Changes in Blocking Characteristics during the First Part of the 21st Century

Abstract

Abstract:A global blocking climatology published by this group for events that occurred during the late 20th century examined the comprehensive list of characteristics that included block intensity. In addition to confirming the results of other published climatologies, they found that Northern Hemisphere blocking was stronger than Southern Hemisphere events and winter events are stronger than summer ones. This work also examined the interannual variability of blocking as related to El Nino. Since this time, there is evidence that the occurrence of blocking has increased globally. A comparison of blocking characteristics during the first part of the 21st century to those in the late 20th century shows that the number of blocking events and their duration have increased in the Northern and Southern Hemisphere. The intensity of blocking has decreased by about nine percent in the Northern Hemisphere, but there was little change in the intensity of Southern Hemisphere events. Additionally, there is little or no change in the genesis regions of blocking. An examination of variability related to El Nino and Southern Oscillation reveals that the variability found in the earlier work has reversed. This could either be the result of interdecadal variability or a change in the climate.

Changes in the Atmospheric Circulation Conditions and Regional Climatic Characteristics in Two Remote Regions Since the Mid-20th Century

“…The work of reference [65] (and references therein) also show that mean annual precipitation has increased over the USA overall since the middle 20th century. The results in the Belgorod Region are consistent with the increase in blocking across the region, especially into the early 21st century [46,66,67].…”

“…Also, the number of days associated with cold season northwest anticyclone types [6] was acutely reduced for this region. Negative monthly precipitation anomalies in recent years (P4) were related to the sharp increase in the number of stationary anticyclones and blocking occurring over Eastern Europe to Kazakhstan [7,46]. This is reflective of positive height anomalies over the region during the summer season especially ( Figure 3H).…”

“…For southwest Russia this is associated with a stationary 500 hPa anticyclone that forms over Kazakhstan (e.g., references [7,21,24]). Also, since the end of the 20th century, blocking has been more frequent across the NH (e.g., reference [46]). The mean seasonal temperature, precipitation, and measures of variability for the BELF surface data associated with the sub-periods shown in Table 1 for the current [4] flow regime epoch (1957-2017) are shown in Table 2.…”

“…The 500 hPa height anomalies were positive over the Belgorod region for both seasons ( Figure 3G,H). The Atlantic Region was characterized by a 25% and 55% increase in the number of blocking events and days according to reference [46], respectively. This agrees with reference [7] which found that the occurrence of blocking doubled from 1970-2010 from 20 • -60 • W. With these changes in the circulation conditions there were higher average annual temperatures in P4 relative to the 1957-2017 period (+0.9 • C) (e.g., references [7,8,49,50]).…”

“…The decadal epochs for the NAO used here will be negative for the periods 1951-1972 and 1996-2010, while the positive epochs were 1972-1995 and 2011-2017. These were derived by examining the three-month running mean NAO index available from reference [45], and the earlier transitions agree with the published dates suggested in reference [46]. Daily values of this index are used as well and these can be obtained from the NOAA Climate Prediction Center (CPC -http: //www.cpc.ncep.noaa.gov/products/).…”