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

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

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

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

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

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

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

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

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

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

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

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

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

Observed Hadley cell v,w

Structure within the Hadley cell Hadley Cell T = T e In upper troposphere,

Structure within the Hadley cell Hadley Cell T = T e In upper troposphere, J

Observed Hadley cell u v,w Subtropical jets

Structure within the Hadley cell Hadley Cell T = T e Near equator, In upper troposphere, But in lower troposphere, (because of friction ). T~φ 4 Te~φ2Te~φ2

Structure within the Hadley cell Hadley Cell T = T e Near equator, In upper troposphere, But in lower troposphere, (because of friction ). T~φ 4 Te~φ2Te~φ2 Vertically averaged T very flat within cell

Observed Hadley cell T v,w

Structure within the Hadley cell Hadley Cell T = T e Near equator, In upper troposphere, But in lower troposphere, (because of friction ). T~φ 4 Te~φ2Te~φ2 T<T e : diabatic heating ➙ upwelling T>T e : diabatic cooling ➙ downwelling edge of cell

The symmetric Hadley circulation

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

Monsoon circulations (in presence of land) summerwinter

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

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

Heat (energy) transport by the Hadley cell Poleward mass flux (kg/s) = M ut Equatorward mass flux (kg/s) = M lt Mass balance: M ut = M lt = M Moist static energy (per unit mass) E = c p T + gz + Lq Net poleward energy flux F = M ut E ut – M lt E lt = M(E ut – E lt ) But (i)Convection guarantees E ut = E lt at equator (ii) Hadley cell makes T flat across the cell ➙ weak poleward energy transport

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