Meet the researcher - Dr Ali Theobald

Dr Ali Theobald

Climate researcher

What inspired you to become a climate researcher and at what age was this?

I’ve always been fascinated by the weather and the natural environment. I like to know why things are the way they are. Growing up in Northern England you get used to very changeable weather! I remember as a young child, maybe from age 8 or so, watching the weather presenters at the end of the evening news with my Dad and thinking “I want to do that’! When eventually I got to University and shared this ambition with my lecturer, he said to me ‘you don’t want to present the forecasts, you want to do the science behind the forecasts’ and he was right. I love finding out how things work and I think there’s still so much to discover about weather patterns, climatology and how it all comes together to affect what we experience on the ground.

Ali’s trend analysis found rainfall distribution is changing, which has implications for water markets, crop choice and adaptation.

 Do you have any extended family or other connections with farming or agriculture?

No I don’t have a farming or agricultural background at all, and I’ve never studied agricultural science. I did spend a lot of time on my grandad’s allotment during school holidays though! Agriculture first came to mind when I was in the literature review phase of my PhD and I was reading several papers that were discussing weather types and the autumn break for cropping in southeast Australia. Although my focus was elsewhere, I saw how the results of my research could be directly applicable to agriculture, initially with regards to the trends we were finding that pointed to a decline in autumn rainfall in the Snowy Mountains region.

What made you curious to take on the teleconnections research on rainfall in the Snowy Mountains region?

I had had some experience of this type of research from my Honours project while at university in the UK, where I looked at how the North Atlantic Oscillation, and to a lesser extent ENSO, influenced the climate of western Europe. I found it fascinating to investigate how ocean and atmospheric processes happening thousands of kilometres away could affect weather elsewhere. So when I saw this PhD advertised I knew it was something that I was very keen to get into. Luckily I got the job!

Rainfall is becoming more intense in the summer months in the Snowy Mountains catchment

A PhD research project is regarded as a long, tedious and sometimes tortuous process. How difficult was it to address the novel attributes of the study, rather than simply duplicating another method of a similar study done elsewhere?

It certainly is a marathon undertaking and can be a real rollercoaster. But, it’s a real privilege being able to spend your time dedicated to a research question. And addressing the novel attributes of the study was the exciting bit – getting to do something that hasn’t been done before! Previous studies had combined surface and mid-level atmospheric data into typing schemes, but in this study we classified weather types from the surface right up to the jet stream. And that allowed the discovery of sometimes very subtle, but important differences in synoptic types. I was able to use existing techniques but adapt them for multi-level analyses. Discovering boosted regression trees was a bit of a revelation to me, and allowed us to actually quantify the relative influence of each teleconnection. As far as I know, we were the first to apply this technique to climate research.

The Snowy Mountains area is the catchment for some of the largest irrigation investment in Australia, with the temporary water markets being driven by runoff in higher catchments. What are the key cycles that you would keep an eye on if you were an irrigator?

At shorter timescales, the Southern Oscillation Index (SOI) and Indian Ocean Dipole (IOD) stand out as quite influential, particularly for synoptic types originating from the NE and NW – these are also directions that can bring greater rainfall totals (because you have a more tropical influence and generally more moisture) and warm rain on snow events, so may drive larger run-off. Positive SOI and negative IOD events are associated with greater moisture transport to the Snowy Mountains. The Southern Annular Mode (SAM) is very influential for synoptic types originating in the Southern Ocean, so, in particular, the cold fronts and cut off lows during winter. In addition, a positive SAM allows for the influx of tropical moisture. I would definitely keep track of the Pacific Decadal Oscillation (PDO) too because that will give you an idea of whether we’re in a longer period of wetter or drier conditions, but also whether the shorter ENSO cycle is likely to be modulated or not (by that I mean enhanced or suppressed depending on whether the two cycles are in sync or not).

Irrigation in the Murrumbidgee Irrigation Area (image courtesy Colly irrigation)

The PDO analysis from this analysis is intriguing. Can you explain the results in the wavelet analysis (in the journal article) of your various climate drivers?

The wavelet analyses were chosen as a way to look at how the influence of each teleconnection on synoptic type frequency varies through time. The results show that the PDO exerts an influence on synoptic types, particularly those originating from the NE and Southern Ocean at inter-decadal timescales, so between 16-32 years. This is an anti-phase influence (the arrows point to the left), which means that the frequency of those weather types increases when the PDO is in its negative phase and vice versa. This in turn means that more rainfall would be expected from those weather types when the PDO is negative. And we know from the literature that, in much the same way that La Nina generally brings wetter conditions, so too does a negative PDO. Interestingly, the PDO plots in Figure 6 in the paper also show some coherence at shorter timescales (4-8 years), in particular with the NE synoptic types, and this is interpreted as demonstrating the modulating effect of the PDO on the ENSO cycle at those shorter timescales. For instance, when there is a La Nina and a negative PDO, rainfall tends to be enhanced even more. Whereas, if those two teleconnections were out of phase (one in its wet phase, one in its dry phase), the result can be suppression of rainfall.

The trend analysis on synoptic type shows rain events becoming heavier and distribution changing from winter to summer dominant patterns. This has implications for water inflows, water markets and cropping. Is it fair to say that theoretical responses to a changing climate seem to be already at work in this particular location? (I.e. tropics moving further south)

Yes, I think that would be a fair conclusion to draw. Our work showed an increase in tropical moisture, particularly along the north-west pathway, over the past few decades, which fits with rain events becoming heavier (due to more moisture-laden tropical air). In addition, the trend towards a more positive SAM means that winter rainfall derived from the passage of cold fronts over the region has declined and is likely to continue to do so in future.

With a La Niña event now strengthening in 2020-21, based on historical analysis undertaken, the run-off and dam inflow in the Snowy catchments are likely to be further enhanced – even with a neutral Indian Ocean Dipole currently decaying?

Yes, that is likely. Climatology shows that atmospheric moisture content generally increases during a La Niña event, especially in tropical latitudes which are the source of moisture of most rainfall events that generate run-off and inflows in the Snowy catchments. The warmer, tropical air also generates warm rain-on-snow events, which can lead to increased melting of the snowpack. Although the Indian Ocean Dipole is currently neutral, if warmer sea surface temperatures occur off the coast of northwest Western Australia in conjunction with a La Niña event, this may also result in increased summer rainfall across southeast Australia due to advection of tropical moisture along trough lines.

Any questions, feedback or want to read a copy of Ali’s paper?

Email us at: farmerforecast@agecon.com.au

Iconic location of Craig’s Hut in the Snowy mountains (image courtesy Ken Duncan).