Dynamical balances and tropical stratospheric upwelling

Transcription

1 Dynamical balances and tropical stratospheric upwelling Bill Randel and Rolando Garcia NCAR Thanks to: Qiang Fu, Andrew Gettelman, Rei Ueyama, Mike Wallace, plus WACCM group at NCAR.

2 Background: Well-known seasonal cycle in tropical tropopause temperature, forced by upwelling Tropical upwelling explained as a result of wave-driven stratospheric circulation (from extratropics) Yulaeva et al (1994), Holton et al (1995), Rosenlof (1995) Larger in NH winter because of stronger stratospheric forcing But need wave-driving to reach deep into tropics (Plumb and Eluszkiewicz, 1999) Tropical waves may also be important seasonality tied to tropical wave response to convection Boehm and Lee (2003), Dima et al (2005; 2007), Kerr-Munslow and Norton (2006), Norton (2006)

3 Temperature, ozone and upwelling at 17.5 km 10 N 10 S temp and ozone in phase, approximately in quadrature with upwelling w*q Ozone is a response to upwelling: Randel et al., JAS, 2007 upwelling ERA40 w* Both temperature and ozone respond to seasonal cycle in w* but what forces the seasonal cycle in upwelling?

4 Science questions: How strong is tropical upwelling? What is the detailed vertical structure within TTL and above? (how good are reanalyses?) How is this partitioned locally (deep convection vs. clear sky)? What are the dynamical balances within TTL? * Note thermodynamic balance is mainly a response to dynamic forcing What forces the seasonal cycle in upwelling? (and hence temperature and ozone). What are the contributions from the tropics and extratropics? What causes increased upwelling in climate change experiments?

5 This talk: Compare estimates of upwelling from: thermodynamic balance momentum balance ERA40 and NCEP reanalyses Diagnose momentum balance for upwelling at 100 hpa tropical vs. extratropical wave forcing Examine upwelling trends in WACCM

6 Thermodynamic balance estimates of w* use accurate radiative transfer code, with input temps from GPS climatology, and climatological trace gases combine with continuity equation, solve iteratively to get w*q should be accurate in stratosphere (Q dominated by radiation) (some uncertainties for cloud effects near tropopause)

7 Estimates of tropical upwelling from downward control (momentum balance plus continuity) Haynes et al, continuity EP flux divergence sensitive calculation: dependent on EP fluxes in low latitudes proportional to 1/f

8 w* annual cycle at 100 hpa (ERA40 data) w*q w*m ERA40 w*

9 w* annual cycle at 100 hpa (NCEP data) NCEP w* m reasonable w*q NCEP w* problematic

10 latitude-time variation in upwelling w* Q ERA40 w* contours: 0.25 mm/sec

11 latitudinal structure of annual mean w* at 100 hpa note differences in subtropics w*m w*q ERA40 w*

12 vertical structure of annual mean w* 15 o N-S most confidence in w* Q in stratosphere Q clear Sky = 0 w*m ERA40 w* 1) Zonal mean upwelling is continuous across TTL 2) Good agreement between w* and w* M (use momentum balance to diagnose forcing )

13 Clear sky, clouds, and zonal mean upwelling inferred strong upwelling above convection of tropical area from radiative calculations from reanalysis and w*m

14 contribution of separate terms in EP flux to calculated w* M at 100 hpa w* M u v u w du/dt v T result: momentum flux u v is the dominant term

15 Climatological EP fluxes EP flux divergence in subtropics mainly associated with tropospheric baroclinic waves

16 seasonal variation in subtropical wave forcing JFM Equatorial planetary waves JAS * how much of the subtropical forcing comes from tropical waves (versus extratropics)?

17 eddy fluxes associated with tropical planetary waves (Dima et al., 2005) note balance of Hadley v* with d/dy (u v ) u v < 0 strong annual cycle of tropical waves u v > 0

18 What drives the annual cycle of subtropical d/dy (u v )? climatological u v at 100 hpa extratropical waves equatorial planetary waves extratropical waves result: a combination of extratropical eddies and equatorial planetary waves

19 estimate contributions from tropical / extratropical u v (set tropical wave fluxes = 0 over 15 o N-S) climatological u v at 100 hpa extratropics extratropical waves equatorial planetary waves total extratropical waves tropics (15 N-S) result: extratropics (baroclinic eddies) contribute to time-mean upwelling tropical planetary waves mainly drive annual cycle at 100 hpa

20 Summary 1) Reasonable agreement between w* m, w* Q, w* (at 100 hpa) 2) 100 hpa w* m in balance with subtropical u v convergence - u v associated with extratropical baroclinic eddies and tropical planetary waves. - annual mean upwelling primarily due to extratropics - seasonal cycle at 100 hpa mainly due to tropical waves

21 Models suggest an increase in stratospheric tropical upwelling (Brewer-Dobson circulation) in future climates ~2% / decade increase Butchart et al., 2006

22 Upwelling balance in WACCM, and long-term trends: 100 hpa w* Climatology w*m w*

23 Annual mean upwelling over 15 N-S w*m Qclear sky=0 w*

24 Climatological EP flux in WACCM Overall balance in WACCM very similar to observations

25 Upwelling trends for (CCMval Ref1) deseasonalized anomalies Model ENSO events w* w*m R=0.84

26 trends in w*m Temperature trends 15 N-S note there is not a simple relation between w* and T trends

27 What causes the trends in w*m? EP flux trends Increase in equatorial planetary wave fluxes Similar result for JAS (not shown) Conclusion: for WACCM Ref1, increased upwelling results mainly from stronger equatorial planetary waves

28 Trends in equatorial planetary wave fluxes WACCM 100 hpa u v

29 Summary: Dynamical balances in WACCM are very similar to observations - subtropical EP fluxes due to midlatitude baroclinic waves plus equatorial planetary waves In WACCM Ref1, trends in tropical upwelling are associated with stronger equatorial planetary waves (associated with warmer, moister tropical troposphere). Note transient increases are also observed for ENSO events.

30 Thank you

31 Ozone seasonal cycle has similar vertical structure to temperature ozone temperature temps from SHADOZ stations and zonal mean GPS data

32 Background: Well-known seasonal cycle of tropical tropopause temperature: Annual cycle amplitude (K) from GPS data Vertical profile at equator 4 cold point Dark line: GPS light lines: radiosondes

33 A large annual cycle above the tropopause also occurs for ozone SHADOZ data at Nairobi SHADOZ ozonesonde measurements over km normalized annual cycle amplitude SHADOZ stations HALOE satellite narrow maximum above tropopause Ozone is also a response to seasonal cycle in upwelling Randel et al., JAS, 2007

34 Interannual changes in upwelling tropopause temperature anomalies anomalies in calculated upwelling over 20 N-S NCEP ERA40 r=-0.53 (I am surprised)

35 Interannual changes in upwelling HALOE satellite data r=.72 tropopause temperature anomalies anomalies in calculated upwelling over 20 N-S NCEP ERA40 r=-0.53 (I am surprised)