Extratropical Continental Heatwaves in a Changing Climate


Climate change is the critical challenge facing humanity. The impacts of global warming will predominantly be experienced on continents, where the rapidly increasing risk of severe heatwaves represents an existential threat to societies and ecosystems. The exceptional summers of 2018 (the UK’s joint hottest on record) and 2019 (which featured the UK’s hottest-ever day) represent ominous portents of the future.

Fundamental understanding of continental heatwaves remains limited, despite the severe impacts on human health, wildfire risk and food production. Average warming over continents is up to 60% larger than over oceans (Sutton et al, 2007), and this enhanced warming is tightly linked to ocean temperatures and land aridity (Byrne & O’Gorman, 2018). But how climate change will affect extreme continental temperatures and heatwaves is highly uncertain, particularly in the extratropical regions poleward of 30N/S that are home to a large portion of the world’s population.

The key hypothesis underlying this project is that the response of extratropical continental heatwaves to climate change is controlled by ocean temperatures and land aridity. Moisture transport and convection in the atmosphere link the surface climates over continents and oceans, such that energy profiles over continents and oceans are tightly coupled (Byrne & O’Gorman, 2013). This energy constraint underpins recent quantitative theories for how average and extreme tropical continental temperatures respond to climate change (Byrne & O’Gorman, 2018; Byrne, 2021). Yet the potential of this energy constraint to unlock the problem of extreme extratropical temperatures in a changing climate is untested and unknown.

The goal of this IAPETUS2 project is to transform understanding of extratropical heatwaves in a changing climate. The project will build on recent studies – led by Dr Michael Byrne – of tropical temperatures in a changing climate (Byrne & O’Gorman, 2018; Byrne, 2021), and will combine new climate simulations and theory to tackle three key objectives:
A. Determine the relative influences of ocean temperatures vs land aridity in controlling extratropical continental heatwaves using a novel set of climate simulations.
B. Develop the first quantitative theory for extratropical continental heatwaves.
C. Diagnose the physical drivers of uncertainties in extratropical heatwave projections across state-of-the-art climate models.


To address Objective A, climate simulations will be used to quantify how extratropical continental heatwaves are influenced by (i) oceans – specifically surface temperature variability – and (ii) land aridity. To isolate these ocean vs land influences on heatwaves, a unique suite of simulations will be performed using the Isca climate model. Not only will these innovative simulations advance fundamental understanding of heatwave dynamics, they will also develop valuable skills for the PhD student in coding, scientific modelling and high-performance computing.

The simulations designed to address Objective A will establish the mechanistic understanding required to derive the first quantitative theory for extratropical continental heatwaves (Objective B). The new theory will build on influential theories for tropical continental temperatures (Byrne & O’Gorman, 2018; Byrne, 2021) and combine dynamic and thermodynamic influences on heatwaves into a unified framework.

In the final year of the project, physical insights into the processes controlling extratropical heatwaves developed during Objectives A and B will be applied to understand the drivers of heatwave changes and uncertainties in state-of-the-art climate model projections (Objective C). The newly-released CMIP6 simulations (Eyring et al, 2016) represent the best estimates of how climate will change in the future, though heatwaves projections remain highly uncertain. A necessary first step towards more reliable heatwave projections is to identify and quantify the physical origins of uncertainty across the CMIP6 simulations. Differences in land aridity dominate uncertainty in projections of average continental warming (Byrne & O’Gorman, 2013), and the hypothesis here is that aridity is also the main driver of extratropical heatwave uncertainty – Objective C will systematically test this hypothesis.

Project Timeline

Year 1

Year 1 will involve a literature review to allow you to develop understanding of heatwave dynamics and physical aspects of climate change. The ingredients of a new theory for continental heatwaves will also be built up. You will also start using the Isca climate model and setting up the simulations of continental heatwaves in a changing climate.

Year 2

Year 2 will focus on (a) running the set of idealised climate simulations to investigate continental heatwaves in different climates and (b) using the simulations to test and refine the new theory for heatwaves. You will draft a research article on this portion of the project and present the key results at an international conference, potentially the American Geophysical Union’s annual meeting in San Francisco.

Year 3

Year 3 will involve applying the insights into heatwave dynamics developed in Years 1 and 2 to state-of-the-art climate simulations from the CMIP6 archive. This will involve substantial Big Data analyses with the objective of narrowing the large uncertainty in future heatwave projections. You will attend and present your research at an international conference on extreme weather events.

Year 3.5

Year 3.5 will focus on writing the PhD thesis and drafting a research article on the analyses conducted during Year 3.

& Skills

The student will be trained on several aspects of physical climate science including atmospheric dynamics, heatwaves, climate modelling and climate change. The student will also be trained in highly sought-after technical skills in computational modelling, high-performance computing, and Big Data analyses.

References & further reading

[1] Byrne & O’Gorman (2013): Land-ocean warming contrast over a wide range of climates: convective quasi-equilibrium theory and idealized simulations,
Journal of Climate, vol. 29, pp. 9045–9061.[2] Byrne & O’Gorman (2018): Trends in continental temperature and humidity directly linked to ocean warming, Proceedings of the National Academy of Sciences, vol. 115, pp. 4863–4868.[3] Byrne (2021): Amplified warming of extreme temperatures over tropical land, Nature Geoscience, in press, (pre-print available at https://www.essoar.org/doi/abs/10.1002/essoar.10505497.1)[4] Fischer & Knutti (2015): Anthropogenic contribution to global occurrence of heavy-precipitation and high-temperature extremes, vol. 5, pp. 560–564.[5] Sutton et al (2007): Land/sea warming ratio in response to climate change: IPCC AR4 model results and comparison with observations. Geophysical Research Letters, vol. 34, L02701.

Further Information

For further information, please contact Dr Michael Byrne by email mpb20@st-andrews.ac.uk.

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