Frosts appear to correlate with El Niño
Incidence and severity of frosts appear to be highly correlated to cycles of the El Niño Southern Oscillation (ENSO).
Frost data is often hard to come by and relies heavily on the diligence of the observer network and can be very site dependent.
For accurate assessments, air temperature is measured in a shaded enclosure called a Stevenson Screen at a height of approximately 1.2 metres above the ground.
Typically, minimum temperatures of two degrees celsius in the screen equate to frost conditions at ground level.
Lower temperatures mean more severe frost.
In a recent desktop study, staff from the Bureau of Meteorology reviewed the trend in frost incidence at six locations in the NSW grain belt since 1961 and correlations with cycles of the ENSO based on sea surface temperature analysis.
The sites were Hay, Wagga Wagga, Cowra, Dubbo, Gunnedah and Moree.
Analysis revealed a decline in the number of frosts at all four sites from 1961-2010 by approximately 10 per cent each decade, the exception being Wagga Wagga.
The most significant decline was in Cowra, which averaged a decline of nine frost days per decade; least significant was Wagga Wagga which averaged a decline of only 0.9 days per decade.
El Niño patterns typically had greatest influence on the number of frosts and the severity of frosts in a year, particularly in the southern sites, although impacts were also observed at both Gunnedah and Moree.
El Niño (dry) years are generally associated with warmer day time temperatures and La Niña (wet) years with cooler days.
While the analysis showed less frost days, El Nino years also appear to extend the potential frost period in a season and may affect anthesis in winter cereals.
Climate forecast products such the Bureau’s Predictive Ocean Atmosphere Model (POAMA) become more accurate as we enter the earlier winter months.
Current POAMA forecasts indicate continued warming of Pacific sea surface temperatures, indicative of El Niño conditions and a shift in Indian Ocean temperatures, indicative of a positive Indian Ocean Dipole.
While it is wise to keep watching these forecast predictors, it is common knowledge now that such patterns have historically reduced the probability of winter and spring rain in the Murray Darling Basin.