An Index of Summer Rainfall for Queensland’s Grazing Lands

Abstract

AbstractA historical rainfall index, relevant to the grazing industries of Queensland, Australia, is described. We refer to our index as the Queensland grazing lands rainfall index (QGLRI), which is a long-term (1890/91?present) time series of austral summer (November?March) rainfall, spatially averaged over a region we define as the Queensland grazing lands region. We argue that our QGLRI better represents historical summer rainfall variability faced by the majority of the grazing industry in Queensland than does area-averaged statewide rainfall. The geographical boundaries of our region were chosen to 1) better represent the spatial patterns of land use, settlement, and livestock densities and 2) coincide with spatial patterns of airmass dominance. The selected region covers 59% of Queensland?s mainland area but carries more than 80% of the state?s livestock. The region?s boundaries also closely match the mean summer location of the boundaries of the ?tropical maritime Pacific? air mass. The selected 5-month season (November?March) was chosen based on summer rainfall dominance, seasonal climatic effects restricting pasture and animal growth, and pasture management implications such as burning and the risk of overgrazing. We find that this season also corresponds to the timing of tropical maritime airmass dominance. The remaining regions of Queensland, far-northern and far-western Queensland, also correspond to well-defined dominant air masses, with properties that are markedly different from those of the tropical maritime Pacific air mass. We demonstrate that the rainfall regime in far-northern Queensland makes a strong contribution to statewide totals, resulting in statewide summer rainfall having lower variability than our QGLRI.