Southern Hemisphere (SH) Stratospheric Warming Events (SWEs) are usually associated with a negative phase of the tropospheric Southern Annular Mode (SAM) during the following summer. In contrast, using ensemble climate model simulations we show that the anomalously high ozone concentrations typically occurring during SWEs can force periods of persistent positive tropospheric SAM in austral spring by increasing lower stratospheric static stability and changing troposphere-to-stratosphere wave propagation. Eventually, the tropospheric SAM switches sign to its negative phase in late spring/early summer, but this ‘downward propagation’ of the stratospheric signal does not occur in simulations without seasonal cycle. We find that the downward propagation is forced both dynamically by adiabatic heating and radiatively by increased shortwave absorption by ozone due to the seasonal cycle. Capturing this ozone forcing mechanism in models requires the inclusion of interactive ozone, which has important implications for the predictive skill of current seasonal forecasting systems.

The Amundsen Sea Low (ASL) is a distinctive feature of the Southern Hemisphere (SH) high latitude atmospheric circulation, regulating regional Antarctic climate, meridional heat transport, ocean circulation, and sea-ice in the Amundsen-Bellingshausen Seas. Most previous research on the ASL has focused on its variability with only a few studies attempting to understand why the climatological ASL exists. These studies have proposed different hypotheses to explain the presence of ASL, however, a clear understanding of the mechanisms responsible for the generation of the ASL remains uncertain. Here we use an atmospheric general circulation model to show that the ASL is a consequence of the interaction between Antarctic topography and the westerly wind jet, with negligible influence from low-latitude teleconnections. A non-rotating fluid flow simulation further suggests that the ASL can be explained by flow separation resulting from the interaction of westerly winds with the topography of Antarctica.

Subtropical Western Boundary Currents (WBCs) are often associated with hotspots of global warming, with certain WBC extension regions warming 3-4 times faster than the global mean. In the Southern Hemisphere strong warming over the WBC extensions has been observed over the last few decades, with enhanced warming projected into the future. This amplified warming has primarily been linked to poleward intensification of the mid-latitude westerly winds in the Southern Hemisphere. Changes in these winds are often thought of as being zonally symmetric, however, recent studies show that they contain strong zonal asymmetries in certain ocean basins. The importance of these zonal asymmetries for the Southern Ocean has not yet been investigated. In this study, we use an ocean-sea-ice model forced by prescribed atmospheric fields to quantify the contribution of projected zonally asymmetric atmospheric changes in generating future ocean warming and circulation changes in the subtropical WBC regions of the Southern Hemisphere. We find that the projected zonally asymmetric component of atmospheric change can explain more than 30% (>2°C) of the SST warming found in the Tasman Sea and southern Australia region and a sizeable fraction of warming in the Agulhas Current region. These changes in SST in both the Indian and Pacific Ocean basins are found to be primarily driven by changes in the large-scale subtropical ocean gyres, which in turn can largely be explained by changes in the surface wind stress patterns.

Changes to the Southern Hemisphere (SH) surface westerlies not only affect air temperature, storm tracks and precipitation; they are also pivotal in controlling global ocean circulation, ocean heat transport, and ocean carbon uptake. Wind-forced ocean perturbation experiments have commonly applied idealized poleward wind shifts ranging between 0.5 and 10 degrees of latitude, and wind intensification factors of between 10 and 300%. In addition, changes in winds are often prescribed ad-hoc without consistently accounting for physical constraints and can neglect important regional and seasonal differences. Here we quantify historical and future projected SH westerly wind changes based on examination of CMIP5, CMIP6 and reanalysis data. Under a high emission scenario, we find a projected end of 21st Century annual mean westerly wind increase of ~10% and a poleward shift of ~0.8° latitude, although there are also significant seasonal and regional variations.