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Rights: University of Waikato
Published 18 June 2008

Sometimes scientists have an idea about how something works but they are not sure until they collect data and analyse it. Dr Peyman Zawar-Reza, from the University of Canterbury, wanted to find out what caused the easterly wind in the Mackenzie Basin. He explains how he used computer modelling rather than other data to answer his question.


My research was involved in the Mackenzie Basin, which is an inter-mountain basin in the Southern Alps. And something interesting would happen in this place, almost like clockwork in summer months, the wind would shift, become really strong, and become really easterly and just pour into the basin over the mountains. And this is something I became interested with, why does this happen, why such a strong flow? Of course, it would be really important for aviation, for fire weather prediction and a whole bunch of other phenomena. But I just wanted to know why this happened. Now to that day, people just explained it as such: that, well, Mackenzie Basin is just 70 kilometres away from the coast, usually air comes in from the ocean, and it takes a few hours for it to come towards the Mackenzie Basin, and the sea breeze would just flow in over the mountains and pour into the basin. This is what we would detect as an easterly, a strong easterly. And I came and tested this on a computer model, so I put topography of New Zealand in a computer model, I put the ocean around it, and sure enough, the model predicted sea breeze forming over the Canterbury Plains and flowing into the basin, and that was good. Well, here is something you can do with the models that you can't do with actual data. If this was actually a sea breeze, what would happen with my computer model if I just removed the ocean around New Zealand, I just made it totally an island surrounded by land? And what happened was really strange. Even without the sea, we still got this wind forming and pouring into the Mackenzie Basin, and then when we looked at it deeper, we found out, no, this was not a sea breeze. It was actually some other sort of wind that was formed by other mechanisms. It was a wind system that was generated by the Southern Alps, the mountain mass, and the basin topography itself. And in the summer, if you have ever been to the basin of the Southern Alps, it gets really hot. The air pressure drops rapidly in the basin during the day, and when the pressure drops, the basin acts like a vacuum cleaner, and it starts sucking air from any direction it can including all the saddles and the mountains that are around it. And that is why, when you go to the region, you usually notice air blasting through all the mountain saddles in the basin – and that is the mechanism that controls the wind, not the sea breeze.

Lloyd Homer, GNS Science
Phillip Capper