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Synoptic climatology of extreme precipitation in alpine Australia

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ABSTRACT

Extreme precipitation over the eastern Australia can be significantly enhanced by topographic interaction with the westerly flow. These extreme events can cause severe flooding, damage and disruption to human activities, yet in some areas they are also an invaluable source of water and snow. In this study, we use rain gauge and snowfall accumulation data to investigate connections between extreme rain and snow in alpine Australia and the large-scale climate. These data have been divided into three geographical locations: the west of the mountains, the ridgeline and the east of the mountains in order to explore the nature of precipitation in each region. The results confirm previous synoptic patterns found earlier in the literature for the western slopes (namely embedded and cutoff lows), and add new insights into the different processes conducive to extreme precipitation on the eastern side of the ranges, which we find to be mostly associated with a blocking structure connected to polar latitudes. Interestingly, our analyses suggests that while a La Niña pattern accompanied by enhanced meridional sea surface temperature (SST) gradients over mid-latitudes is associated with extreme events in the western and high regions, the extreme precipitation composites for the eastern region are associated with a SST pattern that resembles the predominant signal associated with global warming in the Australian region. We hypothesize that while the western and high regions rely on SST gradients, which enhance the westerly jet, the extreme events over the eastern side rely on blocking patterns (anomalous easterlies), which are at least partially responding to the global SST warming with the respective shift of the westerlies to the south. These results help explain why the precipitation over the western side of the Alps is declining more rapidly than the total precipitation observed on the eastern side.

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