A New Blocking Index and Its Application: Blocking Action in the Northern Hemisphere

Abstract

In this paper, a new two-dimensional blocking index is proposed by defining a difference between the daily 500-hPa geopotenial heights at the reference latitude and its north side. The reference latitude is determined by a composite latitude-dependent 500-hPa geopotenial height of blocking events in different seasons and sectors. The new index can take account of the duration, intensity, propagation, and spatial structure of a blocking event. Using this index, the characteristics (frequency, duration, intensity, and preferred occurrence region) of the blocking action in the North Hemisphere (NH) are investigated using a 42-yr sample of blocking events from the NCEP–NCAR reanalyses. It is found that blocking events in the NH are more frequent in the Atlantic–Europe sector than in the Pacific sector in winter and spring and autumn, but more persistent in the Atlantic–Europe sector than in the Pacific sector for all seasons. Blocking events in the Pacific sector tend to have larger amplitudes than the Atlantic counterparts. In addition, it is shown that in the NH independently occurring blocking events are most frequent, but simultaneously occurring blockings are rather rare, indicating that the blocking events in the NH should be a local phenomenon. On the other hand, a comparison with the existing indices [e.g., Tibaldi and Molteni (TM) index] indicates that in summer and autumn the new index shows similar longitudinal dependency of NH blocking events as does the TM index, but it shows two distinct action centers of blocking events in the Atlantic sector in winter and spring (in which the most frequent one is situated more westward) and an eastward blocking action center in the Pacific sector in spring and autumn, compared to the TM index. In addition, it is found that the new blocking index proposed here shows relatively low blocking frequency for all seasons compared to the TM index, especially in the Atlantic sector in spring and in the Pacific sector in winter and spring, which seems to be in agreement with the result obtained by Pelly and Hoskins using the PV–θ index.

The Influence of Atmospheric Blocking on Extreme Winter Minimum Temperatures in North America

“…0 and GHGN , 210 meters per degree latitude. These thresholds are consistent across many studies (Tibaldi and Molteni 1990;Shabbar et al 2001;Barriopedro et al 2006;Diao et al 2006). We focus on blocking events occurring in the northeastern Pacific Ocean (EP; 1608E-1108W; Fig.…”

Winter and summer blocking variability in the North Atlantic region – evidence from long-term observational and proxy data from southwestern Greenland

“…A winter is classified as warm (cold) if the southwestern Greenland temperature index was higher (lower) than the mean plus (minus) one standard deviation. As noted extensively in the literature (e.g., Diao et al, 2006), the highest frequency of blocking is recorded in the northeastern Atlantic and the northeastern Pacific sectors. Both during warm and cold winters, blocking frequency is at maximum in these regions.…”

“…Blocking circulations occur also during the summer season (e.g., Diao et al, 2006). Here we investigate how summer blocking and temperature anomalies in southwestern Greenland are related in order to put the decadal variations of North Atlantic blocking into a long-term context.…”

Winter anticyclonic blocking effects over Europe during 1960–2000 from an ensemble of regional climate models

“…Nevertheless, an additional error source for GCM data analysis (and most probably applicable also to RCM analysis), as discussed by Barriopedro et al (2010), may arise when automatic algorithms are applied to GCM output data without an adequate adaptation of the blocking detection method to the climate of the model. This issue could be partially addressed in a future study with the use of a bi-dimensional blocking index describing the 2-dimensional evolution of blocking events in the RCM outputs, as was also pointed by Diao et al (2006).…”

“…These structures have a tendency to occur in jet-exit re gions, with increased blocking frequency over the northeastern Atlantic and the western United States (Pelly & Hoskins 2003, Tyrlis & Hoskins 2008a. A wide range of climatological studies of blocking events, based on both subjective and objective definitions of blocking, have revealed that the blocking occurrence in these regions seriously affects European weather and climate (Rex 1950, Dole & Gordon 1983, Knox & Hay 1985, Tibaldi et al 1994, Lupo & Smith 1995, Diao et al 2006.…”