Diapositiva 1

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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 Navarra Istituto
Nazionale di Geofisica e Vulcanologia, Bologna,
Italy Centro EuroMediterraneo per i Cambiamenti
Climatici, Bologna, Italy
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The Indo-Pacific climate variability is
identified with the ENSO-monsoon connection
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
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The SINTEXG CGCM it is a atmosphere-ocean-ice
coupled model developed at INGV following the
background of the SINTEX model developed among
the SINTEX EU-project
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
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Setup of the experiments performed in the C20C
framework
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.
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About the ENSO-monsoon connection
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).
ENSO and the Asian summer monsoon are
interactively linked (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.
Other factors influencing the ENSO-monsoon
connection the Eurasian snow cover the Indian
Ocean SSTs
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A mechanism to explain the link between ENSO SST
forcing and the Asian summer monsoon (Kawamura,
1998)
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
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The analysis of the ENSO-monsoon connection has
been performed by means of correlation and
composites analysis based on a selected numbers
of indices
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A precursory signal for the monsoon AMJ SST vs
MTG index (linear correlation)
Amip-SST
Amip-GHG
Coupled model
NCEP HadISST
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The summer season JJA SST vs DMI index (linear
correlation)
Amip-SST
Amip-GHG
Coupled model
NCEP HadISST
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Decadal variability of the monsoon index 11yr
running mean of DMI and IMR
r(Amip-SST NCEP)0.40 r(Amip-GHG NCEP)0.61
IMR
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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
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ENSO-monsoon connection 1976 climate shift
Correlation JJA SST vs DMI (from AMIP-type exp
results)
T30
T42
T106
HadISST ERA40
Cherchi and Navarra, 2006
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About the 1976 climate shift MTG vs AMJ SST
(linear correlation)
1948-1975
1976-2002
Amip-SST
Amip-GHG
Obs
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Composite of JJA TPREP (strong minus weak
monsoon) pre76 post76 in Amip-GHG experiments
1948-1975
1976-2002
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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 Positi
ve 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)
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Composites (JJA SST Indian summer monsoon
rainfall)
Cherchi et al., 2006
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Coupled Manifold technique (Navarra and Tribbia,
2005) A new statistical method to detect the
portion of co-variability between 2 climatic
fields

• The coupled manifold may be used to
• compute the of variance of an atmospheric field
  linked to another atmospheric field, and the
  reverse
• 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)
• identify one-way (forced manifold) and two-way
  (coupled manifold) relations between the fields
  considered

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Coupled manifolds technique used to distinguish
the percentage of variance of Indian
precipitation due to the Indian Ocean SSTA and
other forcing
of variance of TrIndOc SST linked to TrPacOc SST
Observations
Coupled model
Cherchi et al., 2006
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Understanding of the mechanisms involved in the
ENSO-monsoon-Indian Ocean Dipole mode interactions
Correlation of Indian Monsoon Rainfall vs Indian
Ocean SST
Obs
Model
Total SST
Forced SST
Free SST
Cherchi et al., 2006
the shaded pattern is significant at 95
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EOFS of Forced Free SSTA
Cherchi et al., 2006
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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)
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