Windevaporation feedback

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Wind-evaporation feedback the abrupt onset of
monsoons

• William Boos January 3, 2008
• MIT Monsoon Institute, Honolulu
• ... with thanks to Kerry Emanuel

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Outline

• Monsoon onset in axisymmetric models
• Observations
• Summary / conclusions

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Outline

• Monsoon onset in axisymmetric models
• Observations
• Summary / conclusions

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Zonal-mean climatological circulation
January
July
October
April
90 S 0 90
N latitude (degrees)
90 S 0 90
N latitude (degrees)

• Dima and Wallace (2003)

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Solsticial OLR

• Ascent occurs primarily in monsoon regions,
  with possible exception of Pacific

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Central question

• Why does the tropospheric overturning circulation
  on Earth respond nonlinearly to the seasonal
  cycle of solar forcing?
• ... or equivalently, why is monsoon onset
  abrupt?

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Central hypothesis

• Meridional circulation and zonal winds intensify
  abruptly due to WISHE (Wind-Induced Surface Heat
  Exchange)

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WISHE convective quasi-equilibrium
A traditional view
height
condensation level
d(T,q,v)
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Aquaplanet model (MITgcm)

• Axisymmetric (latitude-height) partial sphere,
  64º N - 64º S
• resolution 1º horizontal, 25 hPa vertical
• LW SW radiation scheme
• No moisture-radiation feedbacks
• moist convection (Emanuel 1991)
• aquaplanet, prescribed SST

SST (K)
? similar to Pauluis and Emanuel (2004)
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Phase diagram, moist GCM
weak SST forcing
no WISHE
time
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Model state during onset
? circulation does not reach highly nonlinear,
angular momentum conserving regime
? ? -?yM
SST max

• surface heat fluxes are strongly influenced by
  the circulation

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Summary

• Idealized, axisymmetric models
• WISHE can produce an abrupt onset of solsticial
  flow for weak thermal forcings
• Response for strong thermal forcings involves
  consideration of angular momentum conserving
  dynamics... another talk

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Outline

• Monsoon onset in axisymmetric models
• Observations
• Summary / conclusions

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(No Transcript)
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Seasonal evolution of jet
Following Krishnamurti et al. (1981), define

• jet index

at 850 hPa over Arabian Sea
data NCEP/NCAR Reanalysis, 1979-2005
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Moist static energy budget

• Moist static energy budget
• h cpT Lvq gz
• If we neglect horizontal advection and time
  tendency of h (can test a posteriori)
• where Qnet

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Composite moist static energy source(vertically
integrated, atm. only)
data NCEP Reanalysis (1979-2005)
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Change in Q compared to change in ?
data NCEP Reanalysis (1979-2005)
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Time evolution of mean Q and ?
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Summary

• Idealized, axisymmetric models
• WISHE can produce an abrupt onset of solsticial
  flow for weak thermal forcings
• Response for strong thermal forcings involves
  consideration of angular momentum conserving
  dynamics... another talk
• Observations
• Changes in baroclinic flow during abrupt onset of
  Somali jet are consistent with WISHE feedback
• Changes in stratification and/or h advection may
  be important for weaker changes seen over land