4th International CLIVAR Climate of the 20th Century Workshop, 13-15th March 2007, Exeter, UK

Sensitivity of the Indo-Pacific climate variability to

different forcings in XXth century simulations

Annalisa Cherchi and Antonio NavarraIstituto Nazionale di Geofisica e Vulcanologia, Bologna, Italy

Centro EuroMediterraneo per i Cambiamenti Climatici, Bologna, Italy

Outline of the talk

Model used & Experiments performed in the C20C framework

Simulation of the ENSO-monsoon connection in the experiments performed

Analysis of the decadal variability of the connection (1976 climate shift)

The role of the Indian Ocean

Conclusions

The Indo-Pacific climate variability is identified with the ENSO-monsoon connection

The SINTEXG CGCMit is a atmosphere-ocean-ice coupled model developed at INGV following the background of the SINTEX model developed among the SINTEX EU-project

Atmosphere: Echam4.6Developed at MPI in Hamburg

Spectral model Horizontal resolution: T30, T106

Vertical resolution: 19 sigma layersMPI parallel version

(Roeckner et al., 1996)

Ocean: OPA 8.2Developed at LODYC in Paris

Finite difference modelVertical resolution: 31 levels

(10 in the first 100m)Horizontal resolution: 2x1.5deg

(around the Eq 0.5deg)(Madec et al., 1998)

Coupler: OASIS 2.4Developed at CERFACS in

ToulouseMessage passing based on MPI2

(Valcke et al., 2000)

Ice: LIM(Louven laneuve sea-Ice systeM)

called by OPA routines(Fichefet et al., 1999)

Winds & fluxes

SST

Sea-Ice cover

For a description of the mean climate simulated by the model see the web page: https://www.cmcc.it/web/public/ANS/models/ingv-

sxg

XXth simulations: the atmospheric model is integrated with prescribed radiative forcings (GHGs, ozone and sulfate aerosols) from 1870 to 2002 following the IPCC directives (20C3M experiments). The GHGs prescribed are: CO2, CH4,N2O, CFC11 and CFC12.

AMIP-type experiments

Echam4.6 at T42 resolution forced with observed SST &

sea ice (HadISST) 1871-2002

(6 members)

Coupled experiment

SINTEXG at T42 resolution forced with GHGs (XXth

century simulation)

Setup of the experiments performed in the C20C framework

AMIP-GHG-type experiments

Echam4.6 at T42 resolution forced with observed SST & sea ice (HadISST) and with

GHGs (XXth simulations)1871-2002

(6 members)

Other factors influencing the ENSO-monsoon connection:the Eurasian snow coverthe Indian Ocean SSTs

ENSO and the Asian summer monsoon are interactively linked (Webster and Yang, 1992)

The Asian summer monsoon is strongly influenced by the thermal contrast between the Indian Ocean and the South Asian land mass and by the Tibetan Plateau (e.g. Webster, 1987; Li and Yanai, 1992)

On interannual timescale the ASM is influenced by ENSO (e.g. Rasmusson and Carpenter, 1983; Webster and Yang, 1992).

An important component in the connection is the Walker circulation with the strongest updraft over Indonesia and the western Pacific Ocean in correspondence of the warm pool.

About the ENSO-monsoon connection:

Warm episode in winter-spring

attenuation of the Walker circulation (convection suppressed over the Northern Tropical Indian Ocean and Maritime continents)

anomalous cyclonic circulation to the west of the Tibetan Plateau (Rossby-type response to convective heating)

decreased land surface temperature over central Asia to the north-west of the Indian subcontinent

reduced land-ocean thermal contrast

weakening of the Asian summer monsoon

The reverse is supposed to occur after a cold episode in winter-spring

A mechanism to explain the link between ENSO SST forcing and the Asian summer monsoon (Kawamura, 1998)

The analysis of the ENSO-monsoon connection has been performed by means of correlation and composites analysis based on a selected numbers of indices

DMI (Dynamical Monsoon Index)Mean JJA zonal wind shear (u850-u200) averaged over 40-110E, Eq-20N (Webster and Yang, 1992)

DMI

How to measure and identify El Nino: NINO3 index (monthly SST anomalies averaged in the area 5S-5N and 150W-90W)

MTG (Meridional thermal gradient)(H200-500)(20-40N)-(H200-500)(Eq-20N) averaged in JJA(Kawamura, 1998)

A precursory signal for the monsoon:AMJ SST vs MTG index (linear correlation)

Amip-SST Amip-GHG

Coupled model NCEP & HadISST

The summer season:JJA SST vs DMI index (linear correlation)

Amip-SST Amip-GHG

Coupled model NCEP & HadISST

Decadal variability of the monsoon index:11yr running mean of DMI and IMR

DMI Amip-GHGAmip-SSTNCEP

r(Amip-SST – NCEP)=0.40r(Amip-GHG – NCEP)=0.61

r(Amip-SST – CRU)=0.61r(Amip-GHG – CRU)=0.63

IMR

The ENSO-monsoon connection has a remarkable decadal variability and this relationship weakened in recent decades (Kumar et al., 1999)

Possible causes for those changes:1) seasonality of the ENSO cycle (Kawamura et al., 2003)2) the Indian Ocean Dipole Mode (Ashok, et al., 2001)3) the global warming: an ISM normal despite El Nino conditions for a south-eastward shift of the Walker circulation (Kumar et al., 1999 Science), increase of the ground temperature over the Eurasian continent and consequent increase of the land-sea thermal contrast (Ashrit et al., 2001) or for increase of moisture supply from the Indian Ocean due to increased surface temperatures (Kitoh et al., 1997)4) natural decadal variability

Amip-SSTAmip-GHGObs

NINO3 vs IMR (19 yr sliding window)

NINO3 vs DMI (19 yr sliding window)

Correlation JJA SST vs DMI (from AMIP-type exp results)

ENSO-monsoon connection: 1976 climate shift

pre-1976 post-1976

Cherchi and Navarra, 2006

T30

T42

T106

HadISST &ERA40

About the 1976 climate shift: MTG vs AMJ SST (linear correlation)

1948-1975 1976-2002

Amip-SST

Amip-GHG

Obs

Composite of JJA TPREP (strong minus weak monsoon) pre76 & post76 in Amip-GHG experiments

1948-1975

1976-2002

About the role of the Indian Ocean:

The role of the TIO SST as active or passive element for the ISM has been a controversial issue: Tropical Indian Ocean SST may be considered as a passive element of the ISM system at interannual time scale (Webster et al., 1998) Modelling studies have shown that the Indian Ocean does significantly affect ISM rainfall (e.g. Yamazaki, 1988; Meehl and Arblaster, 2002) and that the annual cycle of SST in the Indian Ocean is crucial for a realistic simulation of the Indian summer monsoon (Shukla and Fennessy, 1994) Positive SST anomalies over the Arabian Sea during the spring preceding the monsoon season are precursors for above normal precipitation over India (e.g. Rao and Goswami, 1988; Clark et al., 2000)

The discovery of the Indian Ocean Dipole Mode (IODM, Saji et al., 1999; Webster et al., 1999), as an important mode of variability of the Indian Ocean itself, suggested the possibility of interactions between this mode of variability and the ISM. The issue is still controversial:Positive IODM events enhance ISM rainfall (Ashok et al., 2001; Li et al., 2003) Positive IODM events are linked to dry conditions over the Indian subcontinent (Webster et al., 2002; Meehl et al., 2003)

Model experiments results have confirmed that positive (negative) Indian Ocean dipole events may reduce the influence of an El Nino (La Nina) event on the Indian monsoon (Ashok et al., 2004)

Composites (JJA SST & Indian summer monsoon rainfall)

Observations

Coupled model

Cherchi et al., 2006

Coupled Manifold technique (Navarra and Tribbia, 2005)

A new statistical method to detect the portion of co-variability between 2 climatic fields

1)(SS'ZS'A

'1

1

1)'( ii

K

ii uuSS

The problem is: Z=ASwhere A becomes: with

The coupled manifold may be used to:i) compute the % of variance of an atmospheric field linked to another atmospheric

field, and the reverse ii) separate Z in Zfor (subspace where variation of one field are connected to

variations of the other field) and Zfree (a subspace where variations are indipendent)

iii) identify one-way (“forced manifold”) and two-way (“coupled manifold”) relations between the fields considered

% of variance of TrIndOc SST linked to TrPacOc SST Observations Coupled model

Observations Coupled model

% of variance of India TPREP linked to TrIndOc SST

Coupled manifolds technique used to distinguish the percentage of variance of Indian precipitation due to the Indian Ocean SSTA and other forcing

Cherchi et al., 2006

Correlation of Indian Monsoon Rainfall vs Indian Ocean SST

Obs ModelTotal SST

Forced SST

Free SST

Cherchi et al., 2006

Understanding of the mechanisms involved in the ENSO-monsoon-Indian Ocean Dipole mode interactions

the shaded pattern is significant at 95%

EOFS of Forced & Free SSTA

Obs ModelEOF1 Forced

EOF2 Forced

EOF1 Free

Cherchi et al., 2006

Conclusions

Atmospheric and coupled model are able to capture in a realistic way the direct impact of anomalous SST forcing associated with ENSO on the South Asian summer monsoon

The connection is better simulated when the GHG forcings are included

The decadal variability of the monsoon indices considered is realistically simulated by the atmospheric model, when the GHGs are included the linear correlation with the observed field is larger

The decadal variability of the ENSO-monsoon connection is captured by the model, as well as its weakening observed in recent decades

The changes observed after 1976 in the ENSO-monsoon connection are realistic in the atmospheric model experiments, especially in the Amip-GHG experiments

After 1976 the relationship between the Indian Ocean SST and the Asian monsoon index is stronger

TIO SSTA influence precipitation over India

Local effects & remote effects (influence from the Tropical Pacific Ocean) of the TIO SSTA on the ISM have been separated by means of the coupled manifold technique

The EOF analysis of the “forced” and “free” SSTA in the TIO is used to analyze the variability of the TIO and its link with the TPO (the link between the TIO & the TPO is weak in the model)

THANK YOU !