Advancing prediction capability by understanding the links between anticyclones and high impact weather in the tropics

Forecasters in Africa routinely look out for the position, strength and orientation of anticyclones when they want to forecast high impact weather such as heavy precipitation. However, anticyclones have not been studied extensively by researchers, and they are not well understood. This project aims to create a theoretical understanding of how anticyclones influence severe weather over Africa, and to use this understanding to improve forecasting methods for the continent.

We want to investigate the link between anticyclones and high impact weather. We want to use this understanding to challenge and develop the next generation of AI-based global weather predictions, in order to improve daily and weekly forecasts for Africa. New AI models show excellent promise to be run operationally and therefore this project will provide a bridge between operational weather prediction, theory of climate dynamics, and AI-based modelling.

The project will address the following research questions:
1. What is the climatology (track, location, speed, strength) of anticyclones at lower and upper levels across the tropics?
2. Which role do anticyclones play in the formation of high impact weather events?
3. What controls the strength of the anticyclones?
4. How well are we able to forecast these links using state-of-the-art km-scale Met Office model simulations, and the new generation of ML-based models?

The proposed project plan:

Year 1: Characterising anticyclones
We will start by tracking anticyclones in 40 years of ERA5 reanalysis data using mean sea level pressure (MSLP) as well as potential vorticity (PV) at upper levels. Based on this data set we will characterise anticyclones based on their size, intensity, speed, orientation, seasonality, and location to produce a global climatology. We will then use a precipitation data set to investigate the link between anticyclones and heavy precipitation. We want to understand whether the role of the anticyclones is entirely passive and the “active” contribution to rainfall formation comes from the low-pressure systems, or if part of the role of the anticyclones is to transport moisture toward the equator, thus contributing to the formation of heavy rainfall. Several studies have shown that midlatitude Rossby waves in the Northern and Southern Hemisphere can play an important role in the formation of heavy rain in Central Africa and Northern Australia. We will investigate the mechanism at play here. This study will provide forecasters in the tropics with the underpinning knowledge for what they have been seeing in charts for many years and hopefully provide them with additional information that will facilitate forecasting high impact weather in the tropics.

Year 2: Controls of the strength of anticyclones
The climatology of MSLP shows that the strongest anticyclones occur in the summer hemisphere. However, we know that the strongest descending branch of the regional Hadley circulation occurs in the winter hemisphere. This conundrum is a hindrance to our understanding of anticyclones. We will investigate what controls the strength of the anticyclones, carrying out this analysis on ERA5 data. The tools we will use are the circulation budget and the omega equation.

Year 3: Predictability and scale interactions
After gaining a more fundamental understanding of the dynamics and the role of anticyclones in driving high impact weather in the tropics we want to investigate how well these processes are represented in cutting-edge Met Office simulations, and in new generation AI-based forecast models. In addition, we want to investigate at which temporal and spatial scales the greatest predictability lies in representing these links.

There will be the opportunity to visit the Ghana Meteorological Agency, in addition to regular online discussions. We will be collaborating with Tess Parker, an expert in anticylones, and Michael Barnes, an expert in midlatitude dynamics; both work in Australia.