dynamical balances and tropical stratospheric upwelling bill randel and rolando garcia ncar thanks...
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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.
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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)
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Temperature, ozone and upwelling at 17.5 km
temp and ozone in phase, approximately
in quadrature withupwelling
Ozone is a responseto upwelling:Randel et al.,
JAS, 2007
Both temperature and ozone respond to seasonal cycle in w* but what forces the seasonal cycle in upwelling?
upwelling
w*Q
10 N – 10 S
ERA40 w*
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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?
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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
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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)
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Estimates of tropical upwelling from ‘downward control’(momentum balance plus continuity)
EP flux divergence
sensitive calculation: • dependent on EP fluxes in low latitudes
• proportional to 1/f
+ continuity
Haynes et al, 1991
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w* annual cycle at 100 hPa (ERA40 data)
w*m
w*Q
ERA40 w*
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w* annual cycle at 100 hPa (NCEP data)
NCEP w*problematic
NCEP
w*m
reasonable
w*Q
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contours: 0.25 mm/sec
latitude-time variation in upwelling
w*QERA40 w*
-0.5-0.5
-0.5
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latitudinal structure of annual mean w* at 100 hPa
w*m
w*Q
note differences in subtropics
ERA40 w*
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vertical structure of annual mean w* 15o N-S
1) Zonal mean upwelling is continuous across TTL
2) Good agreement between w* and w*M (use momentum balance to diagnose forcing )
Qclear Sky = 0
most confidencein w*Q in
stratosphere
ERA40 w*w*m
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Clear sky, clouds, and zonal mean upwelling
of tropical area
from reanalysisand w*mfrom radiative
calculations
inferredstrong upwellingabove convection
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contribution of separate terms in EP flux to calculated w*M at 100 hPa
w*M
v’T’dU/dt
u’w’
u’v’
result: momentum flux u’v’ is the dominant term
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Climatological EP fluxesEP flux divergence
in subtropicsmainly associatedwith troposphericbaroclinic waves
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JFM JAS
seasonal variation in subtropical wave forcing
* how much of the subtropical forcing comes from tropical waves (versus extratropics)?
Equatorial planetary waves
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eddy fluxes associated with tropical planetary waves (Dima et al., 2005)
u’v’ < 0
u’v’ > 0
note balance ofHadley v* with
d/dy (u’v’)
strong annualcycle of
tropical waves
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What drives the annual cycle of subtropical d/dy (u’v’) ?
result: a combination of extratropical eddies and equatorial planetary waves
climatological u’v’ at 100 hPa
extratropicalwaves
equatorialplanetarywaves
extratropicalwaves
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tropics (15 N-S)
extratropics
result: extratropics (baroclinic eddies) contribute to time-mean upwelling tropical planetary waves mainly drive annual cycle at 100 hPa
climatological u’v’ at 100 hPa
extratropicalwaves
equatorialplanetarywaves
extratropicalwaves
total
estimate contributions from tropical / extratropical u’v’
(set tropical wave fluxes = 0 over 15o N-S)
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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
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Models suggest an increase in stratospheric tropical upwelling (Brewer-Dobson circulation) in future climates
Butchart et al., 2006
~2% / decade increase
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Upwelling balance in WACCM, and long-term trends:
100 hPa w* Climatology
w*
w*m
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Annual mean upwelling over 15 N-S
w*
w*m
Qclear sky=0
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Climatological EP flux in WACCM
Overall balance in WACCM very similar to observations
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Upwelling trends for 1950-2003 (CCMval Ref1)
w*
w*m
Model ENSO eventsdeseasonalized anomalies
R=0.84
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1950-2003 trends in w*mTemperature trends 15 N-S
note there is not a simplerelation between w* and T trends
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What causes the trends in w*m ?
EP flux trends1950-2003
Increase in equatorialplanetary wave fluxes
Similar resultfor JAS
(not shown)
Conclusion: for WACCM Ref1, increased upwelling results mainly from stronger equatorial planetary waves
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WACCM 100 hPa u’v’
Trends in equatorialplanetary wave fluxes
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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 withstronger equatorial planetary waves (associated with
warmer, moister tropical troposphere). Note transient increases are also observed for ENSO events.
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Thank you
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Ozone seasonal cycle has similar vertical structure to temperature
ozone temperature
temps from SHADOZ stationsand zonal mean GPS data
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Well-known seasonal cycle of tropical tropopause temperature:
cold point
Annual cycle amplitude (K)from GPS data
4
Vertical profile at equator
Dark line: GPSlight lines: radiosondes
Background:
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A large annual cycle above the tropopause also occurs for ozone
SHADOZozonesondemeasurementsover 1998-2006
SHADOZ data at Nairobi
normalized annual cycle amplitude17.5 km
HALOE satellite
SHADOZ stations
narrow maximumabove tropopause
Ozone is also a responseto seasonal cycle in upwelling
Randel et al., JAS, 2007
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Interannual changes in upwelling
anomalies incalculatedupwellingover 20 N-S
tropopausetemperatureanomalies
ERA40NCEP
r=-0.53 (I am surprised)
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Interannual changes in upwelling
anomalies incalculatedupwellingover 20 N-S
tropopausetemperatureanomalies
ERA40NCEP
r=-0.53 (I am surprised)
r=.72
HALOE satellite data