The influence of Arctic sea ice variability on the summer North Atlantic Oscillation (SNAO)

Abstract:

The summer North Atlantic Oscillation (SNAO), which is strongly related to changes in Atlantic and European summer storm tracks and the latitudinal position of the jet stream, exerts a strong influence on rainfall, temperature, and cloudiness and is related to summer extremes, such as droughts and floods, mainly in Europe. Reconstructions suggest that the SNAO was mainly negative during the last several centuries until the mid-twentieth century when it entered a positive phase, and climate model projections have suggested a predominantly positive SNAO under future global warming. However, during the recent decade, the SNAO has mainly been in a negative phase, along with a southerly shift in the jet stream, accompanied by wet and cool summers in northwest Europe. Sea surface temperatures in the North Atlantic (related to the Atlantic Multidecadal Oscillation, AMO) strongly influence the SNAO, and the current positive phase of the AMO has likely played some role in the recent downturn of the SNAO. Additionally, we found a consistent association between winter/spring Arctic sea ice concentration (SIC), particularly in the Labrador and Nordic seas, and the SNAO over the last decades based on observations. However, since the 1990s the strength of the correlations with the regional SIC has changed, weakening over the Labrador Sea and strengthening over Barents Sea. This is particularly evident during the last decade. Possibly this is a response to the rapid changes in Arctic sea ice. To test this, a new tree-ring based reconstruction of the SNAO as well as CMIP5 model runs are used to examine the influence of Arctic sea ice on the summer atmospheric circulation over northwestern Europe in a long-term context.