Relationships between viticulture and large-scale climate drivers in Australia

Abstract

Annual winegrape phenology timing is largely influenced by temperatures occurring during the growing season. High temperatures, associated with advanced winegrape maturity timing, can alter winegrape composition, with a skewed sugar to acid ratio, leading to wines that are high in alcohol. Conversely, low temperatures delay or inhibit maturity, with grapes not sufficiently developing sugars, colour, and flavour characteristics. Substantial changes to maturity timing can also complicate vineyard and winery planning, with issues related to staffing, equipment, and available space for processing incoming grapes.

Australia’s seasonal weather patterns are influenced by coupled oceanic- atmospheric interactions, known as large-scale climate drivers, with El Niño – Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) being the two climate drivers used in this research. Both of these climate drivers influence temperatures across many of Australia’s wine regions. El Niño and IOD positive phases increase the chance of unusually hot and dry weather. La Niña and IOD negative phases as associated with an increased likelihood of cool, wet weather. These event phases can occur uniquely or in tandem due to oceanic-atmospheric teleconnections. When occurring together, ENSO and IOD combined events tend to have a stronger impact than when occurring alone.

This thesis examined changes to winegrape maturity timing in a changing climate, explored the relationship between maturity timing and viticultural climatic indices, and evaluated the impact of ENSO and IOD on both winegrape maturity timing and viticultural climatic index values. No previous work has been done regarding climate driver impact on winegrapes in Australia.

An important winegrape maturity data set (DOYM) was the main viticultural variable used in this thesis. Trends in DOYM over time were assessed, with maturity found to be occurring earlier in the season coincident with increasing trends in growing season temperatures. Trends in maturity of some cultivars were at a greater magnitude than other cultivars, leading to a shortening of the harvest period, or a vintage compression; however, this was not true for all regions and cultivars.

In order to link DOYM with climatic variables, nine viticultural climatic indices were tested against DOYM data to determine which index had the best correlation to maturity timing. For this portion of the research, two springtime temperature indices were created and it was found these two indices were best correlated to maturity timing.

Annual DOYM and index values were then grouped by ENSO and IOD phase and evaluated for deviation from the mean. Research revealed that DOYM and springtime temperature index values do vary significantly during ENSO and IOD phases, with the largest deviations from the mean occurring during combined ENSO and IOD phases. The IOD was shown to have more impact on values than ENSO, with ENSO-only phases alone found to have little impact on the variables and regions studied. During combined events, DOYM varied by 7 to 28 days from the mean maturity date, which would have implications for grape quality and vineyard management. Similarly, springtime temperature index values during combined event phases were found to be greater than one standard deviation from the mean; however, impact varied by region and phase, with opposing phases not necessarily having an opposite but equivalent response.

Research undertaken for this thesis was the first of its kind, exploring the relationships between winegrape maturity timing, viticultural climatic indices, and climate drivers. Methods developed and used in this thesis are applicable to studies involving other crops and regions worldwide. Findings presented in this thesis reveal important insights into how climate drivers impact winegrape phenology timing, with implications for improving seasonal forecasting and aiding in vineyard management.