1 Introduction Changes in the Southern Hemisphere extratropical atmospheric circulation are dominated by the Southern Annular Mode (SAM, Rogers & Loon, 1982; Thompson & Wallace, 2000). The positive phase of the SAM is characterized by a poleward shift of the westerlies with higher sea level pressure in the midlatitudes and lower sea level pressure at high latitudes. The SAM has exhibited a significant positive trend during austral summer prior to 2000 (Banerjee et al., 2020; Marshall, 2003, 2007).