Wave activity flux : a parameter for the prediction of the onset of blocking for the southern hemisphere?

This study discuss preliminary results on the identification of an important sensitivity of the planetary Rossby wave propagation to the onset of wintertime blocking systems over the South Pacific Ocean through the analysis of wave activity flux fields.

Two blocking episodes observed on distinct longitudes over the South Pacific during the austral winter of 1995 were selected to study the impact of their life cycle on the propagation of barotropic Rossby waves. A barotropic model linearized around meridionally varying basic states referring to the periods immediately prior to the onset of the blocking systems was employed. The model forcing are represented by divergence anomalies positioned in subtropical and tropical latitudes on the Southern Hemisphere. The numerical results showed, for all situations, local maxima on the vorticity anomalies and wave activity flux on the longitudinal band where the blocking systems developed subsequently.

Even though only two cases were analyzed, the blocking episodes studied here presented typical characteristics of the wintertime Southern Hemisphere blocks in terms of structure, duration and longitudinal location. Since the prediction of the onset of blocking is still a difficult task, these preliminary results should encourage further investigations on how well Global Circulation Models represent the planetary-wave/blocking interaction and the usefulness of the wave activity flux for the extended prediction of the onset of blocking patterns over the South Pacific.