multiscale processes in the tropics april 27- may 1, 2009, banff role of mean state and local...

Multiscale processes in the tropics

April 27- May 1, 2009, Banff

Role of mean state and local air-sea interaction on the propagation of intraseasonal oscillations

R. S. AjayamohanCanadian Center for Climate Modelling and Analysis, Victoria, Canada

Collaborators: H. Annamalai, W. J. Merryfield, J. J. Luo, J. Hafner, T. Yamagata



Outline Brief Introduction

Boreal summer intraseasonal oscillations (BSISO)

Role of mean-state and local air-sea interaction on BSISO propagation

Partial coupling experiments

Few Results; How crucial is local Air-Sea interaction in the simulation of intraseasonal oscillations in a CGCM ?

Summary



Tropical intraseasonal variability (20-90 Tropical intraseasonal variability (20-90 days)days)Summer Summer

Winter Winter

Rainfall standard deviation (mm/day)



Motivation

Relative strengths of slowly varying boundary forcing and northward propagating MJO (Boreal Summer IntraSeasonal Oscillations ) determine the seasonal mean monsoon. [E.g. Goswami

and Ajayamohan (2001), Krishnamurthy and Shukla, 2007]

Recent studies highlights the importance of coupled evolution of

SST, circulation and precipitation in the Indian Ocean in simulating the correct phase and amplitude of BSISO.

[E.g. Woolnough et al. (2000), Sengupta et al. (2001), Fu. et al. 2002;2003, Waliser et al 2004, Jiang.

et al. 2004]

Large-scale modes of variability (ENSO, IOD) influences intraseasonal variability. [Ajayamohan et al., 2008, 2009]



ISO Characteristics: Evolution of BSISO. Note the

clear northeastward propagation of precipitation

anomalies.

Ajayamohan and Goswami, 2007, JAS


April 27- May 1, 2009, BanffWhy convection moves poleward ?

Several theories and hypothesis in the literature to explain poleward propagation. [Review articles in Lau and Waliser, Wang and references therein]

Cyclonic vorticity at low-levels and associated boundary layer convergence must be maximum north of convection maximum to initiate poleward propagation of BSISO.

Summer mean flow and mean boundary layer humidity is the key factor.

Large easterly vertical shear seen over monsoon region is an important factor, north of equator.

Near the equator, asymmetric specific humidity contributes to the northward shift of the convection.

Intraseasonal variation of SST. Warm (cool) SST ahead of enhanced (suppressed) convection.



A schematic representation of the evolution and northward propagation of the meridional circulation associated with the 30-60 day mode in the meridional plane. The thin arrows indicate anomalous Hadley circulation. The thick vertical arrow indicates the location of the center of the boundary layer moisture convergence, while the thick horizontal arrow indicates the direction of poleward motion of the cloud band. The thin solid (dotted) line indicates the phase of the relative vorticity at 850 hPa (divergence at 925 hPa) with positive (negative) phase being above (below) the base line. The location of clear sky conditions is shown by the sun-like symbol.

Why

Convection

Moves

Northward ?

A simple model

Warm SST and Convection over EIO, intensifies TCZ.

Ascending motion over EIO and descending motion and clear sky over MT.

Cyclonic ζ and associated BLMC is maximum north of max: convection.

Convection moves northward.

After 10 days, convection reaches ~10N.

Both MT and EIO under subsidence and clear sky.

BLMC north of convection

Active monsoon, convection over MT.

Clear sky over EIO. Anticyclonic ζ and

subsidence over EIO. BLMC north of

convection.

Convection moves to foothills of Himalayas.

Clear conditions over EIO also moves northward.

Decrease in subsidence, continued clear sky conditions, raises SST as net heat flux at surface becomes positive, causing convection to break-out.

Convection builds up to become maximum in another 10 days.

From Lau & Waliser, ISO Book



Role of local Air-Sea Interaction on BSISO propagation

Recent studies emphasize the crucial role of air-sea interactions in defining the observed phase structure of BSISO.

Warm (cool) SST ahead of enhanced (suppressed) convection with a time lag of 7-10 days.

Positive SST anomaly can account for enhanced moisture perturbation through enhancing evaporation and result in BLMC north of active phase of convection.

SEIO is a very important region where BSISO amplification and re-initiation takes place [Fu and Wang (2004), Wang et al. (2006)]



CMAP Rainfall & Reynolds SSTA, Ajayamohan et al., 2008

SST leads precipitation

TRMM Rainfall & TMI SSTA, Wang et al., 2006

SST leads



Incoherent propagation of BSISO precipitation during positive Incoherent propagation of BSISO precipitation during positive IOD yearsIOD years

CMAP Intraseasonal CMAP Intraseasonal Variance Variance

during contrasting IOD yearsduring contrasting IOD years

Role large scale SST mode of variability on BSISO propagation

Ajayamohan et al., 2008, 2009



大気海洋結合循環モデル（ SINTEX-F1 CGCM ）

Every 2 hrs

T106L19

2.2

OCEAN: OPA8.2 ORCAR2 Grid 20X1.50 Eq-0.5 Level 31

Earth Simulator

Non-flux adjustment

5ATMOSPHERE: ECHAM4 T106 L19

EU-Japan Collaboration 　（日欧協力）

海洋大気

カップラー

Coupler OASIS 2.4

T106L192 x 0.52, L31

No flux correction, no sea ice model



SINTEX-F1 JJAS Climatology

and BSISO Variance

CGCMs No: CGCMs No:

gfdl_cm2_1 2 miroc3_2_medres 14

ingv_echam4 3 inmcm3_0 15

mpi_echam5 4 miroc3_2_hires 16

mri_cgcm2_3_2a 5 ncar_pcm1 17

iap_fgoals1_0_g 6 csiro_mk3_5 18

gfdl_cm2_0 7 csiro_mk3_0 19

cccma_cgcm3_t47 8 ncar_ccsm3_0 20

cccma_cgcm3_1_t63 9 ipsl_cm4 21

miub_echo_g 10 giss_model_e_r 22

cnrm_cm3 11 giss_model_e_h 23

giss_aom 12 bcc_cm1 24

bccr_bcm2 13 SINTEX 25



Regressed precipitation wrt to a base time series at EIO.



Regressed SSTA wrt to a base time series at EIO.



Simulation of BSISO Characteristics by SINTEX-F1

Regressed filtered anomalies of precipitation (mm.day−1) averaged over the domain mentioned above. Regression is calculated with respect to a base region 70-90E;12-22N.

SINTEX-F1

CMAP



JJAS SST anomalies at southeast IO

Summer monsoon is punctuated by vigorous intraseasonal oscillations in the form of active and break spells. These oscillations influence the seasonal mean monsoon.

Influence of IOD on poleward propagation of boreal summer intraseasonal oscillations (BSISO) is studied using observations and a 220-year simulation of a coupled ocean-atmospheric model.



Regressed filtered anomalies of precipitation (mm.day−1) averaged over 70-95E. Positive IOD years are associated with disorganized or incoherent poleward

propagation.

CMAP

SINTEX-F1



Shading: JJAS SSTA Vectors: Divergent component of vertically integrated

(1000hPa to 300 hPa) moisture transport anomalies.

Mean-State Changes



+ ve IOD yrs

- ve IOD yrs

All yrs

SINTEX-F1SPH averaged over 70-

95E

U850-U200 averaged over 40-95E

Changes in Mean- State



Arrows : convection

Contours : vorticity

BLMC is north of maximum convection in all phases leading to coherent propagation.



Arrows : convection

Contours : vorticity

BLMC is not always north of maximum convection leading to incoherent propagation.



SST-Precipitation Lead-Lag Correlations for SST-Precipitation Lead-Lag Correlations for contrasting IOD Yearscontrasting IOD Years

Observation

SINTEX-F1



Partial Coupling: atmosphere sees specified SSTs instead of interactive SSTs in specific regions

In these experiments the specified SSTs consist of model climatological SSTs obtained from a fully coupled control run (SSTA = 0)

How crucial is the SST-Precipitation lead relationship How crucial is the SST-Precipitation lead relationship in simulating BSISO propagation in this CGCM?in simulating BSISO propagation in this CGCM?



Contours: SST

Shaded: PRCP



Contours: 850hPa Divergence

Shaded: PRCP



Conclusions

Both mean state and local air-sea interaction seems to play a role in BSISO propagation enabling boundary layer convergence to be ahead of convection.

In the sensitivity experiments, preliminary results suggest that local air-sea interaction provides a modest amplification of BSISO.



Regressed filtered precipitation anomalies

averaged over 70-95E as a

function of latitude and time lag from a 200 year simulation of a coupled

ocean-atmosphere model. [SINTEX-F1]

Ajayamohan, Rao, Luo and Yamagata, 2008



SINTEX-F1

Negative IOD Years

Positive IOD Years



• IOD is defined as a dipole mode in SST anomalies in Indian Ocean coupled to zonal winds and convection. [ Saji et al.

(1999) ; Yamagata et al. (2004 Review) ]

• Coupled ocean-atmosphere Phenomenon with cool (warm) SST anomalies in southeastern IO with warm (cool) SST anomalies in western IO.

• Impact on seasonal and interannual climate variations.



Negative IOD Years

Positive IOD Years



At any time, cyclonic (anticyclonic) vorticity at 850hPa is to the north of negative (positive) precipitation anomalies.

The cyclonic (anticyclonic) vorticity at 850hPa is associated with convergence (divergence) of moisture in the boundary layer.

The atmospheric circulation driven by the diabatic heating associated with the zonally oriented cloud band in the presence of background mean flow with easterly vertical shear produces a cyclonic vorticity with a maximum about 3oN of the center of the cloud band.

Cyclonic vorticity drives frictional convergence in the planetary boundary layer and leads to higher moisture convergence north of the cloud band.

Meridional gradient of the mean boundary layer moisture also helps in making moisture convergence larger to the north of the cloud band.

This leads to convection center to move northward.

Why convection moves northward ?



Composite wavelet spectrum of precipitation anomalies

averaged over 70-95E;10S-Eq (SINTEX-F1)

multiscale processes in the tropics april 27- may 1, 2009, banff role of mean state and local...

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