THE INTERDECADAL VARIABILITY OF NORTHERN HEMISPHERE BLOCKING

During the middle of the previous century, most studies have defined blocking subjectively based on surface and/or mid-tropospheric observations of typical flow configurations. Following the traditional Rex (1950a) criterion, a blocking event can be identified by a split-flow regime in the middle troposphere as a double-jet detectable over more than 45o in longitude and persisting for more than 10 days. Since then, there have been several modifications to the original Rex definition, requiring smaller durations or extensions (Treidl et al., 1981) as well as restrictions in latitude location (White and Clark, 1975) in order to exclude semi-permanent subtropical anticyclones. More recently, numerous criteria have been proposed in order to identify objectively atmospheric blocked flows. Most of them are based on zonal flow indices computed from meridional height gradients at the middle troposphere (Lejenäs and Økland, 1983 (henceforth LO83); Tibaldi and Molteni, 1990 (henceforth TM90); Trigo et al., 2004). Other methodologies detect blocking events as positive height anomalies in the mid-troposheric flow persisting for several days (e.g., Dole and Gordon, 1983) or from normalized indexes based on daily height projections over mean blocking patterns (Liu, 1994; Renwick and Wallace, 1996). The most recent methods combine traditional subjective and objective criteria, as those used by Lupo and Smith (1995) (henceforth LS95) or Wiedenmann et al. (2002) (henceforth WI02), or use indexes derived from dynamical properties related to blocking patterns, as the meridional potential temperature (θ) gradient on a potential vorticity (PV) surface representative of the tropopause (Pelly and Hoskins, 2003) or negative anomalies of vertically integrated potential vorticity within the 500-150 hPa layer (Schwierz et al., 2004). As a result, several long-term studies focused on North Hemisphere blocking events have been published (Rex, 1950a, b; LO83; Dole and Gordon, 1983; TM90; Tibaldi et al., 1994; LS95; WI02). However, some of them are usually confined to certain regions or limited to single seasons. Moreover, the behavior of the North Hemisphere (NH) blocking has been traditionally described in terms of frequency, duration and favored occurrence regions, not considering other characteristics such as genesis location, intensity and size. On the other hand, not many studies include variability, especially at interdecadal scales (e.g., Chen and Yoon, 2002). __________________________________________________ *Corresponding author address: Anthony R. Lupo, Dept. Soil, Environmental, and Atmospheric Sciences, 302 ABNR Building, University of Missouri-Columbia, Columbia, MO 65211. E-mail: LupoA@missouri.edu. Some authors have reported that blocking occurrence may be affected by NH large-scale patterns as the North Atlantic Oscillation (NAO) (e.g., Shabbar et al., 2001). The relationship between blocking and the El Niño-Southern Oscillation (ENSO), however, has been widely discussed (e.g., Renwick and Wallace, 1996; Watson and Colucci, 2002; WI02). Nevertheless, this linkage has been derived for certain regions or single seasons and the ENSO-related variability, if any, has not been clearly established. Thus, this research has three objectives: i) to design an objective automated methodology to provide a complete characterization of single blocked flows as well as a tracking algorithm to identify persistent blocking patterns, solving some previous lacks to date; ii) to obtain the longest record of NH blocking occurrences and parameters providing a robust long-term climatology; iii) to examine the low-frequency blocking variability (from interannual to interdecadal time scales) and the role played by the main NH teleconnection patterns and ENSO.