Abstract
Recent studies point to the sensitivity of mid-latitude winter climate to Arctic sea ice variability. However, there remain contradictory results in terms of character and timing of Northern Hemisphere large-scale circulation features to Arctic sea ice changes. This study assesses the impact of realistic late autumn eastern Arctic sea ice anomalies on atmospheric wintertime circulation at mid-latitudes, pointing to a hidden potential for seasonal predictability. Using a dynamical seasonal prediction system, an ensemble of seasonal forecast simulations of 23 historical winter seasons is run with reduced November sea ice cover in the Barents-Kara Seas, and is compared to the respective control seasonal hindcast simulations set. A non energy-conserving approach is adopted for achieving the desired sea ice loss, with artificial heat being added conditionally to the ocean surface heat fluxes so as to inhibit the formation of sea ice during November. Our results point to a robust atmospheric circulation response in the North Pacific sector, similar to previous findings on the multidecadal timescale. Specifically, an anticyclonic anomaly at upper and lower levels is identified over the eastern midlatitude North Pacific, leading to dry conditions over the North American southwest coast. The responses are related to a re-organization (weakening) of west-Pacific tropical convection and interactions with the tropical Hadley circulation. A possible interaction of the poleward-shifted Pacific eddy-driven jet stream and the Hadley cell is discussed. The winter circulation response in the Euro-Atlantic sector is ephemeral in character and statistically significant in January only, corroborating previous findings of an intermittent and non-stationary Arctic sea ice-NAO link during boreal winter. These results aid our understanding of the seasonal impacts of reduced eastern Arctic sea ice on the midlatitude atmospheric circulation with implications for seasonal predictability in wintertime.
Highlights
The Arctic region (60° N–90° N) has undergone profound changes during the past decades, one of which is the nearsurface accelerated warming with respect to the global average warming (IPCC report 2019)
The mean sea level pressure anomalies indicate a weak positive North Atlantic Oscillation (NAO)-like circulation response over the Euro-Atlantic sector, this response is statistically significant in January only (Fig. 5)
This circulation pattern stands in contrast with the negative NAO circulation response to eastern Arctic sea ice loss identified in a number of (Peings and Magnusdottir 2014; Nakamura et al 2015, Ruggieri et al 2017)—but not all (e.g. Orsolini et al 2012)—previous studies
Summary
The Arctic region (60° N–90° N) has undergone profound changes during the past decades, one of which is the nearsurface accelerated warming with respect to the global average warming (IPCC report 2019). Diminishing Arctic sea ice cover is ascribed a central role in shaping this “Arctic amplification”, while the exact role of this loss in shaping mid-latitude winter climate remains hotly debated. Sea ice loss is apparent in all seasons, with most wintertime loss occurring in the Barents Sea (March, 27%) and most summertime loss occurring in the East Siberian Sea (September, 22%) (Onarheim et al 2018). Even though pan-Arctic sea ice loss is most pronounced during summer, from a seasonal prediction perspective, November is interesting because Barents-Kara sea ice conditions at this time have. Been shown to be a source of predictive skill for European winter climate (Koenigk et al 2016; Scaife et al 2014). Boreal winter is the time of the year when the climatological turbulent heat fluxes at high latitudes are strongest due to the large temperature difference between the warm ocean and cold atmosphere
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