Changes in Global Blocking Character in Recent Decades

Abstract

A global blocking climatology published by this group for events that occurred during the late 20th century examined a comprehensive list of characteristics that included block intensity (BI). In addition to confirming the results of other published climatologies, they found that Northern Hemisphere (NH) blocking events (1968–1998) were stronger than Southern Hemisphere (SH) blocks and winter events are stronger than summer events in both hemispheres. This work also examined the interannual variability of blocking as related to El Niño and Southern Oscillation (ENSO). Since the late 20th century, there is evidence that the occurrence of blocking has increased globally. A comparison of blocking characteristics since 1998 (1998–2018 NH; 2000–2018 SH) shows that the number of blocking events and their duration have increased significantly in both hemispheres. The blocking BI has decreased by about six percent in the NH, but there was little change in the BI for the SH events. Additionally, there is little or no change in the primary genesis regions of blocking. An examination of variability related to ENSO reveals that the NH interannual-scale variations found in the earlier work has reversed in the early 21st century. This could either be the result of interdecadal variability or a change in the climate. Interdecadal variations are examined as well.

Changes in the Dynamics of the Northern Hemisphere Atmospheric Circulation and the Relationship to Surface Temperature in the 20th and 21st Centuries

“…D67 (and also [12]) suggest blocking may occur more frequently in each hemisphere simultaneously. Using the results of [2] suggest this is the case. The work of [2] uses a strict criterion for identifying blocking events compared to DKV46 or D67.…”

“…al. [2] (and references therein). Figure 1c is an example of Type 3 NH flow using ECM 10a showing two amplified waves including a blocking event over Eastern Europe.…”

“…Focusing on the beginning of the 21st century ( Figure 2), a transition to an epoch dominated by meridional flow, especially those classified as Type 4, was underway. Lebedeva et al [1] correlated this period to a period of decreased AO Index values, and [2] showed that blocking in both the NH and SH has increased relative to the latter part of the 20th century. However, using a 10-year running average as a filter on the anomalous occurrences from 1899-2018 mean for particular types reveals that there were three decadal-scale circulation epochs ( Figure 3).…”

“…Additionally, an independent archive of blocking events from 1970 to 2018 was accessed via the website [29] (see also the work of [2]) for the analysis below. The blocking criteria used in [2], and references therein, is stricter than the D67 criterion, which considered amplified ridging to be a blocking process. The teleconnection indexes, specifically, the Arctic Oscillation (AO) and Antarctic Oscillation (AAO) were downloaded from the website [30].…”

“…For the analysis of global of hemispheric atmospheric flow regimes, many teleconnection or blocking indexes have been developed and are currently used (e.g., [1,2]) both operationally and in research. The study of atmospheric circulation and teleconnections in the Northern Hemisphere (NH) began in the 1940s (e.g., [3,4]).…”