The Influence of Atmospheric Blocking on the Rossby Wave Propagation in Southern Hemisphere Winter Flows

抄録

The influence of blocking systems on the propagation of planetary Rossby waves is investigated over the South Pacific. Two episodes of blocked flow observed during the winter of 1995 in the Southern Hemisphere were selected for this purpose. Rossby wave activity is analyzed with the aid of a barotropic model linearized around zonally and meridionally varying basic states obtained from the National Centers for Environmental Prediction Center (NCEP)/National Center for Atmospheric Research (NCAR) Reanalysis Project data. Basic state flows corresponding to the blocking events, and to the seven-day period immediately prior to (after) their onset (decay) were used. The forcing, represented by an anomalous divergence pattern, is placed in two different regions—tropics and subtropics. Numerical results are discussed based on the linear theory for Rossby wave propagation. Fields of stationary wavenumber (K_s) for barotropic Rossby waves are computed for the basic state flows. Wave activity fluxes are calculated for the perturbation fields.
The spatial distribution of K_s showed the jet streams as potentially efficient waveguides, as indicated by the agreement between the position of the jet stream axes and the relative maxima of K_s. A tendency for increasing values of K_s in the high latitudes (south of 65°S) was obtained along the poleward flank of the longitudinal zone where the blocking systems were positioned. This suggests a possible enhancement on Rossby wave activity along sub-polar regions during these blocking events. In contrast, for the periods preceding and subsequent to the blocking systems, values of K_s at high latitudes did not show a tendency to increase, characterizing a normal situation in which local maximum values of K_s are confined to the mid- and subtropical latitudes. The numerical results suggest that the planetary Rossby wave propagation can be influenced by this alteration in the latitudinal distribution of higher values of K_s during the life cycle of the blocking systems. A transition, from subtropical to sub-polar, of the main sector of wave propagation was obtained between the pre-blocking and blocking situations, and with the subtropical Rossby wave activity regaining importance after the decay of the blocking events.
Moreover, for situations preceding the blocking systems, a region of maximum values on wave activity flux was obtained on the longitudinal zone where the blocking patterns would develop subsequently, indicating a possible participation of stationary Rossby wave propagation on the onset of the blocking episodes.