The impact of Arctic late summer sea ice variability on mid-latitude autumn and winter weather.

The influence of Arctic summer sea ice area on autumn and winter climate in the extra-tropic Northern hemisphere is investigated. During the last decades, the Arctic has warmed more than twice the global average rate, a phenomenon called the Arctic Amplification (AA). AA involves a remarkable decrease of the Arctic sea ice cover and a decrease of the latitudinal temperature gradient. Both are supposed to influence the global energy overturning circulation system, including atmospheric mid-latitude waves and the polar cell. The present study analyzes how the Arctic September sea ice cover influences the temperature and pressure in the Northern hemisphere extra-tropics in the following months and seeks explanations for this phenomenon in the atmospheric circulation. To obtain good results, the possible connection of year to year variabilities was studied, and underlying external and internal feedbacks were excluded as much as possible. Three approaches were taken: Firstly, real world data, such as temperature and surface pressure, expressed by ERA-interim from the year 1979 to 2014, were regressed on the September sea ice area. Secondly, the state of the art climate model CESM with the atmospheric model components CAM4 and CAM5 was used to study and compare the same regressions as in ERA, but for preindustrial conditions. Thirdly, to verify the chain of cause and effect, the consequences of three forced low-ice scenarios of the CESM climate model were compared to a control run. The first two datasets showed that the Arctic sea ice variabilities are not homogenous throughout the Arctic. Two regions, the Beaufort to East Siberian Sea (Be-ES) region and the Barents-Kara Sea (Ba-Ka) region, with rather independent sea ice area anomaly time series, were observed. Therefore, the climatological response in the two regions was studied independently. The CESM model runs with forced sea ice conditions give some insight of the winter responses to summer sea ice reduction. In autumn the open water in the Arctic act as heat sources, leading to anomalously warm local temperatures. The local warming leads to extending air and anomalously low surface pressure in the Arctic. This reduces the strength of the polar cell and can lead to a strengthening of the Siberian High in winter. In ERA-interim and CESM CAM4 & 5 the negative September sea ice anomalies persist into the late autumn for Be-ES and through the whole winter for Ba-Ka. Autumn responses to the sea ice anomalies are similar between the three datasets for the two regions, while the winter responses contradict each other. Negative September sea ice anomalies in the Be-ES region seem to induce an East Arctic high pressure in autumn, bringing cold conditions to North Siberia. Ba-Ka region sea ice anomalies induce a low pressure over its region in autumn, a pattern opposing the one induced by the Be-ES region. The ERA-interim winter response to the Ba-Ka sea ice anomaly shows an anomalously high pressure over West Russia, bringing cold conditions to Central and East Eurasia. This pattern is refound in literature, but is opposed by the CESM model runs with CAM4 and CAM5.