Precipitation.
There are good records for rainfall that go back hundreds of years worldwide and in New Zealand since before 1900. These give a record of the past and can indicate the future but the future is not so easy to predict.
Rainfall relies on many different factors and in particular wind direction which in turn is driven by pressure systems and the influences of El Nino and La Nina.
One thing is certain and that is, for every degree rise in temperature the amount of moisture in the atmosphere rises by a significant 8%. This has two effects, the first is that when it rains it rains harder and the second is that the atmosphere is more dense and when the wind blows the increased density of the air can have a more destructive effect.
Kevin Trenberth the internationally renowned climate scientist, who is a Kiwi working in the USA,
said this
Rainfall relies on many different factors and in particular wind direction which in turn is driven by pressure systems and the influences of El Nino and La Nina.
One thing is certain and that is, for every degree rise in temperature the amount of moisture in the atmosphere rises by a significant 8%. This has two effects, the first is that when it rains it rains harder and the second is that the atmosphere is more dense and when the wind blows the increased density of the air can have a more destructive effect.
Kevin Trenberth the internationally renowned climate scientist, who is a Kiwi working in the USA,
said this
"The answer to the oft-asked question of whether an event is caused by climate change is that it is the wrong question. All weather events are affected by climate change because the environment in which they occur is warmer and moister than it used to be.
Climate change from human influences is difficult to perceive and detect because natural weather-related variability is large. Even with a significant climate change, most of the time, the weather is within previous bounds. However, human-induced climate change is persistent and tends to be in one direction, at least insofar as the increases in greenhouse gases are concerned" (IPCC 2007).
Climate change from human influences is difficult to perceive and detect because natural weather-related variability is large. Even with a significant climate change, most of the time, the weather is within previous bounds. However, human-induced climate change is persistent and tends to be in one direction, at least insofar as the increases in greenhouse gases are concerned" (IPCC 2007).
He also added
"Whether or not these values are accepted, the key point is that the anthropogenic climate change effect is not zero or negligible, nor is it large relative to the mean, but it is systematic".
So the increase in temperature and moisture are with us everywhere and effect all weather events to some degree.
To help explain how increased moisture in the atmosphere becomes a flood Kevin Trenberth said this
To help explain how increased moisture in the atmosphere becomes a flood Kevin Trenberth said this
“Typical storms reach out a distance of about three to five times the radius of the rain dimension, and gather in the water vapour, to produce precipitation.” Kevin Trenberth.
To illustrate here are some graphics to illustrate how a low pressure system works..
Heat rises and causes a low pressure system.
Heat rises and sucks air in from a large area in a cyclonic system.
This illustrates how rising air from a low pressure system draws moisture from a large surface area and this concentrates the water into a exceptionally heavy rainfall event.
Changing weather patterns in New Zealand.
Researching the weather in New Zealand Georgina Griffiths the senior climate scientist for NIWA came to the conclusion that Westerly winds were increasing.
The small section of text from her report is just a paragraph of a key point in a detailed piece of research.
The small section of text from her report is just a paragraph of a key point in a detailed piece of research.
Figure: Time series of annual Trenberth Z2 Index (Christchurch – Campbell Island pressure difference). Trend line (dashed) has slope of 0.35 hPa/decade with a 95% confidence interval of ±0.27 hPa/decade. To put this change in perspective, the 1971-2000 climatological annual Christchurch – Campbell pressure difference is 8.4 hPa, so over the 1970-2012 period, this pressure difference (and therefore mean westerly wind) has increased by about 1.5hPa or about 18%. Seasonally, the largest increase has been in winter (~40%), which is what the climate models predict.
This graph from the NIWA Grifiths research shows the steady increase in Westerly winds.
The outcome of the increase in Westerly winds is that we have more wet days on the West coast and more dry days on the east coast.
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There is also an increase in dry days on the East coast.
The Mountain range down the middle of New Zealand forces the moisture laden air from the West up high where it cools and drops its moisture as rain on the west coast. As the clouds continue and drop on the Eastern side they are drier and as they warm up again they no longer have the capacity to deliver rain.
The Mountain range down the middle of New Zealand forces the moisture laden air from the West up high where it cools and drops its moisture as rain on the west coast. As the clouds continue and drop on the Eastern side they are drier and as they warm up again they no longer have the capacity to deliver rain.
These are only small sections from the research which results in the NIWA Griffiths prediction charts below.
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It is important to remember that these charts are for the mid level prediction of CO2 and we are currently running at a level much worse than could have been predicted. The outcome is likely to be much more extreme and to happen earlier.
The overall outcome for the populated and farmed area of the East side of NZ is for much higher temperatures and prolonged periods with very little rain.
Not a good scenario for farming and food production, or for our wildlife.
The overall outcome for the populated and farmed area of the East side of NZ is for much higher temperatures and prolonged periods with very little rain.
Not a good scenario for farming and food production, or for our wildlife.