Winter Jet Stream Clustering over Australia and its Role in Extreme Precipitation Events

Abstract. Jet streams substantially affect the surface weather, including extreme events. Even small changes in the position and strength of the jet stream can have a substantial impact on weather systems, and thus recent studies have linked many extreme weather events to the characteristics of the jet stream. Therefore, it is important to better understand the variability of the jet stream and how it specifically and mechanistically affects surface weather. Despite this importance, there is limited research on jet stream variability in the Southern Hemisphere. In this study, we explore the variability of the wintertime jet stream over Australia by applying a K-means clustering approach to vertically-averaged wind speed fields. Five jet stream configurations are defined, and it is shown that the wintertime jet stream over Australia exhibits substantial variability, particularly in the longitudinal location of the wind speed maximum. The clustering approach enables us to analyze patterns of upper-level forced vertical motion for the five jet configurations and how precipitation is related to these patterns. The strongest ascent occurs just south of the jet maximum, coinciding with anomalously wet regions. Furthermore, we define extreme precipitation events as those exceeding the 99.5^th percentile for Adelaide, Brisbane, Hobart, Melbourne, Perth, and Sydney using precipitation data from the Australian Gridded Climate Data (AGCD). In each of these capital cities, a higher percentage of extreme precipitation events is associated with one or two of the jet configurations, and in most of the cities, these events are linked to increased upper-level forced upward motion in the vicinity of the jet stream aloft. Extreme precipitation events are often associated with either a particularly strong jet stream acting as a waveguide for surface weather systems or a weaker jet stream disturbing the upper-level waveguide and leading to potential vorticity streamers, which have recently been linked to heavy precipitation events.