On the dynamics and impacts of the summertime Southern Hemisphere dominant wave mode

Medium‐scale waves (zonal wave numbers 4–7) dominate the summertime atmospheric state of the midlatitude Southern Hemisphere (SH), with zonal wave number 5 (ZW5) playing the most prominent role. However, despite a recent swell of interest in medium‐scale waves, more than two decades and several generations of atmospheric reanalysis products have elapsed since the most recent systematic analysis of ZW5 and its impacts. Here, we provide a comprehensive overview of ZW5 and explore its links to weather extremes over 40 SH summer seasons. To support these analyses, we develop and apply a new two‐dimensional index that provides more precise descriptions of ZW5 propagation and intensity than traditional metrics do. The five centers of activity associated with ZW5 typically propagate eastward in a latitude band centered near 50°S at periods of approximately 12 days, with well‐defined life cycles. Baroclinic generation of ZW5 patterns mainly occurs downstream of the Andes. ZW5 signals then mature over the south Indian Ocean before succumbing to barotropic decay over the South Pacific. Geopotential anomalies during ZW5 events are associated with substantial variations in surface temperature, total column ozone, and precipitation. ZW5 also contributes significantly to the development of concurrent extremes in the SH midlatitudes. As an example, we present a case study of concurrent severe heat waves in southeast Australia and South America during December 2019, which occurred during a prolonged ZW5 event that lasted for about 7 days. Anomalous horizontal advection and subsidence associated with this persistent ZW5 event led to distinct increases in surface temperature over both regions. Further analysis of ZW5‐associated extreme events suggests relationships between this dominant wave mode and recurrent wave packets.