Aspects of a Northern Hemisphere Atmospheric Blocking Climatology

Abstract

A comprehensive climatology of Northern Hemisphere blocking is described based on a PV-wavebreaking index at the latitude of the climatological storm track and using the 40-yr ECMWF Re-Analysis (ERA-40) dataset. The general characterization of blocking regions is in agreement with most other studies, though more detail is provided here. In the annual average, blocking is most prevalent in the large region from the eastern Atlantic Ocean through Europe to central Asia with a secondary region in the central and eastern Pacific Ocean. Using a blocking criterion with the requirement for both longitudinal extent and temporal persistence, the peak in the frequency is in the Scandinavian region, where 24% of the days are characterized by blocking. In the east Pacific maximum, the corresponding number is 7%. However, there is considerable and very important interannual variability. The decay rate in the number of blocking events lasting at least a specified number of days is significantly less over Europe than elsewhere. However, the average intensity of blocking episodes is slightly higher in the east Pacific. The mean annual cycle of blocking is quite complex. Over most of Europe it continues through the year, with maximum intensities in the autumn and winter. To both the east and west, over the western Atlantic and Asia, there are two periods in the year of highest blocking frequency. Similar two-cycle behavior is found in the eastern Pacific region. The relationship of blocking with the storm track and the mean planetary-scale geopotential ridges is considered, and the evidence that blocking is a particular phenomenon with its own nonlinear dynamics is discussed.

Overview of the synoptic and pollution situation over Europe during the EUCAARI-LONGREX field campaign

“…The described blocking was located between 50 • N-60 • N, the latitude where most blocking events occur. The mean duration of blocking events over Europe was found to be between 7.6-8 days (Tyrlis and Hoskins, 2008). Thus, the blocking event in May 2008 which lasted 12 days, was exceptionally persistent.…”

“…4). Several climatologies of blocking events have shown that the European Atlantic region, at the end of the Atlantic storm track, is the region with the highest frequency of blocking events followed by the eastern Pacific region (e.g., Barriopedro et al, 2006;Tyrlis and Hoskins, 2008). The annual cycle shows maximum frequencies in the cold season.…”

Atmospheric circulation and storm events in the Baltic Sea

“…Circulation Type 3 is meanwhile completely opposite to others. This regime is often referred to as Scandinavian blocking and is characterized by a strong positive height anomaly over Northern Europe [50]. Scandinavian blocking occurs on average 24.5% of the time during winter [51] and plays an important role in the Euro-Atlantic weather regime along with NAO and Atlantic Ridge.…”

Projected Change—Atmosphere

“…In the Atlantic-European sector blocking occurs more frequently over the North Atlantic in winter but more frequently over Europe in summer (e.g. Tyrlis and Hoskins 2008). Blocking is a major contributor to intraseasonal variability in the extratropics and can lead to seasonal climate anomalies over large parts of Europe (e.g.…”