Another hint for a changing stratospheric circulation after 2001

Harald Bönisch (1), Andreas Engel (1), Thomas Birner (2), Peter Hoor (3)

(1) Institute for Atmospheric and Environmental Sciences, J. W. Goethe University, Frankfurt am Main, Germany

(2) Department of Atmospheric Science, Colorado State University, Fort Collins, CO, USA

(3) Max Planck Institute for Chemistry, Air Chemistry department, Mainz, Germany

The Extratropical UTLSCommunity workshop at the National Center for Atmospheric Research, Boulder, Colorado

19-22 October 2009

Another hint?

Decreases in stratospheric water vapor after 2001: Links to changes in the tropical tropopause and the Brewer-Dobson circulation(Randel et al., JGR, 2006)

Another hint?

Decreases in stratospheric water vapor after 2001: Links to changes in the tropical tropopause and the Brewer-Dobson circulation(Randel et al., JGR, 2006)

Enhanced stratospheric circulation?

Adapted from Schmidt et al. (2001)

Residual circulation

Mixing barriers

Lower

Lower-most

Large-scalemixing processes

Increasing upper branch of the Brewer-Dobson circulation?

Mean age of air in the midlatitudes above 30 hPa

Engel et al., Nature Geosciences, 2008

Slope = +0.24 ±0.22 years

Enhanced stratospheric circulation?

Adapted from Schmidt et al. (2001)

Residual circulation

Mixing barriers

Lower

Lower-most

Large-scalemixing processes

Observations: Higher N2O in the LMS

In-situ aircraft measurements▬ Before 2001▬ After 2001

Winter (February)STREAM (1997) vs. SPURT (2003)

Summer (July)STREAM (1998) vs. SPURT (2003)

Diagnostic: N2O-O3 Correlation

Concept of Plumb and Ko, JGR, 1992

tropicalreference correlationupper envelope

extratropicalreference correlationlower envelope

Isentropes

Diagnostic: N2O-O3 Correlation

Enhanced isentropic transport relative to the downwelling leads to steeper (more tropical) correlation

▬ Tropics ▬Extratropics

(No Vortex)--- Isentropes

In-mixing of tropospheric

air

In-mixing of tropical stratospheric air

Seasonality of the residual circulation

Winter (DJF)strongest downwelling

Summer (JJA)weakest downwelling

Seasonality of the N2O-O3 correlation

270 290 310 270 290 310 270 290 310

Winter (February)STREAM (1997)

Spring (May)STREAM (1996)

Summer (July)STREAM (1998)

Seasonality

strongest downwelling

weakestdownwelling

strongest influence of downwelling

phase shifted by ~3-4 month

Changing N2O-O3 correlationbefore and after 2001

270 290 310 270 290 310 270 290 310

SPURT (2003) SPURT (2002) SPURT (2003)

270 290 310 270 290 310 270 290 310

Winter (February)STREAM (1997)

Spring (May)STREAM (1996)

Summer (July)STREAM (1998)

steepergradient

s

Before 2001

After 2001

Changing N2O-O3 correlationbefore and after 2001

Hypothesis:More tropical air masses in the LMS after 2001 caused by the intensification of the lower BD

circulation branch

Other reasons?

Changing O3 photochemistry in the stratosphere

Can not explain higher N2O mixing ratios in the LMS. Unlikely in the LS because of long effective lifetime for O3 there.

Changing influence of arctic vortex O3-depletion on trace gas composition in the LS, especially in spring

Other reasons?O

3-de

plet

ion

[DU

]

Year

Arctic vortex ozone depletion between 380 K and 550 K calculated from HALOE methane und ozone profiles, numbers from Tilmes et al. (2004)

SPURT

(May)

SPURT(April)

STREAM

(May)

Summary and Outlook

Enhanced tropical upwelling after 2001 (Randel et al., 2006) is coupled with an intensification of the lower BD circulation branch.

higher N2O mixing ratios in the LMS after 2001 steeper N2O-O3 gradients in the LMS after 2001 non-decreasing mean age of air above 30 hPa

Including more aircraft campaigns Looking for changes of other tracers and correlations Looking for changes of dynamical parameters, e.g. EP-flux

Summary and Outlook

=> Changes in the BD might be coupled with changes of the TIL and tropopause structuree.g. “Residual circulation and tropopause structure”Thomas Birner

=> Chemical composition in the LMS above the ExTL is dominated by the lower branch of the BD circulation.e.g. “Transport timescales and surface source regions of UTLS air from START-08 tracer correlations”Eric Ray, F. Moore, K. Rosenlof

Thank you for your attention