Backgrounder

Backgrounder

Is New Zealand climate becoming more volatile?

Severe floods this year in Manawatu and Bay of Plenty are leading many to ask whether New Zealand climate is becoming more variable. MAF Policy recently asked NIWA to undertake a preliminary study of New Zealand climate ‘volatility’.

We use the term ‘volatility’ here to refer to the variance about the mean climate – for example, the day-to-day variation within a month, or the month-to-month variation within a year. Are there any trends in these measures of variability in the New Zealand climate?

To find out, we need to look at volatility on two levels.

  • What is happening at individual locations where the climate is measured?
  • What is happening to global or regional climate that might be causing local changes?

Global and local influences

New Zealand climate is affected by atmospheric circulation changes (pressure patterns) influencing wind flows across the country, and by sea temperatures both locally and globally. Variations of these influences occur on daily, seasonal, and multi-year timescales, and are in turn influenced by longer-term trends, such as global warming.

The climate of a particular place depends not only on these global influences, but also on local exposure characteristics, such as proximity to the sea or location in relation to nearby hills, which affect the climate.

If the changes in volatility we see in individual climate station records can be linked to changes in the global or regional climate, then we can better understand why local changes are occurring.

Example

One of the measures of wind flow patterns over New Zealand is known as the Z2 index. It measures differences in atmospheric pressure between Christchurch and Campbell Island south of New Zealand. The higher the pressure difference, the more ‘vigorous’ the wind flow.

The figure below shows the increase in the annual standard deviation of the Z2 index from 1960 to 2003. Note that the trend line does not show an increase in mean pressure – it shows that the dispersion of daily atmospheric pressure around the mean pressure (i.e., the volatility) has been greater in recent years.

This index suggests that there has been an increase in east-west annual volatility – more day-to-day variability within each year, in recent years, than there was in the early 1960s. There have been more days of extreme westerlies and more days of extreme easterlies.

Has this change (and perhaps others like it) had any impact on local climate? We will come back to this question in future issues of The Climate Update.

Trend in the annual standard deviation of the daily Z2 index (Christchurch minus Campbell Island pressure difference) for the period 1960–2003.


Route 72, inland mid Canterbury. (Photo: Alan Porteous)