Isentropic flows and monsoonal circulations

Transcription

1 Isentropic flows and monsoonal circulations Olivier Pauluis (NYU) Monsoons- Past, Present and future May 20th, 2015 Caltech, Pasadena

2 Outline Introduction Global monsoon in isentropic coordinates Dry ventilation Moist ventilation Role of deserts

3 The general circulation Annual Radiation Budget The atmosphere (and ocean) act to transport energy poleward to balance the imposed latitudinal radiation imbalance. Quizz: Where is the meridional circulation at its strongest?

4 Eulerian-mean circulation January July pressure pressure 70S latitude 70N 70S latitude 70N cint = 2.e10 kg/s mean(,p,t)= Z p 0 v(,p 0,t) dp0 g

5 Ferrel cells Hadley cells Polar cell 45S Equator 45N Polar cell Eulerian-mean circulation exhibits the classic three- Exhibits three cell structure Cross-equatorial Hadley dominates It implies a separation between tropics and extra tropics

6 Dry isentropic circulation 360kJ January 360kJ July DSE Z Z Z DSE 270kJ 70S latitude cint = 2.e10 kg/s 70N 270kJ 70S latitude 70N (, 0 )= 1 T Z T 0 Z psurf 0 Z 2 0 a cos g vh( 0 (,,p,t))d dp dt

7 Why the circulation in eulerian and isentropic coordinates are in the opposite direction? v < 0 v > 0 longitude In the midlatitudes, the flow is highly turbulent: the meridional velocity alternates between poleward and equatorward at all levels.

8 In the stormtracks: Eulerian-mean circulation v p < 0 at low pressure v < 0 v > 0 Isobaric surface v p > 0 at high pressure longitude In the midlatitudes, the flow is highly turbulent: the meridional velocity alternates between poleward and equatorward at all levels. This idealized eddies is associated with a poleward flow at high pressure/low level, and equatorward flow at high level

9 In the stormtracks: Isentropic circulation ρ θ v > 0 at high θ v < 0 Cold Warm v > 0 Potential temperature surface ρ θ v < 0 at low θ longitude Thickness variations are such that the upper isentropic layer encompass larger fraction of the poleward flow. Such pattern also corresponds to a net poleward energy mass transport.

10 Isentropic flow and eddy mass transport ρ θ v > 0 at high θ v < 0 v > 0 Potential temperature surface ρ θ v < 0 at low θ longitude The mass flux on isentropic surfaces can be written as: Total mass flux ρ θ v θ = ρ θ θ v θ + ρθ 'v' θ Transport by mean circulation Eddy transport The mass transport by the eddies is in the opposite direction to the mean wind.

11 Parcel trajectories (eulerian) mean velocity Equator Midlatitudes Pole The isentropic circulation better capture the parcels trajectories in presence of eddies. There is no artificial separation between tropics and midlatitudes It directly relates mass transport and transport of dry static energy (enthalpy + geopotential). but it does not include water vapor transport.

12 Moist isentropic circulation January July 360kJ 360kJ MSE Z Z Z MSE 270kJ 270kJ 70S 70N 70S 70N latitude cint = 2.e10 kg/s latitude (, 0 )= 1 T Z T 0 Z psurf 0 Z 2 0 a cos g vh( 0 (,,p,t))d dp dt

13 Why the circulation on moist isentropes is larger? Dry isentrope v < 0 v > 0 longitude

14 In the stormtracks: Circulation on moist isentropes Dry isentrope v < 0 v > 0 Moist isentrope Moist air moving poleward longitude Moist isentropes found in the upper troposphere also intersects the Earth s surface. Such situation corresponds to a poleward flow of warm, moist air near the surface.

15 Circulation on dry isentropes The global circulation has two poleward components in the midlatitudes: an upper tropospheric branch of high θ e -θ; Moist branch: additional mass flow on moist isentropes an a lower branch of warm, most air with high θ e - low θ, which ascent into the upper troposphere within the stormtracks. Mass transport is comparable in each branch.

16 Dry ventilation of the tropical atmosphere 360kJ DSE 270kJ 70S latitude 70N kJ<DSE<297kJ

17 Isentropic Thickness Mass Flux divergence kJ<DSE<297kJ

18 Dry ventilation of the tropical atmosphere 360kJ 360K DSE 270kJ 270K 70S latitude 70N kJ<DSE<307kJ

19 Isentropic Thickness Mass Flux divergence kJ<DSE<307kJ

20 Dry ventilation of the tropical atmosphere 360kJ 360K DSE 270kJ 270K 70S latitude 70N kJ<DSE<317kJ

21 Isentropic Thickness Mass Flux divergence kJ<DSE<317kJ

22 Dry ventilation of the tropical atmosphere 360kJ 360K DSE 270kJ 270K 70S latitude 70N kJ<DSE<327kJ

23 Isentropic Thickness Mass Flux divergence kJ<DSE<327kJ

24 Dry ventilation of the tropical atmosphere 360kJ 360K DSE 270kJ 270K 70S latitude 70N kJ<DSE<342kJ

25 Isentropic Thickness Mass Flux divergence kJ<DSE<342kJ

26 Dry ventilation of the tropical atmosphere 360kJ 360K DSE 270kJ 270K 70S latitude 70N kJ<DSE<357kJ

27 Isentropic Thickness Mass Flux divergence kJ<DSE<357kJ

28 Moist ventilation of the tropical atmosphere 360kJ 360K MSE 270kJ 270K 70S latitude 70N kJ<MSE<327kJ

29 Isentropic Thickness Mass Flux divergence kJ<MSE<327kJ

30 Moist ventilation of the tropical atmosphere 360kJ 360K MSE 270kJ 270K 70S latitude 70N kJ<MSE<347kJ

31 Isentropic Thickness Mass Flux divergence kJ<MSE<347kJ

32 Moist ventilation of the tropical atmosphere 360kJ 360K MSE 270kJ 270K 70S latitude 70N kJ<MSE<357kJ

33 Isentropic Thickness Mass Flux divergence kJ<MSE<357kJ

34 34 318kJ<DSE<327kJ

35 35 318kJ<DSE<327kJ

36 36 313kJ<DSE<322kJ

37 Quizz: Where is the meridional circulation at its strongest?

38 Quizz: Where is the meridional circulation at its strongest? At about 30 latitude in Summer hemisphere and 40 latitude in winter hemisphere

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40 Conclusion Computing circulation on isentropic surfaces offer a way of connecting mass and energy transport. Zonally averaged circulations tend to oversimplify the actual flows. This is especially true for Monsoonal circulations. Tropical ventilation is very different over land and oceans: Ocean: equatorward transport of cold dry air, poleward transport of warm, moist air. Land: moisture transport from crossequatorial flow combined with dry air inflow 40 from nearby deserts