Zonal Wave 3 Pattern in the Southern Hemisphere 1 generated by tropical convection 2 3 4

17 18 A distinctive feature of the Southern Hemisphere (SH) extratropical atmospheric circulation 19 is the quasi-stationary zonal wave 3 (ZW3) pattern, characterized by three high and three 20 low-pressure centers around the SH extratropics. This feature is present in both the mean 21 atmospheric circulation and its variability on daily, seasonal and interannual timescales. 22 While the ZW3 pattern has significant impacts on meridional heat transport and Antarctic 23 sea ice extent, the reason for its existence remains uncertain, although it has long been 24 assumed to be linked to the existence of three major land masses in the SH extratropics. 25 Here we use an atmospheric general circulation model to show that the stationery ZW3 26 pattern is instead driven by zonal asymmetric deep atmospheric convection in the tropics, 27 with little to no role played by the orography or land masses in the extratropics. Localized 28 regions of deep convection in the tropics form a local Hadley cell which in turn creates a 29 wave source in the subtropics that excites a poleward and eastward propagating wave train 30 which forms stationary waves in the SH high latitudes. Our findings suggest that changes in 31 tropical deep convection, either due to natural variability or climate change, will impact the 32 zonal wave 3 pattern, with implications for Southern Hemisphere climate, ocean circulation, 33 and sea-ice. 34 35 Introduction 36 The quasi-stationary ZW3 pattern is a prominent feature in the SH extratropical circulation, 37 with significant impacts on Antarctic sea-ice1,2, meridional heat and momentum transport3 38 and CO2 uptake4. The ZW3 pattern is evident in the time mean but exhibits seasonal 39 variations in location and significant variability in both amplitude and phase at sub-monthly to 40 monthly timescales5,6. ZW3 exhibits a quasi-stationary pattern with small longitudinal 41 movement (between 15-25 degrees) between austral autumn and austral winter3. Previous 42 studies have suggested that the quasi-stationary ZW3 pattern evident in the time-mean 43 circulation in the SH extratropics is linked to the land-ocean distribution in the SH mid44 latitudes, in particular, the presence of three separated land masses and three ocean 45 basins2,3,7–10. This conjecture seems plausible given the presence of the annual mean ZW3 46 surface pressure ridges on or near the southern flank of the three continents and troughs in 47 the three ocean basins between them3,7. However, a similar stationary ZW3 pattern is also 48 present in the Northern Hemisphere (NH) extratropics11, where there is no obvious threefold 49 symmetry in the land-ocean distribution. Therefore, the mechanisms responsible for the 50 generation of this stationary ZW3 pattern in the SH extratropics require further examination. 51