African Easterly Wave Characteristics: Climate Variability and Trends

The frequency of extreme Sahelian rainstorms has been shown to have more than tripled in the past few decades. Along with the increasing occurrences of extreme events, the mean intensity of these storms has also increased. These changes are shown to be correlated with an enhancement of baroclinicity over the Sahel and an increase in synoptic activity due to anthropogenic warming. Although African Easterly Waves (AEWs) are the dominant synoptic weather systems and modulate rainfall over the region, those changes, nonetheless, have hardly been studied in the context of AEW variability such as intensity and frequency. Understanding the connection between the extreme rainfall events and synoptic AEWs on the decadal timescale could improve our fundamental knowledge of the climate system and provide guidelines for the predication of extreme rainfall events. The variability of AEW also has implications for tropical cyclone activity over the Atlantic. Therefore, the objective of this study is to explore the variability of AEW characteristics on decadal timescales and to investigate the possible connection between such variability and the frequency of extreme rainfall events.

This study uses dynamical fields from the ERA-Interim reanalysis. As a proxy for convection and rainfall intensity, we utilize brightness temperature and rainfall estimates from satellite observations. AEWs are represented by tracking individual vorticity maxima. We use curvature vorticity at multiple levels to account for the variability of AEW structures and also to ensure the algorithm tracks coherent waves. Basic AEW characteristics such as phase speed, scale and frequency are calculated. In addition, the intensity of the waves is measured in both curvature vorticity and rainfall to capture different behaviors. Time series of these characteristics are constructed to detect long-term trends. Preliminary results show changes of AEW frequency, although the signs are sensitive across regions, suggesting a shift of preferred storm tracks.

The second part of the research will attempt to explore changes in wave behaviors under the changing environmental conditions due to global warming. For example, studies have shown that the Sahara is warming more rapidly than the rest of Africa and this has caused an increase in the meridional temperature gradient. This enhanced baroclinicity and the associated strengthening of the mid-level easterly jet could change the characteristics of AEWs. It could be hypothesized that those favorable conditions strengthen the average intensity of AEWs and contribute to the increase of extreme rainfall events. The results from this study are expected to improve our understanding on AEW characteristics in a changing climate. Such knowledge could shed light on the projections of organized convection and extreme rainfall events that current climate models can not explicitly resolve.