The possible links between the Barents-Kara sea-ice area, Ural blocking, and the North Atlantic Oscillation.

We investigated the possible links between the Barents-Kara sea-ice area (SIA), Ural blocking, and the North Atlantic Oscillation (NAO) in December-January (DJ) and February-March (FM) using the ERA5 data for the period December 1979-March 2022. The Barents-Kara SIA loss in December is correlated with an increase in geopotential height at 500 hPa (Z500), mean sea-level pressure (MSLP), and the frequency and intensity of blocking over the Ural in DJ. The Barents-Kara SIA loss in December is also associated with the weakening of the stratospheric polar vortex in FM (particularly in mid-February) and the negative NAO index. However, our results show that persistent Ural blocking occurs during the transition from a neutral or positive NAO index to its negative phase. Indeed, a significant decrease in NAO index leads to the development of an area of instantaneous blocking (IB) and positive Z500 anomalies over the Ural. Persistent Ural blocking contributes significantly to Barents-Kara SIA loss, with a peak decline about seven days after the onset of Ural blocking. The onset of persistent Ural blocking also precedes the weakening of the stratospheric polar vortex by about one month. This implies that the negative correlation between the Barents-Kara SIA loss in December and the NAO index in FM might be caused by the weakening of the stratospheric polar vortex, which itself is induced by persistent Ural blocking. We conclude that the Barents-Kara SIA loss in December can be viewed as a sign rather than the cause of changes in atmospheric circulation over the high-latitude North Atlantic in succeeding months, because the Barents-Kara SIA also largely responds to Ural blocking and the NAO. [ABSTRACT FROM AUTHOR]