Chapter 8

1. Chapter 8 Atmospheric-Ocean Interactions: El Niño and Tropical Cyclones

2. Figure CO: Chapter 8, Atmosphere-Ocean Interactions © ultimathule/ShutterStock, Inc.

3. The Oceans • Cover about 70% of the earth’s surface • Cover an even higher percentage in the tropics • Exchange large amounts of energy and water with the atmosphere at the surface • Give water vapor to the atmosphere • Give energy to the atmosphere in the form of latent heating • Exchange energy with the atmosphere by convection and conduction • Advect large amounts of energy from the equator towards the poles

4. The rate of energy transfer between atmosphere and oceans • Is greater when the temperature difference is greater. • Is in the direction from the higher to the lower temperature. • Is greater when the wind speed is greater • For latent heating, rates are greatest when saturation vapor pressure in the atmosphere is largest (high temperatures).

5. Energy Gains and Losses • When the oceans give up energy (negative, blue, purple) the atmosphere gains energy from the oceans • When the oceans gain energy (positive, red, yellow) the atmosphere loses energy to the oceans • Oceans supply energy to storms in middle latitudes • Oceans warm in summer and cool in winter

6. Figure 01: The energy gains and losses of Earth’s oceans

7. Figure 02: Energy transport versus latitude

8. The Sea Surface • Is the part of the ocean that interacts directly with the atmosphere. • Has a temperature called the sea surface temperature (SST) actually measured a few feet below the surface at the intake level of a ship. Cold water is used by ships for air conditioning. • The skin temperature is the temperature right at the surface.

9. Temperature in the Oceans • Generally there are three layers • The surface zone has the highest temperatures • Sometimes called the well-mixed (by waves and convection) zone • The thermocline is a zone of rapidly decreasing temperature as depth increases • In the deep zone the temperature is slightly above freezing • Salt water freezes at a temperature lower than fresh water

10. Figure 03: Temperature vs depth in ocean

11. Temperature Profiles for Different Latitudes • The tropics have the steepest thermocline, because sea surface temperatures are greatest there • The middle latitudes have the deepest surface layer • Polar regions have surface temperatures near freezing

12. Figure 04: the depth and steepness of the thermocline are functions of latitude

13. Water, salt water, and air • Salt water is denser than fresh water. • Icebergs, made of fresh water, float. • Colder water is only very slightly denser than warmer water. • Pressure in the oceans increases downward by 1 atmosphere about every 35 feet. • Density is nearly constant in the oceans, whatever the depth.

14. Sea Surface Temperatures • Are highest in the tropics, lowest at the poles • In middle latitudes and subtropics, are higher on east coasts than west coasts • In polar regions, lowest temperatures on east coasts • In tropical regions, higher temperatures on west coasts • Are highest in the equatorial western Pacific and the Indian Ocean • Correspond to warm and cool ocean currents

15. Figure 05: SST distributions across the globe. (Reproduced from Kara, Wallcraft, Hurlburt, J. Atmos. Oceanic Technol., 20 [2003]: 1616–1632.

16. Surface Pressure, Surface Wind, and Ocean Currents • Subtropical highs are persistent enough to create persistent anticyclonic wind flow • These winds create gyres of anticyclonic ocean surface currents • Ocean currents are bounded by land

17. Figure 06: Ocean currents.

18. Figure 07: SST satellite image of the Gulf Stream Courtesy of SSEC, University of Wisconsin-Madison

19. Figure 08: North Atlantic gyre

20. Wind and the Ekman Spiral • Friction between the air and the sea surface forces the air to move • The Coriolis force turns the water to the right (NH) or left (SH) • Moving water influences the layer of water beneath • The entire pattern is called the Ekman spiral • On average, water moves to right (NH) or left (SH) in Ekman transport

21. Figure 09: Ekman spiral

22. Cold Current, West Coast and Upwelling • Ekman transport moves water away from the shore • The water must be replaced • Replacement water comes from below the thermocline in the process called upwelling • Mixing and cold water brings nutrients close to the surface and favors sea life

23. Figure 10: Ekman winds

24. El Niño • Named “The (boy) child” for the season of most common occurrence. • Is a common but short-lived feature, but occasional episodes last for months or a year or more. The episodes are what we call El Niño today. • El Niño is a phenomenon that affects the entire Pacific Ocean and weather around the globe.

25. Figure 11: Temperature anomalies Courtesy of University of Washington, Joint Institute for the Study of the Atmosphere and Ocean (JISAO)

26. Figure 14: El Niño weather Source: NOAA

27. Characteristics of El Niño • Abnormal warming of the waters off Ecuador and Peru. • Upwelling ceases • Warm waters come from the western Pacific • Trade winds weaken

28. Figure 12: Trade winds & El Niño, T-storms

29. Figure 15: USA snowfall during El Niño Impacts of El Nino on Snowfall by Angel, Jim, Image courtesy of the Midwestern Regional Climate Center, Illinois State Water Survey

30. Figure T01: Global Impacts of Five Major El Niño Events

31. La Niña • Generally, opposite conditions to El Niño • Also described as an enhancement of normal conditions • Abnormal cooling of ocean waters in the eastern Pacific • Upwelling is enhanced • Trade winds are stronger

32. Figure 17: La Niña weather Source: NOAA

33. How to get the latest information about El Niño • Google on “ENSO diagnostic discussion” • Choose the first entry, the National Climate Prediction Center • Look at the latest discussion and the weekly update • ENSO is “El Niño Southern Oscillation”

34. Other Oscillations • The Pacific Decadal Oscillation (PDO) • The North Atlantic Oscillation (NAO) • The Arctic Oscillation

35. Figure 18: NAO Reproduced from www.ldeo.columbia.edu/NAO, Courtesy of Martin Visbeck

36. Tropical cyclones: what are they? • Hurricanes in waters of North and Central America • Typhoons in the western Pacific • Cyclones in the Indian Ocean and Southern Hemisphere • All have sustained winds of 65 knots or 74 mph

37. Tropical cyclones: what are they? • From space, they look like large circular swirls of clouds several hundred km in diameter • The most highly organized and destructive weather systems on Earth • A grouping of a large number of thunderstorms with a circulation about a center of low pressure.

38. Figure 19: Hurricane Isabel Courtesy of CIMSS/University of Wisconsin-Madison

39. Figure 20: Andrew damage Courtesy of NOAA

40. What’s inside a hurricane • At the center of low pressure is the eye, 8 to 80 km across, often almost entirely clear of clouds • Surrounding the eye is an eye wall, a narrow, circular, rotating region of intense thunderstorms and strong upward motion • Spiral bands of thunderstorms and cumulus clouds extend outwards from the eye wall.

41. Figure 21A: Hurricane Mitch satellite images Courtesy of CIMSS/University of Wisconsin-Madison

42. Figure 22: Inside eyewall © Chris Sattlberger/Photo Researchers, Inc.

43. Figure 21B: Hurricane Mitch satellite images Courtesy of CIMSS/UW-Madison

44. Figure T02: The Most Damaging Tropical Cyclones to Affect the United States 1900–2009

45. Hurricanes: How do they form? • Atmosphere and ocean interact to fuel a hurricane • Latent heating near the surface when strong winds evaporate large amounts of water • Energy is transferred from the warmer water to the cooler atmosphere • Updrafts in cumulus clouds transport energy upward

46. Figure 27: Cross section of processes involved in fueling a hurricane

47. Where do hurricanes form? • Where sea surface temperatures are 80°F or higher • No closer to the equator than 5° latitude: the Coriolis effect is needed for rotation about the cyclone center • Where there is little or no vertical wind shear to tilt the center of the storm • Where a disturbance already is present

48. Figure 23: Hurricane paths and SST Courtesy of NASA

49. Figure 25: Swirling winds Adapted from Nese, J. and Grenci, L., A World of Weather: Fundamentals of Meteorology. Kendall/Hunt, 1998,

50. Figure B01: Hurricane hunter plane Courtesy of UCAR/NSF/NOAA