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Lecture13 oct23-bb(1)


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Lecture13 oct23-bb(1)

  1. 1. Lecture  13 General Circulation Jet stream El Nino and La Nina 1
  2. 2. Hadley cell Thermal cell
  3. 3. Hadley cell Light, variable winds at the equator are known as Doldrums Similarly, little wind at 30o N and S is known as the Horse Latitudes 3
  4. 4. Polar cell Thermal cell What is difference between two convergent Lows: ITCZ and subpolar Low? 4
  5. 5. Ferrel cell Dynamic cell Ferrel cell is not successful in explaining the westerlies aloft. 5
  6. 6. Scales of Atmospheric Motions Time and space scale of atmospheric motions Typical size Global scale 5000 km Synoptic scale 2000 km Mesoscale 20 km Microscale 2m Typical life span 6
  7. 7. Semi-permanent pressure areas and seasonal pressure areas 7 Fig. 7-26, p. 190
  8. 8. Semi-permanent Pacific High Semi-permanent Bermuda High 8
  9. 9. Semi-permanent Icelandic Low Semi-permanent Aleutian Low 9
  10. 10. Seasonal pressure areas: Canadian High Seasonal pressure areas: Siberian High 10
  11. 11. Semi-permanent pressure areas and seasonal pressure areas 11 Fig. 7-26, p. 190
  12. 12. at the center of a surface low, the air converges, and then must rise H L at the center of a surface high, the air diverges, and must come from aloft due to sinking motion 12
  13. 13. Zonal distribution of precipitation: 0oN---low pressure cloudy 30oN---high pressure sunny 45-60oN---low pressure cloudy Polar latitudes---high pressure clear 13
  14. 14. Weather associated with The Pacific and Bermuda Highs Pacific High (1) moves northward during summer produced strong subsidence inversion on eastern side (2) during winter, it moves south allowing polar fronts to bring precipitation to SW US 14 Bermuda High transports moist, warm subtropical air to US and southern Canada This air can be unstable
  15. 15. Weather associated with The Pacific and Bermuda Highs 15
  16. 16. Jet Steams
  17. 17. Polar Jet Situated at about 10 km AGL over the polar front (30-70oN) Subtropical Jet Situated above the subtropical highs at about 13 km AGL(20-50oN) 17
  18. 18. 18 1 knot = 1.151 miles per hour
  19. 19.    often have troughs and ridges generally have a maximum (jet streak) in the base of the trough transport heat pole ward (cold air south and warm air north) 19
  20. 20. Jet stream formation Polar Jet 10 8 200 mb 500 mb PGF 700 mb 6 5 km km 4 2 Warm Cold 0 EQ 45oN Large temperature Gradient at surface 20 NP
  21. 21. Jet stream formation Polar Jet Q: How is the subtropical jet formed? 21
  22. 22. Which position is fast? B A 22
  23. 23. Jet stream formation Subtropical Jet -formed on pole ward side of Hadley cell -created largely through the conservation of angular momentum -angular momentum = mVr m=mass V=velocity r=radius V1 -conservation of angular momentum m1V1 r1 m 2V 2 r2 constant r1 m1 V2 r2 m2 23
  24. 24. El Nino and La Nina
  25. 25. • Coastal divergence results in upwelling as cold water rises to replace surface water   brings cold water from deep ocean to the surface cold, nutrient-rich water rises to replace the surface water (good for fishing)
  26. 26. Walker Circulation  East-west circulations caused by continent and topography PGF Typical Walker Circulation rising air is over the western Pacific, and sinking air is over the eastern Pacific 26
  27. 27. Abnormal Walker Circulation  El Nino year occurs with an abnormal Walker circulation Circulation during an El Nino PGF El Nino refers to eastern movement of warm water from the western equatorial Pacific to the eastern equatorial Pacific 27
  28. 28. The animation of sea surface temperature (SST) anomalies to the right shows the unusual warming that occurred during the 97-98 event. Note that an anomaly is a departure from some "normal" value. An anomaly can be either positive (warm) or negative (cold)
  29. 29. El Nino related to the abnormal Walker circulation Spanish name for a little boy during certain years, the coastal waters near Peru were abnormally warm, causing unfavorable fishing conditions. This would occur during the Christmas period occur every 3-7 years, lasting about one year La Nina related to the strong Walker circulation Spanish name for a little girl 29
  30. 30. Previous El Niño Years 1902-1903 1905-1906 1911-1912 1914-1915 1918-1919 1923-1924 1925-1926 1930-1931 1932-1933 1939-1940 1941-1942 1951-1952 1953-1954 1957-1958 1965-1966 1969-1970 1972-1973 1976-1977 1982-1983 1986-1987 1991-1992 1994-1995 1997-1998 2002-2003 2009- 30
  31. 31. Sea surface temperature (SST) in a normal year Low pressure dominates the western Pacific over warm water, and high pressure dominates the eastern Pacific over cold water. 31 Fig. 7-32, p. 196
  32. 32. SST in an El Nino year during an El Nino year, the SSTs in the eastern Pacific become more warmer than in a normal year high pressure shifts from the eastern Pacific to the western Pacific low pressure shifts from the western Pacific to the eastern Pacific this shift in surface pressure is called El Nino Southern Oscillation (ENSO) 32 Fig. 7-32, p. 196
  33. 33. El Nino can have a dramatic effect on weather around the world, creates both precipitation and temperature anomalies. These changes are referred to as the teleconnections 33
  34. 34. ONI Oceanic Nino Index
  35. 35. Nino Region 3.4
  36. 36. Oceanic Niño Index (ONI) • The ONI is based on SST departures from average in the Niño 3.4 region, and is a principal measure for monitoring, assessing, and predicting ENSO. • Defined as the three-month running-mean SST departures in the Niño 3.4 region. Departures are based on a set of improved homogeneous historical SST analyses (Extended Reconstructed SST – ERSST.v3b). The SST reconstruction methodology is described in Smith et al., 2008, J. Climate, vol. 21, 2283-2296.) • NOAA’s operational definitions of El Niño and La Niña are keyed to the ONI index.
  37. 37. NOAA Operational Definitions for El Niño and La Niña El Niño: characterized by a positive ONI greater than or equal to +0.5 C. La Niña: characterized by a negative ONI less than or equal to -0.5 C. By historical standards, to be classified as a full-fledged El Niño or La Niña episode, these thresholds must be exceeded for a period of at least 5 consecutive overlapping 3-month seasons. CPC considers El Niño or La Niña conditions to occur when the monthly Niño3.4 OISST departures meet or exceed +/- 0.5°C along with consistent atmospheric features. These anomalies must also be forecasted to persist for 3 consecutive months.
  38. 38. ONI (oC): Evolution since 1950 The most recent ONI value (August – October 2011) is -0.4oC. El Niño neutral La Niña