Training Course – NWP-PR: Teleconnections and interannual variability of the atmosphere 1...

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


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


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)



The Southern Oscillation (and its links to El Niño)

Walker and Bliss (1932); Bjerknes (1969)

SOI: Tahiti – Darwin SLP

Nino3.4 SST


ENSO teleconnections: rainfall and temperature



The North Atlantic Oscillation

Walker and Bliss (1932)

Van Loon and Rogers (1978)

Positive NAO phase Negative NAO phase



The Indian Ocean Zonal Mode (or I.O. Dipole)

Saji et al. (1999), Saji and Yamagata (2003)

Webster et al. (1999)


Seminal papers: Wallace and Gutzler 1981

The Pacific /North American(PNA) pattern:correlations



The Pacific /North American(PNA) pattern:

composites



Eastern Atlantic(EA) pattern:composites


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


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


Seminal papers: Hoskins and Karoly 1981

a) height (lon, z) at 18Nb) 300hPa vorticityc) 300hPa height

Response to a tropical heat source (15N):


Seminal papers: Simmons Wallace Branstator 1983

First barotropic normal mode at four times during a half-cycle:

East. Atlantic phasePNA phase


Seminal papers: Sardeshmukh and Hoskins 1988

Definition of Rossby wave sourcegenerated by near-equatorial heating


Seminal papers: Sardeshmukh and Hoskins 1988

Response to heating in trop. West Pacific

Rossby wave source

and abs. vorticity


Non-linear models: Charney and DeVore 1979

Multiple steady states of very-low-order barotropic model with wave-shaped bottom topography


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


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


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


‘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


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:


Variability of tropical rainfall: EOF comparison

GPCP S3 S4

EOF 1

EOF 2


Predictability of teleconnections in Sys4: Nino3.4, IOD SST

Nino3.4DJF

IODSON


Predictability of telecon. in Sys4: South Africa rain (DJF)


Predictability of telecon. in Sys4: East Africa rain (SON)


Predictability of teleconnections in Sys4: Sahel rain (JJA)


Predictability of teleconnections in Sys4: PNA, NAO (DJF)

PNA

NAO


Predictability of telecon. in Sys4: Europe T_2m (MAM)


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.

Training Course – NWP-PR: Teleconnections and interannual variability of the atmosphere 1...

Documents

Transcript of Training Course – NWP-PR: Teleconnections and interannual variability of the atmosphere 1...