Abstract
Abstract The wintertime Northern Hemisphere (NH) atmospheric circulation response to current (2007–12) and projected (2080–99) Arctic sea ice decline is examined with the latest version of the Community Atmospheric Model (CAM5). The numerical experiments suggest that the current sea ice conditions force a remote atmospheric response in late winter that favors cold land surface temperatures over midlatitudes, as has been observed in recent years. Anomalous Rossby waves forced by the sea ice anomalies penetrate into the stratosphere in February and weaken the stratospheric polar vortex, resulting in negative anomalies of the northern annular mode (NAM) that propagate downward during the following weeks, especially over the North Pacific. The seasonality of the response is attributed to timing of the phasing between the forced and climatological waves. When sea ice concentration taken from projections of conditions at the end of the twenty-first century is prescribed to the model, negative anomalies of the NAM are visible in the troposphere, both in early and late winter. This response is mainly driven by the large warming of the lower troposphere over the Arctic, as little impact is found in the stratosphere in this experiment. As a result of the thermal expansion of the polar troposphere, the westerly flow is decelerated and a weak but statistically significant increase of the midlatitude meanders is identified. However, the thermodynamical response extends beyond the Arctic and offsets the dynamical effect, such that the stronger sea ice forcing has limited impact on the intensity of cold extremes over midlatitudes.
Highlights
The observed decrease of the Arctic sea ice is one of the most obvious signs of the current global climate change and has been larger than the projections of the Intergovernmental Panel on Climate Change (Solomon et al 2007) for the past few years (Stroeve et al 2007, 2012)
First we examine the average atmospheric response over the entire winter season (DJF) and investigate the mechanisms of the different responses obtained in each experiment
We have documented the response of the latest version of the National Center for Atmospheric Research (NCAR) atmospheric model, CAM5, to current and projected Arctic sea ice loss
Summary
The observed decrease of the Arctic sea ice is one of the most obvious signs of the current global climate change and has been larger than the projections of the Intergovernmental Panel on Climate Change (Solomon et al 2007) for the past few years (Stroeve et al 2007, 2012). Owing to cryospheric feedback processes, the observed surface warming is greater over the Arctic than in other regions of the globe (Serreze and Francis 2006; Screen and Simmonds 2010). This so-called Arctic amplification effect is associated with a strong decrease of the sea ice extent, especially during summer and fall (Serreze et al 2007; Stroeve et al 2011). A decline of the Arctic sea ice can significantly impact other components of the climate system, including the Northern Hemisphere (NH) atmospheric circulation. Some studies (Magnusdottir et al 2004; Deser et al 2004, 2007) have
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