Simulation of Indian Summer Monsoon: Role of Oceans, Mountains and Aerosols
Ravi S Nanjundiah
Centre for Atmospheric and Oceanic Sciences,
Indian Institute of Science,
Bengalooru

ravi@caos.iisc.ernet.in
Thanks to : J Srinivasan, A Chakraborty, Sulochana Gadgil and P Francis,
Problems addressed
Role of Oceans: is the SST-rainfall relationship simulated correctly by AGCMs?
What implication does this have for interannual variability

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

What impact do absorbing aerosols have on the strength of the Indian Summer Monsoon?
Overview of the Model
We use the atmospheric component of Seasonal Forecast Model of Experimental Climate Prediction Center (ECPC)
The model is run at T-63 resolution ~ 1.8 resolution
Uses the Relaxed Arakawa Cumulus parameterization of Moorthi and Suarez (1992) – without downdrafts
Shortwave radiation using the method of Chou (1994)
Longwave radiation using the method of Chou and Suarez (1992)
Using the Simplified SiB (SSiB) for vegetation/land-surface
SST from OIV2 data of Reynolds and Smith (1998)
Is the SST – rainfall relationship correct?
Some studies (Kang and Shukla,2006, Wang et al 2005 )have suggested that SST-rainfall in AGCMs could be wrong and hence simulation of interannual variability could be in error.
In observations, over most parts of the world rainfall and SST are positively correlated
Over Indo-Pacific region the correlation is low and over some parts negative.
Most Models do not simulate the negative correlation
SST-Rainfall Relationship: Another View
Studies by Gadgil et al (1984) and Graham and Barnett (1987) showed that there is a threshold around 27.5 above which convection could be sustained in the tropics.

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

Over Indo-Pacific the SST is generally above the threshold

SST-Rainfall Relationship: Models


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

How is the ENSO-monsoon connection in most models?
Observed Linkage to El-Nino
In observations the correlation between Nino index (negative of Nino3.4 SST anomaly) to ISMR is about 0.39

Years such as 1988 have excess rainfall in conjunction with positive Nino Index
Years such as 1986 1987 and 2002 are years of low ISMR when Nino index is negative

How does the model simulate this?
Linkage to El-Nino in SFM
In SFM the correlation between Nino index to model's ISMR is about 0.78 – much stronger than observed (0.39)
Able to simulate 1988 as an excess rainfall year in conjunction with positive Nino Index
Also able to simulate 1986 1987 and 2002 as years of low ISMR when Nino index is negative
Fails to simulate near normal rainfall in 1997 and excess of 1994 (both years with negative Nino index)
How is this linkage in other models? --- The AMIP Experience
The AMIP Experience
The ENSO-ISMR link varies across models
UKMO and DNM have a high correlation
ECMWF is negatively correlated to ENSO
How does this impact their ability to simulate Interannual Variability of ISMR?
AMIP: Years of Strong and Weak ENSO Forcing
Strong ENSO Forcing Weak ENSO
(1988) (1994)
Almost models get ISMR right during 1988 : year with a strong La-Nina
Almost models get ISMR wrong during 1994 : year with a weak El-Nino
Thus the problems with SST-rainfall relationship cannot explain this
Is there some other factor that most models are missing out?

The EQUINOO Connection
EQUINOO: Equatorial Indian Ocean Oscillation – the atmospheric component of Indian Ocean Dipole
During the positive phase of EQUINOO convection over WEIO increases and over EEIO decreases
Positive phase favourable for ISMR
Gadgil et al (2004) found that a combination of EQUINOO and ENSO could explain almost all extremes.
1994 was a year with weak ENSO and positive EQUINOO
How do models simulate this?
EQUINOO in AMIP Models : 1994
Most models can get the local response to SST anomalies over the equatorial Indian Ocean i.e. EQUINOO signal, correct in 1994
Excess rain over WEIO and less rain over EEIO
The linkage between EQUINOO & Indian Monsoon appears to be wrong


EQUINOO in SFM: 1994 & 1997
Response over EEIO reasonable; weaker over WEIO
Correlation with EQUINOO only 0.05 as compared to obs correlation of 0.4
ISMR response to EQUINOO not correct (as in most other models)
Too strong a response to ENSO and too weak a response to EQUINOO – a systematic problem in AGCMS which needs to be addressed
Summary : Role of Oceans

SST rainfall relationship in most models appears to be realistic

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

How Does Orography Affect Monsoons?
What role does Orography play in modulating the monsoon?
Is onset related to presence of orography?

Is the Somali Jet solely due to presence of African orography?
Monsoon Onset And Orography
Simulations with the NCMRWF model T-80 resoluton (both SAS and Kuo schemes)
Variations perturbations of orography:
role of East and West Himalayas
role of african orography
remote forcing from American orography
Date of Onset (over the entire Indian region) related to
Surface enthalphy crossing a threshold
occurrence of large-scale upward motion
Removal all (global) orography delays onset
West Him. orography more effect than East Him
What happens to Monsoons and Low Level Jet in Absence of African Orography?
Some previous studies consider Somali Jet to be a western boundary current due to African orography (Anderson 1976)
Krishnamurti et al (1976) considered role of land-ocean contrast, heat sources and orography
Mawson and Cullen suggested that western boundary forcing is important for occurrence of Somali Jet
Slingo et al (2004) suggest that absence of African orography would cause major changes in the hydrological cycle
Most linear modelling studies show that African orography as western boundary is essential for Somali Jet
What happens in a non-linear model such as an AGCM ?
Would Indian Monsoon be weaker in absence of African Orography ?
Monsoon rainfall actually increases in absence of African orography
Does the Somali Jet Collapse in Absence of African Orography?
In all simulations, there exists a low-level jet over the Indo-African region
In all simulations the cross-equatorial turning is most prominent at 40E even in the absence of African orography
Cross equatorial flow a little weaker around 40E in the absence of African orography
Is the Somali Jet then a western boundary current?
Low Level Jet in a Aquaplanet Simulation
An aquaplanet simulation with an SST maxima of 302K at 90E and 20 N (to mimic the Indian heat source)
The axis of the cross equatorial jet still occurs around 40-50E about 30 degrees away from the region of heating.
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 (effect of rotation)
Existence of African orography amplifies the cross-equatorial flow
What happens to the strength of the Westerly Jet?
Strength of the Westerly Jet to the north of equator is related to the monsoonal heat source.
Stronger the rainfall higher the winds.
Why are winds stronger in Absence of African Orography?
Non-linear effects tend to dominate
In absence of African orography larger mass convergence from African continent
Large mass convergence leads to higher moisture convergence
Leads to higher rainfall and stronger winds
Summary: Role Of Mountains
Presence of Western Himalaya advances the date of onset
Presence of African orography reduces the monsoonal rainfall
Position of cross-equatorial jet (at around 40E) a response to monsoonal heating
Presence of African mountains at the same location enhances this flow
Strength of the westerly jet related to the str

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