On the Decline of Wintertime Precipitation in the Snowy Mountains of Southeastern Australia

Abstract

Data from a precipitation gauge network in the Snowy Mountains of southeastern Australia have been analyzed to produce a new climatology of wintertime precipitation and airmass history for the region in the period 1990-2009. Precipitation amounts on the western slopes and in the high elevations (.1000 m) of the Snowy Mountains region have experienced a decline in precipitation in excess of the general decline in southeastern Australia. The contrast in the decline east and west of the ranges suggests that factors influencing orographic precipitation are of particular importance. A synoptic decomposition of precipitation events has been performed, which demonstrates that about 57% of the wintertime precipitation may be attributed to storms associated with ''cutoff lows'' (equatorward of 458S). A further 40% was found to be due to ''embedded lows,'' with the remainder due to Australian east coast lows and several other sporadically occurring events. The declining trend in wintertime precipitation over the past two decades is most clearly seen in the intensity of precipitation due to cutoff lows and coincides with a decline in the number of systems associated with a cold frontal passage. Airmass history during precipitation events was represented by back trajectories calculated from ECMWF Interim Reanalysis data, and statistics of air parcel position were related to observations of precipitation intensity. This approach gives insight into sources of moisture during wintertime storms, identifying ''moisture corridors,'' which are typically important for transport of water vapor from remote sources to the Snowy Mountains region. The prevalence of these moisture corridors is associated with the southern annular mode, which corresponds to fluctuations in the strength of the westerly winds in southeastern Australia.

Trends in synoptic circulation and precipitation in the Snowy Mountains region, Australia, in the period 1958–2012

“…Several other studies report similar cool-season declines across SEA and south-western Australia (e.g. Chubb et al 2011;Pook et al 2012), however the time period in many of these studies coincided with the end of the Millennium Drought. Consideration of the cool-season as a whole can mask…”

“…A clear June-September precipitation maximum in the region has traditionally been observed with mean monthly precipitation during these cooler months typically twice as high as during the warmer seasons (Chubb et al 2011). The region has experienced a series of severe droughts and floods over the last century of observational precipitation records -a classical feature of Australia's variable climate.…”

“…A high degree of intra-annual variability is present in the synoptic classification of weather systems. Winter and spring are typically the wettest seasons, when precipitation is heavily influenced by the interaction between the prevailing mid-latitude westerly airflow and orographic effects (Chubb et al 2011;Ummenhofer et al 2009b;Cai and Cowan 2008;Pook et al 2006), and frontal and closed low systems dominate. However, Theobald et al (2015) revealed the importance of warm-season precipitation to the generation of inflows, with more than 40% of precipitation days ≥ 10 mm occurring during the warmer months.…”

Drivers of precipitation stable oxygen isotope variability in an alpine setting, Snowy Mountains, Australia

“…This was complimented by a limited number of more spatially distributed, opportunistic sampling of sites accessed by vehicle or helicopter. Targeted synoptic types included those identified by Chubb et al [2011] (cutoff lows and embedded lows with and without cold fronts), plus inland troughs. A disturbed southwesterly event was also sampled, which was characterized by convective precipitation cells embedded within a southwesterly airstream.…”

“…To further investigate the potential influence of synoptic conditions on isotopic values, we analyzed the air parcel tracks. Our five-event detailed data set included a cutoff low (Event 2), cold front (Events 6 and 9), a disturbed southwesterly (Event 7), and an inland trough (Event 12) and represents the major precipitation-bearing synoptic types for the region (aside from east coast lows) identified by Chubb et al [2011]. The plan view moisture pathway origin ( Figure 3) and vertical pathway (see supporting information) suggest that aspects of the within-and between-event variability (or lack thereof) appear to bear a strong relationship to the variability in the geographic moisture origin and pathway.…”