The Atmosphere: Part 6: The Hadley Circulation

1. The Atmosphere:Part 6: The Hadley Circulation • Composition / Structure • Radiative transfer • Vertical and latitudinalheat transport • Atmospheric circulation • Climate modeling Suggested further reading: James, Introduction to Circulating Atmospheres (Cambridge, 1994) Lindzen, Dynamics in Atmospheric Physics (Cambridge, 1990)

2. Calculated rad-con equilibrium T vs. observed T pole-to-equator temperature contrast too big in equilibrium state (especially in winter)

3. Zonally averaged net radiation Diurnally-averaged radiation Observed radiative budget Implied energy transport: requires fluid motions to effect the implied heat transport

4. Roles of atmosphere and ocean net ocean atmosphere Trenberth & Caron (2001)

5. Rotating vs. nonrotating fluids Ω Ω f > 0 Ωsinφ u φ f = 0 f < 0

6. Hypothetical 2D atmosphere relaxed toward RCE 2D: no zonal variations Annual mean forcing — symmetric about equator φ

7. A 2D atmosphere forced toward radiative-convective equilibrium Te(φ,p) (J = diabatic heating rate per unit volume)

8. Hypothetical 2D atmosphere relaxed toward RCE Above the frictional boundary layer, φ Ω r a Absolute angular momentum per unit mass φ

9. Angular momentum constraint Above the frictional boundary layer, In steady state, φ

10. Angular momentum constraint Above the frictional boundary layer, In steady state, φ Or 2) but m constant along streamlines v=w=0 Either 1) (if u=0 at equator) T = Te(φ,z)

11. Angular momentum constraint Above the frictional boundary layer, In steady state, φ Or 2) but m constant along streamlines v=w=0 Either 1) (if u=0 at equator) T = Te(φ,z)

12. Near equator, solution (2) no good at high latitudes u finite (and positive) in upper levels at equator; angular momentum maximum there; not allowed solution (1) no good at equator Hadley Cell T = Te

13. Near equator, solution (2) no good at high latitudes u finite (and positive) in upper levels at equator; angular momentum maximum there; not allowed solution (1) no good at equator Hadley Cell T = Te

14. Observed Hadley cell v,w

15. Structure within the Hadley cell In upper troposphere, Hadley Cell T = Te

16. Structure within the Hadley cell In upper troposphere, J Hadley Cell T = Te

17. Observed Hadley cell v,w u Subtropical jets

18. Structure within the Hadley cell In upper troposphere, Near equator, Hadley Cell T = Te But in lower troposphere, (because of friction ). T~φ4 Te~φ2

19. Structure within the Hadley cell In upper troposphere, Near equator, Hadley Cell T = Te But in lower troposphere, Vertically averaged T very flat within cell (because of friction ). T~φ4 Te~φ2

20. Observed Hadley cell v,w T

21. Structure within the Hadley cell In upper troposphere, Near equator, Hadley Cell T = Te But in lower troposphere, T>Te: diabatic cooling ➙ downwelling (because of friction ). T~φ4 Te~φ2 edge of cell T<Te: diabatic heating ➙ upwelling

22. The symmetric Hadley circulation

23. JJA circulation over oceans (“ITCZ” = Intertropical Convergence Zone)

24. Monsoon circulations(in presence of land) winter summer

25. Satellite-measured (TRMM) rainfall, Jan/Jul 2003

26. Satellite-measure (TRMM) rainfall, Jan/Jul 2003 ITCZ South Asian monsoon deserts

27. Heat (energy) transport by the Hadley cell Poleward mass flux (kg/s) = Mut Moist static energy (per unit mass) E = cpT + gz + Lq Net poleward energy flux F = MutEut – MltElt = M(Eut – Elt) • But • Convection guarantees • Eut = Elt at equator • (ii) Hadley cell makes T flat across the cell • ➙ weak poleward energy transport Equatorward mass flux (kg/s) = Mlt Mass balance: Mut = Mlt = M

28. Roles of atmosphere and ocean net ocean atmosphere Trenberth & Caron (2001)