Planetary and Synoptic Scale Interactions in Southeast Pacific Blocking Using Potential Vorticity Diagnostics: More Evidence for the Paucity of Wave- Wave Interactions in Southern Hemisphere Blocking

Climatological studies of blocking in the Southern Hemisphere (SH) demonstrate that blocking events are less common (e.g. Lejenas, 1984; Renwick 1998), and are weaker (e.g. Wiedenmann, et al., 2002; hereafter WLMT02) throughout the SH when compared to their NH counterparts. WLMT02 implied that the relative roles in the interaction between planetary-scale and synoptic-scale waves may partially explain the relative paucity of SH blocking. However, the same study demonstrated that blocking events in the South Pacific sector occur with equal frequency and persistence as those in the northern Pacific. Trenberth and Mo (1985) also suggested that the difference in climatological behavior of blocking events between the two hemispheres may be a result of differences in the dynamics that develop and maintain blocking events. Briefly, the dynamic support for blocking events comes from the influx of anticyclonic vorticity advection into the blocking region by an amplifying synoptic-scale wave, however a few studies (e.g., Tsou and Smith, 1990; Alberta et al., 1991; Lupo 1997) suggested a role for temperature advections as well. The dynamic forcing mechanisms that contribute to the growth and maintenance of blocking events have also been partitioned into synoptic, planetary-scale, and scale interaction processes in several studies as well (e.g., Tsou and Smith, 1990; Tracton, 1990; Marques and Rao, 1999; Colucci, 2001). Then, studies have demonstrated that the importance of synoptic and planetary-scale forcing was different for the growth and maintenance of north Atlantic and north Pacific blocking events (e.g., Nakamura et al., 1997; Colucci, 2001). Nakamura et al. (1997) found that north Atlantic blocking events are primarily dependent on planetary-scale processes while north Pacific events are more dependent on synoptic-scale fluxes of potential vorticity (PV) for growth and maintenance. Lupo and Smith (1995b) (hereafter LS95b) and Colucci (2001) found that north Atlantic blocking events were also dependent on interactions between synoptic and planetary-scale processes. Lupo (1997) also demonstrated the importance of synoptic-scale processes relative to the planetary-scale component in the north Pacific region. Recently, Burkhardt and Lupo (2005) demonstrate that __________________________________________________ *Corresponding author address: Anthony R. Lupo, Department of Soil, Environmental, and Atmospheric Sciences, 302 E Anheuser Busch Natural Resources Building, University of Missouri-Columbia, Columbia, MO 65211. Email: [email protected]. in the SH, the synoptic-scale was crucially important to the growth, intensification, and maintenance of two southeast Pacific blocking events. They show, however, that there is a strong negative correlation between the synoptic-scale and scale interaction components. They speculate that this may indicate that in the SH, the wave-wave interactions between the planetary and synoptic-scales may not be mutually beneficial (non-linear), and thus, this may explain why blocking events there are weaker, less persistent, and less frequently than NH events. In the NH, the synoptic-scale and scale interaction terms are frequently of the same sign. The objective of this study was to examine another SH blocking case study in order to provide additional case study evidence in support of Burkhardt and Lupo (2005). The blocking event chosen for study here occurred in the south Pacific during July 2001.