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.
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)
Temperature, ozone and upwelling at 17.5 km
10 N – 10 S
w*Q
upwelling
ERA40
temp and ozone in
phase, approximately
in quadrature with
upwelling
Ozone is a response
to upwelling:
Randel et al.,
JAS, 2007
w*
Both temperature and ozone respond to seasonal cycle in w*
but what forces the seasonal cycle in upwelling?
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?
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
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)
Estimates of tropical upwelling from ‘downward control’
(momentum balance plus continuity)
Haynes et al, 1991
+ continuity
EP flux
divergence
sensitive calculation:
• dependent on EP fluxes
in low latitudes
• proportional to 1/f
w* annual cycle at 100 hPa (ERA40 data)
w*Q
w*m
ERA40 w*
w* annual cycle at 100 hPa (NCEP data)
NCEP w*m
reasonable
w*Q
NCEP w*
problematic
latitude-time variation in upwelling
ERA40 w*
w*Q
-0.5
-0.5
-0.5
contours: 0.25 mm/sec
latitudinal structure of annual mean w* at 100 hPa
note differences
in subtropics
w*m
w*Q
ERA40 w*
vertical structure of annual mean w* 15o N-S
most confidence
in w*Q in
stratosphere
Qclear 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 )
Clear sky, clouds, and zonal mean upwelling
inferred
strong upwelling
above convection
of tropical area
from radiative
calculations
from reanalysis
and w*m
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
Climatological EP fluxes
EP flux divergence
in subtropics
mainly associated
with tropospheric
baroclinic waves
seasonal variation in subtropical wave forcing
JFM
Equatorial
planetary waves
JAS
* how much of the subtropical forcing comes from tropical waves
(versus extratropics)?
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
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
estimate contributions from tropical / extratropical u’v’
(set tropical wave fluxes = 0 over 15o N-S)
climatological u’v’ at 100 hPa
extratropics
extratropical
waves
total
equatorial
planetary
waves
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
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
Models suggest an increase in stratospheric tropical upwelling
(Brewer-Dobson circulation) in future climates
~2% / decade increase
Butchart et al., 2006
Upwelling balance in WACCM, and long-term trends:
100 hPa w* Climatology
w*m
w*
Annual mean upwelling over 15 N-S
w*m
Qclear sky=0
w*
Climatological EP flux in WACCM
Overall balance in WACCM very similar to observations
Upwelling trends for 1950-2003 (CCMval Ref1)
deseasonalized
anomalies
Model ENSO events
w*
w*m
R=0.84
1950-2003 trends in w*m
Temperature trends 15 N-S
note there is not a simple
relation between w* and T trends
What causes the trends in w*m ?
EP flux trends
1950-2003
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
Trends in equatorial
planetary wave fluxes
WACCM 100 hPa u’v’
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.
Thank you
Ozone seasonal cycle has similar vertical structure to temperature
ozone
temperature
temps from SHADOZ stations
and zonal mean GPS data
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
A large annual cycle above the tropopause
also occurs for ozone
SHADOZ
ozonesonde
measurements
over 1998-2006
SHADOZ data at Nairobi
17.5 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
Interannual changes in upwelling
tropopause
temperature
anomalies
anomalies in
calculated
upwelling
over 20 N-S
NCEP
ERA40
r=-0.53 (I am surprised)
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)