Variability of African Easterly Wave Structures

In this study, we explore the structural variability of African easterly waves (AEWs) and their interaction with equatorial and extra-tropical waves. An Empirical Orthogonal Function (EOF) analysis was applied on appropriately-filtered fields to isolate the dominant wave patterns. Three variables were used with EOF analysis; brightness temperature derived from satellite observation (T_b EOF) and meridional wind from reanalysis data at 700 (v700 EOF) and 200 hPa (v200 EOF). The structure of the waves is extracted by projecting the fields of interest onto the principle components of the EOF patterns.

The T_b EOF shows a confined AEW circulation centered around the ITCZ and characterized by more precipitation compared to climatology. In striking contrast to the T_b EOF, the v700 EOF is distinguished by a meridionally broad AEW circulation that appears to interact with equatorial and extratropical waves. In particular, while the peak in circulation is centered at 10°N, there is marked cross-equatorial flow that suggests the presence of a mixed Rossby-gravity wave (MRG) structure. A distinct upper-level MRG structure is also captured by the v200 EOF. The associated lower-level structure resembles an AEW-like flow pattern with an anomalously large wavelength. The upper-level MRG also appears to be closely tired to the mid-latitude Rossby waves in the Southern Hemisphere.

Our results highlight the pronounced structural variability of AEWs and their interaction with other synoptic waves. This suggests the variability of AEW structure and activity could be modulated by, in addition to the large-scale environment, other synoptic waves in the region.