Optimal Arctic sea ice concentration perturbation in triggering Ural blocking formation

Abstract

Utilizing the Community Atmospheric Model version 4, the role of Arctic sea ice concentration (SIC) in triggering the Ural Blocking (UB) event formation on subseasonal timescale is investigated. Taking eight zonal flows as the reference states, the optimal Arctic SIC perturbations that trigger zonal flows into UB events on subseasonal timescale are obtained with the conditional nonlinear optimal perturbation approach. The numerical results show that the Arctic SIC decline in the Greenland, Barents and Okhotsk Seas can trigger zonal flows into UB events on a timescale of four pentads (20 days). Further diagnosis shows that the SIC decline in these regions locally warms the low troposphere via diabatic processes in the first pentad. Then, dynamic processes, such as temperature advection, modulate the temperature in the middle troposphere and weaken the meridional temperature gradient between the Arctic and mid-latitudes upstream of the Ural sector. The weakened meridional temperature gradient further decelerates the background zonal flow near the Ural sector and triggers UB formation in four pentads. This investigation highlights the role of Arctic SIC in the Greenland, Barents and Okhotsk Seas in triggering the formation of UB events on subseasonal timescale. The spatial characteristics of the optimal SIC perturbations may provide scientific support for SIC target observations in capturing the precursor of UB formation and improving UB prediction skills on subseasonal timescale.