The Synoptic Decomposition of Cool-Season Rainfall in the Southeastern Australian Cropping Region

Abstract

Daily rainfall during the April–October growing season in a major cropping region of southeastern Australia has been related to particular types of synoptic weather systems over a period of 33 yr. The analysis reveals that cutoff lows were responsible for at least 50% of all growing-season rainfall and accounted for 80% of daily rainfall events exceeding 25 mm per station. The proportion of rainfall contributed by cutoff lows varies throughout the growing season. It is highest in austral autumn and spring (55% and 57%, respectively) and falls to a minimum in July (42%). By way of contrast, the total contribution of all types of frontal systems to growing-season rainfall is about 32%, although the monthly value reaches a maximum of 41% in July when mean cutoff rainfall reaches a minimum. Rainfall associated with fronts is strongly concentrated in the lower range of daily falls (less than 10 mm per station). Frontal rainfall is found to be more consistent from year to year than is cutoff rainfall. The number of cutoff lows per season is highly variable, and there is a significant correlation between the number of cutoff days and atmospheric blocking in the region south of Australia in each month of the growing season. The mean amount of rainfall per cutoff day is also variable and has declined by approximately 0.8 mm over the analysis period. An understanding of the mechanisms controlling year-to-year variability of cutoff rainfall is therefore an important step in improving seasonal forecasts for agriculture in southeastern Australia.

Impact of Surface Forcing on Southern Hemisphere Atmospheric Blocking in the Australia–New Zealand Sector

“…They are associated with a split of the westerly flow in the upper troposphere into two separate branches ), which favors the detachment of closed low pressure systems from the main westerly flow and equatorward ''steering'' in these branches. This accounts for the initially counterintuitive finding of Pook et al (2006), who report significant positive correlations between days with cutoff rainfall in southeastern Australia and atmospheric blocking in the Australian sector. This role of wintertime atmospheric blocking in southeastern Australia is thus distinct from the more well-known association with summer heat waves in Australia (Hudson et al 2011;Pezza et al 2012) and Europe (e.g., Sillmann and Croci-Maspoli 2009;Coumou and Rahmstorf 2012, and references therein).…”

“…Significant precipitation events in the SH midlatitudes during the cool season are often associated with cutoff low pressure systems, as demonstrated for southern Australia (Pook et al 2006;Risbey et al 2009;Grose et al 2012), southern South America (e.g., Campatella and Possia 2007), and southern Africa (Singleton and Reason 2007). Such closed lows in the midtroposphere, detached from the westerly flow of the jet stream, occur more frequently around the three midlatitude SH continental landmasses during the cool season (Fuenzalida et al 2005;Reboita et al 2010, and references therein).…”

“…A detailed description of CAM3 can be found in Collins et al (2006a) and Hurrell et al (2006). Other model-specific aspects relevant to this study are described in Hack et al (2006b) in relation to the model's simulation of the hydrological cycle, tropical Pacific variability (Zelle et al 2005;Deser et al 2006), and specifically the climate of the Australian region (Ummenhofer et al , 2009.…”

“…The blocking index (BI) predominantly used in this study is based on Pook and Gibson (1999) and is calculated as follows:…”

“…Such closed lows in the midtroposphere, detached from the westerly flow of the jet stream, occur more frequently around the three midlatitude SH continental landmasses during the cool season (Fuenzalida et al 2005;Reboita et al 2010, and references therein). Using synoptic decomposition for southeastern Australian rainfall events, Pook et al (2006) found 50% of the total cool-season (April-October) rainfall and 80% of high-rainfall events to be associated with cutoff low systems. Much of the decline in rainfall in southeastern Australia over recent decades can be attributed to a decline in cutoff low rainfall Risbey et al 2013).…”