Department of Physics, University of Toronto. Thanks: James Anstey, Stephen Beagley, Erich Becker, Michaela Hegglin, Paul Kushner

Transcription

1 Stratospheric Residual Circulation and Tropopause Structure Thomas Birner Theodore G. Shepherd Department of Physics, University of Toronto Thanks: James Anstey, Stephen Beagley, Erich Becker, Michaela Hegglin, Paul Kushner

2 OUTLINE Intro on Coupling Stratosphere Tropopause Residual Circulation near Subtropical Jet Thermal Structure of Tropopause Region (Tropopause Inversion Layer, TIL) Coupling of Residual Circulation and TIL

3 Stratosphere Tropopause Coupling Stratosphere Troposphere Coupling usually involves the Tropopause: S T = S TP + TP T Tropopause Variability and Trend more strongly related to Stratosphere than to Troposphere (e.g. Seidel & Randel, 2006; Son et al., 2006; Son et al., submitted) Link between Tropical and Polar Tropopause through residual circulation (Thuburn & Craig, 2000; Wong & Wang, 2003)

4 from Vallis (2006)

5 weak circulation strong circulation control run Thuburn & Craig (2000)

6 ERA40

7 ERA40

8 from Vallis (2006)

9 downwelling tropospheric circulation

10 DATA Canadian Middle Atmosphere Model T47L71, i.e. vertical resolution near tropopause ~ 1 km ERA40 on model levels (T159L60), i.e. vertical resolution near tropopause ~ 0.8 km

11 CMAM, EP Flux Div & Res. Stream Fct., January

12 CMAM, EP Flux Div & Res. Stream Fct., January Div F > 0 & recirculation, i.e. equatorward mass flux Stagnation

13 ERA40, EP Flux Div & Res. Stream Fct., January

14 Transformed Eulerian (~ Residual) Mean Momentum Equation: EP Flux Divergence meridional PV Flux (on isentropes) Flux Gradient Relationship for PV, i.e. downgradient (diffusive) mixing:

15 Conceptual Model of Rossby Wave Breaking & Mixing stratosphere tropopause irreversible yp < 0 troposphere latitude = 0 longitude downgradient: v/p/ > 0 v < 0, i.e. equatorward (polerly) mass transport (S T)

16 Conceptual Model of Rossby Wave Breaking & Mixing tropopause stratosphere irreversible yp < 0 latitude troposphere = 0 longitude downgradient: v/p/ > 0 v < 0, i.e. equatorward (polerly) mass transport (S T)

17 CMAM, January, 330 K from instantaneous data! Flux Gradient Relation still holds locally!

18 CMAM, EP Flux Div & Res. Stream Fct., January

19 st Conclusions (1 Part) equatorward residual mass flux through subtropical tropopause (S T) associated with poleward PV flux (positive EP flux divergence) driven by local dynamics of breaking Rossby waves in agreement with trajectory studies (e.g. Bourqui & Wernli, 2002) implications for tropopause structure (see below)

20 Thermal Structure of the (Extratropical) Tropopause Region

21 Annual Mean 45 N from High Resolution Radiosoundings Dashed: U.S. Standard Atmosphere Dotted: Conventional Average Full: Tropopause Based Average Birner 2006, JGR

22 Annual Mean 45 N from High Resolution Radiosoundings Dashed: U.S. Standard Atmosphere Dotted: Conventional Average Full: Tropopause Based Average Tropopause Inversion Layer (TIL) Birner 2006, JGR Buoyancy Frequency Squared

23 Zonal Averages, N2 & Isentropes Sondes ('98 '02), Tropopause Based Winter (DJF) Summer (JJA) 10 4 s 2 this is N only! Birner 2006, JGR

24 Zonal Averages, N2 & Isentropes CMAM (free running, equilibrated) DJF JJA 10 4 s 2 this is South to North Pole! Birner et al. 2006, GRL

25 Zonal Averages, N2 & Isentropes ERA40 ('98 '02), Tropopause Based DJF JJA 10 4 s 2 this is South to North Pole! Birner et al. 2006, GRL

26 Coupling of Residual Circulation and Thermal Structure of the Tropopause Region (TIL)

27 CMAM, EP Flux Div & Res. Stream Fct., January

28 Transformed Eulerian (~ Residual) Mean Thermodynamic Equation Residual Vertical & Meridional Velocities Vertical Convergence Vertical Advection Diabatic Heating (mainly radiative in the stratosphere) Diabatic Contribution

29 Vertical Structure of residual vertical Velocity and Static Satbility CMAM ERA40 ( ) Tropopause

30 Annual Cycle of residual vertical Velocity Structure and Static Satbility

31 Annual Cycle of residual vertical Velocity Structure and Static Satbility Time Scale ~ 70 days Lag ~ 2 months

32 Transformed Eulerian (~ Residual) Mean Thermodynamic Equation Newtonian Cooling Approximation

33 N2rad & corresponding Isentropes DJF JJA 10 4 s 2

34 Simulated Equilibrium Response N2 & Isentropes (after ~ 100 days) DJF JJA 10 4 s 2

35 Simulated Equilibrium Response N2 & Isentropes (after ~ 100 days) DJF JJA 10 4 s 2 Double Tropopause

36 Zonal Averages, N2 & Isentropes CMAM (free running, equilibrated) DJF JJA 10 4 s 2 this is South to North Pole! Birner et al. 2006, GRL

37 Annual Cycle of residual vertical Velocity Structure and Static Satbility

38 Radiative response to water vapor and ozone in the lowermost stratosphere dashed lines indicate applied perturbations Randel et al., in press (JAS): Tropopause Inversion Layer from GPS radio occulations

39 Radiative Mechanism H2O enhanced in TIL (compared to background stratosphere) due to Stratosphere Troposphere Exchange Mixing Layer Radiative Cooling just above the Tropopause

40 45 60N H2OTP 60 90N Winter: weak relationship between residual circulation and N2max Summer: H O radiative 2 tropopause, i.e. pronounced N2max Winter: strong relationship between residual circulation and N2max Summer: N2max not very distinct H2OTP

41 nd Conclusions (2 Part) GCMs reproduce strong static stability maximum (TIL) just above extratropical tropopause: implies large scale cause One candidate seems to be the vertical structure of the residual circulation (especially in Winter) H O radiative effects seem to be main 2 cause of TIL in summer

42

43 Anticorrelation of Tropical and Polar Tropopause

44