Chapter 11: Atmospheric/Oceanic Circulation

1. Chapter 11: Atmospheric/Oceanic Circulation Objectives: Driving force Meridional circulation cells Hadley, Ferrel & Polar cells Surface winds & sea-level pressure wind, precipitation and temperature Surface and deep oceanic circulation UNBC

2. Atmospheric circulation is the large-scale movement of air. • The large-scale structure of the atmospheric circulation varies from year to year, but the basic structure remains fairly constant. • Individual weather systems – mid-latitude weather may occur "randomly“. However, the average of these systems - the climate - is quite stable. UNBC

3. Primary High-Pressure and Low-Pressure Areas • Equatorial low-pressure trough • Subtropical high-pressure cells • Subpolar low-pressure cells • Polar high-pressure cells UNBC

4. Driving force • At higher lat., solar energy flux spreads over a wider area => less solar energy per unit area. • Earth emits outgoing radiation. • Net radiation = (incoming solar rad.) • - (outgoing rad.) . UNBC

5. Annual incoming solar rad. Annual outgoing terrestrial rad. • Net radiation has deficit poleward of 37°, & surplus equatorward of 37°. • This means poles should keep cooling while tropics keep warming. Since this is not happening, other processes must operate to maintain net energy balance at each lat. • Atm. & oc. circulation (climate & weather) due to unequal lat. distr. of energy. UNBC

6. Single Convection cell in a non-rotating Earth • Imagine the earth as a non-rotating sphere with uniform smooth surface characteristics. • the sun heats the equatorial regions much more than the polar regions. In response to this, two huge convection cells develop. UNBC

7. Farrell Cell polar Cell UNBC

8. Hardly Cell • Atmosphere is heated in the equator => Air becomes less dense and rises => Rising air creates low pressure at the equator. • Air cools as it rises because of the lapse rate => Water vapor condenses (rains) as the air cools with increasing altitude => Creates high rainfall associated with the Intertropical Convergence Zone in the tropics (ITCZ). • As air mass cools it increases in density and descends back to the surface in the subtropics (30o N and S), creating high pressure. UNBC

9. Polar cell and Farrell cell • In the pole area, the surface is much cold, especially in winter. This results in increased air density near the surface => higher pressure. The higher density and pressure lead to divergence => surface air moves towards tropic. The cold air from pole will meet the warm air from Tropic around to form “Pole Front Zone. • For mass conservation, there are aloft circulations corresponding the surface circulations, which forms two cells, called Pole cell and Farrell cell. UNBC

10. A idealized pattern of surface wind without rotation of the earth UNBC

11. The pattern of surface wind with the rotation of Earth UNBC

12. The three-celled model vs. reality: the bottom line • Hadley cells are close approximations of real world • Ferrel and polar cells do not approximate the real world • Model is unrepresentative of flow aloft • Continents and topographic irregularities cause model oversimplification UNBC

13. A) Idealized winds generated by pressure gradient and Coriolis Force. B) Actual wind patterns owing to land mass distribution.. UNBC

14. Sfc. winds converg. towards Eq., deflected by Coriolis => Easterlies (NE & SE Trades) The sfc. winds converg. at the ITCZ (Intertropical Convergence Zone). Rising air => clouds. region called “doldrums” by sailors. Rising air at ITCZ spreads poleward, sinking at 30° (high p belts = subtropical highs). region called “horse latitudes” (horses thrown overboard or eaten by sailors). UNBC

15. High p at poles => sfc.air flows equatorward; Deflected by Coriolis => Polar Easterlies converg. to the Subpolar Low (low p at 60°) Coriolis deflects air flowing from subtropical high to subpolar low => Westerlies Polar cell 60°-90°, Ferrel cell 30°-60° Where mild air from Ferrel cell meets cold air from polar cell => polar front Hadley & polar cells are thermally driven, but Ferrel cell is a thermally indirect cell. UNBC

16. Horse LatitudesAround 30°N we see a region of subsiding (sinking) air.  Sinking air is typically dry and free of substantial precipitation. Many of the major desert regions of the northern hemisphere are found near 30° latitude.  E.g., Sahara, Middle East, SW United States. • Doldrums Located near the equator, the doldrums are where the trade winds meet and where the pressure gradient decreases creating very little winds.  That's why sailors find it difficult to cross the equator and why weather systems in the one hemisphere rarely cross into the other hemisphere.  The doldrums are also called the intertropical convergence zone (ITCZ). UNBC

17. Surface winds & sea-level pressure (SLP) January UNBC

18. Surface winds & sea-level pressure (SLP) July UNBC

19. N.Hem.: Bermuda (Azores) High, Pacific High, Icelandic Low, Aleutian Low. July: Bermuda High & Pacific High stronger & further north. Icelandic Low & Aleutian Low weaken & shift northward. ITCZ shifts northward Jan.: Highs over continents in N.Hem., lows over contin. in S. Hem. (monsoon effect). July: Lows over continents in N.Hem., highs over contin. in S. Hem. UNBC

20. The Sahel reflects seasonal migration of the ITCZ UNBC

21. Shifts in the ITCZ affect the Sahel UNBC

22. Hadley, Ferrell, and Polar cells are major players in global heat transport of the south-north. They are called Latitudinal circulation, causedby latitudinal difference of incident solar radiation. Longitudinal circulation, on the other hand, comes about because water has a higher specific heat capacity than land and thereby absorbs and releases heat less readily than land UNBC

23. The Upper Troposphere • Two important points: • Pressure decreases more rapidly in cold air • Temperature in lower troposphere decreases poleward • Creates upper level pressure gradients UNBC

24. Pressure and Height UNBC

25. 500 mb heights decrease poleward • pressure gradient stronger during the winter • 500 mb heights are higher in the summer than in the winter UNBC

26. Westerly Winds in the Upper Atmosphere • the existence of the upper level pressure gradient  air is being • pushed toward poles • Coriolis effect deflects upper air  Westerlies dominate upper troposphere • Strongest during winter • explains why storms move eastward, • flight times UNBC

27. The Polar Front and Jet Streams • Gradual change in temperature with latitude does not always occur • Steep temperature gradients exist between cold and warm air masses • polar front - marks area of contact, steep pressure gradient  polar jet stream • polar jet stream - fast stream of air in upper troposphere above the polar front • stronger in winter, affect daily weather patterns • Low latitudes  subtropical jet stream UNBC

28. Rossby Waves • Rossby waves - largest of these planetary wave patterns • seasonal change – fewer, more well-developed waves in winter, with • stronger winds • instrumental in meridional transport of energy and storm development UNBC

29. The Walker circulation is caused by the pressure gradient force that results from a high pressure system over the eastern pacific ocean, and a low pressure system over Indonesia. When the Walker circulation weakens or reverses, an El Niño results. The Southern Oscillation Index (SOI) is calculated from the monthly or seasonal fluctuations in the air pressure difference between Tahiti and Darwin. UNBC

30. UNBC

31. The Walker circulation, which spans almost half the circumference of Earth, pushes the Pacific Ocean’s trade winds from east to west, generates massive rains near Indonesia, and nourishes marine life across the equatorial Pacific and off the South American coast. Changes in the circulation, which varies in tandem with El Niño and La Niña events, can have far-reaching effects. UNBC

32. Jet streams • Jet streams: air currents thousands of km long, hundreds of km wide, a few km thick (centred near tropopause). • Max speed > 200 km/hr. • Polar jet stream near polar front, separating cold air from mild air. Jet stream turning south => cold air moves south. UNBC

33. So there is sinking air around 30 degree, which forms divergence region on surface and convergence region aloft. The convergence between cold air with warm air can cause a great temperature gradient in this region, and further causing a large gradient in pressure => speeding the air flow => cause the jet. UNBC

34. Subtropical jet stream at ~30° • Jet streams meander, polar jet may merge with subtropical jet. • Polar jet may also branch into 2. UNBC

35. 2 mechanisms for jet streams • Where polar cell meets Ferrel cell, or Ferrel cell meets Hadley, airs of different T meet => large T gradient => large p gradient => geostrophic winds. 2) As air moves from low to high lat., its circular orbit shrinks. => orbiting speed incr. (conservation of angular momentum; e.g. spinning skater moves arms towards body). UNBC

36. Sea breeze • Daytime: land warms more than sea => rising air & low p on land. Air flows from sea to land. UNBC

37. Land breeze • Night: Land cools more than sea. => Sinking air & high p over land. Air flows from land to sea. UNBC

38. Valley breeze & mountain breeze • Daytime: At same elevation, air on mountain slope heated more than air over valley => low p over mountain slope => air flows upslope from valley (valley breeze). • Night: Air on mountain slope cooled more than air over valley => mountain breeze. UNBC

39. Chinook wind • Chinook: warm, dry wind on eastern slope of Rockies. • Western slope: condensation => release of latent heat. Moisture lost from precip. • Descending wind on eastern slope => warming from compression. UNBC

40. Monsoons • largest synoptic scale winds on Earth • A seasonal reversal of wind • Asian monsoon which is characterized by dry (wet), offshore (onshore) flow conditions during cool (warm) months • Orographic lifting leads to high precipitation totals UNBC

41. Monsoons • Winter: continents cool more than oc. => sinking air & high p over continent • Summer: continents warm more than oc. => rising air & low p over continent • Most prominent with the massive Asian land mass. UNBC

42. Winter monsoon UNBC

43. Summer monsoon UNBC

44. Meridional cells & precip. • Northward shift of cells during summer, & southward shift during winter => precip. changes. UNBC

45. Oceanic Circulation UNBC

46. thermocline-> Temperature T • Vertical profile: • Solar radiation absorbed within 100m of sea surface. • Wind => surface mixed layer of 50-200m, (T is nearly uniform). • Thermocline occurs between 200-1000m depth: T decr. rapidly with depth. • Below thermocline, T decr. very slowly to 0-3oC at oc. bottom. Mixing layer UNBC

47. Thermocline: The thermocline is the transition layer between the mixed layer at the surface and the deep water layer. In the thermocline, the temperature decreases rapidly from the mixed layer temperature to the much colder deep water temperature. • The mixed layer and the deep water layer are relatively uniform in temperature, while the thermocline represents the transition zone between the two. UNBC

48. South North Vertical temperature section in Atlantic UNBC

49. UNBC

50. Surface currents • Gyres: Large horizontal circulation cells. UNBC