Convectively Coupled Kelvin Waves and the MJO in

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

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? AM 2 model work w/ Sobel, Maloney & Bellon Master’s thesis of Marshall Stoner

Convectively Coupled Equatorial Waves What sets speed? Moist 1 st baroclinic mode? (gross moist stability: Neelin, Emanuel, etc) Dry 2 nd baroclinic mode? (Mapes, Majda, etc) Observations show clear 2 nd 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. 0 K Ratio of grid-scale to convective (simplified Betts-Miller) precipitation sets the GMS See Frierson (2007) for more detail

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 See Frierson (2007 b) for more detail Longitude

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: Tokioka values: Higher entrainment => harder to warm upper troposphere Stronger trigger => more unstable GMS also reduced

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

CCKW Vertical Structures In full GCM, many cases show 2 nd baroclinic mode structures (unlike in simplified GCM) Shallow -> deep -> 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

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

Phase Speed Determination? Estimated equivalent depths versus GMS: Circled cases have clear 2 nd baroclinic structure 1 st baroclinic mode seems to explain phase speed Presence/absence of 2 nd baroclinic mode doesn’t appear to have effect

Phase Speed Determination? 2 nd baroclinic mode and cloud-radiative forcing effects on GMS Stratiform phase => higher GMS Shallow phase => lower GMS Mode structure effect on GMS averages to zero, and are small near center of the wave CRF changes have small effect everywhere

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 AM 2 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)

MJO in GFDL AM 2 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 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 AM 2 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 AM 2 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 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)

Intraseasonal Composites of structure: QOBS Connected to midlatitude wave trains, smaller scale Flat More similar to observed MJO? When WISHE is suppressed, QOBS ISV (left) remains, while Flat ISV (right) disappears M. S. thesis work of Marshall Stoner (2010)

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)

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

WISHEful Thinking Evaporation composites for Flat (zonally symmetric) and Flat + Walker cell Flat 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