Disruption of the European Climate !
Seasonal Clock in a warming world!
Christophe Cassou (CNRS-Cerfacs)!
Julien Cattiaux (CNRS-Météo-France)!
Link between temperature extremes and atm. circulation (1)!
•  W-Europe temp. extremes are associated with persistent High over North-Sea/Scandinavia, which blocks
the westerlies (easterlies anomalies) Rex (1950), Slonosky et al. (2001), Cassou et al. (2005), Sillmann et al. (2012), among others
2m-Temp. anom."
2m-Temp. anom."
Feb. 6-12, 2012!
Cold spell!
SLP anomalies (NCEP)*!
MODIS image!
H
Low-level (850hPa) !
wind anomalies!
“Le Moscou-Paris express”!
1st Weather forecast bulletin
broadcast on December, the
17th , 1946 on the national
unique channel French TV!
“Le Moscou-Paris express”
= blocking!
Paul Douchy!
© Radio Diffusion Francaise (RDF,
Telejournal), Meteorologie Nationale!
“Le Moscou-Paris express”!
H
1st Weather forecast bulletin
broadcast on December, the
17th , 1946 on the national
unique channel French TV!
“Le Moscou-Paris express”
= blocking!
Paul Douchy!
© Radio Diffusion Francaise (RDF,
Telejournal), Meteorologie Nationale!
Link between temperature extremes and atm. circulation (1)!
•  W-Europe temp. extremes are associated with persistent High over North-Sea/Scandinavia, which blocks
the westerlies (easterlies anomalies) Rex (1950), Slonosky et al. (2001), Cassou et al. (2005), Sillmann et al. (2012), among others
•  Blocking is a recurrent pattern throughout the year: it induces cold episodes in winter / warm in summer.
2m-Temp. anom."
2m-Temp. anom."
Feb. 6-12, 2012!
Cold spell!
Jul. 20-25, 2006!
heat wave!
SLP anomalies (NCEP)*!
MODIS image!
H
H
Low-level (850hPa) !
wind anomalies!
* Correlation between summer 2006 and winter 2012 pressure pattern = 0.67!
Link between temperature extremes and atm. circulation (2)!
•  W-Europe temp. extremes are associated with persistent High over North-Sea/Scandinavia, which blocks
the westerlies (easterlies anomalies) Rex (1950), Slonosky et al. (2001), Cassou et al. (2005), Sillmann et al. (2012), among others
•  Blocking is a recurrent pattern throughout the year: it induces cold episodes in winter / warm in summer.
2m-Temp. anom."
2m-Temp. anom."
Feb. 6-12, 2012!
Cold spell!
Jul. 20-25, 2006!
heat wave!
SLP anomalies (NCEP)*!
MODIS image!
Definition of a daily anomalous
SLP index (SLPi)!
Low-level (850hPa) !
wind anomalies!
* Correlation between summer 2006 and winter 2012 pressure pattern = 0.65!
Seasonal relationship between N-Europe SLP and wind anomalies (1)!
Fraction of U@850hPa variance explained* by PSLi (all 365 days)!
•  Sea-level pressure anomalies over North-Sea/Scandinavia explain ~60% of the daily variance (correlation
equal to ~0.75) of the low-level zonal wind (at 850hPa) over the Western/Central Europe
* Fraction of explained variance = square of the correlation coefficient!
Seasonal relationship between N-Europe SLP and wind anomalies (2)!
Winter days
Fraction of U@850hPa variance explained* by PSLi (all 365 days)!
Summer days
•  Sea-level pressure anomalies over North-Sea/Scandinavia explain ~60% of the daily variance (correlation
equal to ~0.75) of the low-level zonal wind (at 850hPa) over the Western/Central Europe
WHATEVER THE SEASON
* Fraction of explained variance = square of the correlation coefficient!
Seasonal relationship between N-Europe SLP and W-Europe temp. (1) !
Winter days
Summer days
•  The SLP-temperature relationship is SEASONALLY DEPENDENT, especially over western Europe
Data: 2-meter temperature from NCEP-NCAR (shading) and weather station (dots) from the ECA&D datasets!
Seasonal relationship between N-Europe SLP and W-Europe temp. (2) !
Winter days
Definition of a daily anomalous 2-m
temperature index (T2Mi)!
Summer days
•  The SLP-temperature relationship is SEASONALLY DEPENDANT, especially over western Europe
Data: 2-meter temperature from NCEP-NCAR (shading) and weather station (dots) from the ECA&D datasets!
The European climate seasonal Clock (1)!
January
SLPi/T2Mi regression:
-1.4oC/10hPA
Amplitude of the regression coef.!
between SLPi and T2Mi indices!
July
SLPi/T2Mi regression:
+2oC/10hPA
0.66oC/10hPa!
1.33oC/10hPa!
2oC/10hPa!
The European climate seasonal Clock (2)!
Neg. Reg/Corr.
Pos. Reg/Corr.
January
SLPi/T2Mi regression:
-1.4oC/10hPA
July
SLPi/T2Mi regression:
+2oC/10hPA
0.66oC/10hPa!
1.33oC/10hPa!
2oC/10hPa!
The European climate seasonal Clock from several observational estimates!
Neg. Reg/Corr.
Pos. Reg/Corr.
Observations
January
SLPi/T2Mi regression:
-1.4oC/10hPA
July
SLPi/T2Mi regression:
+2oC/10hPA
Data: NCEP-NCAR (dots) and NOAA-20CR reanalysis + weather station from ECA&D for observational uncertainty estimates (shading)!
Objective definition of the summer/winter seasons (starting dates)!
  The winter starting date is defined as
the day where the correlation between PSL
and West Europe Temp. indices crosses 0
January
SLP/T2M regression:
-1.4oC/10hPA
July
SLP/T2M regression:
+2oC/10hPA
Data: NCEP-NCAR reanalysis (dots)!
  The summer starting date is defined as
the day where the correlation between PSL
and West Europe Temp. indices crosses 0
1850-2010 historical evolution of the summer starting date!
20CR"
NCEP"
31st of March [1980-2010]!
•  Summer starts 11 days earlier in the 2000s than in the 1960s corresponding to a trend equal to
~2.5 days/decade (NCEP-NCAR and NOAA-20CR reanalysis, ECA&D weather station data)
Use of climate models to assess the origins of the trend
•  detection: test the compatibility with internal climate variability
•  attribution to specific forcings (natural and/or anthropogenic) if detectable.
The CNRM-CM5 model seasonal clock!
Observations
CNRM-CM5 model
(Voldoire et al. 2013)
Detection study for the summer starting date!
10-member Historical
•  PiControl : 1000-yr of simulation
with fixed 1850 external forcings
(GHG, aerosols, solar and volcano)
20CR"
PiCtl
NCEP"
Estimation of the internal
variability
•  Historical : 10 members of
1850-2005 simulations with observed
time-evolving forcing
12 April
31st of March [1980-2010]!
Possible outcome in the
presence of observed
forcings
•  Observed estimates within the model outcomes in historical simulations
•  Trends for earlier summer in the 2000s well captured in the historical simulations
•  Starting date of the summer at the verge of detectability at the end of the 2000s.
Projected summer starting date based on the RCP8.5 scenario!
10-member Historical
•  PiControl : 1000-yr of simulation
with fixed 1850 external forcings
(GHG, aerosols, solar and volcano)
20CR"
PiCtl
NCEP"
Estimation of the internal
variability
•  Historical : 10 members of
1850-2005 simulations with observed
time-evolving forcing
Possible outcome in the
presence of observed
forcings
•  RCP85 : 5 members of 2005-2100
following the RCP85 IPCC scenario
(high GHG emission scenario :
business as usual).
•  Additional 10 days advance by 2070-2100
Possible outcome with
projected forcings
  The sharpest trend is now! What is the origin of
the ongoing trend in the summer starting date?!
Attribution study for the XXth century summer starting date!
•  PiControl : 1000-yr of simulation
with fixed 1850 external forcings
(GHG, aerosols, solar and volcano)
PiCtl
Ensemble Means
Estimation of the internal
variability
•  Historical : Ensemble Mean of 10
members of 1850-2005 simulations
with observed time-evolving forcing
Estimation of the total
(anthropogenic+natural)
forcings
•  Attribution rums : Ensemble Mean
of 5 members of 1850-2005
simulations with observed timeevolving forcing taken separately:
•  HISTNAT: Natural forcings only
•  HISTGHG: GHG only
•  HISTNAT: GHG+Aerosol only
  No volcanic eruption + solar max [1920-1960]!
  The observed current winter shortening is !
attributable to GHG forcing …!
  … But modulated by anthropogenic aerosols forcings!
Remote influence of Eastern Europe snow cover (1)!
Snow cover
(Eastern Europe)!
  Reduction in Eastern-Western
Europe temp. gradient in March is
responsible for the ongoing earlier
summer: role of Eastern Europe Snow
decline!
Remote influence of Eastern Europe snow cover (2)!
Snow cover
(Eastern Europe)!
  Reduction in Eastern-Western
Europe temp. gradient in March is
responsible for the ongoing earlier
summer: role of Eastern Europe Snow
decline!
Phenological markers and Eastern Europe snow (1)!
Snow cover (Eastern
Europe) (1979-2015)!
  Strong interannual correlation between the date of Riesling grape
budburst in Alsace and Eastern Europe snow cover over 1979-2013!
Phenological markers and Eastern Europe snow (2)!
  Trends in phenology indicators compatible with the trend in summer starting date
estimated from a meteorologically-based metric (Menzel 2006, Rutishauser et al. 2007, Thackeray et
al. 2010, among others)"
Conclusion!
  Novel and objective metrics to define winter/summer climate seasons in Europe, Based on
an intrinsic feature of the circulation-temperature relationship."
  Evidence for summer advance in different observational datasets from 1960s (~ 3 days
per decade) leading to a 10-day earlier onset in the 2000s"
Consistent with the expected response to GHG forcing as assessed from model attribution simulations"
 Consistence with some phenological markers (e.g Menzel et al 2006)"
Complementary interpretation to the traditional evoked mean global warming to explain earlier spring
events"
 Evidence for a remote influence of snow disappearance over Eastern Europe affecting the
seasonal shift of the relationship between atm. circulation and surface temperature over
Western Europe."
 Continuing earlier summer onset expected in the XXIst century"
Assessed from RCP8.5 scenario (10-day additional advance by 2070-2100). "
 Evidences for expected changes in intra-seasonal temperature variability."
Partially explained by changes in zonal temperature gradients."
  No clear trend for winter starting"
This work: Cassou, C. and J. Cattiaux (2016): « Disruption of the European climate seasonal clock in a warming world », !
Nature Climate Change, doi:10.1038/nclimate2969 !