4.4 Theory

1. 4.4 Theory

2. 4.4 Theory (structures) African Easterly Wave trough Peak rainfall ahead of trough 2-3000km Wavelength 11-12oN Relative Vorticity 700hPa (10-5s-1) Average precipitation rate (mm/day)

3. 4.4 Theory (structures) Note distinctive horizontal tilts

4. 4.4 Theory (structures) Note distinctive vertical tilts

5. 4.4 Theory (Instability) Consensus has developed in the community that AEWs arise in association with a linear instability mechanism Recall the Charney-Stern necessary conditions for instability: In the fluid interior and at the surface have opposite signs or has opposite signs in the fluid interior

6. 4.4 Theory (The basic state)

7. 4.4 Theory (The basic state) 925hPa q 315K PV • PV ‘strip’ present on the cyclonic shear side of AEJ. • Strong baroclinic zone 10o-20oN 925hPa qe • High qe strip exists near 15oN See Dickinson and Molinari (2000) for more discussion of PV sign-reversals)

8. 4.4 Theory (The basic state) Note definition of meridional gradient in qgpv

9. 4.4 Theory (see Class Notes) Discussion of Interacting Rossby Waves

10. 4.5 Modeling (Introduction) Different types of modeling are possible: Idealized modeling Climate modeling using a global model Regional modeling (e.g. for case studies) Here modeling of the AEWs and AEJ are both considered.

11. 4.5 Modeling (Idealized)

12. 4.5 Modeling

13. 4.5 Modeling

14. 4.5 Modeling

15. 4.5 Modeling

16. 4.5 Modeling 9

17. 4.5 Modeling

18. 4.5 Modeling (non-linear)

19. 4.5 Modeling Nonlinear life-cycles

20. 4.5 Modeling

21. 4.5 Modeling

22. 4.5 Modeling

23. 4.5 Modeling

24. 4.5 Modeling (Global)

25. 4.5 Modeling (Global)

26. 4.5 Modeling (Global)

27. 5.5 Modeling (Global)

28. 4.5 Modeling (Global)

29. 4.5 Modeling (Regional) Following is an animation of a predicted AEW sequence in WRF (Berry)

30. 5.5 Modeling (Regional)

31. 4.5 Modeling (The Basic State)

32. 4.5 Modeling (The Basic State)

33. 4.5 Modeling (The Basic State)

34. 4.5 Modeling (The Basic State)

35. 4.5 Modeling (The Basic State)

36. 4.5 Modeling (The Basic State)

37. 4.5 Modeling (The Basic State)

38. 5.5 Modeling (The Basic State) 33

39. 4.5 Modeling (The Basic State)

40. 4.5 Modeling (The Basic State)

41. 4.5 Modeling (The Basic State)

42. 4.5 Modeling (The Basic State)

43. 4.5 Modeling (The Basic State)

44. 4.5 Modeling (The Basic State)

45. 4.5 Modeling (Main Points) Basic State: AEJ is maintained by deep moist convection to the south and dry convection to the north (tephigram construction) From a PV-thinking perspective in addition to the surface temperature gradient the AEJ is associated with two distinctive PV anomalies – a negative one linked to the dry convection and a +ve one linked to the moist convection (and to a lesser extent, descent in the heat low circulation) AEWs: Idealised modeling confirms that AEWs can grow via a mixed barotropic-baroclinic instability mechanisms. Normal modes tend to be dominated by barotropic growth. As these modes grow and become more nonlinear they become more dominated by baroclinic growth. Climate modeling studies tend to be more about the models than the processes (biased view). Regional Modeling can be used for process studies.

46. 4.5 Modeling (Main Points) AEWs and AEJ So the observed WAM is characterized by convective processes that maintain the AEJ and processes linked to the growth of AEWs that weaken the AEJ. The observed climate represents the long-term balance of these two aspects of the WAM system. Question: Do we expect an active AEW year to be associated with a strong AEJ or weak AEJ?