Present day and future trends in Northern
Hemisphere blocking
Etienne Dunn-Sigouin
PhD candidate
Columbia University
Reading blocking workshop, April 7th 2016
DJF surface temperature trends (1990-2013)
Leiserowitz et al. 2013
Some key conclusions:
• Half of Americans say global warming is affecting weather in the United
States
• A growing majority of Americans believe weather in the United States
has been “worse” over the past several years
• Half of Americans say weather in their local area has been “worse” over
the past several years
Key questions:
1. Have there been significant changes in blocking in recent decades ?
2. Will blocking occur more less frequently in the future ?
3. Why does blocking change in the future ?
Caveats: Blocking Identification
MERRA DJF 1980 - 2012
Barnes et al. 2012 index
• Different indices produce
different climatologies
Dunn-Sigouin et al. 2013 index
• 3 common types: reversal,
anomaly, hybrid
• Equally valid: Capture
different aspects of blocking
Masato et al. 2013 index
Barnes et al. 2014 JGR
Caveats: Model skill (relevant for future trends)
CMIP5 historical DJF
CMIP5 historical JJA
CLIMATOLOGY
BIAS
CI = 0.05
Masato et al. 2013 JCLIM
• Blocking frequency: Winter underestimation, summer overestimation
• Generally consistent with other indices (e.g. Dunn-Sigouin and Son 2013 JGR)
• Several studies have documented trends in blocking in reanalysis:
•
Weidenmann et al. 2002, Chen and Yoon 2002, Luo and Wan 2005, Barriopedro et al. 2006, Croci-Maspoli et al.
2007, Davini et al. 2012, Barnes et al. 2013, Small et al. 2014, Chen et al. 2015, Golan et al 2015, Horton et al. 2015
• Focus on Barnes et al. 2014: 3 blocking indices, 4 reanalyses, 3 time periods and all
seasons:
No hemispheric wide trends in blocking frequency
Linear trend 1980-2012
Linear trend 1990-2012
INCREASING
DECREASING
Barnes et al. 2014 JGR
• Robust trend if 3/4 reanalyses show significant trend
• No trends in SON linked to arctic amplification and peak sea ice decline
Recent blocking trends (1990-2012) not remarkable in context of last 60 years
Asia (60E-120E) DJF
NCEP
reanalysis
• Large inter annual and
decadal variability
• Differences between each
blocking index
Barnes et al. 2014 JGR
Europe (0-60E) JJA
Different reanalyses can show different trends for the same index
Barnes et al. 2014 JGR
Consistency in local 1990-present day trends in certain studies
Zonal wind: m/s/decade
Wave activity:
10^8 m^2/decade
Stippling: significance
Chen et al. 2015 GRL
• Local increasing trends in DJF over Asia consistent with Barnes et al. 2014
• Similar consistency with Horton et al. 2015
Key points: Present day trends
• No robust hemispheric wide trends in blocking frequency
• SON in particular (AA hypothesis)
• localized increasing and decreasing trends with some consistency across
some studies in certain seasons (Asia DJF)
• Large background variability on inter annual to decadal timescales
Multi-model mean change in future blocking frequency (RCP 8.5, 12 models)
INCREASE
DECREASE
FUTURE CLIM
CMIP5 JJA
future - historical
CMIP5 DJF
future - historical
CI = 0.05
• DJF: Poleward shift over pacific and reductions over Atlantic
• JJA: reductions over oceans and increase over Asia
• ~30-40% reduction in Atlantic JJA
Masato et al. 2012 JCLIM
Consistent changes in blocking frequency for different studies
CMIP5 annual future – historical (RCP 8.5, 12 models)
Shading: # of
models that
agree on
statistically
significant
change
CI = days/year
• Large model spread (shading)
Dunn-Sigouin and Son 2013 JGR
• Amplitude changes differ between studies (~10-20% above)
• Differences in timing of peak Ural blocking change with Masato et al. 2012
Reduction in blocking frequency consistent with CMIP3 models
Barnes et al. 2010 Clim Dyn
• More model agreement on the sign in CMIP3 than CMIP5?
CMIP5 multi-model mean blocking frequency changes scale with forcing
strength?
DJF RCP4.5 - Historical
DJF RCP8.5 - Historical
Cheung and Zhou 2015 JCLIM
• Blocking frequency decreases are gradual and consistent in time
• Caveat: only done with one blocking index
No significant change in multi-model mean blocking duration
Annual Historical
Annual RCP8.5 - Historical
NH
Atlantic
Pacific
• Consistent with CMIP3 results of Barnes et al. 2010
Dunn-Sigouin and Son 2013 JGR
• Individual model studies differ e.g. Sillmann and Croci-Maspoli 2009, Matsueda et
al. 2009
Understanding future trends
• Key features that are consistent across blocking indices and
forcing scenarios:
1. Multi-model mean reduction in blocking frequency over
oceans
2. Multi-model mean increase/eastward extension in
blocking frequency over eastern Europe/Asia
•
Consistent with the historical trend?
3. Large model spread in blocking frequency changes
Linear changes:
Atlantic mean state changes account for certain blocking frequency changes
SON Blocking frequency
• Strengthened and extended
mean jet in future
Historical
Future
Difference - - -
• Same eddies on a strengthened
mean gradient -> less blocking
• Increased downstream variance
(non-linear) accounts for
eastward blocking shift over
europe
Mean
DeVries et al. 2013 Clim Dyn
Variance
Non-linear changes:
Future eastward blocking shift consistent with internal variability
Atlantic SON
• Internal variability:
Stronger jet ->
Eastward blocking shift
Increase
Decrease
Historical
• Model spread: models with
stronger future jets will have
1. Larger blocking reductions
over Atlantic
2. Larger blocking increases
over Europe/Asia
DeVries et al. 2013 Clim Dyn
Non-linear changes:
Future poleward jet shift consistent with reductions in blocking/wave breaking
Annual North Atlantic 0-60W
Barnes and
Hartmann
2010 GRL
Annual North Atlantic 0-60W
Barnes and
Polvani
2013 JCLIM
Summary:
1. Have there been significant changes in blocking in recent decades ?
• No robust hemispheric wide trends in blocking frequency
• localized increasing and decreasing trends with some consistency across studies
in certain seasons (Asia DJF)
2. Will blocking occur more less frequently in the future ?
•
Reduction in blocking over the oceans and an increase over eastern Europe/Asia
•
Large model spread
•
IPCC AR5: “medium confidence blocking (frequency) will not increase”
3. Why does blocking change in the future ?
•
Possible role for mean state changes (linear) and eddy changes (non-linear) or
both