A Systematic Comparison of Tropical Waves over Western and Eastern Equatorial Africa

A systematic comparison of tropical waves over western and eastern equatorial Africa

Fink¹, Andreas H., Andreas Schlueter¹, Roderick van der Linden¹and Joaquim G. Pinto¹.

¹Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany

Low-latitude rainfall variability on the daily to intraseasonal timescale is often related to tropical waves, including convectively coupled equatorial waves, the Madden-Julian Oscillation (MJO), and tropical disturbances (TDs). Despite the importance of rainfall variability for vulnerable societies in tropical Africa, the relative influence of tropical waves for this region is largely unknown. This contribution is a follow-up study using the approach described in Schlueter at al. (2019a,b, J. Climate) for northern tropical Africa and presents the first systematic comparison of the impact of six types of tropical waves on precipitation over western (WEA) and eastern equatorial Africa (EEA), including the associated changes to atmospheric moisture budget terms. The study comprises the two rainy seasons in equatorial Africa, the March–May (MAM) and October–December (OND) seasons and used, apart from the ERA-Interim reanalysis, two satellite-based rainfall products, viz. Tropical Rainfall Measuring Mission (TRMM) and (Climate Hazards InfraRed Precipitation with Stations (CHIPS) data sets, and a relatively dense rain gauge network in EEA only. Tropical wave activity is considered in the 5°N–5°S equatorial belt.

The study documents for the first time a strikingly different importance of the investigated six types of equatorial waves for rainfall variability in WEA and EEA. West of the western Rift mountains, over the Congo Basin, Kelvin waves and TDs dominate precipitation variability at the daily timescale, whereas over the East African Highlands, the slower modes, viz. the MJO and the Equatorial Rossby (ER) waves dominate at the weekly timescale. Over the adjacent tropical eastern Atlantic Ocean, wave activity is higher during MAM due to higher sea surface temperatures and related heavy oceanic rainfall. Consistent with earlier studies, the same statement holds for EEA, whereas in WEA wave activity is strongest in the second, moister rainy season in OND, especially at the Atlantic coast. Comparisons between the different rainfall datasets show very consistent results between TRMM and CHIRPS, whereas the station data indicate similar wave-related fluctuations, but higher mean rainfall during MAM in EEA.

Maps of variations in rainfall, moisture flux and moisture flux convergence, as well as columnar precipitable water (PW) during eight wave phases reveal that the rainfall modulation due to the waves is related to anomalies in the moisture flux and flux convergence. The wet MJO phase in EEA is associated with anomalous low-level westerlies over the Congo Basin after the MJO left Afrika towards the Indian Ocean. Yet, easterly anomalies at the East African coast lead to wetter conditions before the wet MJO phase overpasses the region. Over the EEA highlands, wetter conditions persist after the peak wet phase of the MJO. Westerly anomalies over the Congo Basin are related to an ER wave before its wettest phase crosses the EEA region. The modulation of rainfall by the ER waves is surprisingly high given the equatorial location. It appears that the slow MJO and ER waves lead to a moistening of the EEA highlands, potentially then favoring outbreaks of widespread convection in the course of the afternoon.

The Kelvin waves also modulate low-level moisture fluxes and convergence over the Congo Basin, but the impact on PW is less pronounced than in the MJO and ER case for EEA. Not surprisingly, the wet phase of the Kelvin waves are associated with anomalous convergence in enhanced low-level easterly flow over the Congo Basin. While rainfall fluctuations are strong and significant over the Congo Basin in the TD band, associated changes in the atmospheric moisture fields are mostly non-significant. Both, the origin of the variance in the TD band, i.e. African Easterly waves vs. independently westward moving mesoscale convective systems, and the thermodynamic and dynamic reasons for rainfall modulations, remain elusive. Whereas Mixed Rossby Gravity waves show no discernible signal, Eastward-Inertio Gravity (EIG) waves show significant, anti-symmetric rainfall anomalies straddling the equator. To the best of the knowledge of the authors, the importance of EIG waves for rainfall in the Congo basin has not been shown before. Like in the TD case, no related signal in the investigated atmospheric moisture variables is discernible.

The present study hints at significant, yet different roles of tropical waves over the Congo Basin and the East African Highlands, the demarcation line being the foothills of the western Rift mountains. The bulk of the rainfall modulation can be understood by modifications in the moisture content due to low-level wind changes, yet the causes of large, coherent and significant rainfall variations in the TD and EIG bands are unrelated to this. Irrespective of this, the large-scale wave controls on convection in the entire African equatorial belt suggest potential in their use for rainfall forecasting.