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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)

ehanna@sheffield.ac.uk

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.)