An unusual dry-season precipitation event over West Africa: The role of an extratropical upper-level disturbance for the heat low and a surge in the monsoonal southwesterlies

Between 20 and 22 January 2004 unusual dry-season precipitation was observed over large parts of tropical West Africa including the countries of Ivory Coast, Burkina Faso, Ghana, Togo, Benin, and Nigeria. Precipitation amounts exceeded 40 mm at numerous stations between the Guinea Coast and as far north as 10°N where the long-term January mean is between 1 and 5 mm. At these latitudes, soil moisture recovered from extremely low dry season values and pastures in the open tree savanna greened for weeks after the event. Surface observations between 17 and 20 January reveal unusual hot temperatures and low pressures over southern Burkina Faso and a conspicuous northward excursion of the Intertropical Discontinuity (ITD) from about 7ºN to 13ºN. These observations indicate intensification and a northward shift of the heat low and an unseasonal poleward penetration of moist, monsoonal southwesterlies to the southeast of the heat low. The unusual behavior of the heat low is associated with the penetration of an upper-level trough from the extratropics to low latitudes over North Africa. ECMWF re-analysis data and water vapor images suggest upper-level subsidence and dryness to the south and west of the disturbance. We will discuss possible mechanisms for a linkage between the upper-trough and the low-level heat low such as: (1) High solar irradiation due to low aerosol, water vapor, and cloud content in the subsided air; (2) suppression of low-level vertical mixing through a stabilization of the atmosphere; (3) and advection of adiabatically warmed air from the north. On 20 and 21 January diurnal solar heating and low-level (moisture) convergence along the northward shifted ITD promoted the growth of cumulonimbus clouds that merged to large and long-living convective clusters. Their organization and maintenance during nighttime hours was aided by low inertial stability at the level of the strong subtropical jet to the east of the trough, which allowed a quick horizontal displacement of the convective outflow. On 21 January low-level convergence was augmented by a Harmattan surge following the upper-trough. This study reveals a complex interplay between the extratropical upper-level disturbance, the near-surface wintertime heat low dynamics, and the vertical coupling of the lower and upper troposphere by deep convection.