Lecture 14 7 February 2005 Atmospheric and Oceanic Circulations (continued) Chapter 6

Credit: Wind simply put, wind is the horizontal flow of air in response to differences in air pressure these pressure differences are usually due to uneven solar heating at the surface wind flows because of pressure gradient ‘heat rises’

Four forces that determine winds 1. Gravity - pulls gas molecules close to Earth density & pressure decrease with height 2. Pressure gradient force - the difference in air pressure between areas 3. Coriolis force - deflects wind from a straight line to the right or left depending on hemisphere 4. Friction force - the drag on air flow from the Earth’s surface

Pressure Gradient Force

Pressure Gradient Force and Isobars if there were no other forces acting on wind, it would flow in straight lines (perpendicular to isobars) from high to low pressure zones

Coriolis Force (just the facts) Rotation of Earth acts to deflect any motion from a straight line Deflection is to right (NH) to the left (SH) Coriolis “force” act on a right angle to the motion Coriolis Force is NOT a real “force” but is caused by viewing motion on a rotating planet

Figure Credit: “Earth’s Climate” by W. Ruddiman The Coriolis Force affects air flow in response to pressure gradients in the atmosphere

geostrophic winds - PGF and Coriolis forces are opposite and balanced Credit: the CF deflects the wind to the right until upper troposphere wind flows parallel to isobars ~7km

Friction Force Surface friction reduces wind speed and the effects of the Coriolis force Friction causes winds to move across isobars at an angle The friction force operates only in the bottom km of the atmosphere, and it acts opposite to the direction of motion

PGF + Coriolis + Friction Forces

Figure Credit: “Earth’s Climate” by W. Ruddiman

Show shockwave 27_WindPatDev.swf

Useful things to remember Coriolis Force Acts to the right of motion (left in SH) Not a real force – matter of perspective Geostrophic wind – upper troposphere/ocean Horizontal pressure & Coriolis forces balance Winds go ALONG isobars Surface friction – lower troposphere Enables CROSS isobar flow

Useful things to remember Low pressure cells Less dense –humid Circulation is counter-clockwise (NH) – cyclonic Convergent near ground – rising air masses High pressure cells More dense – dry Circulation is clockwise (NH) – anticyclonic Divergent near ground – descending air masses

Heating & Humidity in Tropics solar heating in the tropics expands air and decreases its density - leading to increased buoyancy How would this change the average molecular weight of air? average molecular weight of air is ~29 g/mol average density of air is 1.3 kg/m^3 what happens to air density if you add water vapor? It also gets more humid (adding water vapor)

Convection on your Stove

Convection on Earth

Warm, moist air rises and is replaced by cooler drier air from other sites Credit: convect.jpeg

as this air rises, it cools and water condenses out, leading to intense precipitation

Credit: A satellite (GOES) view of the ITCZ over the eastern Pacific InterTropical Convergence Zone

the position of the ITCZ tracks the sun (it is in the summer hemisphere) - the location of the ITCZ determines the rainy season in many tropical countries, especially those in Africa the horizontal winds within the ITCZ are calm - the doldrums

The C in ITCZ the intense uplift of air creates horizontal pressure gradients at the surface Credit: NASA JPL as a result, winds converge towards the equator from both hemispheres what about the complete cycle - where does the uplifted air go?

Equator-to-pole cross section of circulation

Hadley cell circulation this circulation refers to the complete circulation of rising air in the tropics, descending air over 30 °N and °S, and trade winds converging at the equator the descending branch of the Hadley circulation brings warm, dry air to the surface leading to high pressure & reduced precipitation

Subtropical high-pressure cells these cells occur where the tropical air descends in either hemisphere

Equator-to-pole cross section of circulation

Figure 6.18 Jet Streams

Show shockwave 04_GlobalWinds.swf

Figure Credit:

Figure Credit: “Earth’s Climate” by W. Ruddiman Monsoon Circulation

Figure Credit: “Earth’s Climate” by W. Ruddiman Monsoon Circulation

Figure Credit: physicalgeography.net Monsoon Circulation

Asian monsoon intense, dry winds flow from the Asian interior in response to the gradient between the continental high pressure & equatorial (ITCZ) low pressure

in summer, the ITCZ shifts north, reversing the pressure gradient – winds flow from the Indian ocean & gain moisture Asian monsoon

Daytime land-sea breeze results from differential heating of land and sea - not from radiation differences - but from the different specific heats of land and water

Nighttime land-sea breeze at night, the land cools more rapidly than the sea and thus overlying air becomes more dense and has a higher pressure

What time of day would you go jogging in Los Angeles? WHY?