The role of clouds in the continuing decline of the Arctic sea ice Irina Gorodetskaya, Bruno Tremblay and B. Liepert AWI, Potsdam, 29 January 2008 Thanks to: J. Francis, K. Stramler, R. Cullather

arctic

Sea ice concentrations Sea ice MAXIMUM: March Sea ice MINIMUM: September Data: HadSST1

Beaufort sea in winter Beaufort sea

frost smoke in winter Frost smoke from a freshly opened lead in winter

land fast ponding ice ponding

September 2006 September 2005 Data Source: National Snow and Ice Data Center (NSIDC), Boulder, Colorado, USA September 2007

x 2007

Arctic Energy Budget Figure by N. Untersteiner.

Ice-Albedo feedback

TOA albedo vs NH sea ice Radiative effectiveness of ice wrt TOA albedo: RE = albedo (100% ice conc) - albedo (0% ice conc) surface albedo for ice surface albedo for ocean winter summer RE (TOA albedo) << RE (surface albedo) due to the presence of clouds over open oceans RE (sfc alb) ~ 0.5 Gorodetskaya et al, Atm-Ocean 2006

Maps of sea ice and snow RE Gorodetskaya et al, Atm-Ocean 2006 NH snow SH sea iceNH sea ice

Reflected SW: total and due to sea ice Gorodetskaya and Tremblay, 2008, for AGU monograph “Arctic sea ice decline:...”, Eds. DeWeaver, Bitz, Tremblay

Summer: cloud forcing offsets sea ice effects on the surface shortwave radiation % estimated from Wang and Key, Science 2003

- Spring: large positive trend Schweiger, GRL Summer: no trend … Cloud cover over the Arctic Ocean:

Belchansky et al Change (days) from to in melt onset: in freeze onset: in melt duration:

Arctic Oscillation recovered and sea ice did not… Overland and Wang, GRL 2005

Total variance in the perennial ice edge attributable to anomalies in forcing parameters, J. A. Francis and E Hunter

Seasonal cycles over Canadian Arctic sector TOVS data

SHEBA

(Zuidema et al. J Atm Sci 2005) Arctic clouds contain liquid the entire year (based on Intrieri et al., JGR 2002; SHEBA data) LIQUID ~ 10 ICE ~ 0.2 Mean optical depth in May:

Lidar depolarization ratios: phase detection 6 May 1998 (Intrieri et al., JGR 2002; Beaufort and Chukchi Seas)

Cloud phase and long-wave: SPRING->SUMMER MayJuneApril

Daily radiative fluxes and albedo Gorodetskaya, Tremblay, Liepert, to be submitted to GRL

Daily downwelling LW and sfc temperature Gorodetskaya, Tremblay, Liepert, to be submitted to GRL

Zoom on the melt onset: Gorodetskaya, Tremblay, Liepert, to be submitted to GRL

Cloud base temperature April and mid MayMarch and early May Winter Summer late August-early September Gorodetskaya, Tremblay, Liepert, to be submitted to GRL

Downwelling longwave flux depending on liquid water path and cloud base temperature CBT  = 1 - exp(-  o LWP) F LW =   T e 4 (Stephens, 1978) T e =CBT Gorodetskaya, Tremblay, Liepert, to be submitted to GRL

Changes between seasonal modes CBT, o C LWP, g m -2  F(CBT), W m -2  F(LWP), W m -2 Winter -235 early spring mid-May Summer September

Conclusions from SHEBA study The timing of the melt onset is determined by the increase in downwelling LW rather than decreased surface albedo Major contribution to the increase in the downwelling LW flux comes from the increase in the cloud base temperatures at the end of spring and the fact that clouds contain large amount of liquid Longer melt period in the Arctic Pacific sector in the beginning of the 21st century compared to the 1980-s is similarly associated with larger downwelling LW flux at the end of summer/early fall due to increased cloudiness and warmer cloud temperatures

Sea ice thickness from NCAR CCSM3 21st century run Holland, Bitz, Tremblay, GRL 2006 Absorbed SW and ocean heat transport

CCSM3: temperature, clouds, and radiative fluxes in the 21 st century Gorodetskaya and Tremblay, 2008, for AGU monograph “Arctic sea ice decline:...”, Eds. DeWeaver, Bitz, Tremblay

Atmospheric changes responsible for increased downwelling LW Gorodetskaya and Tremblay, 2008, for AGU monograph “Arctic sea ice decline:...”, Eds. DeWeaver, Bitz, Tremblay

Seasonal changes in radiative fluxes albedo clouds LW down SW down Gorodetskaya et al. 2008, J. Climate

Arctic Energy Budget Figure by N. Untersteiner.

Conclusions Clouds are thought to provide the “umbrella” protecting the Arctic Ocean surface from increased solar flux absorption due to the sea ice melting However... Sea ice has a robust effect on planetary albedo despite the mitigating effect of clouds Clouds actively contribute to the present sea ice decline by increasing downwelling longwave radiation Increase in cloud SW cooling is limited by LWP Future increase in atmospheric and thus cloud base temperatures will allow cloud LW warming to increase even more

1-layer sea ice thermodynamic model: ice thickness and concentration Predicts: Ts, Ti, h, SIC Forced with: CCSM3 radiation, atm T, ocean heat flux

simulated ice thickness for standard and perturbed forcing

simulated ice albedo

ice concentration

increased LW down smaller sea ice area increased SW and LW absorbed by the ocean increased ice bottom melt

Conclusions NCAR CCSM3 model predicts seasonally ice-free Arctic by 2100 together with more cloud formation, more liquid water in clouds, increased cloud LW warming and cloud SW cooling Experiments with a sea ice thermodynamic model show that increased LW cloud forcing can explain nearly 40% of the sea ice thinning in the NCAR CCSM3 model during 21st century Strong SW cloud cooling during summer compensates but does not cancel the effect of increased LW forcing The ice albedo feedback is initiated by the increased LW flux, while the oceanic heat flux is fixed at level

Thus we should not rely on clouds to prevent disappearance of the Arctic sea ice …

Temperature profile within the ice

SHEBA expedition: Surface Heat Budget of the Arctic Ocean October 1997-October 1998

Changes annual mean sea ice extent at the end of the 21st century Arzel, Fichefet, Goosse, Ocean Modelling 2006

paleoclimate theories M. Milankovitch, 1941: variations of the astronomical elements and the reflective power of the polar caps => strong oscillations of summer insolation => glacial cycles M. Budyko, 1969: small variations of atmospheric transparency => quaternary glaciations H. Gildor and E. Tziperman, 2000: sea ice is off => glaciers grow; sea ice is on => glaciers withdraw Dansgaard et al, 1989, Alley et al. 1993, Broecker 2000, Denton et al. 2005: displacements of sea ice edge + rapid atmospheric circulation changes => Dansgaard-Oeschger events

modern warming Holland and Bitz 2003: the ice-albedo feedback is one of the major factors accelerating melting of the Arctic sea ice in response to the increase in the globally averaged temperature Groisman et al, 1994: spring snow retreat => enhances spring air temperature increase Hall, 2002: surface albedo feedback accounts for ~1/2 the high-latitude response to CO 2 doubling Winton, 2005: Surface albedo feedback is a contributing, but not a dominating, factor in the coupled-models simulated Arctic amplification => Sea ice and atmosphere work together in changing the surface and TOA net shortwave flux

Sea level pressure TOVS data