Simulation of Indian Summer Monsoon: Simulation of Indian Summer Monsoon: Role of Oceans, Mountains and AerosolsRole of Oceans, Mountains and Aerosols

Ravi S NanjundiahRavi S NanjundiahCentre for Atmospheric and Oceanic Centre for Atmospheric and Oceanic

Sciences,Sciences,Indian Institute of Science, Indian Institute of Science,

BengalooruBengalooru

ravi@caos.iisc.ernet.inravi@caos.iisc.ernet.inThanks to : J Srinivasan, A Chakraborty, Sulochana Thanks to : J Srinivasan, A Chakraborty, Sulochana

Gadgil and P Francis, Gadgil and P Francis,

Problems addressedProblems addressed

Role of Oceans: is the SST-rainfall relationship Role of Oceans: is the SST-rainfall relationship simulated correctly by AGCMs?simulated correctly by AGCMs?

What implication does this have for interannual What implication does this have for interannual variabilityvariability

How does African Orography influence the Indian How does African Orography influence the Indian Monsoon and the Somali Jet?Monsoon and the Somali Jet?

What impact do absorbing aerosols have on the What impact do absorbing aerosols have on the strength of the Indian Summer Monsoon?strength of the Indian Summer Monsoon?

Models used: NCMRWF (@T-80,L18), ECPC-SFM(@T-63,L28)Models used: NCMRWF (@T-80,L18), ECPC-SFM(@T-63,L28)

Is the SST – rainfall relationship Is the SST – rainfall relationship correct?correct?

Some studies (Kang and Shukla,2006, Wang et al Some studies (Kang and Shukla,2006, Wang et al 2005)have suggested that SST-rainfall in AGCMs could be 2005)have suggested that SST-rainfall in AGCMs could be wrong and hence simulation of interannual variability could wrong and hence simulation of interannual variability could be in error. be in error.

In observations, over most parts of the world In observations, over most parts of the world rainfall and SST are positively correlatedrainfall and SST are positively correlatedOver Indo-Pacific region the correlation is low and Over Indo-Pacific region the correlation is low and over some parts negative.over some parts negative.

Most Models do not simulate the negative correlationMost Models do not simulate the negative correlation

SST-Rainfall Relationship: Another ViewSST-Rainfall Relationship: Another ViewStudies by Studies by Gadgil et al (1984) Gadgil et al (1984) and and Graham and Barnett (1987) Graham and Barnett (1987) showed that there is a threshold showed that there is a threshold around 27.5 above which around 27.5 above which convection could be sustained in convection could be sustained in the tropics. the tropics.

However, beyond this threshold However, beyond this threshold SST does not play a major role in SST does not play a major role in modulating convection. Graham modulating convection. Graham and Barnett state that “and Barnett state that “associations associations between SSTs and OLR for SSTs between SSTs and OLR for SSTs above 28C at the different locations above 28C at the different locations generally range between -0.3 to generally range between -0.3 to +0.3 and suggest that the +0.3 and suggest that the dependence level of convection on dependence level of convection on SSTs in this temperature range is SSTs in this temperature range is usually slight. '‘usually slight. '‘Gadgil et al (1984) also show Gadgil et al (1984) also show that correlation between SST and that correlation between SST and Convection is low beyond 28CConvection is low beyond 28C

Over Indo-Pacific the SST is Over Indo-Pacific the SST is generally above the thresholdgenerally above the threshold

Do models get this non-linear relationship?

SST-Rainfall Relationship: ModelsSST-Rainfall Relationship: Models

Most models show a non-Most models show a non-linear behaviour with a linear behaviour with a treshold around 27.5 Ctreshold around 27.5 C

How is the ENSO-How is the ENSO-monsoon connection in monsoon connection in most models?most models?

Observed Linkage to El-NinoObserved Linkage to El-Nino

Observed Nino Index and ISMR

In observations the correlation In observations the correlation between Nino index (negative between Nino index (negative of Nino3.4 SST anomaly) to of Nino3.4 SST anomaly) to ISMR is about 0.39ISMR is about 0.39

Years such as 1988 have Years such as 1988 have excess rainfall in conjunction excess rainfall in conjunction with positive Nino Indexwith positive Nino Index

Years such as 1986 1987 and Years such as 1986 1987 and 2002 are years of low ISMR 2002 are years of low ISMR when Nino index is negativewhen Nino index is negative

How does the model simulate How does the model simulate this?this?

Linkage to El-Nino in SFMLinkage to El-Nino in SFM

Nino index and ISMR in SFM

In SFM the correlation between In SFM the correlation between Nino index to model's ISMR is Nino index to model's ISMR is about 0.78 – much stronger than about 0.78 – much stronger than observed (0.39)observed (0.39)

Able to simulate 1988 as an Able to simulate 1988 as an excess rainfall year in excess rainfall year in conjunction with positive Nino conjunction with positive Nino IndexIndex

Also able to simulate 1986 1987 Also able to simulate 1986 1987 and 2002 as years of low ISMR and 2002 as years of low ISMR when Nino index is negativewhen Nino index is negative

Fails to simulate near normal Fails to simulate near normal rainfall in 1997 and excess of rainfall in 1997 and excess of 1994 (both years with negative 1994 (both years with negative Nino index)Nino index)

How is this linkage in other How is this linkage in other models? --- The AMIP Experiencemodels? --- The AMIP Experience

The AMIP ExperienceThe AMIP Experience

The ENSO-ISMR link varies across modelsThe ENSO-ISMR link varies across models UKMO and DNM have a high correlationUKMO and DNM have a high correlation ECMWF is negatively correlated to ENSOECMWF is negatively correlated to ENSOHow does this impact their ability to simulate Interannual Variability of ISMR?How does this impact their ability to simulate Interannual Variability of ISMR?

AMIP: Years of Strong and Weak ENSO ForcingAMIP: Years of Strong and Weak ENSO Forcing

Strong ENSO Forcing Weak ENSO Strong ENSO Forcing Weak ENSO (1988) (1994)(1988) (1994)

Almost models get ISMR right during 1988 : year with a strong La-NinaAlmost models get ISMR right during 1988 : year with a strong La-NinaAlmost models get ISMR wrong during 1994 : year with a weak El-NinoAlmost models get ISMR wrong during 1994 : year with a weak El-NinoThus the problems with SST-rainfall relationship cannot explain thisThus the problems with SST-rainfall relationship cannot explain thisIs there some other factor that most models are missing out? Is there some other factor that most models are missing out?

The EQUINOO ConnectionThe EQUINOO Connection

EQUINOO: Equatorial Indian Ocean Oscillation – the atmospheric component of EQUINOO: Equatorial Indian Ocean Oscillation – the atmospheric component of Indian Ocean DipoleIndian Ocean DipoleDuring the positive phase of EQUINOO convection over WEIO increases and During the positive phase of EQUINOO convection over WEIO increases and over EEIO decreasesover EEIO decreasesPositive phase favourable for ISMRPositive phase favourable for ISMRGadgil et al (2004) found that a combination of EQUINOO and ENSO could Gadgil et al (2004) found that a combination of EQUINOO and ENSO could explain almost all extremes. explain almost all extremes. 1994 was a year with weak ENSO and positive EQUINOO1994 was a year with weak ENSO and positive EQUINOOHow do models simulate this?How do models simulate this?

EQUINOO in AMIP Models : 1994EQUINOO in AMIP Models : 1994

Most models can get the local response to SST anomalies over the Most models can get the local response to SST anomalies over the equatorial Indian Ocean i.e. EQUINOO signal, correct in 1994equatorial Indian Ocean i.e. EQUINOO signal, correct in 1994

Excess rain over WEIO and less rain over EEIOExcess rain over WEIO and less rain over EEIO

The linkage between EQUINOO & Indian Monsoon appears to be wrongThe linkage between EQUINOO & Indian Monsoon appears to be wrong

EQUINOO in SFM: 1994 & 1997EQUINOO in SFM: 1994 & 1997

Response over EEIO reasonable; weaker over WEIOResponse over EEIO reasonable; weaker over WEIO

Correlation with EQUINOO only 0.05 as compared to obs correlation of 0.4Correlation with EQUINOO only 0.05 as compared to obs correlation of 0.4

ISMR response to EQUINOO not correct (as in most other models)ISMR response to EQUINOO not correct (as in most other models)

Too strong a response to ENSO and too weak a response to EQUINOO – Too strong a response to ENSO and too weak a response to EQUINOO – aa systematic problem in AGCMS which needs to be addressedsystematic problem in AGCMS which needs to be addressed

Summary : Role of OceansSummary : Role of Oceans

SST rainfall relationship in most models SST rainfall relationship in most models appears to be realisticappears to be realistic

Simulation of EQUINOO and its linkage to Simulation of EQUINOO and its linkage to ISMR is a problem in most modelsISMR is a problem in most models

How Does Orography Affect Monsoons?How Does Orography Affect Monsoons?

What role does Orography play in modulating the What role does Orography play in modulating the monsoon?monsoon?

Is onset related to presence of orography?Is onset related to presence of orography?

• Is the Somali Jet solely due to presence of African Is the Somali Jet solely due to presence of African orography?orography?

Monsoon Onset And OrographyMonsoon Onset And Orography

Simulations with the NCMRWF Simulations with the NCMRWF model T-80 resoluton (both SAS model T-80 resoluton (both SAS and Kuo schemes)and Kuo schemes)

Variations perturbations of Variations perturbations of orography:orography:

role of East and West Himalayasrole of East and West Himalayas role of african orographyrole of african orography remote forcing from American remote forcing from American

orographyorographyDate of Onset (over the entire Indian Date of Onset (over the entire Indian region) related to region) related to

Surface enthalphy crossing a threshold Surface enthalphy crossing a threshold (346KJ/kg)(346KJ/kg)

occurrence of large-scale upward occurrence of large-scale upward motion (500 hPa vertical velocity)motion (500 hPa vertical velocity)

All- India onset not MOKAll- India onset not MOK

Removal of all (global) orography Removal of all (global) orography delays onsetdelays onset

West Him. orography more effect than West Him. orography more effect than East HimEast Him

•African orography does not effect date of onset

What happens to Monsoons and Low Level What happens to Monsoons and Low Level Jet in Absence of African Orography?Jet in Absence of African Orography?

Some previous studies consider Somali Jet to be a western Some previous studies consider Somali Jet to be a western boundary current due to African orography (Anderson 1976)boundary current due to African orography (Anderson 1976)

Krishnamurti et al (1976) considered role of land-ocean Krishnamurti et al (1976) considered role of land-ocean contrast, heat sources and orographycontrast, heat sources and orography

Mawson and Cullen suggested that western boundary forcing is Mawson and Cullen suggested that western boundary forcing is important for occurrence of Somali Jetimportant for occurrence of Somali Jet

Slingo et al (2004) suggest that absence of African orography Slingo et al (2004) suggest that absence of African orography would cause major changes in the hydrological cyclewould cause major changes in the hydrological cycle

Most linear modelling studies show that African orography as Most linear modelling studies show that African orography as western boundary is essential for Somali Jetwestern boundary is essential for Somali Jet

What happens in a non-linear model such as an AGCM ?What happens in a non-linear model such as an AGCM ?

Would Indian Monsoon be weaker in absence Would Indian Monsoon be weaker in absence of African Orography ?of African Orography ?

Monsoon rainfall actually increases in absence of Monsoon rainfall actually increases in absence of African orographyAfrican orography

Does the Somali Jet Collapse in Absence of Does the Somali Jet Collapse in Absence of African Orography?African Orography?

In all simulations, there exists a low-level jet over the Indo-In all simulations, there exists a low-level jet over the Indo-African regionAfrican region

In all simulations the cross-equatorial turning is most prominent In all simulations the cross-equatorial turning is most prominent at 40E even in the absence of African orographyat 40E even in the absence of African orography

Cross equatorial flow a little weaker around 40E in the absence Cross equatorial flow a little weaker around 40E in the absence of African orographyof African orography

Is the Somali Jet then a western boundary current?Is the Somali Jet then a western boundary current?

Low Level Jet in a Aquaplanet SimulationLow Level Jet in a Aquaplanet Simulation

An aquaplanet simulation with an SST maxima of 302K at 90E and 20 An aquaplanet simulation with an SST maxima of 302K at 90E and 20 N (to mimic the Indian heat source)N (to mimic the Indian heat source)

The axis of the cross equatorial jet still occurs around 40-50E about 30 The axis of the cross equatorial jet still occurs around 40-50E about 30 degrees away from the region of heating.degrees away from the region of heating.

It appears that Low Level Jet occurs as a response to heat source with It appears that Low Level Jet occurs as a response to heat source with cross-equatorial turning about 30-40 degrees west of the heat source cross-equatorial turning about 30-40 degrees west of the heat source (effect of rotation)(effect of rotation)

Existence of African orography amplifies the cross-equatorial flowExistence of African orography amplifies the cross-equatorial flow

What happens to the strength of the Westerly What happens to the strength of the Westerly Jet?Jet?

Strength of the Westerly Jet to the north of equator is related to the Strength of the Westerly Jet to the north of equator is related to the monsoonal heat source.monsoonal heat source.

Stronger the rainfall higher the winds.Stronger the rainfall higher the winds.

Why are winds stronger in Absence of African Why are winds stronger in Absence of African Orography?Orography?

Non-linear effects tend to Non-linear effects tend to dominatedominateIn absence of African In absence of African orography larger mass orography larger mass convergence from African convergence from African continentcontinentLarge mass convergence Large mass convergence leads to higher moisture leads to higher moisture convergenceconvergenceLeads to higher rainfall and Leads to higher rainfall and stronger windsstronger winds

Summary: Role Of MountainsSummary: Role Of Mountains

Presence of Western Himalaya advances the date of Presence of Western Himalaya advances the date of onsetonsetPresence of African orography reduces the monsoonal Presence of African orography reduces the monsoonal rainfallrainfallPosition of cross-equatorial jet (at around 40E) a response Position of cross-equatorial jet (at around 40E) a response to monsoonal heatingto monsoonal heatingPresence of African mountains at the same location Presence of African mountains at the same location enhances this flowenhances this flowStrength of the westerly jet related to the strength of the Strength of the westerly jet related to the strength of the monsoonal heating monsoonal heating

Absorbing Aerosols and Strength of the Absorbing Aerosols and Strength of the Indian Summer MonsoonsIndian Summer Monsoons

Black Carbon aerosols Black Carbon aerosols heat the lower troposphere heat the lower troposphere and cool the surfaceand cool the surface

The heating effect put into The heating effect put into the NCMWRWF model (T-the NCMWRWF model (T-80 resolution)80 resolution)

Heating included only over Heating included only over the Indian region (from the Indian region (from IndoEx data)IndoEx data)

Heating effect removed Heating effect removed with the onset of monsoonswith the onset of monsoons

Role of Aerosols: Interaction with Cumulus Role of Aerosols: Interaction with Cumulus SchemeScheme

Simulations conducted with Kuo and SAS schemesSimulations conducted with Kuo and SAS schemesBoth show an increase in rainfallBoth show an increase in rainfallWarming of lower troposphere could destablise the atmosphere and Warming of lower troposphere could destablise the atmosphere and lead to increase in rainfalllead to increase in rainfallHowever regions of increase and decrease are different in the two However regions of increase and decrease are different in the two simulationssimulationsImpacts on regional scale could be sensitive to model physics Impacts on regional scale could be sensitive to model physics

Conclusions Conclusions

Non-linear behaviour of SST-rainfall with a threshold Non-linear behaviour of SST-rainfall with a threshold around 27.5 C is simulated by most modelsaround 27.5 C is simulated by most models

Above this threshold convection is not determined by SSTAbove this threshold convection is not determined by SST

EQUINOO – ISMR linkage not realistic in most modelsEQUINOO – ISMR linkage not realistic in most models

West Himalayan orography has major impact on the date West Himalayan orography has major impact on the date of onsetof onset

Occurrence of Somali Jet is a response to monsoonal Occurrence of Somali Jet is a response to monsoonal heatingheating

Presence of African Orography enhances the cross-Presence of African Orography enhances the cross-equatorial flowequatorial flow

Absorbing Aerosols strengthen the Indian monsoon, Absorbing Aerosols strengthen the Indian monsoon, region of enhancement sensitive to model physicsregion of enhancement sensitive to model physics

Thanks to DST, IMD & ISRO for their supportThanks to DST, IMD & ISRO for their support

Thank YouThank You

The Road AheadThe Road AheadThe cause of weak response to EQUINOO in almost all AGCMS The cause of weak response to EQUINOO in almost all AGCMS

needs to be studiedneeds to be studied

Probable cause could be cumulus parameterization schemes:Probable cause could be cumulus parameterization schemes: These need to be systematically validated over the Indian These need to be systematically validated over the Indian

region with ARMEX and BoBMEX dataregion with ARMEX and BoBMEX data Based on these validations the cumulus schemes may Based on these validations the cumulus schemes may

need to be modified/or new ones formulatedneed to be modified/or new ones formulatedPossibly the cumulus schemes operating over land and Possibly the cumulus schemes operating over land and over ocean need to different (a paper in latest Nature over ocean need to different (a paper in latest Nature suggests that partitioning of moisture over land and suggests that partitioning of moisture over land and ocean are very different)ocean are very different)Possible modifications to Boundary Parameterizations Possible modifications to Boundary Parameterizations at low and high wind speeds? at low and high wind speeds?

Excess response to ENSO in AGCMs needs to be reducedExcess response to ENSO in AGCMs needs to be reduced

The problem of cold SSTs in Coupled Models needs to be The problem of cold SSTs in Coupled Models needs to be studiedstudied

Climate Modelling @ CAOSClimate Modelling @ CAOS AGCMS used at CAOS for about 12 years to study monsoonsAGCMS used at CAOS for about 12 years to study monsoonsNCMRWF, SFM from the NCEP hierarchyNCMRWF, SFM from the NCEP hierarchyCCM2, CAM2, & CAM3 from the NCAR stableCCM2, CAM2, & CAM3 from the NCAR stableCoupled Models used at CAOS: CCSM2, CCSM3Coupled Models used at CAOS: CCSM2, CCSM3Climate related Problems addressed with AGCMS:Climate related Problems addressed with AGCMS:

Role of surface hydrology on Seasonal & Intraseasonal Role of surface hydrology on Seasonal & Intraseasonal VariationsVariationsRole of OrographyRole of OrographyRole of cumulus convectionRole of cumulus convectionImpact of Aerosols on Indian Summer MonsoonsImpact of Aerosols on Indian Summer MonsoonsImpact of numerical methods (moisture transport) on Impact of numerical methods (moisture transport) on simulation of Monsoonssimulation of MonsoonsInterannual variability and role of Indian OceanInterannual variability and role of Indian OceanPaleo-climate Studies (Mid-holocene)Paleo-climate Studies (Mid-holocene)

Climate Modelling @CAOSClimate Modelling @CAOSComputational Aspects of Climate Modelling at CAOS:Computational Aspects of Climate Modelling at CAOS:

One of the earliest in the world to adapt parallel One of the earliest in the world to adapt parallel computing to climate modelling (1988 at NAL)computing to climate modelling (1988 at NAL)Involved in successful parallel implementation of Involved in successful parallel implementation of NCMRWF & CCM2 modelsNCMRWF & CCM2 modelsCurrently studying scalability of CAM3 : role of Currently studying scalability of CAM3 : role of message compressionmessage compressionLoad imbalances between components in the Load imbalances between components in the climate system model : balancing of radiational climate system model : balancing of radiational loads.loads.Software re-engineering of the NCMRWF model. Software re-engineering of the NCMRWF model. Portability issues.Portability issues.

Grid-enabling the Climate System Model: Exploiting Grid-enabling the Climate System Model: Exploiting computational resources simultaneously across the country computational resources simultaneously across the country to run Climate System Models to run Climate System Models Interacting with IMD in the development of a seasonal Interacting with IMD in the development of a seasonal forecast system.forecast system.Interaction with industry: Collaboration on some of these Interaction with industry: Collaboration on some of these topics with IBM, Intel etc.topics with IBM, Intel etc.

Are Coupled Models a Panacea?Are Coupled Models a Panacea?-The DEMETER Experience-The DEMETER Experience

DEMETER: A project to compare seasonal forecasts with coupled models DEMETER: A project to compare seasonal forecasts with coupled models during the period 1959 – 2001during the period 1959 – 2001UKMO coupled model could not capture the severe droughts of 66, 68,72UKMO coupled model could not capture the severe droughts of 66, 68,72Could not capture excess monsoons of 1959, 1970 & 1988Could not capture excess monsoons of 1959, 1970 & 1988

Almost all AGCMs could capture the 1988 monsoonAlmost all AGCMs could capture the 1988 monsoon In the last five years the sign of ISMR was consistently wrongIn the last five years the sign of ISMR was consistently wrong

Almost all coupled models in IPCC 20C3M have colder than observed Almost all coupled models in IPCC 20C3M have colder than observed SSTs over the Indian region SSTs over the Indian region