Tropical Grasslands (2007) Volume 41, 203–215

Influence of the El Niño–Southern Oscillation on climate risk
and native pastures in the northern Murray-Darling Basin

J.F. CLEWETT1 AND N.M. CLARKSON2

1 Agroclim Australia, Highfields, Toowoomba, Queensland
2 Toowoomba, Queensland

Abstract

The El Niño–Southern Oscillation (ENSO) is shown to have historically had significant effects on climatic risk concerning seasonal rainfall, minimum and maximum temperatures, evaporation, pasture establishment and growth, and onset of the growing season of native pastures in the northern Murray-Darling Basin of southern Queensland. In this region, the effects of the La Niña pattern of temperatures in the eastern equatorial Pacific Ocean were generally more favourable for pastures, whereas the effects of the El Niño pattern were less favourable. The potential contribution of Southern Oscillation Index (SOI) information to seasonal forecasts was assessed using historical rainfall records from 112 longterm rainfall-recording stations, and temperatures, pan evaporation and modelled pasture growth for 30 locations. Forecast skill was measured by percent change in median values, non-parametric statistical tests and cross-validated skill scores. Forecast skill, spatial homogeneity and temporal consistency of SOI-based seasonal forecasts for pasture growth were greater than for rainfall.
Use of the 3-month average SOI, and to a lesser extent the SOI phase, forecasting systems has historically provided significant forecasting skill for pasture conditions during spring and summer at lead times of up to 2 months. When the average SOI in winter or spring was below –5, the median pasture growth in the following spring-summer season was 20% below the longterm median (mean of the 30 sites in the study region). In contrast, when the SOI was above +5, the median pasture growth was 26% above the long-term median in the following period. Forecasts of climatic conditions in winter (rainfall, temperature, evaporation) were reasonably skilful but estimates of growth of tropical grasses during this period were very low and thus seasonal forecasts of winter growth are not so relevant. The application of seasonal climate forecasts to achieve better pasture management is discussed in relation to improving the productivity and sustainability of grazing systems. Some examples are given of pasture and livestock decisions that could be improved by taking changes in climatic risk into account during El Niño and La Niña episodes.

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