Warming’s effect on wind, rain

A scientist explains what their research has taught them about climate change.

We live on a collection of islands that straddle the cool waters of the Southern Ocean and the warmth of the subtropical Pacific – stretching all the way from the warm beaches of Northland to the rugged and windswept beauty of Stewart Island, with large mountain ranges running down the spines of both Te Ika a Ma¯ ui and Te Wai Pounamu.

This stark combination of geography and topography has a significant influence on how we experience the present impacts of climate change, and what we can expect into the future.

It has been my life’s work to use climate models to make predictions about New Zealand’s future, but even more importantly to try to understand what they are telling us about how the world works. I believe that for us to make important decisions based on model predictions, we need to really understand them, and this matters even more as Artificial Intelligence becomes widespread in our lives.

So, what have I learnt from all this research into predictions?

It turns out that much of our weather is influenced by the behaviour of the southern hemisphere mid-latitude jet.

This relatively narrow band of strong winds stretches from the surface of the Earth to the top of the troposphere at about 12km, increasing steadily in strength with height. It is created primarily by the difference in temperature between the warm tropical and cold polar atmospheres, combined with the rotation of the Earth.

At the surface this jet of wind gives us the westerlies that encircle the Southern Ocean. Higher up it directly affects the routes that aeroplanes take to fly to other countries as quickly as possible.

On average the core of the jet sits south of New Zealand, but it wanders around, lurching wildly. In the winter it is often located directly over New Zealand, but sometimes plunges south to the edge of the sea ice. This jet leads to the common sailor names for areas of the Southern Ocean: the ‘‘roaring forties’’, ‘‘furious fifties’’ and ‘‘screaming sixties’’.

Climate change is having a significant effect on our jet. As we increase greenhouse gases in the atmosphere we warm everywhere, including the land and ocean. New Zealand surface air temperatures have gone up nearly 1 degree in the past 100 years, pretty much like everywhere else.

However, the greatest warming in the

Heavy rain caused the Waiho River at Franz Josef to swell. Sam Dean says the increasing strength of the southern hemisphere mid-latitude jet means stronger westerly winds, with more rain on the west coasts of both the North and South islands. atmosphere isn’t at the surface, not even in the Arctic where so much ice is melting. Thanks to water vapour feedbacks it actually occurs away from the surface in the tropical atmosphere hot spot. In fact, within the tropics the temperature of the free atmosphere appears to be rising roughly 80 per cent faster than the temperature at the Earth’s surface. At the same time manmade ozone depletion has helped to cool the atmosphere over Antarctica.

This increasing temperature difference between the tropics and the South Pole is driving a dramatic increase in the strength of the southern hemisphere jet, and the changing properties of the atmosphere are forcing it to shift southward towards the pole.

Our published research has made a significant contribution to understanding how this happens. And we are predicting more of the same into the future.

The effect of these changes in the jet upon New Zealand depends on the season. In summer and autumn this shift of the jet further south causes a weakening of the westerly winds, with calmer, drier conditions, especially in the north. In the winter and spring, the increasing strength of the jet means stronger westerly winds, with more rain on the west coasts of both islands, and somewhat less rain in parts of the east coast.

This change in rainfall pattern is a direct consequence of the change in wind interacting with the mountain ranges that separate the east from the west and has consequences for water availability. The increasing strength of the jet also means bigger waves on our coasts.

You can see the current projections for New Zealand temperature and rainfall at the website Our Future Climate New Zealand, or view the Ministry for the Environment report on climate change projections for New Zealand.

The summary is that New Zealand’s average climate change can be thought of as a combination of warming with an acceleration and shift of the jet stream driving changes in the wind and rain and waves. Understanding the consequences of our emission of greenhouse gases into the atmosphere remains essential.