Substantial disagreement continues between modeling studies in attributing midlatitude climate extremes to Arctic sea-ice anomalies. This is a result of uncertainties due to internal variability, nonlinear interactions, model biases, or more likely a combination of these effects. In this study, we use large ensembles from two high-top atmospheric general circulation models (SC-WACCM4 and E3SM) to separate the sea ice-forced signal from atmospheric internal variability (noise). Following protocol for the Polar Amplification Model Intercomparison Project (PAMIP), each simulation is prescribed with either pre-industrial, present-day, or future levels of sea-ice concentration, which are associated with global warming projections of 2°C. We use 300 ensemble members per simulation to obtain large sample sizes for robust statistics in the context of internal variability.
While an equatorward shift of the eddy-driven jet is found in boreal winter, the response to future sea-ice loss is small relative to climatology and highly sensitive to the number of ensemble members considered. On average, a sea ice-forced signal in the large-scale circulation cannot be distinguished from atmospheric internal variability in our simulations. A low signal-to-noise ratio is also demonstrated in the stratosphere, where the sign of the polar vortex response can be interpreted differently depending on the ensemble size. However, the local thermodynamic effects are statistically significant with strong surface warming and increases in precipitation found in the vicinity of newly ice-free areas. This warming is generally confined to the Arctic, and there is little response in the midlatitudes. Our results highlight the important role of internal variability in the extratropics and emphasize the need for especially large ensembles (>150-200 members) when assessing the dynamical response to both present-day and future Arctic sea-ice loss. (from https://ams.confex.com/ams/2020Annual/meetingapp.cgi/Paper/367289)
1. Detection of Signal in the Large-
Scale Circulation Response to
Arctic Sea-Ice Decline
Zachary Labe
Yannick Peings and Gudrun Magnusdottir
Department of Earth System Science at the University of California, Irvine
14 January 2020
100th AMS Annual Meeting
@ZLabe
2. LENS (RCP 8.5) Mean
December - Relative to 1981-2010 Climatology
6. Is the circulation response to Arctic sea-ice
loss actually robust in the context of
internal variability?
Global climate change
Northern Hemisphere mid-
latitude weather
Arctic
Amplification
Changes in:
+ Storm tracks
+ Jet stream
+ Planetary waves
Natural Variability
+ Internal modes
+ Solar cycle
+ Volcanoes
Northern Hemisphere cryosphere changes
+ Summer and early fall Arctic sea-ice loss
+ Fall Eurasian snow cover increases
+ Late fall and winter Arctic sea-ice loss
[adapted from Cohen et al., 2014;
Nature Geosciences]
Polar Vortex
7. Global climate change
Northern Hemisphere mid-
latitude weather
Arctic
Amplification
Changes in:
+ Storm tracks
+ Jet stream
+ Planetary waves
Natural Variability
+ Internal modes
+ Solar cycle
+ Volcanoes
Northern Hemisphere cryosphere changes
+ Summer and early fall Arctic sea-ice loss
+ Fall Eurasian snow cover increases
+ Late fall and winter Arctic sea-ice loss
[adapted/changed from
Cohen et al., 2014;
Nature Geosciences]
Polar Vortex
Is the circulation response to Arctic sea-ice
loss actually robust in the context of
internal variability?
8. Complex.
Global climate change
Northern Hemisphere mid-
latitude weather
Arctic
Amplification
Changes in:
+ Storm tracks
+ Jet stream
+ Planetary waves
Natural Variability
+ Internal modes
+ Solar cycle
+ Volcanoes
Northern Hemisphere cryosphere changes
+ Summer and early fall Arctic sea-ice loss
+ Fall Eurasian snow cover increases
+ Late fall and winter Arctic sea-ice loss
[adapted from Cohen et al., 2014;
Nature Geosciences]
Polar Vortex
41. 1. Small signal in dynamical response to sea-ice decline
relative to internal variability and climatology
2. Strong surface warming and increase in precipitation mostly
confined to Arctic Ocean
3. AGCM experiments need even larger ensembles (>200
members) to address the noise
Is the circulation response to Arctic sea-ice
loss actually robust in the context of
internal variability?
42. 1. Small signal in dynamical response to sea-ice decline
relative to internal variability and climatology
2. Strong surface warming and increase in precipitation mostly
confined to Arctic Ocean
3. AGCM experiments need even larger ensembles (>200
members) to address the noise
Is the circulation response to Arctic sea-ice
loss actually robust in the context of
internal variability?
43. 1. Small signal in dynamical response to sea-ice decline
relative to internal variability and climatology
2. Strong surface warming and increase in precipitation mostly
confined to Arctic Ocean
3. AGCM experiments need even larger ensembles (>200
members) to address the noise
Is the circulation response to Arctic sea-ice
loss actually robust in the context of
internal variability?