1. NATS 101-06 Lecture 17 Air Masses

2. Supplemental References for Today’s Lecture Lutgens, F. K. and E. J. Tarbuck, 2001: The Atmosphere, An Introduction to the Atmosphere, 8 th Ed. 484 pp. Prentice Hall. (ISBN 0-13-087957-6)

3. Ocean Currents of World Ahrens Fig. 7.24

4. Summer SST Along West Coast Ahrens Fig 7.24 Prevailing Winds Sea surface temperatures (SST) along West Coast are quite cold during summer, especially off Northern California. Due to upwelling of cold, nutrient rich water by prevailing N winds. Coastal Surface Water

5. Upwelling from Alongshore Winds Ahrens Fig 7.25 Wind pushes surface water southward. Coriolis force deflects water to the right. Cold water from below rises to surface. Fog persists over the cold water.

6. weather.unisys.com Upwelling Regions El Nino 3.4

7. El Nino-Southern Oscillation (ENSO) An important atmospheric-ocean feedback Normal conditions in tropical Pacific: -Warm SST, low SLP, and T-storms in W Pacific -Strong subtropical highs in E Pacific -Easterly winds and cool upwelling water along equator in East Pacific -Prevailing southerly winds off of Peru produce cold upwelling and excellent fishing

8. El Niño-Southern Oscillation (ENSO) Every few years (4-5 years): -Equatorial Central Pacific warms -Low SLP, T-storms shift to Central and East Pacific (the Southern Oscillation) -Trades and southerly winds off Peru weaken -Upwelling ceases off Peru, warming leads to massive kill off of fish. Typically occurs around Christmas (an El Niño event)  Alters global patterns of wind, temp and rain

9. Walker Circulation Pushes water westward Darwin Tahiti Walker Circulation oscillates with a quasi-period of every few years. Oscillation is very evident in SLP records for Darwin and Tahiti. Aguado & Burt, p230

10. Southern Oscillation Index (SOI) SLP SST

12. See current conditions

18. Ahrens Fig. 7.28 El Niño Anomalies

22. El Nino Precipitation Extremes

23. La Nina Precipitation Extremes

24. El Nino Precipitation Extremes

25. La Nina Precipitation Extremes

26. ENSO-Atmosphere Coupling Ocean temperature pattern SLP, winds, storms, etc.Ocean currents, upwelling By observing SST’s and surface winds in equatorial Pacific, we are able to forecast ENSO events with considerable skill (much better once they have started to form). ENSO forecasts lead to skillful seasonal forecasts for the US several months in advance (e.g. 1997-1998 winter).

27. Ocean and ENSO Summary Major Ocean Currents Driven by prevailing wind Upwelling Regions Occurs along west coasts of continents Cold water rises from below to surface Nutrient rich, excellent fishing regions

28. Summary El Nino-Southern Oscillation Occurs every few years Central equatorial Pacific warms Low SLP, T-storms move with warm water Upwelling weakens along Peru coast Can be predicted up to one-year in advance Modulates global patterns of wind, temp, rain

29. What is an Air Mass? Air Mass Large area (>1600 km by 1600 km) of air that contains relatively uniform, horizontal distributions of temperature and moisture.

30. How Air Masses Form If surface air resides in a region for a few days, it acquires the thermal and moisture characteristics of the underlying surface. Source regions for Air Masses are: Big in area [ >>(1600 km) 2 ] Dominated by persistent high pressure and light winds

31. Air Mass Source Regions Contrasting source regions are Continents versus Oceans Tropics versus Poles An Air Mass is designated in terms of its Source Region

32. Air Mass Characteristics Ahrens Table 8.1

33. Air Mass Source Regions for NA Ahrens Fig 8.2

34. Creation of cP Air Mass Williams p22

35. Creation of cP Air Mass Williams p23

36. Creation of mP Air Mass Lutgens & Tarbuck, p 230

37. Lake Effect Snows As cP air flows over the warmer, open Great Lakes, it is warmed and moistened. When the modified cP air flows onshore, prodigious snows of several feet can result. Lutgens & Tarbuck, p 230

38. Creation of mT and cT Air Masses Williams p24

39. Contrasting Air Masses Ahrens Fig 8.9 17 Apr 1976

40. Paths of cP Air Masses Ahrens Fig 8.3

41. cP Air Mass Ahrens Fig 3 p203 24 Dec 1983

42. Modification of cP Air Mass Ahrens Fig 8.4 cP cP mP mP warm ocean

43. mP Air Masses Ahrens Fig 8.7Ahrens Fig 8.5 Air mass modified further as it crosses several mountain ranges of West U.S.

44. cT Air Mass Ahrens Fig 8.1029-30 Jun 1990

45. Winter mT Pacific Air Masses mT mP Ahrens Fig 8.8 “Pineapple Express”

46. Cold Air-Warm Air Movement Williams p23

47. Weather Map with Air Masses Ahrens Fig 8.11

48. Due Today Turn in 4” by 6” card, include Comments on Lecture SID NAME

49. Summary Air Masses Large (>1000 miles) regions with “uniform” temperature and moisture characteristics Classified by Source Region Continental (c) or Maritime (m) Polar (P) or Tropical (T) Source Regions Big in area (>>1600 km by 1600 km) Dominated by light winds (long resident times)

50. Assignment for Next Lecture Topic - Fronts Reading - Ahrens pg 212-219 Problems - 8.12, 8.13