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					Global Scale Winds Chapter 7
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Mariners have known since before Columbus that winds blow easterly off of Africa out to the Atlantic Ocean These winds were called the “trade winds” Columbus used the trade winds to travel out to the New World In addition, mariners knew to avoid areas around 30˚ N/S Latitude where winds were light or calm This area was known as the “horse latitudes”

Ancient Knowledge of Winds

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A conceptual model is a simple representation of reality, while still having all the major features needed Our conceptual model of the global circulation of the atmosphere needs to include or explain:
• • • • • The steady winds observed by mariners The lack of winds in certain regions Regions of low and high precipitation The global pattern of cloudiness The existence of jet streams

Conceptual Models

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What if the Earth had just one simple convection cell in each hemisphere? Rising air would occur near the Equator where solar heating is strongest Sinking air would occur at the Poles where the air is coldest Air would flow from the Poles to the Equator at the surface to replace the rising air at the Equator Air would flow from the Equator to the Poles aloft to replace the air moving away from the Poles

Simplest Model

Simplest Model

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Unfortunately, that fails to explain many surface features For example, if the air flows from North to South in the Northern Hemisphere, why do winds switch between the Trade Winds and the Westerlies at mid-latitudes? The rotation of the Earth prevents this very simple model from existing The Coriolis Force causes air to deflect to the right in the Northern Hemisphere Air is unable to make the entire trip from the poles down to the Equator before turning right and blowing parallel to the Equator

Simplest Model Fails

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It turns out, we need three cells in each hemisphere to account for all the weather features mentioned earlier One convection cell exists in the tropics, called the Hadley Cell Another exists in the polar regions In the mid-latitudes, a coherent convection cell does not exist

The Conceptual Model of Global Circulation

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A jet stream is a swift river of air found in the upper troposphere Two are usually found in each hemisphere: Each jet stream is formed by different processes The radius from the axis of the Earth’s rotation out to the atmosphere is largest at the Equator As one goes towards the Poles, that radius decreases At the Pole, the radius shrinks to zero As air moves poleward, the air has its radius decrease from the axis
• Polar jet stream • Subtropical jet stream

Jet Streams

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Subtropical Jet Stream

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Conservation of Angular Momentum

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Angular momentum = the quantity of mass times rotational velocity times radius Angular momentum is conserved (kept a constant) If radius decreases, the rotational velocity must increase to keep the angular momentum a constant This explains why ice skaters spin faster when they pull their arms in toward them

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The trade winds in the Northern and Southern hemisphere all converge at or near the Equator InterTropical Convergence Zone (ITCZ): Region where trade winds converge at the surface The winds beneath the ITCZ are often light, resulting in the “doldrums” Sinking portion of each Hadley Cell creates high pressure called subtropical highs The subtropical high creates the light winds of the horse latitudes

Hadley Cell

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Air sinks at the Poles, creating the Polar highs Air blows from the Poles down towards the Equator Coriolis force deflects the air to the right, resulting in the polar easterlies The boundary between the polar easterlies and the westerly winds of the midlatitudes is called the polar front The polar front separates cold polar air from more temperate air to the South

Polar Cell

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As we discussed in the last chapter, strong winds should exist above regions where the temperature gradient is large The polar jet stream forms because of this temperature gradient Therefore, the polar jet stream is found above the polar front

Polar Jet Stream

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Air moves away from the subtropical highs at the surface Winds blow up towards the midlatitudes from the South Coriolis Force deflects the winds to the right, resulting in the Prevailing Westerlies (or midlatitude westerlies) The midlatitudes don’t have a complete cell (winds don’t blow down to the subtropical highs aloft like your book shows) If there was that return flow southward aloft, the Coriolis force would deflect those winds to the right and we would observe easterly winds aloft That is not the case - they are westerly!

Midlatitude Circulation

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Winds above 500 mb tend to flow in wavelike patterns Troughs – elongated regions of low pressure (dips) Ridges – elongated regions of high pressure (bumps) Typically 3-5 waves will encircle the Northern Hemisphere – called Rossby Waves Zonal flow – a small amplitude pattern where winds blow mostly west-to-east Meridional flow – a large amplitude (deep troughs and peaked ridges) pattern where cold air moves equatorward and warm air flows poleward

Upper-Air Midlatitude Westerlies

Upper-Air Midlatitude Westerlies

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Zonal Index – the average speed of the west-to-east 500 mb wind between 35˚ and 55˚ latitude High zonal index – zonal flow Low/Negative zonal index – meridional flow Split flow pattern – zonal flow exists near the poles with a meridional flow pattern further to the south
Blocking Patterns – When large pools of cold or warm air get separated from the stronger flow Blocking patterns can persist for extended periods of time resulting in extreme weather events Index cycle – irregular oscillation between patterns (meridional to zonal) Short waves – superimposed smaller waves that travel through Rossby waves

Flow Patterns

Flow Patterns

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There is an energy surplus at the tropics and an energy deficit at the poles The circulations in the atmosphere act to redistribute the heat Therefore, our conceptual model also explains this transfer of heat

Poleward Transport of Energy

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The The The and The

global circulation pattern shifts with the Sun and the seasons ITCZ shifts location during the course of the year following the Sun ITCZ will shift more over land than water because land heats up faster to a higher temperature polar jet stream shifts south in the winter, north in the winter

Seasonal Variations

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Other highs and lows of the general circulation also shift and change in strength during the course of the year Heat lows – low pressure that appears due to extreme heating of the Earth’s surface

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Monsoon – a seasonal shift in wind direction The shift in wind direction also causes a shift in precipitation amounts Most famous example is over India In Summer, the Himalayan mountains heat up and results in winds blowing off the Indian Ocean onto the continent The warm, moist air rises orographically Clouds and copious rainfall results in the Summer In the winter, the Siberian High develops over the Himalayas, and the winds reverse direction With winds blowing from land and sinking down off the mountains, the precipitation shuts off

Seasonal Variations The Monsoon


				
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