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Faculty of Mathematics


Whether you want to predict the Earth's climate or forecast its daily weather, you need to understand the behaviour of the fluids that make up atmosphere and oceans. Professor Peter Haynes of the Atmosphere-Ocean Dynamics Group at DAMTP is one of the mathematicians who provide this crucial understanding. His work in the field has recently earned him a Fellowship of the Royal Society.

Haynes is interested in all things that move within the Earth's atmosphere and its oceans. He has worked on a range of topics, from the propagation and dissipation of waves, to the mixing and transport of chemical species, such as ozone, water vapour and chlorine- and bromine-containing compounds (that can act to destroy ozone). His research has provided generic, theoretical insights into the fluid dynamical processes involved, but it has also focussed on real observations and particular applications.

I am in the business of simplification. I try to reduce systems to find out what the minimal ingredients are that are important for the atmosphere and the ocean. PeterHaynes

In all cases, the aim is to construct mathematical models that can accurately describe a physical process while being simple enough to provide insight and be used in practice. "I am in the business of simplification," says Haynes. "A state-of-the-art climate model incorporates many, many different processes. But if you look at a particular phenomenon it's never clear exactly which of these processes are vital. I try to reduce systems to find out what the minimal ingredients are that are important for a particular atmospheric or oceanic process."

Loading the weather dice

An example of the scope of Haynes' work involves the stratosphere. Extending from roughly 10km to 50km above the Earth's surface, the stratosphere is the second-lowest layer of the Earth's atmosphere. It envelopes the lowest layer of the atmosphere, called the troposphere, in which most of the processes which we experience as weather occur. Consequently meteorologists have until recently pretty much ignored the stratosphere when producing their forecasts.

"However, over the last twenty years it has been shown that knowing about the state of the stratosphere is helpful for weather forecasting in mid latitudes, that is, in the part of the world where we live," explains Haynes. "For example, in the last couple of winters there have been events called sudden stratospheric warmings: these are disruptions of the circulation in the stratosphere." It seems as though the sudden warmings have some implications for weather: they appear to lead to a spell of dry and cold weather in northern Europe and more mild, wet and windy weather for southern Europe. "It's a probabilistic effect," explains Haynes. "It's not that you can be certain that if you see [a sudden stratospheric warming] then there will be very cold weather in northern Europe. But the warming sort of loads the dice."

Haynes has developed some of the models that describe the interaction between stratosphere and troposphere and the mechanisms by which warmings in the former might impact the weather in the latter. Understanding of the interaction has increased a great deal over the last twenty years and as a result centres such as the Met Office now include information on the stratosphere when producing their seasonal forecasts, both in short term and seasonal forecasting.

"What I am interested in now is whether there might be similar effects in the tropics," says Haynes. "There is a hint that there might be — but the physics of the interaction is likely to be very different. I am interested to understand how such connections might work and if they could be used in forecasting."

Spotting trends

It's not just in weather that the stratosphere has an important effect: our climate, too, appears to be affected by processes that happen within it. The Earth's most abundant greenhouse gas is water vapour. Around 99% of the water vapour contained in the Earth's atmosphere resides in the troposphere, but tiny concentrations of it also exist in the stratosphere.

"The concentration of [stratospheric water vapour] is very low, so you might imagine that it's not important," explains Haynes. "But because [of the way it interacts with radiation], changing these concentrations potentially has a significant effect on surface temperatures. Additionally, water vapour in the stratosphere is important for ozone: the moisture in the stratosphere changes the chemistry of the stratosphere and can change the balance between production and destruction of ozone."

Haynes has been working on two questions posed by stratospheric water vapour. One is to explain what scientists have observed, or think they have observed. "We see fluctuations in stratospheric water vapour and sometimes they have been interpreted as persistent long-term trends," says Haynes. "For example, about ten years ago there was a lot of excitement because it seemed as though water vapour concentrations in the stratosphere had been increasing. But this is a delicate issue: are we seeing a systematic trend or are we just seeing quasi-random variations from one year, or one decade, to the next?"

The problem is compounded by the fact that accurate measurements are very difficult to obtain. "If you have an instrument on an aircraft or balloon, then the instrument goes up through very moist lower parts of the atmosphere before getting to the stratosphere to measure very low concentrations of water vapour and then coming back down again," explains Haynes. "So there is a lot of scope for corruption of measurements."

Haynes has modelled the processes that determine changes in stratospheric water vapour concentrations and shown how recent changes, which are sometimes increases and sometimes decreases, can be explained. It looks now as if claims of an increasing trend are premature. This is an important reminder to scientists working in the field. Rather than scrambling to explain a supposed trend, efforts should go into collecting reliable continuous measurements over many years and then subjecting them to careful analysis.

Stratospheric climate change

Whether or not a trend can currently be identified, there is the question of how water vapour concentrations in the stratosphere will change in the future. Any water vapour contained in the stratosphere originally comes from the surface of the Earth and so must have passed the boundary between troposphere and stratosphere. "The fact that temperatures are very cold [at this boundary] means that only a small amount of water vapour can get through," says Haynes. "So water vapour is controlled by temperature."

This means that global warming can affect stratospheric water vapour concentrations. In turn, higher concentrations of this greenhouse gas in the stratosphere can then feed back into climate change. Haynes and his colleagues at DAMTP are using mathematical models to understand this potential feedback loop. This involves not only the fluid dynamics Haynes is an expert in, but also microphysics and the physics of radiation. "I like the fact that one is forced to learn new bits of physics and think of ways of incorporating them into the models," he says.

Haynes' work on the stratosphere only represents a small part of the research that has earned him a Royal Society Fellowship. The faculty is proud to have hosted most of Haynes' distinguished career. Apart from a two-year stint at the University of Washington, in the Department of Atmospheric Sciences, he has been in DAMTP since he started his PhD in 1980. Between 2005 and 2015 he served as Head of DAMTP. The role enabled him to contribute, not just to the content, but also to the growth and development of the research that takes place here and the people who produce it. "There is an enormous range of topics being covered by research and teaching in DAMTP and as Head of Department one has to do one's best to get a picture of what is going on on across that range — that was very interesting and rewarding."

The Fellowship of the Royal Society recognises these past achievements, but also points the way to the future. "It's great from a personal satisfaction point of view, of course," says Haynes. "But it also encourages me to carry on with my research." Congratulations Peter Haynes!