Atmospheric Circulation (Air-Sea Interaction)

1. Atmospheric Circulation (Air-Sea Interaction) • We live at the bottom of an ocean of air, the atmosphere • The atmosphere and the ocean are interdependent; what happens in one system causes changes in the other • Surface currents in the oceans are directly caused by atmospheric winds

2. Air-Sea Interaction • Differences in solar energy (heating) across the Earth – combined with the Earth’s spin – create winds • Winds drive surface currents and create waves • Likewise, certain atmospheric phenomena are manifested (originate) in the oceans • El Niño – Southern Oscillation • Hurricanes, cyclones

3. Air-Sea Interaction • Earth’s atmosphere is composed mainly of Nitrogen, Oxygen, and Water Vapor • Nitrogen: 78% • Oxygen: 21% • Argon, CO2, Neon, Helium, Methane, others: 1% • Air is never completely dry, however, and water vapor (H2O) can occupy as much as 4% of the air’s volume

4. Air-Sea Interaction • Temperature and humidity determine the density of air masses, which in turn determines whether these air masses will rise or sink • Air containing water vapor is less densethan dry air at the same temperature and density • Also, when heated, air expands and becomes less dense • This means that cold air is denser than warm air and cold, dry air is much denser than warm, moist air

5. Say what? • Warm air is less dense than cold air because increasing temperature results in greater molecular dispersion  Increasing Temperature

6. Say what? (Continued) • Moist air is less dense than cold air because the weight of water vapor (H2O) is less than that of Nitrogen (N2) and Oxygen (O2) • When water vapor increases, the amount of O2 and N2 decreases per unit volume • Molecular weight of O2 = 16 + 16 = 32 • Molecular weight of N2 = 14 + 14 = 28 • Molecular weight of H2O = 1 + 1 + 16 = 18

7. Atmospheric Circulation • Air masses will move from regions of high pressure (dense air) to regions of low pressure (less dense air) • A low pressure zone results from moist and/or warm air • A high pressure zone results from dry and/or cold air • The flow of air from regions of high to low pressure create the winds

8. Atmospheric Circulation • Because warm and/or moist air is less dense, it rises (heat rises) • Likewise, cool and/or dry air is more dense and so it sinks • As air rises, it expands and cools; water vapor in rising, expanding air will condense into clouds because the cooler air is no longer able to hold as much water vapor • Precipitation transfers water vapor (AND HEAT!!) from low to high latitudes

10. Solar Heating Varies with Latitude • The Earth revolves around the sun in an elliptical path • The Earth itself is tilted at an angle of 23.5° • The tilt of Earth’s rotational axis results in the seasons

11. Solar Heating Varies with Latitude • The amount of solar energy reaching the Earth’s surface varies with latitude and season • Because of the Earth’s tilt, solar energy reaching the equator strikes at a low angle, concentrating the radiation in a small area; solar energy reaching the poles, however, does so at a lower angle and so less heat is absorbed in polar zones

12. Near the poles, light filters through more atmosphere and approaches at a low angle, favoring reflection rather than absorption

13. Got albedo? • Albedo is the measure of solar radiation that is reflected back into space • A high albedo indicates that more energy is reflected back into space, while a low albedo indicates that less energy is reflected back to space • Ice and snow (even clouds) increases albedo, and so much of the light that reaches the polar regions is reflected back into space

14. Uneven solar heating and atmospheric circulation • Air is warmed in the tropics and rises • Air is cooled near the poles and falls • It seems logical to suspect then, that air heated in the tropics expands and becomes less dense as it moves towards the poles, where it will cool (and condense) sinking back towards the poles…. • BUT, THIS IS NOT WHAT HAPPENS!

15. HYPOTHETICAL CIRCULATION ON A NON-SPINNING EARTH

16. Enter the Coriolis Effect… • Experience will tell us, however, that winds in the mid-latitudes of the northern hemisphere do NOT flow out of the north, but rather the west • The hypothetical winds described do not resemble the actual wind patterns of the Earth because we have neglected the effect of the Earth’s rotation

17. Enter the Coriolis Effect… • The rotation of the Earth strongly influences the motion of air and water • This effect is named the Coriolis effect after its discoverer, Gaspard Gustave de Coriolis • The Coriolis effect changes the intended path of a moving body • Causes moving objects on Earth to follow curved paths

18. Coriolis: The Example • Imagine you and your friend are on a carousel • You are sitting on the inside of the carousel and your friend is sitting on the outside • You throw a ball to your friend, but are amazed to find that the ball curves sharply to the right and your friend is unable to catch it (and this is not because you throw like a girl…) • Other friends watching from a hot air balloon hovering over the carousel confirm that the path of the ball was in fact straight

19. Coriolis: The Example • If we compare our carousel to the Earth, we know that the Earth will complete a full rotation every 24 hours • People living on the equator however must complete a much larger circumference of rotation than would people in middle and high latitudes • In order for every part of the Earth to complete a rotation in 24 hours, points on the equator MUST travel faster than points near the poles

20. Back to the carousel • When you throw the ball to your friend on the carousel, YOU are traveling slower than he/she is riding on the outside of the carousel • In order for you and your friend to complete a rotation within the same time, the inner riders on the carousel must travel slower than those riding on the outside who must cover more ground in the same amount of time

21. The Coriolis Effect • The equator must travel faster than higher latitudes must travel in order for all regions of the Earth to complete 1 full rotation in 24 hours (people in Anchorage, AK and the equator all experience the same 24-hour day) • Therefore as objects travel from one region of the globe to another, they are subject to changing speeds of travel rst.gsfc.nasa.gov/Sect14Sect14_1c.html

23. The Coriolis Effect Will Keep You Up At Night….

24. The Coriolis Effect Your friend, rotating faster to cover more distance (red line) in same time 15 mph 8 mph You, rotating slower to cover less distance (blue line) in same time Carousel rotating counter-clockwise

25. 15 mph 8 mph You throw ball while moving at 8mph in what you consider to be a straight path Carousel rotating counter-clockwise

26. 15 mph As ball travels, it carries with it this slower motion with it* *The carousel beneath the ball is traveling faster than it 8 mph Carousel rotating counter-clockwise

27. 15 mph As ball travels, it carries with it this slower motion with it* *The carousel beneath the ball is traveling faster than it 8 mph Carousel rotating counter-clockwise

28. 15 mph As ball travels, it carries with it this slower motion with it* *The carousel beneath the ball is traveling faster than it 8 mph Carousel rotating counter-clockwise

29. My art is WAY better, but just in case you want the book’s version…

30. The rotation of the Earth is counter-clockwise • In the northern hemisphere, objects are deflected to the right relative to their path of motion

31. In the southern hemisphere, objects are deflected to the left for the same reason(poles are moving slower than equator)

32. Let’s return to our hypothetical model… • Air does warm, expand and rise along the equator

33. The Coriolis Effect Influences the Movement of Air in the Atmosphere • When the air has traveled ~ one third of the way from the equator to the pole – to about 30°N or 30°S latitude, the air becomes dense enough to sink back towards the surface, completing the loop • But instead of moving directly towards the equator, the wind is deflected it to the right in the Northern Hemisphere, and to the left in the Southern Hemisphere

34. At the equator, warm, moist air rises, resulting in a low pressure zone Deflected to the right As the rising air becomes colder & drier, its density increases, resulting in a high pressure zone

35. The Coriolis Effect Influences the Movement of Air in the Atmosphere • Note that the Coriolis effect does not cause the winds; it only influences the wind’s direction • As air rises at the equator, it will lose water vapor by precipitation caused by expansion and cooling. This drier air travels north or south of the equator and grows denser as it cools

36. Descending air towards equator is deflected to the right of its path of motion Descending air towards equator is deflected to the left (Southern H.) of its path of motion

37. Throwing a monkey wrench into the Coriolis concept… • The tendency of wind to deflect because of the Coriolis effect increases with its speed and with distance from the equator • This means that winds in high latitudes deviate much moreso than do tropical winds occurring at low latitudes moving at the same speed • Likewise, faster winds will be deflected moreso than slower winds in either region

38. General Wind Patterns • This means that there is, in fact, no Coriolis effect at the equator, and hence, no deflection of wind • This is because the change in velocity (speed) of the Earth changes very little near the equator, but changes muchy more at higher latitudes  greater Coriolis effect

39. General Wind Patterns • A column of warm, low density air rises away from the surface and creates a band of low pressure at the equator • The weather in areas of low pressure is characterized by cloudy conditions with lots of precipitation because rising air cools and cannot retain (hold onto) its water vapor • This region is clothed in tropical rain forests

40. General Wind Patterns • A column of cool, dense air moves towards the surface and creates high pressure zones. Descending air is quite dry and so these regions are characterized by dry, clear, fair conditions • Sinking air is very arid (dry) and the great deserts of the world are centered along this band of high pressure (30°N and 30°S)

41. Desert anyone?

42. General Wind Patterns • Sailors have a special term for the calm, equatorial regions where low pressure persists and little winds exist; the doldrums • Sailors also have a special term for the regions within the high pressure band, where winds are light and variable; the horse latitudes • Places between the high and low pressure bands, on the other hand, experience rapidly moving air, and are characterized by strong, dependable winds

43. (Horse latitudes)

44. Winds are named for the direction in which they originate

45. Tropical Cyclones (Hurricanes) • Tropical cyclones are huge rotating masses of low pressure characterized by strong winds and torrential rain • In North and South America, tropical cyclones are commonly called hurricanes • In the western North Pacific, they are called typhoons • In the Indian Ocean, they are called cyclones

46. Tropical Cyclones (Hurricanes) • Tropical cyclones carry tremendous amounts of heat from one region of the world to another • The energy contained in a single hurricane is greater than that generated by all energy sources in the United States in one year! • Hurricanes are powered by the release of water’s latent heat of condensation

47. Recipe for a Hurricane: • Ocean temperature greater than 25°C (77°F), which provides an abundance of water vapor to the atmosphere via evaporation (summer and fall) • Warm, moist air, which supplies vast amounts of heat as the water vapor condenses and fuels the storm • The Coriolis Effect, which causes the hurricane to rotate counterclockwise in the Northern Hemisphere, and clockwise in the Southern Hemisphere

49. Tropical Cyclones (Hurricanes) • In fact, there are no hurricanes that can occur directly over the equator because the Coriolis Effect is zero there • Hurricanes can not form below 8° N or S latitude; Coriolis effect is not strong enough to cause deflection • Hurricane season: June 1 – Nov 30: 97% of cyclones (hurricanes) occur during this time

50. Tropical Cyclones (Hurricanes) • As air rises (in the northern hemisphere), it is deflected to the right • This results in a counter-clockwise rotation Pressure gradient (moving towards the low pressure center) is represented by blue arrows; the Coriolis deflection is represented by red arrows L http://en.wikipedia.org/wiki/File:Hurricane_isabel_and_coriolis_force.jpg