Influence of Arctic Sea Ice Loss in Autumn Compared to That in Winter on the Atmospheric Circulation

Russell Blackport

James Screen

University of Exeter

PAMIP Workshop 26th June 2019

Blackport, R., & Screen, J. A. (2019).Geophysical Research Letters, https://doi.org/10.1029/2018GL081469

Background

• Winter circulation response to projected year-round sea ice loss in coupled models consistent across models and protocols

Screen et al. 2018

Question: To what extent is the winter circulation response driven by autumn sea ice loss?

• Relevant for understanding physical mechanisms as well as for seasonal prediction

• Observed correlations indicate winter response may be driven by Autumn sea ice

• Previously addressed in an AGCM (Sun et al. 2015), but ocean coupling may provide additional mechanisms for lagged response

Model Experiments• HadGEM2-ES model with 1.25° latitude by 1.875° longitude resolution

• 4 ensembles (400 realizations x 5 years):• Present day control: 2008-2012 RCP8.5 forcing

• ‘2C’: 2036-2040 RCP8.5 forcing (target for sea ice loss ensembles)

• ‘2Cice’: As in control, but sea ice albedo modified to achieve year-round sea ice loss similar to ‘2C’ ensemble. Cold deep snow albedo: 0.80 -> 0.05, Snow-free ice albedo: 0.61 -> 0.66

• ‘2CiceASO’: As in control, but sea ice albedo modified to achieve Autumn sea ice loss similar to ‘2C’ ensemble. Cold deep snow albedo: 0.80 -> 0.88, Snow-free ice albedo: 0.61 -> 0.10

Sea Ice Response

Autumn sea ice loss

Year-round sea ice loss

Surface Air Temperature Response

Year-round sea ice loss

Surface Air Temperature Response

Autumn sea ice loss

Year-round sea ice loss

• In response to autumn ice loss, high-latitude warming persists into winter despite little change in SIC/SIE

Zonal Wind Response

Year-round sea ice loss

Zonal Wind Response

Autumn sea ice loss

Year-round sea ice loss

• Winter circulation response not seen in response to sea ice loss in autumn

U700 Response

Autumn sea ice loss

Year-round sea ice loss

Sea Level Pressure Response

Autumn sea ice loss

Year-round sea ice loss

Polar Cap Height Response

Autumn sea ice loss

Year-round sea ice loss

• We find a winter circulation response to year-round sea ice loss similar to previous coupled model experiments

• These are not seen in response to reduced sea ice extent only in autumn

• Our results suggest winter circulation response seen in models is primarily driven by winter sea ice loss

Conclusions

• Only one model used – conclusions could be model dependant

• Pan-Arctic sea ice vs regional sea ice loss

• Are the responses to autumn and winter sea ice loss additive?

• Could test these with additional experiments using PAMIP forcing fields

Caveats and Future Work