Blocking Indices in the Northern Hemisphere: Assessments for 2020 and Trends of Long-Term Changes

In this article, episodes of potential blocking in the Northern Hemisphere (40 degrees-75 degrees N) are determined based on the analysis of deviations of the H500 geopotential at the nodes of a regular geographic grid from their midlatitude values. The cumulative calendar season/year duration of blocking episodes is considered as the TD blocking index. The spatial distribution of seasonal/annual TD indices and their anomalies in 2020 are analyzed (up to 160 days). The highest values of the annual TD index in 2020 were observed on the European continent in the zone 50 degrees-57.5 degrees N (up to 160 days). Anomalies of the TD index in European Russia (ER) and in Western Siberia amounted to +30 days; in the northern regions of Eastern Siberia they lasted more than +40 days. In the Western Hemisphere, positive anomalies in the annual TD index were observed in the East Pacific Ocean south of 50 degrees N (for more than +50 days). Negative anomalies covered the central regions of North America (up -80 days). According to estimates in active blocking sectors, in the first half of 2020, blocking in the European sector (10 degrees W-60 degrees E; 50 degrees-65 degrees N) was weakened relative to the multiyear average. In the summer and autumn seasons, positive anomalies of blocking indices were noted in this region. In the North American sector (100 degrees-160 degrees W; 50 degrees-65 degrees N), negative anomalies were observed in all seasons except for spring. Estimates of the linear trend of blocking indices at the nodes of the regular grid and in general in the latitudinal belt and its sectors were analyzed for 1949-2020 and 1976-2020. On average, negative trends prevailed in all seasons, but the spatial distribution of the trend coefficients varied from season to season. The trend of the annual duration of blocking episodes in the latitude zone 50 degrees-65 degrees N is 1.0 days/10 years and is statistically significant at a 1% level.