Impact of Zonal and Meridional Atmospheric Flow on Surface Climate and Extremes in the Southern Hemisphere

The Southern Annular Mode (SAM) describes the annular or zonal component of the large-scale atmo-spheric circulation in the Southern Hemisphere (SH) extratropics and influences surface climate across the SH. Although this annular flow is dominant in austral summer, in other seasons considerable zonal asymmetries are evident, reflecting a zonal wave 3 (ZW3) pattern. We define an index representing asymmetric flow using the first two leading modes of meridi-onal wind variability in the SH. Two orthogonal ZW3 indices are used together to capture longitudinal shifts in the wave train and their connection to tropical convection. We compare the impacts of the SAM and ZW3 on surface climate by ex-amining composites of temperature and precipitation fields during each season. Impacts on mean and extreme surface cli-mates are assessed. We find that the SAM and ZW3 are not clearly separated modes, but rather, ZW3 modulates the impact of the SAM across the midlatitudes. The SAM influence on regional temperature and precipitation is similar for both mean impacts and extremes. The ZW3 influence on extremes is more varied across indices and does not always reflect the ZW3 impact on mean fields. Notably, amplified ZW3 activity has a significant influence on the number of midlatitude fronts and frontal rainfall, highlighting the importance of considering ZW3 when exploring the surface climate impacts of large-scale SH circulation states, particularly for nonsummer seasons.

Automated summary

The Southern Annular Mode (SAM) describes the annular or zonal component of large-scale atmospheric circulation in the Southern Hemisphere (SH) extratropics and affects surface climate across the SH. In addition to the dominant annular flow in austral summer, zonal wave 3 (ZW3) patterns with zonal asymmetries are evident in other seasons. This study explores the influence of both SAM and ZW3 on surface climate, finding that ZW3 modulates the impact of SAM and plays a significant role in the surface climate impacts of large-scale SH circulation states, particularly in nonsummer seasons.