Lecture 22 -- The Oceans ATMOSPHERE CIRCULATION AND WINDS Coriolis effect Prevailing winds and vertical circulation Zones of pressure, evap. & ppt. Factors modifying global winds -- Differential heating of land and sea Fig. 6.14 RADIATION IMBALANCE -- ultimate driving force of global winds Warm, moist air rising at Equator Cold, dry air sinking at poles If the earth were not rotating, convection cells in the atmosphere would look like this. CORIOLIS EFFECT -- deflects winds (and ocean currents) so that they cannot flow in a straight line easily; tendency to follow a curved path Earth's rotation changes the direction of winds & currents: N. Hem.: deflection always to the right S. Hem.: deflection always to the left Cause -- Different latitudes rotate at different rates Rotational velocity increases from poles to Equator CORIOLIS EFFECT, cont d Winds and ocean currents have both... an initial velocity and direction, and... an initial rotational velocity that depends upon latitude. As they cross latitudes, their directions change because the rotational speed of the Earth's surface i different from what it was at their previous latitudes Result: Apparent deflection to Earth-bound observers. 1
Analogy -- launching a missile Fire from North Pole (no rotation) Earth rotates beneath it Deflection -- to the west (right) Fire from Equator (max. rotation) Moves to higher latitude (lower surface rotation speed) Missile rotating faster than Earth beneath Deflection -- to the east (right) Earth's Rotation -- Coriolis effect Winds (and ocean currents) travelling long distances are deflected from their original course......always to the right in Northern Hemisphere...always to the left in Southern Hemisphere Why does that happen? -- latitudes rotate at different rates 90 0 km/hr 60 800 km/hr 30 1400 km/hr 0 1670 km/hr ww2010.atmos.uiuc.edu/ East & West Coriolis Deflections 2
Amount of wind/current deflection depends on...... time of travel (how long it moves)... distance of travel (how far it moves) Winds and currents are deflected a large amount because they are in continuous motion over long distances. GLOBAL WIND SYSTEM Major zones Trade Winds Westerlies Polar Easterlies Other important features: 1. Convective Circulation / Convection Cells Occur at the Global scale, the continental scale, and the local scale Land surface 3
Other Features of The Earth s Wind System Convergent & divergent zones at boundaries between the three convection cells : Convergent -- air masses come together and rise Divergent -- sinking air; air masses moving apart Descending air creates zones of... high atm. pressure high evaporation -- sinking dry air Ascending air creates zones of: low atm. pressure high rainfall -- rising moist air "Jet Streams" Strong winds in upper troposphere Above boundary zones with converging or diverging air Modification of global wind patterns -- Differential heating of air and land Seasonal &daily heating/cooling cycles Oceans and lakes -- little T change Land areas -- significant T change 4
1. Daily cycle of winds in coastal areas (think Miami Beach) Day Day: onshore winds (peak at mid-afternoon) Night: offshore winds (peak in early morning) Night Fog: What causes it? 1. Advective Fog - offshore warm moist air moves over cold coastal waters (San Francisco) 2. Sea Smoke - cold air from land moves over warmer ocean water 3. Radiative Fog - air warmed during the day cools and condenses during the night 2. Seasonal monsoons (India, Southeast Asia) Summer: Hot continent - rising air above it Creates onshore winds (and rain) Summer (Wet) Winter (Dry) Winter: Cold continent, offshore winds 5
3. Seasonal changes in wind patterns and pressure zones over continents and oceans Semi-permanent seasonal zones of different pressure Summer Ocean (cool) Land (warm) Ocean (warm) Winter Land (cool) Winds (air flow) around pressure zones are deflected due to Coriolis effect: Clockwise around (and out of) High-Pressure cells (Northern Hemisphere) Counterclockwise around (and into) Low-Pressure cells (Northern Hemisphere) These are superimposed on the Trades, Westerlies, etc. 6