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

x2007

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 2004

- Summer: no trend …

Cloud cover over the Arctic Ocean:

Belchansky et al. 2004

Change (days) from 1979-88 to 1989-2001

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, 1980-2004

J. A. Francis and E Hunter

Seasonal cyclesover 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

May JuneApril

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 MayWinter

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(-oLWP)

FLW = Te4

(Stephens, 1978)

Te=CBT

Gorodetskaya, Tremblay, Liepert, to be submitted to GRL

Changes between seasonal modes

CBT,oC

LWP,

g m-2

F(CBT),

W m-2

F(LWP),

W m-2

Winter -23 5

early spring -19 24 14 +100

mid-May -10 21 +34 0

Summer -1 45 +42 +10

September -3 61 -9 0

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 andthe 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 andocean heat transport

CCSM3: temperature, clouds, and radiative fluxes in the 21st century

Gorodetskaya and Tremblay, 2008, for AGU monograph “Arctic sea ice decline:...”, Eds. DeWeaver, Bitz, Tremblay

Atmospheric changesresponsible for increased

downwelling LW

Gorodetskaya and Tremblay, 2008, for AGU monograph “Arctic sea ice decline:...”, Eds. DeWeaver, Bitz, Tremblay

Seasonal changesin radiative fluxes

albedo

clouds

LW down

SW down

Gorodetskaya et al. 2008, J. Climate

Arctic Energy Budget

Figure by N. Untersteiner.

ConclusionsClouds are thought to provide the “umbrella” protectingthe Arctic Ocean surface from increased solar fluxabsorption 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 thicknessfor 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 2000-2010 level

Thus we should not rely on cloudsto 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 CO2 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 changingthe surface and TOA net shortwave flux

Sea level pressure

TOVS data