DARGAN M. W. FRIERSONUNIVERSITY 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

Convectively Coupled Kelvin Waves and the MJO in a Hierarchy

of GCMs

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 Master’s thesis of Marshall Stoner

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)

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)

Convectively coupled Kelvin waves

GMS reduction leads to slower convectively coupled waves:

GMS = 6.9 K GMS = 3.9 K GMS = 3.0 K

See Frierson (2007) for more detail

Ratio of grid-scale to convective (simplified Betts-Miller) precipitation sets the GMS

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…

Longitude

Composited pressure velocity

See Frierson (2007b) for more detail

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 => 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

Moist Static Energy

Vertical profile of MSE in the North West Pacific ITCZ for SAS simulations:

Higher entrainment => harder to warm upper troposphere Stronger trigger => more unstable

GMS also reduced

Tokioka values:

Equatorial Waves in a Full GCM

Phase speeds in SAS simulations:

In Kuo simulations:

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

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

CCKW Vertical Structures

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

Shallow -> deep -> stratiform

See Lin et al (2008) and Frierson et al (submitted) for more detail

Warm over cold temperature anomalies

Gradual moistening of boundary layer/midtroposphere

CCKW Vertical Structures

Depends on convection scheme though!

Least inhibited SAS case => No tilt in omega (but OK temperature)

Most inhibited Kuo case => No tilt in omega, q (but OK temperature)

Kuo simulations never show tilted omega orhumidity.

Only mostinhibited case shows realistictemperatureperturbations

Phase Speed Determination?

Estimated equivalent depths versus GMS:

1st baroclinic mode seems to explain phase speed Presence/absence of 2nd baroclinic mode doesn’t

appear to have effect

Circled cases have clear 2nd baroclinic structure

Phase Speed Determination?

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

Stratiform phase =>higher GMS Shallow phase =>

lower GMSCRF changes have small effecteverywhere

Mode structure effect on GMSaverages to zero, and are small near center of the wave

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?

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:

See SMBF (Nature Geoscience 2008; J. Adv. Modeling Earth Systems in press)

Obs (NCEP) Modified GFDL model Unmodified GFDL model

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 => 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)

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)

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)

Zonally Symmetric Results

Log(variance) spectra: QOBS (left) and “Flat” (right)

Enhanced power in eastward intraseasonal bandConnected to moist Kelvin wave?

More clear dominance of east over westLess connected to Kelvin wave?

M.S. thesis work of Marshall Stoner (2010)

Intraseasonal Composites

Composites of structure:

When WISHE is suppressed, QOBS ISV (left) remains, while Flat ISV (right) disappears

Connected to midlatitude wave trains, smaller scale

More similar to observed MJO?

QOBS Flat

M.S. thesis work of Marshall Stoner (2010)

Mean States

Mean states (solid = QOBS, dashed = flat):

Flat has weaker easterlies, and a double ITCZStandard WISHE likely drives the waves

M.S. thesis work of Marshall Stoner (2010)

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)

Walker Cell Case

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

Surface winds

Log(variance)

Variance avoidssurface westerlyregion?

M.S. thesis work of Marshall Stoner (2010)

WISHEful Thinking

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

Flat

Flat + Walker cell

Both essentially have evaporation leading the wave

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

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