Justin Wettstein (GFI):
Atmospheric forcing of, response to, and positive feedback(s) on future Arctic sea ice loss
Abstract
Internal variability in twenty-first century summer Arctic sea ice loss and its relationship to the large-scale atmospheric circulation is investigated in a 39-member Community Climate System Model version 3 (CCSM3) ensemble for the period 2000--2061. Each member is subject to an identical greenhouse gas (GHG) emissions scenario and differs only in the atmospheric model component's initial condition.
September Arctic sea ice extent trends during 2020--2059 range from -2.0 to -5.7 million square kilometers across the 39 ensemble members, indicating a substantial role for internal variability in future Arctic sea ice loss projections. A similar nearly 3-fold range (-7,000 to -19,000 cubic kilometers) is found for summer sea ice volume trends.
Higher rates of summer Arctic sea ice loss in CCSM3 are associated with enhanced trans-polar drift and Fram Strait ice export driven by surface wind and sea-level pressure patterns. Over the Arctic, the co- varying atmospheric circulation patterns resemble the so-called ``Arctic Dipole'', with maximum amplitude between April and July. Outside the Arctic, an atmospheric Rossby wavetrain over the Pacific sector is associated with internal ice loss variability. Interannual covariability patterns between sea ice and atmospheric circulation are similar to those based on trends, suggesting that similar processes govern internal variability over a range of time scales. Interannual patterns of CCSM3 ice-atmosphere covariability compare well with those in nature and in the newer CCSM4 version of the model, lending confidence to the results. Atmospheric teleconnection patterns in CCSM3 suggest that the tropical Pacific modulates Arctic sea ice variability via the aforementioned Rossby wavetrain. Large ensembles with other coupled models are needed to corroborate these CCSM3-based findings.
Atmospheric response to and feedback(s) on 21st century Arctic sea ice loss
(preliminary results):
Interannual and lower-frequency variability in Arctic sea ice extent is associated with predictable anomalies in the amount of absorbed shortwave radiation during summer. Some of this absorbed solar heat is used to retard sea ice growth in the autumn, but a substantial fraction is re-released to the surface atmosphere in the fall and winter (during the polar night) in the form of longwave radiative and turbulent heat fluxes. A near-surface baroclinic atmospheric circulation results in the stable boundary layer over the Arctic during winter and the particular location of this surface "thermal" low drives additional ice loss via the mechanical driving mechanism described in the first paper. Similar patterns have been noted in atmosphere-only simulations and some observational data, but the fully- coupled nature of this response is only detectable via a holistic analysis of the ocean-ice-atmosphere system in twenty-first century coupled model simulations and the available observations.