Three-Dimensional Structure of Synoptic Waves over West Africa 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 both kinematic and thermodynamic variables to examine the characteristics and variability of AEWs.

Consistent with the literature, a distinct evolution of convection within the AEW is observed with regression maps based on EOFs. However, what previous studies have not clearly shown is a detailed “checkerboard” pattern of convective regions. Our analysis reveals that, in addition to the peak at the intertropical convergence zone, a secondary maximum of precipitating area in the Sahel (20N) is linked to the enhancement of moisture advection from the low-level northern vortex growing at the baroclinic zone. Perhaps the most striking feature in our result is the meridionally broad circulation tied to AEWs in EOFs of meridional wind. While both circulation pattern and energetics demonstrate the dominant AEW structure centered around 10N, the cross-equatorial flow, antisymmetric geopotential signature as well as the associated kinematic fields suggest a mixed Rossby-gravity wave (MRG) straddling at the equator. Granted that the vast majority of studies on MRG focus on the central and western Pacific region, this “hybrid” kind of AEW and MRG over West Africa and the Atlantic has received little attention and more work regarding the nature and causes of this hybrid wave structure is needed. In contrast to the meridionally broad circulation in the kinematic EOFs, the brightness temperature EOF shows a confined circulation centered around 7.5N, in line with the climatologically most active region of 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 the wave structure over West Africa and also its capacity to identify variability of AEWs. This method can be further applied to numerical models to evaluate their ability to capture the AEW structure and activity and therefore shed light on the variability of Sahel rainfall and tropical cyclone activity.