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Training Course – NWP-PR: Teleconnections and interannual variability of the atmosphere 1
Teleconnections and
interannual
variability
of the atmosphere Franco Molteni
European Centre for Medium-Range Weather Forecasts
Training Course – NWP-PR: Teleconnections and interannual variability of the atmosphere 2
Outline
• Teleconnection patterns: Definition from observational datasets Dynamical origin and interpretation
• Explaining interannual variability through teleconnections: Partition of variance: effect of time and ensemble averaging Are teleconnection patterns generated by internal variability and
boundary forcing/coupling the same?
• Predictability of teleconnection/low fr. variability indices Can we predict tropical rainfall? Linear indices: examples from the ECMWF System-4 re-forecasts
Training Course – NWP-PR: Teleconnections and interannual variability of the atmosphere 3
Teleconnections: definitions and examples
Teleconnection:
Relationship between anomalies of opposite sign (and/or different variables) in different regions of the world
Teleconnection pattern:
Regional or planetary scale pattern of correlated anomalies
Teleconnections may be induced either by internal atmospheric dynamics or by coupling with other components of the climate system (mainly the tropical oceans)
Training Course – NWP-PR: Teleconnections and interannual variability of the atmosphere 4
Teleconnections: definitions and examples
The Southern Oscillation (and its links to El Niño)
Walker and Bliss (1932); Bjerknes (1969)
SOI: Tahiti – Darwin SLP
Nino3.4 SST
Training Course – NWP-PR: Teleconnections and interannual variability of the atmosphere 5
ENSO teleconnections: rainfall and temperature
Training Course – NWP-PR: Teleconnections and interannual variability of the atmosphere 6
Teleconnections: definitions and examples
The North Atlantic Oscillation
Walker and Bliss (1932)
Van Loon and Rogers (1978)
Positive NAO phase Negative NAO phase
Training Course – NWP-PR: Teleconnections and interannual variability of the atmosphere 7
Teleconnections: definitions and examples
The Indian Ocean Zonal Mode (or I.O. Dipole)
Saji et al. (1999), Saji and Yamagata (2003)
Webster et al. (1999)
Training Course – NWP-PR: Teleconnections and interannual variability of the atmosphere 8
Seminal papers: Wallace and Gutzler 1981
The Pacific /North American(PNA) pattern:correlations
Training Course – NWP-PR: Teleconnections and interannual variability of the atmosphere 9
Seminal papers: Wallace and Gutzler 1981
The Pacific /North American(PNA) pattern:
composites
Training Course – NWP-PR: Teleconnections and interannual variability of the atmosphere 10
Seminal papers: Wallace and Gutzler 1981
Eastern Atlantic(EA) pattern:composites
Training Course – NWP-PR: Teleconnections and interannual variability of the atmosphere 11
Seminal papers: Horel and Wallace 1981
Correlation of 700hPa height with a) PC1 of Eq. Pacific SSTc) SOI index
Schematic diagram of tropical-extratropical teleconnections during El Niño
Training Course – NWP-PR: Teleconnections and interannual variability of the atmosphere 12
Seminal papers: Gill 1980
a) Near-surfacewind and
vertical motion
b) Near-surface wind and
sea-level pressure
c) Motion in x-z plane along the
Equator
Atmospheric response to near-equatorial heating
Training Course – NWP-PR: Teleconnections and interannual variability of the atmosphere 13
Seminal papers: Hoskins and Karoly 1981
a) height (lon, z) at 18Nb) 300hPa vorticityc) 300hPa height
Response to a tropical heat source (15N):
Training Course – NWP-PR: Teleconnections and interannual variability of the atmosphere 14
Seminal papers: Simmons Wallace Branstator 1983
First barotropic normal mode at four times during a half-cycle:
East. Atlantic phasePNA phase
Training Course – NWP-PR: Teleconnections and interannual variability of the atmosphere 15
Seminal papers: Sardeshmukh and Hoskins 1988
Definition of Rossby wave sourcegenerated by near-equatorial heating
Training Course – NWP-PR: Teleconnections and interannual variability of the atmosphere 16
Seminal papers: Sardeshmukh and Hoskins 1988
Response to heating in trop. West Pacific
Rossby wave source
and abs. vorticity
Training Course – NWP-PR: Teleconnections and interannual variability of the atmosphere 17
Non-linear models: Charney and DeVore 1979
Multiple steady states of very-low-order barotropic model with wave-shaped bottom topography
Training Course – NWP-PR: Teleconnections and interannual variability of the atmosphere 18
The new ECMWF Seasonal fc. system (Sys-4)
OASIS-3NEMO
1/1-0.3 d. lon/lat42 levels
H-TESSEL
IFS 36R40.7 deg (T255)
91 levels
4-D variational d.a.
3-D v.d.a. (NEMOVAR)
Gen. ofPerturb.
System-4CGCM
Initial Con. Ens. Forecasts
Training Course – NWP-PR: Teleconnections and interannual variability of the atmosphere 19
ECMWF System 4: main features
• Operational forecasts
51-member ensemble from 1st day of the month released on the 8th 7-month integration
• Experimental ENSO outlook
13-month extension from 1st Feb/May/Aug/Nov 15-member ensemble
• Re-forecast set
30 years, start dates from 1 Jan 1981 to 1 Dec 2010 15-member ensembles, 7-month integrations 13-month extension from 1st Feb/May/Aug/Nov
Training Course – NWP-PR: Teleconnections and interannual variability of the atmosphere 21
Variability of Z 200hPa in DJF from seasonal ensembles
Standard deviation from 11-member ensembles, DJF 1981/2005
5-daymeans
Seasonal-ensemble
means
Seasonalmeans
St.dev.ratio
Training Course – NWP-PR: Teleconnections and interannual variability of the atmosphere 22
‘Forced’ vs. internal variability in seasonal ensembles
11-member ensembles, DJF 1981/2005
Z 200 hPaEl Niño
composite
La Niñacomposite
5-day-meanEOF 1
EOF 2
Training Course – NWP-PR: Teleconnections and interannual variability of the atmosphere 23
Can we predict tropical rainfall teleconnections?
• Convective clouds decrease solar radiation
at the surface
• On tropical oceans, anomalous divergent
winds tend to increase evaporation to the
west/east of the convective region when
the climatological near-surface zonal
winds are westerly/easterly
• The strength of the feedback depends on
the heat capacity of the “interactive”
surface layer and the phase of the day
Tropical convection tends to occur over warm SST and warm-
and-wet land surface, and it induces a number of feedbacks:
Training Course – NWP-PR: Teleconnections and interannual variability of the atmosphere 24
Variability of tropical rainfall: EOF comparison
GPCP S3 S4
EOF 1
EOF 2
Training Course – NWP-PR: Teleconnections and interannual variability of the atmosphere 26
Predictability of teleconnections in Sys4: Nino3.4, IOD SST
Nino3.4DJF
IODSON
Training Course – NWP-PR: Teleconnections and interannual variability of the atmosphere 27
Predictability of telecon. in Sys4: South Africa rain (DJF)
Training Course – NWP-PR: Teleconnections and interannual variability of the atmosphere 28
Predictability of telecon. in Sys4: East Africa rain (SON)
Training Course – NWP-PR: Teleconnections and interannual variability of the atmosphere 29
Predictability of teleconnections in Sys4: Sahel rain (JJA)
Training Course – NWP-PR: Teleconnections and interannual variability of the atmosphere 30
Predictability of teleconnections in Sys4: PNA, NAO (DJF)
PNA
NAO
Training Course – NWP-PR: Teleconnections and interannual variability of the atmosphere 31
Predictability of telecon. in Sys4: Europe T_2m (MAM)
Training Course – NWP-PR: Teleconnections and interannual variability of the atmosphere 32
Conclusions
• Teleconnections may be induced either by internal atmospheric
dynamics or by coupling with other components of the climate system
(mainly the tropical oceans).
• Although a linear interpretation of teleconnection is adequate in most
cases, non linear effects should not be neglected.
• Prediction of trop. rainfall teleconnections requires accurate modelling
of heat-flux feedbacks from the ocean mixed layer and land surface.
• In the ECMWF seasonal fc. System-4, teleconnections related to ENSO
are well modelled in the tropical and extra-trop. Pacific region, fairly
well over Africa and South America, less so over South Asia.
• Predictability over Europe/Atlantic: mostly limited during winter,
higher in other seasons when internal variability is reduced.