Three-Dimensional Structure of African Easterly Waves based on Empirical Orthogonal Functions

The three-dimensional structure of African Easterly Waves (AEWs) is revisited with an Empirical Orthogonal Function (EOF) approach. The structure of AEWs is extracted by projecting the wind fields from reanalysis data and satellite-derived brightness temperature onto the principle components associated with EOF patterns of appropriately filtered brightness temperature and circulation parameters. This EOF approach isolates the most dominant wave structure and through lagged regression analysis we can explore the associated evolution of the AEWs.

In this talk we present results of EOF analysis conducted with two representative variables of AEWs, 700 h Pa meridional wind and brightness temperature. The EOFs explored in this study are found to be physical and robust as determined by a series of domain sensitivity tests.

Consistent with the literature, the meridional wind EOF reveals a meridionally broad circulation associated with AEWs. In addition, as shown by Kiladis et al (2006), a distinct evolution of convection within the wave is observed at 10°N: convection occurs in northerly flow over East Africa, shifts into the trough around the Greenwich and eventually into southerljes behind the trough as the wave moves over the Atlantic. However, a detailed “checkerboard” pattern of convective regions is also observed, which has not clearly been seen in previous studies. The brightness temperature EOF also shows a similar evolution of convection relative to the wave, but the circulation associated with it is mostly confined to south of the African easterly jet and is more strongly coupled with convection. These findings motivate more in-depth investigation of the variability in AEW structures using an EOF approach.

This study highlights the advantage of EOF approach to document AEW structure and also its capacity to identify different characteristics of AEWs. We will pursue this approach in future work to shed light on the nature and causes of AEW-variability.