Future projections of growing degree days and frost in New Zealand and some implications for grape growing
Examination of two daily temperature-based indices (growing degree days and number of days of frost), calculated from gridded data for the period 1974–2003 and from projected data for the periods 2020–2049 and 2070–2099, shows that the percentage of New Zealand’s land area potentially suitable for growing grapes is likely to increase significantly over the coming century. This paper firstly describes a method of interpolating maximum and minimum air temperature data measured or estimated at 143 base climate stations throughout New Zealand for every day of the period 1974–2003 onto a regular 0.05° latitude / longitude grid using a thin plate smoothing spline model. An analysis of the interpolation error using 20 validation sites shows that the average root mean square error is 1.2 °C for daily maximum air temperature and 1.6 °C for daily minimum air temperature. The gridded daily temperature data are then scaled using seasonal downscaled climate change scenarios from the CSIRO9 transient coupled atmosphere-ocean General Circulation Model (AOGCM) to produce projected daily maximum and minimum temperatures for the periods 2020–2049 and 2070–2099. Lastly, an analysis of current and future growing degree and frost days, based on the gridded temperature data for the three 30-year periods, is performed.