The major wind systems

Review of last lectureReview of last lecture• 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 surface currents? In the case of Ekman spiral, what is the direction of surface current relative to surface wind? Two types of ocean upwelling

Semipermanent Pressure CellsSemipermanent Pressure Cells

• Instead of cohesive pressure belts circling the Earth, semipermanent cells of high and low pressure exist; fluctuating in strength and position on a seasonal basis.

• These cells are either dynamically or thermally created.

• Sinking motions associated with the subtropical highs promote desert conditions across specific latitudes.

• Seasonal fluxes in the pressure belts relate to the migrating Sun (solar declination).

For NH winter:

1. Aleutian and Icelandic lows

2. Siberian and Bermuda-Azores highs

3. South Pacific, Atlantic, Indian highs

South Pacific high

South Atlantic high

South Indian high

For NH summer:

1. Tibetan low

2. Hawaiian and Bermuda-Azores highs

3. South Pacific, Atlantic, Indian highs

South Pacific high

South Atlantic high

South Indian high

High pressure: Desert and DroughtHigh pressure: Desert and DroughtGlobal distribution of deserts (all near high pressure cells)

The 2011 East African droughtThe 2011 East African drought

Video: Horn of Africa drought 2011Video: Horn of Africa drought 2011

http://www.youtube.com/watch?v=HXYFvx0SAMI

• Ocean surface currents – horizontal water motions• Transfer energy and influence overlying atmosphere• Surface currents result from frictional drag caused by

wind - Ekman Spiral

General circulation of the oceansGeneral circulation of the oceans

• Water moves at a 45o angle (right) in N.H. to prevailing wind direction

• Due to influence of Coriolis effect

• Greater angle at depth

• 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

Global surface currentsGlobal surface currents

• 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.

Ocean upwellingOcean upwelling

• Coastal upwelling: Due to Coriolis effect

• Equatorial upwelling: Due to Coriolis effect

Foehn Winds over the AlpsFoehn Winds over the Alps• A Foehn wind is a type of dry

down-slope wind that occurs in the lee (downwind side) of a mountain range.

• They are initiated when mid-latitude cyclones pass to the southwest of the Alps.

• Most common in winter• Air undergoes compressional

warming, and can raise temperature by as much as 30 oC.

• Central Europe enjoys a warmer climate due to Foehn winds.

Foehn Winds

• Central Europe enjoys a warmer climate due to Foehn winds.

Chinook WindsChinook Winds

• Chinooks are similar to Foehn winds but happen on the eastern side of the Rocky Mountains and form when low pressure systems occur east of the mountains.

• Most common in winter• Can exceed 90 mph.• Warm up the temperature for

western Great Plains

Chinook winds

Effects on 1988 Winter OlympicsEffects on 1988 Winter Olympics• Chinook winds melted the snow• For the first time, Winter Olympics extended to 16 days• Sent a ski jumper flying into a camera tower

• Occur in California during the transitional seasons, especially autumn, when high pressure is located to the east.

• Often contribute to the spread of wildfires.

Santa Ana WindsSanta Ana Winds

Santa Ana winds

Santa Ana winds and Santa Ana winds and spread of wildfirespread of wildfire

Katabatic WindsKatabatic Winds

• Katabatic winds warm by compression but originate when air is locally chilled over high elevations. The air becomes dense (with low temperature) and flows downslope.

• Can reach 60-120 mph.

• Common along Antarctica and Greenland ice sheets.

• Also referred to as Boras winds of the Balkan Mountains and Mistral winds in France.

Sea and Land BreezesSea and Land Breezes• Caused by temperature

differences between land and sea. Strongest in spring/summer.

• During the day (night) land is hotter (colder) than water.

• A thermal low develops over the warmer region.

• Air converges into the low, ascends, and produces clouds and possibly precipitation.

Sea breeze over HawaiiSea breeze over Hawaii

Valley and Mountain BreezesValley and Mountain Breezes

• Diurnal variation similar to a land/sea breeze occur in mountainous areas and are called valley and mountain breezes.

• Mountains facing the sun heat more intensely than shaded valley areas. This develops a thermal low during the day which produces a valley breeze.

• At night the situation reverses producing a mountain breeze.

SummarySummary• Semipermanent pressure cells. High pressure causes

desert and drought.• What drives the ocean surface currents? In the case of

Ekman spiral, what is the direction of surface current relative to surface wind? Two types of ocean upwelling

• Foehn winds over the Alps and Chinook winds over the Rockies. Warm up the eastern slope

• Santa Ana winds. Caused by high pressure over the mountain. Spread wildfires

• Katabatic winds• Sea and land breezes. Caused by temperature difference

between land and sea.• Valley and mountain breezes. Caused by temperature

difference between mountain slope and valley.