Impacts: Canterbury’s changing climate
Storm damage to pine plantation Canterbury image: Sonny Whitelaw
The current and future climate of the Canterbury region
“All aspects of the climate of Canterbury are dominated by the influence of the Southern Alps on the prevailing westerly airflows. These prevailing westerlies result in a steep precipitation gradient eastward from the western ranges. Five main climate zones can be distinguished in Canterbury:
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The plains, with prevailing winds from the north-east and south-west, low rainfall, and a relatively large annual temperature range by New Zealand standards.
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The eastern foothills and southern Kaikoura Ranges, with cooler and wetter weather, and a high frequency of north-west winds.
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The high country near the main divide, with prevailing north-west winds, abundant precipitation, winter snow and some glaciers particularly towards the south.
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Banks Peninsula and the coastal strip north of Amberley, with relatively mild winters, and rather high annual rainfall with a winter maximum.
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The inland basins and some sheltered valleys, where rainfall is low with a summer maximum, and diurnal and annual temperature ranges are large.
Although north-west winds are not frequent on the plains they are an important consideration, due to the exceptional evaporation that occurs on north-westerly days. Irrigation is necessary in most parts of the plains during the growing season due to the relatively low rainfall received there.” – NIWA
Warmer oceans around New Zealand means there’s more water vapour over the water. Warmer air carries more moisture so winds are able to pick up more moisture as they cross the Tasman, leading to more atmospheric rivers, and resulting precipitation (rain, hail, snow) reaching the West Coast and parts of Southland and atmospheric rivers.
This increases the risks of flooding from rivers that originate in the Southern Alps. Rivers that begin life in the foothills to the east of the main divide, will likely have less rain and dry out more often and for longer periods, however will also be subject to flooding (Fig. 1 Canterbury’s main rivers, Figs. 4, 5 & 6). The natural hydrology of braided rivers has been destroyed or confined (virtually all Canterbury rivers), and the pace is accelerating.
Stronger dryer nor’westerlies combined with higher air temperatures (Fig. 2; current average and Fig. 3 projected under RCP8.5.) leads to a greater number of dry days (Fig. 7), less snow (Fig. 10), and greater evapotranspiration (Fig. 11) (there has been a verified 10% increase globally since 2003), compounding the effect of more frequent and intense droughts.
Simply put, rainy periods will be much rainier, and drier dry periods will be much dryer.
This also increases the risk of wildfires, especially in plantation forests and areas where wetlands have been removed (most of Canterbury).
As heat and moisture are key drivers of weather, by continuing to add greenhouse gases into the atmosphere, we are increasing the risk of more frequent and more powerful extreme weather events including marine heatwaves, hailstorms, more powerful windstorms and tornadoes, and more frequent and intense flooding both rivers and from rainfall.
Average current temperatures
Projected temperature increases (NIWA: 2020)
Current mean rainfall
Current mean number of dry days
Projected changes to rainfall
Projected changes in the number of dry days
Current mean number of snow days
Current mean evapotranspiration
Projected changes in the number of snow days
Projected evapotranspiration deficit accumulation
