IPCC_Managing Risks of Extreme Events.pdf - Climate Access

Changes in Climate Extremes and their Impacts on the Natural Physical EnvironmentChapter 3single stations (see Section 3.3.3 for details), or oceanic variables suchas extremes in non-tide residuals are (if these are located in the vicinityof the main storm tracks) possible proxies for extratropical cycloneactivity. Trend detection in extratropical cyclone variables such asnumber of cyclones, intensity, and activity (parameters integratingcyclone intensity, number, and possibly duration) became possible withthe development of reanalyses, but remains challenging. Problems withreanalyses have been especially pronounced in the Southern Hemisphere(Hodges et al., 2003; Wang et al., 2006). Even though different reanalysescorrespond well in the Northern Hemisphere (Hodges et al., 2003;Hanson et al., 2004), changes in the observing system giving artificialtrends in integrated water vapor and kinetic energy (Bengtsson et al.,2004) may have influenced trends in both the number and intensity ofcyclones. In addition, studies indicate that the magnitude and even theexistence of the changes may depend on the choice of reanalysis (Trigo,2006; Raible et al., 2008; Simmonds et al., 2008; Ulbrich et al., 2009)and cyclone tracking algorithm (Raible et al., 2008).The AR4 noted a likely net increase in the frequency/intensity ofNorthern Hemisphere extreme extratropical cyclones and a polewardshift in the tracks since the 1950s (Trenberth et al., 2007; Table 3.8), andcited several papers showing increases in the number or strength ofintense extratropical cyclones both over the North Pacific and the NorthAtlantic storm track (Trenberth et al., 2007, p. 312) during the last 50years. Studies using reanalyses indicate a northward and eastward shiftin the Atlantic cyclone activity during the last 60 years with both morefrequent and more intense wintertime cyclones in the high-latitudeAtlantic (Weisse et al., 2005; Wang et al., 2006; Schneidereit et al., 2007;Raible et al., 2008; Vilibic and Sepic, 2010) and fewer in the mid-latitudeAtlantic (Wang et al., 2006; Raible et al., 2008). The increase in highlatitudecyclone activity was also reported in several studies of Arcticcyclone activity (X.D. Zhang et al., 2004; Sorteberg and Walsh, 2008; Seppand Jaagus, 2011). Using ship-based trends in mean sea level pressure(MSLP) variance (which is tied to cyclone intensity), Chang (2007) foundwintertime Atlantic trends to be consistent with National Centers forEnvironmental Prediction (NCEP) reanalysis trends in the Atlantic, butslightly weaker. There are inconsistencies among studies of extremecyclones in reanalyses, since some studies show an increase in intensityand number of extreme Atlantic cyclones (Geng and Sugi, 2001; Pacioreket al., 2002; Lehmann et al., 2011) while others show a reduction (Gulevet al., 2001). These differences may in part be due to sensitivities of theidentification schemes and different definitions of an extreme cyclone(Leckebusch et al., 2006; Pinto et al., 2006). New studies have confirmedthat a positive NAM/NAO (see Section 3.4.3) corresponds to strongerAtlantic/European cyclone activity (e.g., Chang, 2009; Pinto et al., 2009;X.L. Wang et al., 2009b). However, studies using long historical recordsseem to suggest that some of these links may be statistically intermittent(Hanna et al., 2008; Matulla et al., 2008; Allan et al., 2009) due tointerdecadal shifts in the location of the positions of the NAO pressurecenters (Vicente-Serrano and Lopez-Moreno, 2008; X.D. Zhang et al.,2008). It is unclear to what extent the statistical intermittency impliesthat the underlying physical processes creating the connection act onlyintermittently. A possible influence of the Pacific North America (PNA)pattern on the entrance of the North Atlantic storm track (overNewfoundland) has been reported by Pinto et al. (2011). It should benoted that there is some suggestion that the reanalyses cover a timeperiod that starts with relatively low cyclonic activity in northern coastalEurope in the 1960s and reaches a maximum in the 1990s. Long-termEuropean storminess proxies show no clear trends over the last century(Hanna et al., 2008; Allan et al., 2009; see Section 3.3.3 for details).Studies using reanalyses and in situ data for the last 50 years have notedan increase in the number and intensity of north Pacific wintertimeintense extratropical cyclone systems since the 1950s (Graham and Diaz,2001; Simmonds and Keay, 2002; Raible et al., 2008) and cyclone activity(X.D. Zhang et al., 2004), but signs of some of the trends disagreedwhen different tracking algorithms or reanalysis products were used(Raible et al., 2008). A slight positive trend has been found in northPacific extreme cyclones (Geng and Sugi, 2001; Gulev et al., 2001;Paciorek et al., 2002). Using ship measurements, Chang (2007) foundintensity-related wintertime trends in the Pacific to be about 20 to 60%of that found in the reanalysis. Long-term in situ observations of northPacific cyclones based on observed pressure data are considerablyfewer than for coastal Europe. However, using hourly tide gauge recordsfrom the western coast of the United States as a proxy for storminess,an increasing trend in the extreme winter Non-Tide Residuals (NTR) hasbeen observed in the last decades (Bromirski et al., 2003; Menendez et al.,2008). Years having high NTR were linked to a large-scale atmosphericcirculation pattern, with intense storminess associated with a broad,south-easterly displaced, deep Aleutian low that directed storm trackstoward the US West Coast. North Pacific cyclonic activity has beenlinked to tropical SST anomalies (NINO3.4; see Section 3.4.2) and thePNA (Eichler and Higgins, 2006; Favre and Gershunov, 2006; Seierstadet al., 2007), showing that the PNA and NINO3.4 influence storminess,in particular over the eastern North Pacific with an equatorward shift instorm tracks in the North Pacific basin, as well as an increase in stormtrack activity along the US East Coast during El Niño events.Based on reanalyses, North American cyclone numbers have increasedover the last 50 years, with no statistically significant change in cycloneintensity (X.D. Zhang et al., 2004). Hourly MSLP data from Canadianstations showed that winter cyclones have become significantly morefrequent, longer lasting, and stronger in the lower Canadian Arctic overthe last 50 years (1953-2002), but less frequent and weaker in the south,especially along the southeast and southwest Canadian coasts (Wanget al., 2006). Further south, a tendency toward weaker low-pressuresystems over the past few decades was found for US East Coast wintercyclones using reanalyses, but no statistically significant trends in thefrequency of occurrence of systems (Hirsch et al., 2001).Studies on extratropical cyclone activity in northern Asia are few. Usingreanalyses, a decrease in extratropical cyclone activity (X.D. Zhang et al.,2004) and intensity (X.D. Zhang et al., 2004; X. Wang et al., 2009) overthe last 50 years has been reported for northern Eurasia (60-40°N) witha possible northward shift with increased cyclone frequency in the higherlatitudes (50-45°N) and decrease in the lower latitudes (south of 45°N),164