Understanding impacts of climate change on the tropical rain belt and its variability over Africa

This project will aim to understand the processes controlling climate change in the tropical rain band over Africa (often referred to as the “inter-tropical convergence zone”), how this is represented in models and the implications for climate projections.

Motivation

The tropics cover 40% of Earth’s area, including 36% of its land, and are home to 40% of its people. Africa is the continent with the largest tropical land mass, and its rapidly growing population includes many of the people who are most vulnerable to climate change. Rainfall in the tropics is dominated by the tropical rain-band that encircles the earth (Figure 1). To first order the rain-band tracks the seasonal variation in solar heating, reaching its northernmost extent in the northern hemisphere summer, and its southernmost extent in northern hemisphere winter. As a result of this movement, rainfall from the tropical rain-band directly affects not only the lush tropical forests of the Congo, but also regions such as the southern reaches of the Sahara. The rain-band’s movement leads to the annual West African monsoon rains on which populations in the Sahel depend, and the bi-annual rains of East Africa, which vary enormously across the region. Regional climate projections of rainfall in the tropics are highly uncertain and in many regions we do not know if it will get wetter or drier. This PhD will tackle this fundamental problem using a range of state-of-the-art global and regional models and observations to understand how climate change affects the tropical rain-belt, with a focus on Africa.

Figure 1: The tropical rain belt over West Africa and the Atlantic (image credit: NOAA NWS National Hurricane Centre).

Air ascends in the tropical rain-band, causing clouds and rain, and descends further north or south, causing the great deserts of the world, including the Sahara. Together these form the important large-scale pattern of circulation known as the “Hadley circulation”. Even in simple configurations of climate models without continents there is a diverse response of the tropical rain-band to climate change, which carries through to simulations of the real world. Understanding these changes and the processes governing them is made much more challenging by the fact that the cumulonimbus storms that generate ascent and rainfall in the tropics are not explicitly resolved in global models, which have grid-spacings of approximately 100km and means their effects are normally represented by simplifications known as parametrisations. Recently the £20 million Future Climate for Africa (FCFA) programme has run the first simulations, referred to as “CP4A”, that have a small enough grid-spacing (approximately 4km) to explicitly capture these storms, and these are now being complemented by new larger-domain shorter-duration simulations. This is a step-change in modelling and the high resolution of such runs fundamentally changes the representation of convection and rainfall, and so provides a unique opportunity for new understanding. This PhD will examine a range of models to generate new understanding of changes in the tropical rain-band and identify the main drivers, with assessment against relevant observations to evaluate models and processes.

 

Objectives

The project will address the following scientific objectives: (1)  How will climate change affect the tropical rain-band, especially over Africa, and how does this depend on internal atmospheric processes, and coupled processes such as land and ocean change? (2)  How does an explicit representation of convective processes affect projected rain-band changes? (3)  How do changes seen in models relate to those observed in reality, and can this be used to constrain the diverse predictions from models?

Depending on the interests of the candidate, further objectives could include: (i) identification of the role of non-greenhouse gas forcings such as aerosols and land-use change; (ii) investigation of changes in inter-annual, or intra-annual, variability, in mean rainfall or extremes; (iii) investigating the relationship between the position, intensity and size of the rain-band and the width of the tropical belt (i.e. locations of subtropical dry zones).

These objectives will be tackled using the latest observations and a range of state-of-the-art models, including reanalyses (a blend of observation and model), simulations of past and future global climate from the latest climate models (the Sixth Coupled Model Intercomparison Project), and recent/ongoing convection-permitting UM simulations.

 

Your skills and Training

You will benefit from the training offered by the Panorama DTP. You will also benefit from expertise at the Hadley Centre, by working with a Met Office supervisor. The project will capitalise on the recent production of CMIP6 simulations from world-leading centres, as well as state-of-the-art convection-permitting simulations, and you will learn to handle such large datasets using modern programming tools and techniques.

 

Wider Opportunities

By joining the Institute for Atmospheric Science within the School of Earth and Environment, you will be joining a large, diverse and vibrant group of researchers, including a sizeable group working on tropical dynamics, with a track record of leadership in major projects such as HyCRISTAL, AMMA2050 and GCRF African SWIFT. This track record was recently recognised in a prestigious 2021 Queens Anniversary Prize. Leeds is home to the headquarters of the National Centre for Atmospheric Science, a centre distributed across leading UK universities. The Priestley International Centre for Climate and water@Leeds provide opportunities for wider collaboration across campus. Finally, ICAS and the universities’ formal partnership with the Karlsruhe Institute for Meteorology (KIT, Germany) provide the opportunity for collaboration with another leading group in tropical meteorology. Although fieldwork is not needed, it is possible that opportunities for fieldwork will arise during the PhD, alongside the usual opportunities for conferences and meetings.