Winds and Global Circulation Chapter 5 Winds and Global Circulation

Introduction Reason for winds goes back to concepts of insolation and radiation Differential (unequal) heating of Earth’s surface by latitude and at smaller scales leads to variations in pressure  gradient between locations  air motion Like opening a bottle or can of beer Earth’s rotation plays role in direction of flow

How Pressure Leads to Air Motion

So What is Pressure? Force per unit area Dad says, “Have you checked the air in your tires lately?” – can measure because higher pressure in tire exerts force on gauge In day-to-day life, why do we feel atmospheric pressure? Air has mass and is being pulled downward by Earth’s gravity

Measurement of Pressure Various units – in tire example, use pounds per square inch; also kg/cm2, pascals (Pa), millibars (mb), cm or mm of Mercury, inches of Mercury Standard sea level pressure = 101,320 Pa = 1013.2 mb = ~ 76 cm Hg = 760 mm Hg = 29.92 in Hg Measure using barometer – mercury or aneroid

Mercury Barometer Very accurate! Hg in a glass tube Tube placed in dish of Hg Hg flows out of tube due to gravity  a vacuum at top Hg stops flowing when air pressure pushing on Hg in dish = pull of gravity on Hg in tube As pressure on Hg in dish increases, Hg in tube rises

Pressure Change with Altitude What do you notice if you swim to the bottom of a pool? Same concept with atmosphere – less of atmosphere above you as altitude increase, so less pressure On weather maps, use a formula to adjust pressure readings to sea level, which enables pressure analyses (isobars)

Wind Air motion or flow of air with respect to surface, usually only horizontal movements Measured in meters per second or miles per hour (1 m/s approximately = 2 mph) Speed determined using anemometer (most common is cup anemometer) Direction determined with wind vane and always given as direction from which wind is coming (wind blowing from NW to SE = northwesterly wind); also use degrees

Measuring Wind Speed and Direction Wind vane and anemometer Compass directions/ degrees

Forces Affecting Wind Many features affect wind (buildings, hills, valleys, etc.); think of swirling motion in a football/baseball stadium Three main forces determine wind speed and direction on larger scales: 1. Friction 2. Pressure gradient force 3. Coriolis effect

Friction “Drag” due to a surface Causes wind speeds to decrease and has minor influence on wind direction If looking at a profile of wind vs. height, would see a half-U shape, with slower speeds near the surface and increasing values higher in atmosphere Negligible at high levels

Pressure Gradient Force Due to difference in pressure (a gradient) between locations PGF “pushes” air from areas of higher to lower pressure A stronger gradient (greater difference in pressure) between locations produces stronger winds Therefore, affects both direction and speed If only PGF, then wind is perpendicular to isobars, but of course this is too simple....

Pressure Gradient Force

Pressure Gradient Force

Coriolis Effect Deflection of motion of an object (including wind) from its path So mainly affects direction Due to rotation of Earth In Northern Hemisphere, deflection is to the right of pressure gradient force, while in Southern Hemisphere, deflection is to the left of PGF Most deflection at poles, least at equator

Coriolis Effect

Local Winds Friction and PGF have greatest effect on small-scale winds; Coriolis negligible Some local winds include: Convective winds: heating causes pressure gradient; at low levels, wind flows toward warm region (convergence), and at high levels, wind flows away from warm region (divergence) Mountain and valley breezes: during day, mountainsides heated causing air flow up valleys; opposite effect at night

Local Winds (cont’d) Land and sea breezes: due to specific heat attributes (land heats and cools faster than water); during the day, land heats, and air flow from cooler water surface towards land (sea breeze); opposite at night, because water warmer than land surface (land breeze)

Cyclones and Anticyclones Areas of low pressure Air spirals inward and upward (convergence) Due to Coriolis, air moves counterclockwise in NH and clockwise in SH Associated with cloudy weather and precip Anticyclones Areas of high pressure Air spirals outward and downward (divergence) Due to Coriolis, air moves clockwise in NH and counterclockwise in SH Associated with fair weather

Winds in Cyclones (L) and Anticyclones (H)

SUN How is heat transported from the Equator to the Poles? Earth Cold High Pressure 0o 30oN 60oN 30oS 60oS 90oN Warm Low Pressure SUN Earth

L Warm air rises at the equator producing Low pressure and flows towards the poles 0o 30oN 60oN 30oS 60oS 90oN L

H L H Cold air sinks at 30o N and S latitude Creating high pressure 30oS 60oS 90oN Cold air sinks at 30o N and S latitude Creating high pressure (Subtropical High pressure) H L H

H L H Northeasterly and southeasterly surface winds flow from the subtropical high pressure belts (30o N and S) to the low pressure belt (ITCZ) at the equator (calm winds: doldrums) westerly surface winds flow from the subtropical high pressure belts towards higher latitudes 0o 30oN 60oN 30oS 60oS 90oN H L H

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L H L H L westerly surface winds are forced to rise around 60o N and S latitude when they encounter cold polar easterly winds from the poles resulting in Subpolar Low pressure (SPL) belts 0o 30oN 60oN 30oS 60oS 90oN L H L H L

H L H L H L H cold air sinks at the poles producing polar high (PH) pressure regions 0o 30oN 60oN 30oS 60oS 90oN L H L H L H

Three-cell Model

2-D Glance at Surface Semi-permanent pressure cells around globe Called semi-permanent, because location and intensity vary with season (Fig. 5.17, p. 164) Seasonal shifts lead to Movement of Intertropical Convergence Zone (ITCZ) – moves northward in July, southward in January – why? Monsoon: shift in wind direction from offshore flow in winter (dry season) to onshore flow in summer (wet season)

Semi-permanent Pressure Cells

Winds Aloft As you move farther from the Earth’s surface, friction has less impact, so can focus mainly on PGF and Coriolis Geostrophic wind: wind that moves parallel to isobars and at a right angle to the PGF (Fig. 5.22a) Flow is initially in direction of PGF, but in Northern Hemisphere, Coriolis deflects motion to the right At some point, PGF = Coriolis (sum = 0), so speed and direction of air flow no longer changes See Fig. 5.22b for illustration of final two points

Winds Aloft

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Other Characteristics of Winds Aloft General pattern: Weak easterly winds in tropics (Equatorial Easterlies) High pressure at Tropics of Cancer and Capricorn, westerly winds to Arctic and Antarctic Circles (Westerlies) Spiraling motions from Circles to poles

Specific Features of Winds Aloft Rossby waves Undulations or waves in Westerlies which move cold air toward equator and warm air toward poles Primary mechanism for poleward heat transfer Polar front Sharp boundary between cold polar air and warm tropical air Jet streams Narrow zones (tube-like) of very fast wind speeds (center has highest speeds) Occur along strong pressure gradients Two affecting US – polar jet and subtropical jet

Rossby Waves Smooth westward flow of upper air westerlies Develop at the polar front, and form convoluted waves eventually pinch off Primary mechanism for poleward heat transfer Pools of cool air create areas of low pressure

Ocean Circulation Why are we talking about the oceans during the same class at winds? Atmospheric circulation drive the direction of surface ocean currents Ocean currents also act as heat transfer mechanisms (help global energy balance) General pattern is warm currents on eastern flank of continents and cold currents on western flank (Gulf Stream vs. California current)

Ocean Currents Upwelling is vertical movement in the oceans Important because it brings nutrients to upper levels of ocean Occurs along western edges of continents – one of strongest is along South America What does this have to do with weather and climate? Teleconnection – relationship between circulations in one region and weather/climate in another region; impacts vary across globe and even on same continent

Ocean Currents

El Nino El Niño-Warmer than normal waters in the Equatorial Pacific La Niña-Cooler than normal waters in the Equatorial Pacific

Normal vs. El Nino and Associated Weather Effects