Influence of the Tropics on the Southern Annular Mode

Perturbations in the southern annular mode (SAM) are shown to be significantly correlated with SST anomalies in the central tropical Pacific during austral winter and SST anomalies in the eastern tropical Pacific during austral summer. The SAM signature in the Pacific sector resembles a tropically forced Rossby wave train, the so-called Pacific–South American pattern, while the signature in the Indian Ocean sector is a zonally elongated meridional dipole. Thus, the SAM contains strong zonally asymmetric variability and tends to behave differently in the Eastern and Western Hemispheres, with internal dynamics prevailing in the Indian Ocean sector and the forced response to tropical SST anomalies exerting a strong influence in the Pacific sector. The tropically forced component of the SAM in the Pacific sector is related to a geographically fixed active Rossby wave source to the east of Australia within the core of the subtropical jet. In addition to the well-documented positive trend in summer, the SAM also exhibits a negative wintertime trend since 1979, characterized by prominent geopotential height increases over the high latitudes. In both seasons, SAM trends are closely linked to long-term trends in tropical Pacific SST that are independent of the canonical eastern Pacific ENSO variability. Although the SAM is an intrinsic pattern of high-latitude variability, the SAM index reflects the superposition of both high-latitude and tropically forced variability.