DARGAN M. W. FRIERSON

1
Convectively Coupled Kelvin Waves and the MJO in
a Hierarchy of GCMs

• DARGAN M. W. FRIERSON
• UNIVERSITY OF WASHINGTON, DEPARTMENT OF
  ATMOSPHERIC SCIENCES
• COLLABORATORS MARSHALL STONER, DAEHYUN KIM,
  JIALIN LIN, IN-SIK KANG, MYONG-IN LEE, ADAM
  SOBEL, ERIC MALONEY, GILLES BELLON

2
Outline

• What sets speed/structure of convectively coupled
  equatorial waves?
• In a simplified GCM
• Modeling work with SNU group
• What is required to generate a MJO-like
  structure?
• AM2 model work w/ Sobel, Maloney Bellon
• Masters thesis of Marshall Stoner

3
Convectively Coupled Equatorial Waves

• What sets speed?
• Moist 1st baroclinic mode? (gross moist
  stability Neelin, Emanuel, etc)
• Dry 2nd baroclinic mode? (Mapes, Majda, etc)
• Observations show clear 2nd baroclinic structure
  (Kiladis et al 2009)

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CCKWs in a Simplified GCM

• Convectively coupled Kelvin waves (CCKWs)dominate
  tropical variability in a simplified GCM

Unfiltered Hovmoller diagram of precipitation at
the equator
In this model, gross moist stability controls
the speed of these waves
Model of Frierson, Held Zurita-Gotor
(2006) Plot from Frierson (2007)
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Convectively coupled Kelvin waves

• GMS reduction leads to slower convectively
  coupled waves

GMS 6.9 K
GMS 3.9 K
GMS 3.0 K
Ratio of grid-scale to convective (simplified
Betts-Miller) precipitation sets the GMS
See Frierson (2007) for more detail
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Simplified Moist GCM CCKWs

• These CCKWs are powered by evaporation-wind
  feedback
• Likely not true in reality in Indian Ocean
• Vertical structure is purely first-baroclinic
  mode
• Unrealistic

Composited pressure velocity
Longitude
See Frierson (2007b) for more detail
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Equatorial Waves in a Full GCM

• Experiments with SNU atmospheric GCM
• Run over observed SSTs, realistic geography
• Simplified Arakawa-Schubert (SAS) and Kuo
  convection schemes
• Varying strength of convective trigger
• Tokioka entrainment limiter for SAS
• Higher Tokioka parameter gt least entraining
  plumes are eliminated
• Moisture threshold for Kuo
• From always triggering convection to 95 RH
  required

See Lin, Lee, Kim, Kang and Fri. (2008, J Clim)
Fri. et al (submitted) for more
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Moist Static Energy

• Vertical profile of MSE in the North West Pacific
  ITCZ for SAS simulations
• Higher entrainment gt harder to warm upper
  troposphere
• Stronger trigger gt more unstable
• GMS also reduced

Tokioka values
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Equatorial Waves in a Full GCM

• Phase speeds in SAS simulations
• In Kuo simulations

• Wavespeed decreases with stronger moisture
  trigger
• Simulated equivalent depths scale with gross
  moist stability

See Lin, Lee, Kim, Kang and Fri. (2008, J Clim)
Fri. et al (submitted) for more
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CCKW Vertical Structures

• In full GCM, many cases show 2nd baroclinic mode
  structures (unlike in simplified GCM)

Shallow -gt deep -gt stratiform
Gradual moistening of boundary
layer/midtroposphere
Warm over cold temperature anomalies
See Lin et al (2008) and Frierson et al
(submitted) for more detail
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CCKW Vertical Structures

• Depends on convection scheme though!

Kuo simulations never show tilted omega
or humidity. Only most inhibited case shows
realistic temperature perturbations
Least inhibited SAS case gt No tilt in omega
(but OK temperature)
Most inhibited Kuo case gt No tilt in omega, q
(but OK temperature)
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Phase Speed Determination?

• Estimated equivalent depths versus GMS
• 1st baroclinic mode seems to explain phase speed
• Presence/absence of 2nd baroclinic mode doesnt
  appear to have effect

Circled cases have clear 2nd baroclinic structure
13
Phase Speed Determination?

• 2nd baroclinic mode and cloud-radiative forcing
  effects on GMS

Stratiform phase gt higher GMS
Shallow phase gt lower GMS
CRF changes have small effect everywhere
Mode structure effect on GMS averages to zero,
and are small near center of the wave
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Open Questions

• Reasons for second baroclinic mode structure
• And why seen in some fields more easily than
  others?
• Applicability to other models?
• Need for thorough comparisons of composites
• Relation to changes in mean precipitation?

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MJO in GCMs

• Work with Sobel, Maloney, Bellon using GFDL AM2
  model w/ realistic geography
• First crank up Tokioka entrainment limiter to
  get a better MJO simulation

Obs (NCEP)
Modified GFDL model
Unmodified GFDL model
See SMBF (Nature Geoscience 2008 J. Adv.
Modeling Earth Systems in press)
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MJO in GFDL AM2 Model

• Ratio of variance in eastward/westward
  intraseasonal bands 2.6 for modified GFDL model
• Less than the observed value of 3.5, but larger
  than nearly all models in Zhang et al (2006)
  comparison
• Higher entrainment in convection scheme gt more
  sensitivity to midtropospheric moisture
• Next test role of evaporation-wind feedbacks in
  driving the modeled MJO
• Set windspeed dependence in drag law formulation
  to globally averaged constant value

See SMBF (Nature Geoscience 2008 J. Adv.
Modeling Earth Systems in press)
17
Evap-Wind Feedback in Modeled MJO

• MJO greatly weakened when evaporation-wind
  feedback (EWF) is turned off!

With EWF
Without EWF
See SMBF (Nature Geoscience 2008 J. Adv.
Modeling Earth Systems 2009)
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MJO in Aquaplanet AM2

• What is required to have a MJO-like structure in
  a model?
• Land-sea contrast?
• Zonal asymmetry/Walker cell?
• Evaporation-wind feedback?
• Experiments with Neale Hoskins aquaplanet AMIP
  boundary conditions
• QOBS Flat
• GFDL AM2 model with Tokioka modification

M.S. thesis work of Marshall Stoner (2010)
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Zonally Symmetric Results

• Log(variance) spectra QOBS (left) and Flat
  (right)

Enhanced power in eastward intraseasonal
band Connected to moist Kelvin wave?
More clear dominance of east over west Less
connected to Kelvin wave?
M.S. thesis work of Marshall Stoner (2010)
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Intraseasonal Composites

• Composites of structure
• When WISHE is suppressed, QOBS ISV (left)
  remains, while Flat ISV (right) disappears

QOBS
Flat
Connected to midlatitude wave trains, smaller
scale
More similar to observed MJO?
M.S. thesis work of Marshall Stoner (2010)
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Mean States

• Mean states (solid QOBS, dashed flat)
• Flat has weaker easterlies, and a double ITCZ
• Standard WISHE likely drives the waves

M.S. thesis work of Marshall Stoner (2010)
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How about Flat a Walker cell?
Surface winds
Now mean westerlies over much of the
tropics Will WISHE still be important?
(standard theory assumes mean easterlies)
M.S. thesis work of Marshall Stoner (2010)
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Walker Cell Case

• MJO-like variability still exists (although
  weaker)
• Again it disappears if WISHE is suppressed

Log(variance)
Variance avoids surface westerly region?
Surface winds
M.S. thesis work of Marshall Stoner (2010)
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WISHEful Thinking

• Evaporation composites for Flat (zonally
  symmetric) and Flat Walker

Flat Walker cell
Flat
Both essentially have evaporation leading the wave
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Open Questions

• What sets scale, speed of the MJO-like
  phenomenon?
• Related to Kelvin wave at all, or a moisture
  mode?
• Advection of dry air by WWBs Rossby cyclones
  appears to be important in setting speed as well
  as WISHE
• Comparisons with other models (including CRMs)
• Similar mechanisms acting? (mechanism denial
  experiments in a range of models)
• Compare composites as well as spectra
• Understanding of how/when different mechanisms
  can power waves can help our interpretation of
  observations

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Conclusions

• Convectively coupled waves in simple and full GCM
  are affected by gross moist stability
• Full GCM shows second baroclinic mode
  characteristics
• MJO-like structures can exist in aquaplanet model
• Zonally symmetric or with Walker cell
• More realistic ISV is powered by WISHE in mostly
  traditional manner