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
Multiscale processes in the tropics
April 27- May 1, 2009, Banff
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
Multiscale processes in the tropics
April 27- May 1, 2009, Banff
Tropical intraseasonal variability (20-90 Tropical intraseasonal variability (20-90 days)days)Summer Summer
Winter Winter
Rainfall standard deviation (mm/day)
Multiscale processes in the tropics
April 27- May 1, 2009, Banff
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]
Multiscale processes in the tropics
April 27- May 1, 2009, Banff
ISO Characteristics: Evolution of BSISO. Note the
clear northeastward propagation of precipitation
anomalies.
Ajayamohan and Goswami, 2007, JAS
Multiscale processes in the tropics
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.
Multiscale processes in the tropics
April 27- May 1, 2009, Banff
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
Multiscale processes in the tropics
April 27- May 1, 2009, Banff
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)]
Multiscale processes in the tropics
April 27- May 1, 2009, Banff
CMAP Rainfall & Reynolds SSTA, Ajayamohan et al., 2008
SST leads precipitation
TRMM Rainfall & TMI SSTA, Wang et al., 2006
SST leads
Multiscale processes in the tropics
April 27- May 1, 2009, Banff
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
Multiscale processes in the tropics
April 27- May 1, 2009, Banff
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
Multiscale processes in the tropics
April 27- May 1, 2009, Banff
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
Multiscale processes in the tropics
April 27- May 1, 2009, Banff
Regressed precipitation wrt to a base time series at EIO.
Multiscale processes in the tropics
April 27- May 1, 2009, Banff
Regressed SSTA wrt to a base time series at EIO.
Multiscale processes in the tropics
April 27- May 1, 2009, Banff
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
Multiscale processes in the tropics
April 27- May 1, 2009, Banff
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.
Multiscale processes in the tropics
April 27- May 1, 2009, Banff
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
Multiscale processes in the tropics
April 27- May 1, 2009, Banff
Shading: JJAS SSTA Vectors: Divergent component of vertically integrated
(1000hPa to 300 hPa) moisture transport anomalies.
Mean-State Changes
Multiscale processes in the tropics
April 27- May 1, 2009, Banff
+ ve IOD yrs
- ve IOD yrs
All yrs
SINTEX-F1SPH averaged over 70-
95E
U850-U200 averaged over 40-95E
Changes in Mean- State
Multiscale processes in the tropics
April 27- May 1, 2009, Banff
Arrows : convection
Contours : vorticity
BLMC is north of maximum convection in all phases leading to coherent propagation.
Multiscale processes in the tropics
April 27- May 1, 2009, Banff
Arrows : convection
Contours : vorticity
BLMC is not always north of maximum convection leading to incoherent propagation.
Multiscale processes in the tropics
April 27- May 1, 2009, Banff
SST-Precipitation Lead-Lag Correlations for SST-Precipitation Lead-Lag Correlations for contrasting IOD Yearscontrasting IOD Years
Observation
SINTEX-F1
Multiscale processes in the tropics
April 27- May 1, 2009, Banff
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?
Multiscale processes in the tropics
April 27- May 1, 2009, Banff
Contours: SST
Shaded: PRCP
Multiscale processes in the tropics
April 27- May 1, 2009, Banff
Contours: 850hPa Divergence
Shaded: PRCP
Multiscale processes in the tropics
April 27- May 1, 2009, Banff
Multiscale processes in the tropics
April 27- May 1, 2009, Banff
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.
Multiscale processes in the tropics
April 27- May 1, 2009, Banff
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
Multiscale processes in the tropics
April 27- May 1, 2009, Banff
SINTEX-F1
Negative IOD Years
Positive IOD Years
Multiscale processes in the tropics
April 27- May 1, 2009, Banff
• 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.
Multiscale processes in the tropics
April 27- May 1, 2009, Banff
Negative IOD Years
Positive IOD Years
Multiscale processes in the tropics
April 27- May 1, 2009, Banff
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 ?
Multiscale processes in the tropics
April 27- May 1, 2009, Banff
Multiscale processes in the tropics
April 27- May 1, 2009, Banff
Composite wavelet spectrum of precipitation anomalies
averaged over 70-95E;10S-Eq (SINTEX-F1)
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