Spatial variation in springtime temperature index values during ENSO and IOD events shows non-equivalent phase response for viticultural regions in Australia

Seasonal variations in winegrape production are intimately connected with growing season weather, with unusually hot or cold temperatures impacting grape and subsequent wine composition. El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) phases impact weather across Australia, particularly at the start of the Southern Hemisphere winegrape growing season; however, impacts are spatially and temporally variable. Temperature-based viticultural climatic indices (e.g. daily maximum springtime temperature) summarise growing season conditions, which allows for inter-annual and inter-regional comparison of conditions and can be used to assess changes to temperature that occur during ENSO and IOD events. This analysis investigated variations in values of a viticultural index, summed daily maximum springtime temperature (SONmax), relative to ENSO-only, IOD-only and ENSO and IOD combined events (ENSOIOD), with the objective of determining whether or not SONmax values changed during the winegrape growing season. Representative sites in 18 viticultural regions in Australia that included a range of climates were selected for analysis. Regional SONmax response to ENSO, IOD, and ENSOIOD event phases was variable. Opposing phases of an event (e.g. IOD positive and IOD negative phases) differed in spatial impact and strength of impact, with some regions responding disproportionately to opposing phases. SONmax values recorded during ENSOIOD event phases showed the most deviation from the mean, suggesting combined ENSOIOD events caused greater SONmax anomalies than either an ENSO or IOD event alone; however, the magnitude of the anomalies differed between regions and varied by phase. Cluster analysis by event phase showed that while some regions consistently had a similar variance of SONmax values relative to other regions, different regions were inconsistent in response. Regional correlation strength and direction (positive or negative) was also related to event and phase, with response to warm and cool phases being non-equivalent. This highlights that impacts of opposing phases are not equal-but-opposite in strength and also vary regionally. Improving regional understanding of impact and prediction for ENSO, IOD, and particularly ENSOIOD would therefore be useful for seasonal planning of viticultural management, as these events can often be predicted before the start of the winegrape growing season in Australia.