Inter-Annual Variability and Large-Scale Controls of African Easterly Wave Activity

Over 70% of all Atlantic tropical cyclones (TCs) are directly or indirectly related to African Easterly Waves (AEWs) (Russell et al. 2016). That same study noted a link between the interannual variability of AEWs and Atlantic TC activity. In addition, AEWs have wide ranging effects on the weather and climate of sub-Saharan Africa. It is therefore important to understand the physical mechanisms that control their year-to-year variability.

Here we define AEW activity using 2-10 day filtered eddy kinetic energy (EKE). We examine the interannual variability of AEWs using empirical orthogonal functions (EOFs) of the seasonal mean EKE fields. The leading modes are regressed against various climatological indices and variables including El Nino Southern Oscillation (ENSO), sea surface temperatures (SSTs), large scale wind patterns, and the genesis numbers (GNs) presented in Russell et al. (2016). We also use a composite analysis method to examine if there are differences between the structure of AEWs during years when the leading modes of variability are at their respective peaks.

Preliminary results indicate that the three leading EOF modes represent around 50% of AEW variability. The EOFs highlight regional shifts and amplification/suppression of AEW activity in response to the large scale forcing. The leading mode of variability, which accounts for around 30% of the total, is strongly linked to ENSO, and moderately correlated with GN. These, and other results, will be described in our presentation.