Intensification of the shallow BDC branch: mechanism and consequences for Age of Air

Transcription

1 Intensification of the shallow BDC branch: mechanism and consequences for Age of Air H. Garny1,2, M. Dameris1, W. Randel2, G. Bodeker3, R. Deckert1 T. Birner4 1 Deutsches Zentrum für Luft und Raumfahrt (DLR), Oberpfaffenhofen, Germany 2 Hella Garny NCAR, Boulder, USA 3 Bodeker Scientific, Alexandra, New Zealand 4 Colorado State University, Fort Collins, USA. BDC workshop-2012

2 Motivation: Changes in the BDC Models versus Observations Models: Increase in the stratospheric meridional circulation common result in CCMs, so is decrease in AoA. Models agree well in increase in lower stratospheric tropical upwelling, less well in changes at higher latitudes and altitudes (i.e. deep branch).

3 Motivation: Changes in the BDC Models versus Observations Observations: Models: Increase in the stratospheric meridional circulation common result in CCMs, so is decrease in AoA. Models agree well in increase in lower stratospheric tropical upwelling, less well in changes at higher latitudes and altitudes (i.e. deep branch). No change in AoA in middle stratosphere (Engel et al 2009). Negative trends in tropical LS ozone indicate increased upwelling (Randel&Thompson, 2011). Residual circulation transit time trends from JRA 25 (Bönisch et al 2011). =>weak evidence indicates changes in the shallow but not in the deep branch

4 Motivation: Changes in the BDC Models versus Observations Observations: Models: Increase in the stratospheric meridional circulation common result in CCMs, so is decrease in AoA. Models agree well in increase in lower stratospheric tropical upwelling, less well in changes at higher latitudes and altitudes (i.e. deep branch). No change in AoA in middle stratosphere (Engel et al 2009). Negative trends in tropical LS ozone indicate increased upwelling (Randel&Thompson, 2011). Residual circulation transit time trends from JRA 25 (Bönisch et al 2011). =>weak evidence indicates changes in the shallow but not in the deep branch in general, evidence for shallow branch changes stronger than for deep branch changes Need to understand better what we look at in both models and observations to resolve the apparent discrepancies! Problem: BDC can not be measured directly!! Only tracers can be measured, and from them AoA is derived need to understand relation between AoA and residual circulation, and their impacts on tracers (that are measured) better!

5 Part 1: Changes in the Residual circulation in models Strengthening in most models both in the deep and shallow branch. Hemisphere-wide circulation (deep branch) Secondary circulation (shallow branch)

6 Mechanism for strengthening of the shallow branch Using CCM E39C A. Upper boundary at 10 hpa, no NOGW paramerterization, OGW after Miller et al (1989). Changes only in shallow branch! Due to low lid? low lid does not necessarily lead to lack of deep branch changes, see Talk by Felix Bunzel. E39CA Trend resid. streamfunction [kg/s/a] Due to GW parameterization? Some studies (McLandress and Shepherd, 2009 and Garcia and Randel, 2008) show importance of GW for deep branch

7 Drivers of Tropical Upwelling Trends Difference 2000s 1960s Pressure [hpa] Diff from transient Simulation From Time-slices: SST trop. SST GHG relative Difference in tropical Upwelling / decade [%]

8 Pressure [hpa] Zonal Wind Temperature Drivers of Tropical Upwelling Trends

9 Mechanism of shallow branch strengthening Subtropical jets are shifted upward => upward shift of region of wave dissipation => intensification of shallow branch! Annual mean trend in EP flux Changes in tropical wave generation might intensify response in cases (depending on SST change pattern), but cannot be the main factor. Robust mechanism! Only dependent on tropical mean SST change. Attribution also holds in high top model, see Sophie Oberländer's Talk on Friday. Mechanism in agreement with Shepherd & McLandress, 2011 More details in: Garny et al., JAS, 2011

10 Part 2: Age of Air and Residual Circulation Transit time Age of Air residual circulation transport + effects of Mixing How to distinguish the two? use Residual circulation Transit time (RCTT) (Birner&Bönisch 2011): hypothetical age if there was only transport by the residual circulation calculate backward trajectories driven by the residual winds (v*, w*) in the latitude pressure plain. terminate where trajectories hit the tropopause. The time elapsed is the 'transit time' at the location the trajectory was started. From Birner & Bönisch, 2011

11 Age of Air and RCTT from Model data Calculate 'Residual Circulation Transit time' (RCTT) from global Model allows comparison with Age of Air Note: This in ongoing work and results are PRELIMINARY Mean Age of Air AoA calculated from 3D trajectories run online in the model (advection scheme ATTILA) Residual Circulation Transit time RCTT calculated from 2D trajectories driven by the resid. circ. (v*,w*) only 10 year annual mean from CCM E39CA

12 Difference Age of Air and RCTT: effects of Mixing (+?) Mean Age of Air RCTT AoA RCTT What is this term? Modification in RCTT due to two way mixing (i.e. can be negative!)

13 Aging due to Mixing AoA - RCTT 1. AoA > RCTT almost in entire stratosphere => effects of mixing lead to older air

14 Aging due to Mixing AoA - RCTT RCTT / AoA 1. AoA > RCTT almost in entire stratosphere => effects of mixing lead to older air 2. The additional aging is largest in the subtropics / mid latitudes. The largest contribution to AoA of mixing induced aging also in the subtropical lower stratosphere (up to 70%!) => Recirculation caused by mixing older air in subtropics / mid latitudes

15 Aging due to Mixing AoA - RCTT RCTT / AoA 3. Residual circulation transit time contributes most (60 70%) to AoA in tropics and high latitudes, i.e. in regions of strong upwelling and downwelling but little wave induced mixing.

16 Aging due to Mixing AoA - RCTT RCTT / AoA 4. Younger air in lowermost stratosphere in high latitudes => Mixing along isentrops in lowermost stratopshere: Mixes young tropical air with old extratropical air leads to younger air in extratropics

17 Age of Air and RCTT: relation The meridional circulation is driven by wave dissipation Wave dissipation wave breaking / stirring Mixing Thus, the Residual circulation transit time and aging due to mixing are not independent! Assume RCTT is proportional to aging by mixing (here A_Mix ): RCTT = f * A_Mix (with f dependent on lat, height) Then AoA = RCTT + A_Mix = (1-1/f) RCTT = f2 * RCTT thus AoA is proportional to RCTT, or RCTT/AoA = constant calculate standard deviation in annual mean (RCTT/AoA) over 10 years to get a measure of linearity between AoA and RCTT

18 Age of Air and RCTT: relation E39CA Mean(RCTT/AoA) Std(RCTT/AoA) / Mean(RCTT/AoA) < 10% MPI-Echam6 10yrs of TS1990 Mean(RCTT/AoA) Std(RCTT/AoA) / Mean(RCTT/AoA) < 10% everywhere ~10-20% => Ratio RCTT/AoA is close to constant in MPI Echam6, fluctuations of 10 20% in E39CA (might be due to variability in transient run, in particular QBO nudging, or noisier AoA from trajectories) => Small fluctuations indicate a linear relation between AoA and RCTT, i.e. the aging by mixing scales with variability in the residual circulation

19 Age of Air and RCTT: future changes Thanks to Felix Bunzel for providing data! ΔResidual Circulation [kg/s/year] E39CA MPI-Echam6 TS2050-TS1990 increase in shallow branch of the BDC increase in shallow and deep branch of the BDC

20 Age of Air and RCTT: future changes MPI-Echam6 TS2050-TS1990 ΔResidual Circulation E39CA ΔRCTT/ ΔAoA ΔAoA Relative contribution of changes in Transit time and Mixing to changes in AoA RCTT change only in tropics, contributes most there. Younger air in extratropics due to changes in aging by mixing. contribution to AoA change similar to contribution to mean AoA: RCTT dominated in low and high latitudes, Mixing induced aging in subtropics / mid latitudes.

21 Δ(RCTT/AoA) /Mean(RCTT/AoA) Age of Air and RCTT: relation E39CA =>For changes only in the shallow branch in E39CA, the ratio changes in high latitudes in the tropical LS by ~10% MPI-Echam6 TS2050-TS1990 => For changes in deep+shallow branch in MPI Echam6, changes in the ratio RCTT/AoA are smaller 10%, but significant in tropics and Northern mid latitudes

22 Summary Part 1: Robust mechanism for changes in the shallow branch, driven by increases in the mean tropical SSTs. OUTLOOK: How about deep branch? Investigate role of Gravity waves, and the sensitivity to the choice and setting of the GW parametrization. Part 2: Age of Air determined by residual circulation in tropics and high latitudes, by Mixing in mid latitudes Changes in AoA due to both changes in residual circulation Transit time and in aging by mixing, depending on structure of BDC change Year to year variations and long term changes in the ratio of RCTT to AoA are mostly < 10 % => indicates that AoA changes due to the residual circulation and due to Mixing scale approximately linearly, with small local deviations. OUTLOOK: further explore physical meaning of aging by mixing term and relation of residual circulation transit time and AoA impact on tracers link to observations