Axisymmetric Theory and the Interactive, Asymmetric Monsoon

1
Axisymmetric Theory and the Interactive,
Asymmetric Monsoon

• Nikki Privé
• 11 April 2005

2
What is a monsoon?

• Seasonal reversal of wind direction
• Generally accompanied by heavy rains during summer

3
July Surface Flow and Precipitation Over Asia
mm/day
ERA-40 long term mean
4
Observed Zonal Mean Monsoon Flow
5
Theories of Monsoon Circulation

• Linear
• Gill (1980) model
• Atmospheric response to localized tropical
  heating on a ?-plane
• Rodwell and Hoskins (1995)
• Linear model with specified heating can reproduce
  many features of the observed monsoon flow
• Rossby waves induce subsidence to the west of the
  monsoon, creating east-west asymmetry
• Nonlinear
• Plumb and Hou (1992)
• Examined axisymmetric circulation induced by
  local subtropical forcing
• Threshold behavior between weak, linear
  circulation and strong, global angular momentum
  conserving circulation
• Emanuel (1995), Zheng (1998)
• Extended axisymmetric theory to a moist
  atmosphere
• Threshold can be framed in terms of subcloud
  moist entropy

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Driving Questions

• How well does the axisymmetric theory apply to
  the interactive, asymmetric monsoon?
• Does the meridional circulation exhibit threshold
  behavior?
• Is the meridional circulation nonlinear (angular
  momentum conserving)?

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Axisymmetric Theory - Threshold Behavior
Thermal Equilibrium (weak forcing)
u
Plumb and Hou (1992)
EQ
Angular Momentum Conserving (strong forcing)
u
EQ
Critical threshold is the vanishing of upper
tropospheric absolute vorticity.
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Driving Questions

• How well does the axisymmetric theory apply to
  the interactive, asymmetric monsoon?
• Does the meridional circulation exhibit threshold
  behavior?
• Is the meridional circulation nonlinear (angular
  momentum conserving)?
• What determines the extent and location of the
  monsoon?

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What Determines the Location of the Monsoon?
Angular Momentum Conserving
Thermal Equilibrium
Thermal Equilibrium
EQ
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What Determines the Location of the Monsoon?
Angular Momentum Conserving
Thermal Equilibrium
Thermal Equilibrium
EQ
Zero vertical shear of zonal wind
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What Determines the Location of the Monsoon?
Angular Momentum Conserving
Thermal Equilibrium
Thermal Equilibrium
EQ
Zero vertical shear of zonal wind
Zero meridional gradient of subcloud moist static
energy
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What Determines the Location of the Monsoon?
Angular Momentum Conserving
Thermal Equilibrium
Thermal Equilibrium
EQ
Subcloud moist static energy
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Implications of Theory of Monsoon Location

• The impact of various mechanisms on the location
  and extent of the monsoon can be studied by
  examining their impact on the subcloud moist
  static energy.
• Caveat The large-scale circulation interacts
  with the subcloud thermodynamics, so this theory
  is diagnostic, not prognostic.

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Driving Questions

• How well does the axisymmetric theory apply to
  the interactive, asymmetric monsoon?
• Does the meridional circulation exhibit threshold
  behavior?
• Is the meridional circulation nonlinear (angular
  momentum conserving)?
• What determines the extent and location of the
  monsoon?

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Model Progression
Axisymmetric (2D)
OCEAN
LAND
Zonally Symmetric (3D)
OCEAN
LAND
Zonally Asymmetric (3D)
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Continental Model Setup
North
South
16N
EQ
Over land, the surface forcing is determined by
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Submonsoonal Case
Circulation kg/s
Zonal wind m/s
moist static energy (h), J/kg
Precipitation mm/day
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Monsoonal Case
Circulation kg/s
Zonal wind m/s
1000 mb h J/kg
Precipitation mm/day
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Threshold Behavior?
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Threshold Behavior?
CONTINENTAL
AQUAPLANET
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Angular Momentum Conservation?
CONTINENTAL
AQUAPLANET
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Axisymmetric Theory and the 2D Monsoon

• A transition between a weak, local meridional
  circulation and a strong, global circulation is
  observed as the forcing is increased.
• A sharp threshold in the circulation strength is
  not observed in the presence of a continent.
• The strong, global circulations are close to
  angular momentum conserving, and are nonlinear in
  nature.

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Model Progression
Axisymmetric (2D)
OCEAN
LAND
Zonally Symmetric (3D)
OCEAN
LAND
Zonally Asymmetric (3D)
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3D Monsoonal Case
Circulation kg/s
Zonal wind m/s
1000 mb h J/kg
precipitation mm/day
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2D Monsoonal Case
Circulation kg/s
Zonal wind m/s
1000 mb h J/kg
Precipitation mm/day
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Impact of Eddies on Subcloud Moist Static Energy
J/kg
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Threshold Behavior in 3D?
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Angular Momentum Conservation in 3D?
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Axisymmetric Theory in the Presence of Eddies

• Zonal mean meridional circulation transitions
  between a weak, local circulation and a strong
  global circulation with increased forcing.
• Threshold behavior of the circulation strength is
  observed.
• Eddies weaken the conservation of angular
  momentum in the meridional circulation.

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Model Progression
Axisymmetric (2D)
OCEAN
LAND
Zonally Symmetric (3D)
OCEAN
LAND
Zonally Asymmetric (3D)
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1000 mb Winds and Precipitation
mm/day
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500 mb winds and ?
Pa/s
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Precipitation With Realistic Ocean SSTs
mm/day
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1000 mb Winds and Moist Static Energy
Realistic Ocean SSTs
J/kg
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1000 mb Winds and Moist Static Energy
Uniform Warm Ocean
J/kg
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1000 mb Moist Static Energy with Coastal Walls
Realistic Ocean SSTs
J/kg
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Precipitation, Case with Coastal Walls
Realistic Ocean SSTs
mm/day
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500 mb ? with Coastal Walls
Realistic Ocean SSTs
Pa/s
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Axisymmetric Theory in a Zonally Asymmetric Setup

• No transition from local to global circulation
  occurs with a uniform, warm ocean.
• A cross-equatorial SST gradient is needed to
  induce a global meridional circulation.
• Characteristics of a linear response (Rodwell and
  Hoskins (1996)) are present in the large-scale
  circulation.
• Zonal asymmetry of forcing strongly impacts the
  strength and extent of the monsoon through
  advection of low moist static energy air.

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January Precipitation over Australia
mm/day
ERA-40 long term mean
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July Surface Flow and Precipitation Over Asia
mm/day
ERA-40 long term mean
42
January Subcloud Moist Static Energy, Australia
J/kg
ERA-40 long term mean
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July Subcloud Moist Static Energy, Asia
J/kg
ERA-40 long term mean
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Conclusions

• Axisymmetric theory remains applicable in the
  presence of an interactive continent in a purely
  axisymmetric setup.
• Axisymmetric theory also holds reasonably well
  in the presence of eddies.
• Large-scale asymmetry of the continental forcing
  significantly lessens the applicability of
  axisymmetric theory.
• The poleward limit of the meridional circulation
  is collocated with the maximum in subcloud moist
  entropy.
• Advection of low moist static energy from the
  ocean and midlatitudes strongly impacts the
  monsoon location and extent.

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Final Thoughts

• Care should be taken when attempting to apply
  axisymmetric theory to highly asymmetric
  situations.
• The moist static energy may be used as a
  diagnostic tool to evaluate the first-order
  impact of different mechanisms upon the monsoon.

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Acknowledgements

• Thesis committee
• Alan Plumb
• Kerry Emanuel
• Elfatih Eltahir
• John Marshall
• Other assistance
• Bill Boos, Jean-Michel Campin, Will Heres,
• Chris Hill, Ed Hill, Rob Korty, Greg Lawson,
• Olivier Pauluis, Jeff Scott