east Polar front – the boundary between the mild air towards the equator and the cold air near the pole (Subpolar lows) Polar easterlies – air behind the polar front that is deflected to the west by the Coriolis force

Figure 7.21: The idealized wind and surface-pressure distribution over a uniformly water-covered rotating earth. Watch this Active Figure on ThomsonNow website at www.thomsonedu.com/login. Fig. 7-21, p. 185

Average Surface Winds and Pressure: The Real World Semipermanent highs and lows – two areas of highs and lows per hemisphere Bermuda high & Pacific high Icelandic low & Aleutian low Siberian high

Figure 7.22: Average sea-level pressure distribution and surface wind-flow patterns for January (a) and for July (b). The heavy dashed line represents the position of the ITCZ. Fig. 7-22a, p. 188

Fig. 7-22b, p. 189

The General Circulation and Precipitation Patterns Where would you expect regions of high and low precipitation? Heavy precipitation Regions close to ITCZ, polar fronts, subpolar lows

Westerly Winds and the Jet Stream Jet streams – regions of very fast-moving air in the upper atmosphere due to high pressure at the equator and low pressure near the poles

Winds and Upwelling Upwelling – rising of cold water from below Wind flow parallel to the coastline is deflected to the right by what force? Cold water replaces this

El Niño and the Southern Oscillation El Niño – a natural event that occurs around Christmas. Upwelling weakens along the west coast of South America. Major events can cause great hardship Southern Oscillation – seasaw pattern of high and low pressures switching on opposite sides of the Pacific Ocean La Niña – cold water episodes opposite El Niño

El Niño and the Southern Oscillation

Other Atmosphere-Ocean Interactions North Atlantic Oscillation Arctic Oscillation Pacific Decadal Oscillation