Modulation of Precipitation over West Africa by Equatorial Waves

Equatorial waves often couple with deep convection and this way modulate rainfall on the synoptic to subseasonal timescale throughout the tropics. Until now, however, no comparative study of the influence of all the different wave types on precipitation has been performed, specifically for West Africa.

To fill this gap, the following wave types are analyzed for the extended West African monsoon seasons April to October: (1) the Madden-Julian Oscillation (MJO), (2) Kelvin waves, (3) equatorial Rossby waves, (4) eastward-propagating inertio-gravity waves, (5) mixed Rossby-gravity waves and (6) tropical disturbances/African Easterly Waves. The different wave types are filtered in the wavenumber-frequency domain using outgoing longwave radiation. Eight different wave phases are defined from a phase diagram calculated from the time-derivative of the filtered wave signals following Yasunaga and Mapes (2012). Subsequently, composites of dynamical and thermodynamical fields for each wave phase of the different wave types are produced using the ERA Interim reanalysis from the European Centre for Medium-Range Weather Forecasts. Finally, the influence of equatorial waves on West African precipitation is quantified using three rainfall datasets: (1) the gauge-adjusted three-hourly combined microwave-infrared estimates from the Tropical Rainfall Measuring Mission (TRMM) 3B42 V.7 for the period 1998–2016, (2) the Karlsruhe African Surface Station Database, which provides daily data from an extended rain gauge network for a period from 1979 to 2013, and (3) the gauge-calibrated, infrared based Climate Hazards Group InfraRed Precipitation with Station data V.2 (CHIRP/S) precipitation estimation, also for the period from 1979 to 2013.

All aforementioned wave types significantly influence West African rainfall and dynamics. The analysis of upper- and lower-level divergence, wind, and geopotential height largely confirms the structural characteristics expected from wave theory. The influence of the MJO and equatorial Rossby waves reaches far into the subtropics due to an interaction of the tropical and extratropical regimes. Station and satellite data confirm the influence on local precipitation, although significant differences exist in how strongly the wave types couple with precipitation and on which time scale. All equatorial waves – except the eastward-propagating inertio-gravity waves – modulate environmental conditions known to determine the frequency of occurrence, the intensity, and degree of organization of precipitation systems over West Africa. Favorable conditions for organization of convection prevail in Kelvin waves, African Easterly Waves, and the MJO. Contrarily, equatorial Rossby waves and partially the mixed Rossby-gravity waves tend to trigger large-scale rainfall by dynamic lifting.

The results emphasize the importance of a correct representation of equatorial waves in numerical weather prediction products for West Africa. Additionally, the statistical relationships found here indicate that statistical forecast models can be improved by including equatorial wave activity.