3.5.4 Blocking

Blocking events, associated with persistent high-latitude ridging and a displacement of mid-latitude westerly winds lasting typically a week or two, are an important component of total circulation variability on intra-seasonal time scales. In the NH, the preferred locations for the blocking are over the Atlantic and the Pacific (Tibaldi et al., 1994), with a spring maximum and summer minimum in the Atlantic-European region (Andrea et al., 1998; Trigo et al., 2004). Observations show that in the Euro-Atlantic sector, long-lasting (>10 day) blockings are clearly associated with the negative NAO phase (Quadrelli et al., 2001; Barriopedro et al., 2006), whereas the blockings of 5 to 10 day duration exhibit no such relationship, pointing to the dynamical links between the life cycles of NAO and blocking events (Scherrer et al, 2006; Schwierz et al., 2006). Wiedenmann et al. (2002) did not find any long-term statistically significant trends in NH blocking intensity. However, in the Pacific sector, Barriopedro et al. (2006) found a significant increase from 1948 to 2002 in western Pacific blocking days and events (57 and 62%, respectively). They also found less intense North Atlantic region blocking, with statistically significant decreases in events and days. Wiedenmann et al. (2002) found that blocking events, especially in the North Pacific region, were significantly weaker during El Niño years.

In the SH, blocking occurrence is maximised over the southern Pacific (Renwick and Revell, 1999; Renwick, 2005), with secondary blocking regions over the southern Atlantic and over the southern Indian Ocean and the Great Australian Bight. The frequency of blocking occurrence over the southeast Pacific is strongly ENSO-modulated (Rutllant and Fuenzalida, 1991; Renwick, 1998), while in other regions, much of the interannual variability in occurrence appears to be internally generated (Renwick, 2005). A decreasing trend in blocking frequency and intensity for the SH as a whole from NRA (Wiedenmann et al., 2002) is consistent with observed increases in zonal winds across the southern oceans. However, an overall increasing trend in the frequency of long-lived positive height anomalies is evident in the reanalyses over the SH in the 1970s (Renwick, 2005), apparently related to the introduction of satellite observations. Given data limitations, it may be too early to reliably define trends in SH blocking occurrence.