Geographical Patterns of Sea-Ice Retreat in the Transition to a Seasonally Ice-Free Arctic

The September sea-ice extent minimum is influenced by summertime processes, primarily thermodynamic, as well as dynamic processes during the previous winter. In this paper, we focus on the effect of the large-scale winter mean sea-ice circulation on the following September minimum sea-ice extent and, in particular, how coastal divergence in the western and eastern Arctic dictates the geographical pattern of retreat. We compare the Community Climate System Model 4 (CCSM4) and the Community Earth System Model 1 Large Ensemble (CESM-LE) with sea-ice motion vectors from NOAA/NSIDC for the past three decades. We find that the large-scale mean winter atmospheric circulation in CCSM4 is characterized by a positive Arctic Oscillation (AO) index with a broad Transpolar Drift Stream, sea-ice divergence in the western Arctic and a large negative bias in its Arctic mean sea-level pressure (around -7 mb). On the other hand, CESM-LE has a mean winter sea-ice circulation more similar to observations in its late 20th century climate and no bias in its mean sea-level pressure, although there are some spatial differences.We use a Lagrangian ice trajectory model to quantify the amount of ice divergence along the Alaskan and Eurasian coastlines by backtracking the September sea-ice extent minimum ice edge to its position throughout the previous year. We find that the sea-ice retreat in CCSM4 occurs mostly on the Pacific side of the Arctic Ocean with more coastal divergence in the Chukchi and Beaufort seas - in line with the more positive AO. On the other hand, the sea-ice retreat is found to be more symmetric around the North Pole in CESM-LE. Given that a positive trend in the AO index is a robust feature of Global Climate Models participating in CMIP5, our results suggest that sea ice will continue to retreat preferentially from the Pacific sector, as has been observed in the last decade.