Abrupt evolution of the summer Northern Hemisphere annular mode and its association with blocking

Abstract

Using reanalysis data from the National Centers for Environmental Prediction‐National Center for Atmosperic Research, Boulder, Colorado, for the period from 1958 to 2005, we statistically analyzed the relationships of the summer Northern Hemisphere annular mode (summer NAM) with hemispheric‐scale anomalous summer weather and the occurrence of blocking highs. The anomalous positive NAM (low‐pressure anomaly in the Arctic and high‐pressure anomaly in midlatitudes) accounts well for the hemispheric‐scale weather associated with anomalous blocking between the polar and subtropical jets, whereas blocking rarely occurs during negative NAM periods. The double jet stream structure is more evident during periods of anomalous positive NAM than during periods of negative NAM. The surface temperatures associated with the anomalous positive NAM clearly show Europe to be hot and East Asia to be cool, as was the case during the anomalous summer of 2003. The occurrence of a positive summer NAM is therefore consistent with the hemispheric‐scale anomalous summer weather associated with blocking in 2003. We investigated the abrupt evolution of atmospheric patterns and the geographic distribution of blocking highs associated with the development, maintenance, and decay periods of an anomalous positive NAM. During the development period, blocking tends to occur over Europe and the Atlantic Ocean, but no significant blocking signature is evident over eastern Eurasia. During the maintenance stage, blocking tends to occur in the Far East. During the decay stage, blocking over the Pacific region is obvious. This longitudinal migration of blocking phenomena may be used to predict the evolution through time of the NAM.

A possible cause of the AO polarity reversal from winter to summer in 2010 and its relation to hemispheric extreme summer weather

“…Ogi et al (2004) and Tachibana et al (2010) demonstrated that in winter, but not in summer, the SV NAM accords well with the AO defined by Thompson and Wallace (2000) and used by the Climate Prediction Center of the U.S. National Oceanic and Atmospheric Administration (NOAA/CPC). Ogi et al (2005) and Tachibana et al (2010) also demonstrated that the SV NAM successfully captures anomalous summertime weather conditions associated with blocking anticyclones, such as the hot summer in Europe in 2003, whereas the original AO of Thompson and Wallace (2000), mainly reflects atmospheric variabilities in winter and cannot capture such a hot summer. Therefore, Ogi et al (2005) redefined the summertime SV NAM as the summer AO.…”

“…These findings are in agreement with previous studies. For example, Tachibana et al (2010) reported that a blocking anticyclone over the Atlantic sector that induces blocking over the Russian Far East is associated with a long-lasting, strongly positive AO caused by wave-mean flow interactions. As a result of these interactions, the positive AO pressure pattern can continue for a long time.…”

Quasiresonant amplification of planetary waves and recent Northern Hemisphere weather extremes

“…In the meantime, several other unusually strong regional summer extremes already have occurred in recent years that make it questionable that only a purely stochastic mechanism of extremes is at work (8). The global observations attest that these extremes, such as the Russian heat wave in 2010 and the record heat wave in the United States in 2011, persisted over nearly the whole summer-which is not inherent in ordinary blockings with a characteristic e-folding time of about 5-7 d-and were in fact of hemispheric scale: a stable anomalous atmospheric circulation pattern enveloped the whole NH (4,(9)(10)(11)(12)(13)(14).…”

Role of quasiresonant planetary wave dynamics in recent boreal spring-to-autumn extreme events

“…However, a number of severe summer extremes have already occurred since then in the NH, particularly in Europe. The anomalous atmospheric circulation patterns accompanying these extremes were of hemispheric scale (14)(15)(16)(17)(18)(19)(20)(21)(22). They encircled the entire NH and persisted over nearly the whole summer, which is fundamentally different from conventional regional blocking events with about a 10-d life span (23).…”

“…Based on the NH annular mode (NAM) (see ref. 24), Tachibana et al (14) showed that an anomalously strong positive summer NAM (as occurred specifically during the Branstator (26) proposed a mechanism of generation of predominantly zonally oriented Rossby wave trains. He showed that a sufficiently intense quasizonal subtropical jet could act as a waveguide for a quasistationary zonal wave number 5.…”