greenland blocking index 1851-2015: a regional climate change...
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Greenland Blocking Index 1851-2015:a regional climate change signal
Edward Hanna & Richard Hall (Dept. Geography, Uni. Sheffield)
Tom Cropper (Scientific Figure Design, Sheffield)
John Cappelen (Danish Meteorological Institute, Copenhagen)
SPARC/WCRP/WWRP Workshop on Atmospheric Blocking, Reading, 6-8 April 2016
Greenland Blocking Index (GBI) (Fang 2004, Hanna et al. 2013) is mean 500 hPa (mb) geopotential height over this region
(60-80N, 20-80W)
Greenland Blocking Index is strongly (negatively) linked with North Atlantic Oscillation: here we show summer (JJA) series
Hanna et al. (2013, 2014)
From David Stephenson’s (Exeter) NAO webpages (e.g. high NAO usually low GBI):
20CRv2c GBI homogenisation
20CRv2c-NCEP/NCAR GBI splicing
Summer (JJA) GBI: 1851-2015
High incidence of high summer (JJA) GBI values since 2007
Winter (DJF) GBI: 1851-2015
December GBI variability
High incidence of extreme GBI December values since 2001
Summer/JJA (2010-2015)-(1981-2000) near-surface air temperatures
October-January (2010-2015)-(1981-2000) near-surface air temperatures
Francis and Vavrus 2015Hartmann 2015
Overland (2015)
Multi-year composite 700 hPa geopotential height composites of: JJA/summer 2007-2015 (left)JJA climatology for 1981-2010 (centre)JJA anomaly for 2007-2015 (right)Anomalies are relative to 1981–2010 mean. Units are in m. Data are from the NCEP–NCAR Reanalysis through the NOAA/Earth Systems Research Laboratory.
Overland, Francis, Hanna, Wang (2012, Geophys. Res. Lett.), updated.
GBI impacts on mid-latitude weather:Lower-troposphere temperature (top) and precipitation (bottom) anomalies for winter (left) and summer (right) for 10 highest minus 10 lowest GBI years during 1851-2015. Based on Twentieth Century Reanalysis v2c data. Note cold winters/wet summers over UK.
Summary
• New homogenised Greenland Blocking Index monthly & seasonal time series, 1851-2015.
• Significant GBI increases in all seasons since 1981, with strongest monthly increases in July/August.
• Recent clustering of high GBI summer values (7/top 11 values in 165 years occurred since 2007).
• The higher summer GBI is linked with more frequent meridional anomalies in polar jet stream since 2007.
• GBI has become significantly more variable in December which has seen a significant clustering of extreme (high and low) GBI values since 2001.
• This may be attributed to destabilisation of early winter jet stream arising from large autumn sea-ice losses and increased ocean->atmosphere heat fluxes in recent years, also linked with sometimes more extreme negative Arctic Oscillation and North Atlantic Oscillation December values.
Homogenised GBI monthly time series 1851-2015 is available from:
http://www.sheffield.ac.uk/geography/staff/ hanna_edward/gbi
New paper in press with Int. J. Climatol.(Hanna, Cropper, Hall, Cappelen, 2016)
The end – thank you – questions?
after Francis 2014
Composite Greenland Temperatures(Hanna et al. 2012 Environ. Res. Lett.)
CGT2 (1961-2015) = mean of Upernavik, Aasiaat, Ilulissat, Sisimiut, Nuuk, Paamiut, Narsarsuaq, Qaqortoq& Tasiilaq.
CGT3 (1895-2015) = mean of Upernavik, Ilulissat, Nuuk, Narsarsuaq & Tasiilaq.
CGT3 enables comparative analysis of last decade warming with 1930s/40s warm period.
CGT seasonal trends (degC)
Season 1981-2014
(CGT2)
1981-2014
(CGT3)
1991-2014
(CGT2)
1991-2014
(CGT3)
2001-2014
(CGT2)
2001-2014
(CGT3)
DJF 6.1 6.0 6.5 6.2 1.3 1.8
MAM 3.6 3.2 3.1 2.7 -0.8 -0.8
JJA 2.4 2.1 2.2 2.2 0.6 0.7
SON 2.0 1.8 1.4 1.1 -0.7 -0.9
June 700 mbheight
anomaly 2007-2012
Arctic Dipole and
Greenland Blocking
Ridge
Loss of sea ice
Loss of Glacial Ice
Poor weather
Early May-June snow melt
Overland, Francis, Hanna & Wang (2012) Geophys. Res. Lett.
Hanna et al. (2012, Int. J. Climatol.)