Persistence of the Southern annular mode on regional spatial scales

Abstract:

The Southern annular mode (SAM) is the leading empirical orthogonal function of zonal mean zonal wind in the Southern hemisphere. The SAM features a dipolar pattern of anomalous zonal winds and corresponds to the meridional shift of the eddy-driven jet. The zonal index (i.e., the time series associated with the SAM) is characterized by temporal persistence, which a zonally symmetric simple model involving a positive eddy-jet feedback has been proposed to explain. However, it remains open to debate that whether the annular structure of the SAM reflects zonally coherent fluctuations or zonally symmetric statistics of variability.

Using reanalysis data, the present study seeks to explore the annularity of the SAM and provide a new interpretation of the persistence of the SAM on regional spatial scales. Longitude-time lag regression shows that, during periods of high zonal index, the dipolar structure of anomalous zonal wind is zonally confined, rather than zonally symmetric. The persistence of the SAM derives from in-phase anomalies associated with zonally propagating wave packets. At a fixed longitude, the dipolar structure can barely persist longer than the lifespan of individual eddies. The downstream development of in-phase anomalies is responsible for hemispheric-scale persistence of the dipolar pattern. The implication of the wave-packets perspective of SAM for climate projection is further discussed.