8.4.5 Atmospheric Regimes and Blocking

Weather, or climate, regimes are important factors in determining climate at various locations around the world and they can have a large impact on day-to-day variability (e.g., Plaut and Simonnet, 2001; Trigo et al., 2004; Yiou and Nogaj, 2004). General Circulation Models have been found to simulate hemispheric climate regimes quite similar to those found in observations (Robertson, 2001; Achatz and Opsteegh, 2003; Selten and Branstator, 2004). Simulated regional climate regimes over the North Atlantic strongly similar to the observed regimes were reported by Cassou et al. (2004), while the North Pacific regimes simulated by Farrara et al. (2000) were broadly consistent with those in observations. Since the TAR, agreement between different studies has improved regarding the number and structure of both hemispheric and sectoral atmospheric regimes, although this remains a subject of research (e.g., Wu and Straus, 2004a) and the statistical significance of the regimes has been discussed and remains an unresolved issue (e.g., Hannachi and O’Neill, 2001; Hsu and Zwiers, 2001; Stephenson et al., 2004; Molteni et al., 2006).

Blocking events are an important class of sectoral weather regimes (see Chapter 3), associated with local reversals of the mid-latitude westerlies. The most recent systematic intercomparison of atmospheric GCM simulations of NH blocking (D’Andrea et al., 1998) was reported in the TAR. Consistent with the conclusions of this earlier study, recent studies have found that GCMs tend to simulate the location of NH blocking more accurately than frequency or duration: simulated events are generally shorter and rarer than observed events (e.g., Pelly and Hoskins, 2003b). An analysis of one of the AOGCMs from the MMD at the PCMDI found that increased horizontal resolution combined with better physical parametrizations has led to improvements in simulations of NH blocking and synoptic weather regimes over Europe. Finally, both GCM simulations and analyses of long data sets suggest the existence of considerable interannual to inter-decadal variability in blocking frequency (e.g., Stein, 2000; Pelly and Hoskins, 2003a), highlighting the need for caution when assessing blocking climatologies derived from short records (either observed or simulated). Blocking events also occur in the SH mid-latitudes (Sinclair, 1996); no systematic intercomparison of observed and simulated SH blocking climatologies has been carried out. There is also evidence of connections between North and South Pacific blocking and ENSO variability (e.g., Renwick, 1998; Chen and Yoon, 2002), and between North Atlantic blocks and sudden stratospheric warmings (e.g., Kodera and Chiba, 1995; Monahan et al., 2003) but these connections have not been systematically explored in AOGCMs.