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
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
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)
from Vallis (2006)
weak circulation strong circulation control run Thuburn & Craig (2000)
ERA40
ERA40
from Vallis (2006)
downwelling tropospheric circulation
DATA Canadian Middle Atmosphere Model (CMAM) @ T47L71, i.e. vertical resolution near tropopause ~ 1 km ERA40 on model levels (T159L60), i.e. vertical resolution near tropopause ~ 0.8 km
CMAM, EP Flux Div & Res. Stream Fct., January
CMAM, EP Flux Div & Res. Stream Fct., January Div F > 0 & recirculation, i.e. equatorward mass flux Stagnation
ERA40, EP Flux Div & Res. Stream Fct., January 1998-2002
Transformed Eulerian (~ Residual) Mean Momentum Equation: EP Flux Divergence meridional PV Flux (on isentropes) Flux Gradient Relationship for PV, i.e. downgradient (diffusive) mixing:
Conceptual Model of Rossby Wave Breaking & Mixing stratosphere tropopause irreversible mixing @ yp < 0 troposphere latitude = 0 longitude downgradient: v/p/ > 0 v < 0, i.e. equatorward (polerly) mass transport (S T)
Conceptual Model of Rossby Wave Breaking & Mixing tropopause stratosphere irreversible mixing @ yp < 0 latitude troposphere = 0 longitude downgradient: v/p/ > 0 v < 0, i.e. equatorward (polerly) mass transport (S T)
CMAM, January, 330 K from instantaneous data! Flux Gradient Relation still holds locally!
CMAM, EP Flux Div & Res. Stream Fct., January
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)
Thermal Structure of the (Extratropical) Tropopause Region
Annual Mean Climatology @ 45 N from High Resolution Radiosoundings Dashed: U.S. Standard Atmosphere Dotted: Conventional Average Full: Tropopause Based Average Birner 2006, JGR
Annual Mean Climatology @ 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
Zonal Averages, N2 & Isentropes Sondes ('98 '02), Tropopause Based Winter (DJF) Summer (JJA) 10 4 s 2 this is 24 72 N only! Birner 2006, JGR
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
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
Coupling of Residual Circulation and Thermal Structure of the Tropopause Region (TIL)
CMAM, EP Flux Div & Res. Stream Fct., January
Transformed Eulerian (~ Residual) Mean Thermodynamic Equation Residual Vertical & Meridional Velocities Vertical Convergence Vertical Advection Diabatic Heating (mainly radiative in the stratosphere) Diabatic Contribution
Vertical Structure of residual vertical Velocity and Static Satbility CMAM ERA40 (1998 2002) Tropopause
Annual Cycle of residual vertical Velocity Structure and Static Satbility
Annual Cycle of residual vertical Velocity Structure and Static Satbility Time Scale ~ 70 days Lag ~ 2 months
Transformed Eulerian (~ Residual) Mean Thermodynamic Equation Newtonian Cooling Approximation
N2rad & corresponding Isentropes DJF JJA 10 4 s 2
Simulated Equilibrium Response N2 & Isentropes (after ~ 100 days) DJF JJA 10 4 s 2
Simulated Equilibrium Response N2 & Isentropes (after ~ 100 days) DJF JJA 10 4 s 2 Double Tropopause
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
Annual Cycle of residual vertical Velocity Structure and Static Satbility
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
Radiative Mechanism H2O enhanced in TIL (compared to background stratosphere) due to Stratosphere Troposphere Exchange Mixing Layer Radiative Cooling just above the Tropopause
45 60N H2OTP 60 90N Winter: weak relationship between residual circulation and N2max Summer: H O radiative 2 cooling @ tropopause, i.e. pronounced N2max Winter: strong relationship between residual circulation and N2max Summer: N2max not very distinct H2OTP
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
Anticorrelation of Tropical and Polar Tropopause