Blocking anticyclones have been the focus of much research in the past twenty years, with the majority of studies considering only the Northern Hemisphere. Not only are the dynamics behind their persistence of great interest to researchers, but NWP models are unable to forecast block formation and decay with consistent accuracy.

Of those researchers that have considered blocking in the Southern Hemisphere, most have used diagnostics based on conventional meteorological parameters in order to examine the processes behind the formation, maintenance and decay of blocks. Following Hoskins et al. (1985), this study aims to provide a different viewpoint by examining blocking systems using isentropic potential vorticity (PV).

Case studies of blocking anticyclones in the South Pacific region were examined in terms of PV and used to develop diagnostics. These data were obtained from the NCEP/NCAR reanalysis data set (Kalnay and Coauthors, 1996). These new diagnostics help to reveal some of the processes behind Southern Hemisphere blocking, particularly how blocking systems interact with high frequency transient weather systems. When analysed in terms of low-pass filtered PV, the block appears as a long-lived anticyclonic PV anomaly. Higher frequency PV anomalies are deflected and deformed around the block, with a net flux of anticyclonic PV into the block. Hence, there is an interaction between blocking systems and transient weather systems, which helps to maintain blocks in the Southern Hemisphere.

These diagnostics will be applied to NWP forecasts, so that differences between the reanalyses and forecasts can be distinguished. This will help to identify possible reasons for the diminished forecast skill that NWP models have shown when forecasting blocks.

Hoskins, B.J., M.E. McIntyre and A.W. Robertson (1985). On the Significance of Isentropic Potential Vorticity Maps. Q. J. R. Met. Soc., 111, pp 877-946.

Kalnay, E. and Coauthors (1996). The NCEP/NCAR 40-year Reanalysis Project. Bull. Amer. Meteor. Soc., 77, pp 437-471.