What on Earth is happening to our (summer) weather?

Edward Hanna Department of Geography, University of

SheffieldREAP Leeds talk, 20 June 2013

Overview

• Global warming and extreme weather• The jet stream and the North Atlantic

Oscillation• Recent jet stream changes and effects on UK

and Greenland summer weather & climate

http://data.giss.nasa.gov/gistemp/mapsJim Hansen, NASA

There is a surplus of heat in the tropics which is transported poleward by the atmosphere and oceans.

Mid-latitude storms are eddies which stir up warm and cold air masses, and so move heat poleward.

T. Woollings

http://www.ldeo.columbia.edu/NAO/

The Polar Front Jet is associated with the winter NAO

From National Weather Service Weather Forecast Office, http://www.srh.noaa.gov/hun/ • NAO response is due to latitudinal shifts in the jet stream (Woollings,

2010).• In winter, a positive NAO has a stronger south-north pressure

gradient and stronger jet, shifted northwards. Warmer (and wetter) conditions over N and central Europe, Eastern U.S.A. and cooler over Mediterranean, E. Canada and Greenland. In summer jet stream is further north so positive NAO gives sunny, warm in NW Europe.

• A negative NAO has a weaker pressure gradient and a more meandering jet.

1899190519111917192319291935194119471953195919651971197719831989199520012007-3

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Hurrell PC-based seasonal NAOI values: 1899-2012

DJF

MAM

JJA

SON

Year

NAO

I

1899190519111917192319291935194119471953195919651971197719831989199520012007-3

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Hurrell PC-based seasonal NAOI values: 1899-2012

DJFMAMJJASONDJF5MAM5JJA5SON5

Year

NAO

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Possible causes of jet stream variability

• Sea ice extent changes• Snow cover• Atlantic sea surface temperatures• Solar variability• El Niño – Southern Oscillation• Tropical volcanic eruptions• The Quasi-biennial oscillation in the equatorial

stratosphere (high-altitude jet stream) circulation.

http://svs.gsfc.nasa.gov/vis/a000000/a003900/a003998/Minimum_SeaIce_Area_2012_09_16.1080.tif

Sea Ice Cover (Perovich et al. 2012 Arctic Report Card)

Decreasing trends of 2.6%/decade for March and 13.0%/decade for September relative to mean 1979-2000 values. Summers 2002-2012 had unprecedented series of extreme summer ice-extent minima.

Hot Arctic-Cold Continents Hot Arctic-Cold Continents

Added Ocean Heat Storage and Heat Flux from New Sea Ice Free Areas

June 700 mb height

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 and Wang GRL 2012

In Hanna et al. (2012 Int. J. Climatol. and other recent papers we use a more Greenland-specific (than the NAO) measure of air circulation changes, the Greenland Blocking Index (GBI) (Fang, 2004). Defined as the mean 500-hPa height over the Greenland area 60–80°N, 20–80 °W (Figure 1),“which is equivalent to the NAO/AO index but emphasizing more the northern centre of the NAO dipole pattern” (Fang, 2004, p. 131).

Greenland Blocking Index in summer (JJA) since 1948 is strongly (negatively) linked with NAO

Greenland coastal met station summer (JJA) mean surface air temperatures (Hanna et al. 2013, Int. J. Climatol. ).

http://www.nasa.gov/topics/earth/features/greenland-melt.html

Simulated GrIS surface melt extent, 1960-2012 (Hanna et al. 2013)

GrIS has contributed 8.0 mm to GSL since 2002, and its mass loss of 367 Gt/y between Sep 2008 and Sep 2012 was almost twice that for Jun 2002-Jul 2006 (193 Gt/y).

Greenland Ice mass loss from GRACE (Box et al. 2012 Arctic Report Card)

As the HIGH

LATITUDES

warm faster than the

MIDDLE LATITUDES

Jet stream animationfrom Francis and Vavrus, GRL 2012

Big difference (left) in polar vortex and jet stream configuration in last 6 Junes (2007-2012, left) compared with climatology (1981-2010, middle). Right shows difference between 2 plots.

Overland, Francis, Hanna, Wang (2012) Geophys. Res. Lett.

JJA 2012

Warm Arctic-Cold Continents (James E. Overland)We can say that loss of sea ice and snow adds additional heat to atmosphere which pushes toward a greater chance for north<->south flow (Blocking patterns) and weaker, more persistent jet stream at mid-latitudes with greater weather extremes: cold spells, heat waves, flooding, prolonged snowfall, and drought

BUT: it will not happen the same way in every year and location due to mid-latitude chaotic natural variability

Coarse resolution climate models do not capture Blocking Patterns very well

1) Wide Range of September Sea Ice Extent Hindcasts and Predictions

89 ensemble members from 36 CMIP5 models under strongest (RCP8.5) emissions scenario

2) Observed loss is faster than expected by modelsMean never reaches sea ice free limit by 2100

Overland et al. (2013)

Summary* Human-driven global warming is not a uniform process and can spring surprises in regional climate change: may be linked with tendency for UK to have wetter, cooler summers (and for Greenland to have warmer, sunnier summers) in last 6 years.* Year-to-year persistence in recent unusual large-scale weather patterns.* May be influenced by earlier seasonal losses in Arctic sea-ice + snow cover, & changes in soil moisture.* More high-pressure blocking systems in early summer on N American side of (sub-) Arctic in recent summers (2007-2012). These blocks (although transient phenomena) appear to contribute to more persistent, stationary and north<->south jet stream flow.* Mountain terrain may have greater influence on an already weakened jet stream, which may increase blocking effect, e.g. of Greenland in splitting the jet stream. More southerly jet or far northward branch from around top of Greenland may increase cyclone formation on ‘lee side’ over UK.