Can we use forecasts of El Niño and La Niña for frost management in the Eastern and Southern grains belt?

Abstract

Frost damage on winter cereals in Australia is a low frequency but high consequence threshold event with losses up to 100%. In frost prone regions grain growers delay their planting time and adjust the variety maturity selection to minimise the risk of severe frost damage and in doing so they usually incur a yield penalty of 5% to 20%. This trade-off between sowing wheat crops early enough to gain the yield advantage of milder spring conditions at grain fill, but late enough to avoid frost at flowering has long been recognised. Managing this trade-off is difficult because the date of the last frost is highly variable, hence the interest in guidance on the risk for the coming season. There is good reason to expect greater frost risk with the clear night sky associated with El Niño years and less frost risk in La Niña years. This raises the question as to whether information on the likelihood of a season being El Niño or La Niña should be used to adjust planting time and maturity type.

We found that frosts are more frequent and the median date of the last frost is later in El Niño years, whereas in a La Niña year there are fewer frosts and the median date of the last frost is generally earlier. However, the question most grain farmers are interested in is not the number of frosts, or even the median date of the last frost, but the timing of the latest 10-20% of frosts. Unfortunately we found that there was no distinction for this parameter using perfect knowledge of El Niño or La Niña. This suggests caution is warranted when basing frost risk management on imperfect forecasts of the El Niño Southern Oscillation (ENSO) index.

Key Words

Frost risk, seasonal climate forecasts, El Niño Southern Oscillation

Introduction

Frost at the time of flowering (anthesis) and grain filling of wheat is a significant risk across much of the Australian grains belt. Spring provides the best combination of moisture and temperature for anthesis and grain filling. Although spring conditions are usually warm in southern Australia, there are occasional but damaging radiation frosts associated with slow moving high pressure cells preceded by a cold spell that brings in air that is not only cold but also very dry.

Although rainfall is the most common atmospheric element related to the El Niño Southern Oscillation (ENSO), temperature is also influenced by ENSO. Stone et al. (1996) showed that minimum temperatures in spring associated with frost could be predicted using the 5-phase Southern Oscillation Index system. Willcocks and Stone (2000) conducted a comprehensive analysis of the influence of the southern oscillation on frost risk for 12 sites ranging from Biloela in central Queensland to Dubbo in central NSW. Anwar et al. (2007) analysed a range of sites in south-eastern Australia and found that the date of last frost tended to occur later in the year during the negative SOI years, and earlier during the positive SOI years.

Methods

Historical daily minimum temperatures were obtained from seven sites in the grains belt associated with the CLIMARC project which digitised daily temperature back to 1900. Data from 1900 to 2005 was analysed for seven stations across eastern and south-eastern Australia: Goondiwindi (Qld); Gunnedah, Wagga Wagga, and Deniliquin (NSW); Mildura and Nhill (Vic); and Snowtown (SA). The Bureau of Meteorology’s classification of El Niño and La Niña years has been used, which leads to 24 El Niño years, 21 La Niña years and 61 Neutral years in the 106-year record.

Results and Discussion

The trend towards fewer frosts at some locations needs to be considered as part of risk management but is not the focus of this study on ENSO and frost. As expected from basic climate principles, there are generally more frosts in El Niño years (average increase of 20%) and fewer frosts in La Niña years (average decrease of 18%). With the exception of Mildura, the shift in the number of frosts with ENSO is statistically significant.

Conclusion

At the study sites investigated, it was found that frosts are more frequent and the median date of the latest frost is often later in El Niño years, whereas in a La Niña year there are fewer frosts. However, the main interest of grain farmers is not the number of frosts, or even the median date of the last frost, but the timing of the latest 10-20% of frosts. Unfortunately we found that there was little difference in this parameter, and therefore suggest that extreme caution should be used in applying a prediction of future ENSO states for managing frost risk.

References

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