Sophie Oberländer, Ulrike Langematz, Stefanie Meul Institut für Meteorologie, Freie Universität Berlin

Transcription

1 Future changes in the Brewer-Dobson Circulation and possible causes derived from Chemistry-ClimateClimate Model Simulations, Ulrike Langematz, Stefanie Meul Institut für Meteorologie, Freie Universität Berlin SPARC Workshop on the Brewer-Dobson Circulation June 2012, Grindelwald

2 Introduction and Motivation Independent model simulations of future climate project intensified tropical upward mass flux (e.g. Butchart et al., 2010; Li et al., 2010; Okamoto et al., 2011;...) BDC-acceleration due to higher greenhouse gas (GHG) concentrations is expected But: Causes for the simulated trend are still under debate, e.g. changes in generation and propagation of planetary waves, or the role of gravity waves Tropical sea surface temperatures (SSTs) were found to influence the lower branch of the BDC (e.g. Garny et al., 2011) But: Changes in the deep branch still uncertain 2

3 Introduction and Motivation Sensitivity studies separating the single forcings can help to understand underlying mechanisms Here: Chemistry-Climate Model simulations in timeslice mode for present and future climate, independently changing single forcings Separation of changes due to greenhouse gas (GHG) concentrations, sea surface temperatures (SST) and sea ice concentration (SIC) as well as chlorofluorocarbons (CFC) Attention: GHGs and SSTs are connected separation only possible due to prescription of SSTs GHG : effect of rising GHGs on the atmosphere (e.g. radiation, composition) SST : effect of rising GHGs on the SSTs 3

4 Model: EMAC (in FUB-configuration) ECHAM5/MESSy Atmospheric Chemistry Model (Jöckel et al., 2006) Chemistry-Climate Model (CCM) with FUBRad-Radiation parameterization (Nissen et al., 2007) Horizontal resolution: T42 (2.8 x 2.8 ) Vertical resolution: L39 (top in 0.01 hpa, ~80 km) No interactively coupled ocean SST/SIC prescribed Gravity wave parameterization: o Non-orographic - NGW: Hines, 1997 ( model output) o Orographic - OGW: Lott and Miller, 1997 ( no output) 4

5 Timeslice-Experiments Timeslice Simulations: Integration of 20++ years under equal boundary conditions, solar mean radiation Name GHG SST/SIC CFC GHGSST SST tropsst (Tropics) : Future GHG + CFC GHG+SST GHG CFC SST Trop. SST Extra-Trop. SST 5

6 Temperature [K/decade] - DJF Future tropospheric warming (max. 0.6 K/decade) SSTs (mainly tropical SSTs) Future stratospheric cooling (max. 0.8 K/decade) GHGs 6

7 Zonal wind [m/s/decade] - DJF Changes in tropical SSTs determine zonal wind changes UTLS-changes in temperature strong T intensified subtropical jets Winter stratosphere: deceleration of stratospheric polar night jet 7

8 Mass-streamfunction [kg/m/s/decade] DJF Intensification in future masstransport Significant influence from tropical SSTs up to ~10 hpa GHG changes important in upper stratosphere 8

9 Strength of the BDC Tropical upward mass flux Integration of the mass-streamfunction within the turnaround latitudes Reference Simulation (2000) - DJF Tropical upward mass flux: (Holton, 1990) 9

10 Tropical upward mass flux [%/decade] - DJF Intensification in future mass-flux (black) within whole stratosphere Significant influence from tropical SSTs up to ~10 hpa GHG changes important in upper stratosphere Which waves are responsible for the changes? 10

11 Separation of different scale waves Application of the downward-control principle to model data Total forcing (all waves) Resolved waves (EPFlux-divergence) EPFD Unresolved waves Orographic Non-orographic gravity waves gravity waves OGWD NGWD mass-streamfunction from direct calculation (Haynes et al., 1991) mass-streamfunction due to EPFD and NGWD from model data, OGWD as residual 11

12 Tropical upward mass flux [% (direct)] - DJF Resolved waves (EPFD): 90 % in lower and 55% in upper stratosphere Unresolved waves: OGWD: important in upper stratosphere NGWD: growing influence with height (~35% at stratopause) 12

13 Tropical upward mass flux [% (direct)] - DJF Resolved waves (EPFD): Temporal (left) versus spatial (right) scale Largest amount from planetary scale, transient waves 13

14 Tropical upward mass flux [%/decade] - DJF Future change 2095 vs (n=1-3) (n 4) Resolved waves (EPFD): Lower (transient waves) and middle stratosphere (planetary waves) Unresolved waves: OGWD + NGWD: Middle and upper stratosphere 14

15 Tropical upward mass flux [%/decade] - DJF GHG SST Above ~10 hpa: ½ GHG + ½ SST: unresolved (OGWD+NGWD) waves determine mass-flux changes Below ~10 hpa: SSTs determine mass-flux: ½ OGWD + ½ EPFD (transient) 15

16 Concept of Stratospheric Age of Air Here: Mass-streamfunction and upward mass-flux include only residual masstransport through (slow) circulation But: BDC includes both: (fast) mixing & (slow) circulation Therefore : Concept of Stratospheric Age of Air Not shown here in detail, but: Age-of-air-concept points in same direction Mean age [days/decade] DJF 16

17 Summary I: What forces future changes in the BDC? Future intensification in the residual circulation shown from changes in mass-streamfunction and tropical upward mass-flux Major importance of tropical SSTs explaining nearly total future change signal in lower and middle stratosphere BDC-changes due to tropical SSTs not restricted to lower stratosphere deep circulation branch is affected as well Consideration of changes in GHGs and extra-tropical SSTs required to explain total change signal in upper levels 17

18 Summary II: Which waves are responsible? SST -effect GHG -effect Increase in wavepropagation Shallow branch Strengthened T in UTLS-region Strengthening and upward shift of subtrop. jets OGW reach higher up and account for larger drag in mid-latitudes In-situ generation of (stationary) waves upstream the (slightly) intensified stratospheric polar night jet Influences on gravity waves in upper atmosphere Changes in conditions for stationaryplanetary wave propagation Deep branch 18