Dynamics of the Southern Annular Mode during austral spring and summer: stratospheric and tropospheric interactions

The study has examined the mechanisms of the Southern Annular Mode (SAM) during austral spring and summer. The focus has been on the dynamical processes which operate on the seasonal time scales enabling predictability of the SAM. By employing concepts of wave-mean flow interactions, an observational analysis of the interconnections between tropical, extratropical and stratospheric circulations relevant for the dynamics of the SAM has been performed. Firstly, it has been shown that from mid-October until late December the persistence of the SAM is dominated by its downward coupling with the stratospheric polar vortex. Most of the year-to-year changes in the strength of this coupling are determined by the seasonal peak in the upward heat fluxes across the tropopause observed from August to mid-October and, hence, can be estimated in advance. Additional predictability of the springtime SAM arises from the pre-conditioning of the polar vortex during late autumn – early winter when its background state is conductive to upward propagating waves. Dynamical processes which underlie the stratospheric impact on the SAM have been further investigated. Secondly, it has been shown that starting from January the variability of the SAM is dominated by the impact of tropical convective systems over South America, Africa and Indonesia. Persistent variability in the latter is present throughout austral winter and spring and is related to long-lived ENSO-like variability in the Pacific Ocean. However, only upon the solstitial poleward shift of the thermal equator does it begin to affect the SAM. The sensitivity can be understood in terms of the impact of the poleward intruding subtropical highs on the propagation pathways of extratropical waves. It is then argued that the location of the subtropical boundary plays a regulating role in the summer climate system whereby the expanding tropics have enabled a stronger impact of tropical variations on the circulation of the midlatitudes. The results contribute to the growing body of evidence that tropical and stratospheric circulations can have a significant impact on midlatitude weather patterns and suggest that eliminating the model biases would help to achieve improved seasonal forecasts of the SAM.