PNNL

Abstract

A regional perspective is taken to investigate the intriguing ~25-day periodicity in the South Hemisphere (SH) baroclinic annular mode (BAM) and the underlying possible mechanisms. Results show that the 25-day periodicity exists not only in the circumglobally zonal average but also in the regional average of eddy activity as long as the domain of the average is wide enough to accommodate a wave packet. The favorable domain for the ~25 days periodicity is located downstream of the largest baroclinicity in midlatitudes. The wave activity developed there can travel circumglobally several times with a typical group velocity of 25 ms-1, and reenter the domain every 12.5 days, setting the base periodicity. A slower oceanic oscillation at a time scale of ~100 days in the same region appears to select and reinforce the multiples of 12.5 days, giving rise to a pronounced ~25-day periodicity in the wave activity.