Polar Predictability on Seasonal to Decadal Timescales

Ted Shepherd Department of Meteorology

University of Reading

Photos: The desiccation of high Arctic shallow ponds: Smol & Douglas (2007 PNAS)

• The Arctic is warming faster than any region on Earth • Annual mean surface air temperature changes, last 30 years

From GISS web site

• Temperatures have warmed most in late-fall/early-winter • Results from strong seasonality of Arctic sea ice; heat is

released from ocean to atmosphere during this season

From NSIDC web site Screen & Simmonds (2010 GRL)

Surface air temperature trends over 1989-2009 (K/decade), from ERA-Interim (dotted) and station data

(solid) poleward of 70°N

• Coastal zones are experiencing longer periods of open water, leading to increased storm surges and erosion of permafrost coasts • Open water season along Beaufort Coast in north Alaska

Overeem et al. (2011 GRL)

• Increasing intrusion of sea water from storm surges is changing coastal ecosystems • A large storm surge in the Mackenzie Delta in 1999 led to

widespread changes from freshwater (green) to brackish (red) species, unmatched in over 1000 years, which have persisted to the present day

Pisaric et al. (2011 PNAS)

• The impact of a changing Arctic on mid-latitudes is unclear – Could it lead to more wintertime cold-air outbreaks, as in

North America in January 2014? Proximate explanation of this unusual behaviour is meteorological, but a natural question to ask is whether it is a harbinger of things to come (cf. Met Office briefing paper) WWRP-PPP/WCRP-PCPI Joint Workshop on “polar-lower latitude linkages and their role in weather and climate prediction” (Barcelona, Dec 2014)

• Several mechanisms contribute to Arctic amplification, but the most important appears to be the lapse-rate feedback

CMIP5 analysis by Pithan & Mauritsen (2014 Nature Geosci.)

Lapse-rate feedback is positive in polar regions due to the stable boundary layer, and negative at lower latitudes Implies we need good representation of the stable boundary layer

• Observations show two states of the Arctic boundary layer – Cloudy state with little radiative cooling, clear state with

strong radiative cooling (stronger inversion => less cooling)

Result of Arctic air-mass formation (bottom figure) Cloudy state (mixed phase clouds) not well represented in climate models Pithan et al. (2013 Clim. Dyn.)

• A flagship activity of the PPP, covering Arctic and Antarctic – WCRP-PCPI will participate through its three joint

initiatives with the WWRP-PPP • An extended period of coordinated intensive observational

and modelling activities in order to improve polar prediction capabilities on a wide range of time scales – Augmented by preparation and consolidation phases – http://www.polarprediction.net/about-ppp/yopp.html

• Will encourage coupled assimilation in the Arctic on an experimental basis, to guide future reanalyses

• Could connect to MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate), a proposed field experiment of IASC

Year of Polar Prediction (YOPP): mid-2017 to mid-2019

• Winter season: Stratosphere-resolving models can correctly predict the surface response to Stratospheric Sudden Warmings (SSWs) when initialized at the time of the SSW – Figure shows response averaged over 16-60 days after the

SSW, for 20 SSWs from 1970-2009 (model: ensemble of 10)

Sigmond, Scinocca, Kharin & Shepherd (2013 Nature Geosci.)

• Winter season: A key component of wintertime variability is the North Atlantic Oscillation (NAO). Recent efforts suggest there may be some seasonal predictability of the wintertime NAO, but large ensembles are required – Skill involves coupled behaviour of stratosphere, ocean, sea ice – DJF forecasts initialized around November 1, r=0.62 (99% CI)

Scaife et al. (2014 GRL)

• Summer season: Arctic sea-ice prediction a rapidly growing area • SEARCH Sea-Ice Outlook (SIO): most skill comes from the trend • Anomalous years are more difficult to predict; need to determine

where predictability may lie, e.g. in springtime sea-ice thickness

Stroeve et al. (2014 GRL)

Longer timescales: Climate models suggest a strengthening of the wintertime storm track density over the UK

Zappa et al. (2013 J. Clim.)

Mean CMIP5 response to RCP 8.5 in late 21st century

ERA-I climatology Equatorward shift of Icelandic storm track seems to result from Arctic warming (opposite to usual poleward shift of storm tracks) Model spread is largest on the poleward side

Sigmond & Scinocca (2010 J. Clim.)

• Model predictions of wintertime Arctic circulation change are very sensitive to the behaviour of the stratosphere, in the same way as the surface circulation responds to SSWs • Mean sea-level pressure response to doubled CO2 is here

changed by changing the orographic gravity-wave drag (DRAG) (DRAG)

(strengthened stratospheric vortex) (weakened stratospheric vortex)

• SPARC DynVar CMIP5 analysis suggests GHG-induced changes in wintertime Arctic sea-level pressure are affected as much by changes in the stratospheric polar vortex as by tropical upper tropospheric warming or by Arctic surface warming

• Response to stratosphere is NAO-like and opposite in sign to that from tropical warming, consistent with Scaife et al. (2012 Clim. Dyn.) and in the same sense as Sigmond & Scinocca (2010)

Manzini et al. (JGR, in press)

Tropical warming Arctic warming Stratosphere

IPCC AR5

Antarctic

• Climate models suggest Antarctic warming and late-summer sea-ice loss, which are not seen in observations

Marshall et al. (2014 Phil. Trans. Roy. Soc. Lond. A)

• The Arctic and Antarctic are almost mirror images of each other, leading to a hemispheric asymmetry in the oceanic meridional overturning circulation • Transports heat away from the Antarctic, but to the Arctic • Also the Antarctic has experienced the ozone hole, which

has increased the wind-driven upwelling in the Southern Ocean

Marshall & Speer (2012 Nature Geosci.)

• Asymmetry of the oceanic meridional overturning circulation leads to an asymmetry in the response time of SSTs to GHG warming, with a complicated response to the ozone hole – Is this why Antarctic warming has yet to emerge?

Marshall et al. (2014 Phil. Trans. Roy. Soc. Lond. A)

Summary • The Arctic is warming faster than any place on Earth

– Secular changes on decadal timescales are very apparent – Predictability is dominated by climate change itself

• The phenomenon of “Arctic amplification” is robust and well understood, but there are significant quantitative uncertainties – Evidence points to model biases in mixed-phase clouds and

stable boundary layers (also important for weather prediction) • The wintertime NAO may be fairly predictable from late autumn • Stratosphere plays a key role in Arctic wintertime circulation, on

both subseasonal and decadal timescales • Seasonal prediction of late-summer Arctic sea ice extent is a

rapidly growing area; so far, skill mainly comes from the trend • Disagreement between modelled and observed behaviour in the

Antarctic may result from poor representation of the oceanic meridional overturning circulation

• Improve knowledge and understanding of past polar climate variations (up to 100 years)

• Assess reanalyses in polar regions (joint with WWRP-PPP) • Improve understanding of polar climate predictability on

seasonal to decadal timescales (joint with WWRP-PPP) • Assess performance of CMIP5 models in polar regions • Model error (joint with WWRP-PPP) • Improve understanding of how jets and non-zonal circulation

couple to the rest of the system in the Southern Hemisphere

WCRP Polar Climate Predictability Initiative Developing the science to underpin better predictions

Co-leads: Cecilia Bitz (U Washington, USA) & Ted Shepherd (U Reading, UK)

Initiatives