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

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*

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)

EP flux divergence

sensitive calculation: • dependent on EP fluxes in low latitudes

• proportional to 1/f

+ continuity

Haynes et al, 1991

w* annual cycle at 100 hPa (ERA40 data)

w*m

w*Q

ERA40 w*

w* annual cycle at 100 hPa (NCEP data)

NCEP w*problematic

NCEP

w*m

reasonable

w*Q

contours: 0.25 mm/sec

latitude-time variation in upwelling

w*QERA40 w*

-0.5-0.5

-0.5

latitudinal structure of annual mean w* at 100 hPa

w*m

w*Q

note differences in subtropics

ERA40 w*

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

Clear sky, clouds, and zonal mean upwelling

of tropical area

from reanalysisand w*mfrom radiative

calculations

inferredstrong upwellingabove convection

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

Climatological EP fluxesEP flux divergence

in subtropicsmainly associatedwith troposphericbaroclinic waves

JFM JAS

seasonal variation in subtropical wave forcing

* how much of the subtropical forcing comes from tropical waves (versus extratropics)?

Equatorial planetary waves

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

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

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)

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

Butchart et al., 2006

~2% / decade increase

Upwelling balance in WACCM, and long-term trends:

100 hPa w* Climatology

w*

w*m

Annual mean upwelling over 15 N-S

w*

w*m

Qclear sky=0

Climatological EP flux in WACCM

Overall balance in WACCM very similar to observations

Upwelling trends for 1950-2003 (CCMval Ref1)

w*

w*m

Model ENSO eventsdeseasonalized anomalies

R=0.84

1950-2003 trends in w*mTemperature trends 15 N-S

note there is not a simplerelation between w* and T trends

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

WACCM 100 hPa u’v’

Trends in equatorialplanetary wave fluxes

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.

Thank you

Ozone seasonal cycle has similar vertical structure to temperature

ozone temperature

temps from SHADOZ stationsand zonal mean GPS data

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:

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

Interannual changes in upwelling

anomalies incalculatedupwellingover 20 N-S

tropopausetemperatureanomalies

ERA40NCEP

r=-0.53 (I am surprised)

Interannual changes in upwelling

anomalies incalculatedupwellingover 20 N-S

tropopausetemperatureanomalies

ERA40NCEP

r=-0.53 (I am surprised)

r=.72

HALOE satellite data