IAP-26-014

Atmospheric circulation over land in a changing climate

The atmospheric circulation – i.e., the movement of air zonally, meridionally, and vertically – shapes Earth’s climate on large spatial scales and long timescales through its transport of heat, water, and momentum. Circulation also shapes local, day-to-day weather and its variability. Understanding and predicting the atmospheric circulation is a crucial scientific question – particularly over land regions – due to its influence on rainfall patterns, hot and cold temperature extremes, wind-related hazards, and wind-sourced renewable energy.

The atmospheric circulation is changing as the climate warms, driven in part by changing temperature gradients associated with spatial warming patterns (e.g., polar-amplified warming, land-ocean warming contrast). These circulation changes strongly modulate regional responses to global warming. Yet future circulation changes are highly uncertain, with different climate models projecting a wide range of possible responses (Xie et al 2015). As a result, atmospheric circulation is a major source of uncertainty in climate projections (Shepherd et al 2014), hampering efforts to effectively mitigate and adapt to a warming climate. This limited knowledge of circulation changes contrasts with well-established understanding of other aspects of land climate change, including temperature and humidity responses (e.g., Byrne & O’Gorman 2018).

Transforming understanding of the atmospheric circulation in a warming world is one of the “grand challenges” in climate science (Bony et al 2015). This project will examine changes in atmospheric circulation over land using numerical simulations from a hierarchy of climate models. Land circulation systems across a range of scales will be analysed including large-scale divergent flows (intertropical convergence zone, Hadley and Walker circulations), zonal jets (midlatitude and African easterly jets), weather regimes (Lee et al 2023), and regional-scale mesoscale convective systems (Taylor et al 2026). The goals are to: (i) identify robust circulation responses across the simulations; (ii) develop physical understanding of the circulation changes based on principles of atmospheric dynamics and thermodynamics; and (iii) identify the processes driving uncertainty in circulation responses across climate models. The project will advance fundamental understanding of the atmospheric circulation over land, an essential step towards more reliable climate projections at the regional scale.

The project will utilise three sets of climate simulations to investigate three science questions:

1. Robust responses of atmospheric circulation over land to climate change: Using a 100-member ensemble of simulations from the CESM2 global climate model, changes in atmospheric circulation between historical and future periods (under the SSP370 scenario) will be quantified. Focus will be on identifying circulation changes that are radiatively forced and therefore robust across ensemble members. For the robust changes identified (e.g., weakening of the African easterly jet), conceptual models based on established atmospheric dynamics (e.g., thermal wind) will be developed to explain the physical processes underpinning the responses.

2. Fast versus slow drivers of land circulation changes: In response to radiative forcing from greenhouse gases, the climate system evolves on fast and slow timescales (Bony et al. 2013). The ‘slow’ response is mediated by sea-surface temperature (SST) changes over decades and longer, whereas the ‘fast’ response occurs quickly via rapid, greenhouse gas-driven warming of the land surface and adjustments to clouds and atmospheric cooling. To decompose the fast versus slow drivers of land circulation changes, two sets of simulations from CFMIP (Webb et al. 2017) with prescribed SSTs will be analysed. First, changes between a control simulation with historical SSTs (amip) and a perturbed simulation with prescribed +4K SST warming (amip-p4k) will isolate circulation changes driven by the slow SST response to forcing. Second, changes between the amip control and a perturbed simulation with quadrupled CO2 concentration but fixed SSTs (amip-4xCO2) will isolate the role of fast processes in controlling the circulation responses. Together these analyses will reveal the drivers of robust land circulation changes across regions, and the timescales over which these changes are expected to emerge.

3. Surface-to-atmosphere influences on circulation over land: Recent work has demonstrated the key role of soil moisture in modulating overlying mesoscale atmospheric circulations (Barton et al. 2025; Taylor et al. 2026). Yet how this coupling between soil moisture patterns and circulation evolves as the climate warms is an open question. Using state-of-the-art, high-resolution, convection-permitting simulations, the response of mesoscale circulations over land to a warming climate will be quantified. The influence of changes in soil moisture on the circulation responses will be investigated using recent ideas linking soil moisture, temperature gradients, and wind shear (Barton et al. 2025), with the goal of developing a comprehensive understanding of how regional-scale land surface processes shape the dynamics of the atmosphere above, including high-impact events (e.g., severe thunderstorms).

All project analyses will be performed on JASMIN, the UK’s data analysis facility for environmental science, using Python or a similar high-level programming language.

Year 1

Year 1 will involve a literature review to allow the student to develop understanding of atmospheric circulation and physical aspects of climate science. The student will also work with the CESM2 large ensemble data to identify and interpret robust circulation changes over land. The student will attend the annual UK National Climate Dynamics workshop to broaden their understanding of current research in the area and network with other researchers.

Year 2

Year 2 will conclude analyses of the CESM2 data and write-up that research for submission to a peer-reviewed journal. Analyses of the CFMIP simulations to decompose land circulation changes into fast and slow response will begin. The student will present key project results at the European Geosciences Union annual meeting in Vienna.

Year 3

Year 3 will focus on analysing the high-resolution simulations to understand the role of soil moisture in mesoscale circulations. The student will attend and present the research at an international workshop on high-resolution modelling.

Year 3.5

Year 3.5 will focus on writing the PhD thesis and drafting research articles on the analyses conducted during Years 2 and 3.

The project student will be trained in key aspects of physical climate science including atmospheric dynamics, general circulation of the atmosphere, land-atmosphere coupling, and climate change. The student will also be trained in highly sought-after technical skills in high-performance computational analysis.