Dynamics of the West African westerly jet and its association with Sahel precipitation

There are two distinct low-level westerly flow regimes important for transporting moisture into West Africa during the summer. One is the westerly component of the monsoon flow, which results from the Coriolis acceleration acting on the southerly flow across the Guinean coast that is driven by land/sea contrast. The other is the westerly flow over the eastern Atlantic and West African coast around 10°N, which has been identified by satellite observations as a jet. Using the high-resolution ERA40 reanalysis, ERA-Interim reanalysis, and CRU TS3.0 precipitation data, the climatology, dynamics, and variability of this West African Westerly jet (WAWJ), and how it is associated with Sahel precipitation, are investigated.

Based on the strength of the zonal wind speed, five stages of jet development are identified. The WAWJ develops in early June (Stages 1-2), reaches its maximum intensity from late July to early September (Stage 3), and weakens and dissipates during September through mid-October (Stage 4-5). At the mature stage, the WAWJ is located within the Atlantic ITCZ and extends from the surface to 700 hPa, with a maximum zonal wind speed of 5-6 m s^-1 at 925 hPa. In the 6-hourly ERA 40 reanalysis, the jet has a weak diurnal cycle.

Momentum budget analysis reveals that the WAWJ is initiated and maintained when a westerly acceleration region is formed by the superposition of the Atlantic ITCZ and the westward extension of the thermal low, which is associated with the formation of an offshore low over the eastern Atlantic. The development of the offshore low is related to SST warming in the eastern Atlantic. The surface heating budget shows that at the jet maximum the warming is primarily associated with net solar radiative heating at 15°-25°N.

The WAWJ is super-geostrophic at its maximum. While much of the seasonal variation in the WAWJ can be explained by the geostrophic wind, the ageostrophic wind contributes more than 40% at the beginning and ending stages.

The relationship between the WAWJ and Sahel precipitation is examined for August, when both the jet intensity and rainfall are at maximum, from 1958 to 2008. The WAWJ is associated with Sahel precipitation variations on both decadal and interannual time scales, with a strong jet tending to occur during wet years. In wet (dry) decades, the eastward moisture transport towards West Africa is enhanced (reduced), mainly due to the anomalously strong (weak) WAWJ.

The low-level zonal moisture transport across the west coast (15°W, 7.5-10°N) is more highly correlated with Sahel precipitation variability than the meridional transport by the monsoon flow across the Guinean Coast (5°N, 5°W-10°E) does on the decadal scale. During the wet period from 1958-1971, both the WAWJ and the monsoon flow are strong, while for the recovery period after 1988 the WAWJ is strong and the monsoon flow is weak. The regime with a wet Sahel, strong jet, and weak monsoon is also found in individual years, e.g., 1994 and 1999, which suggests that the WAWJ plays a critical role in moisture transport into West Africa that is independent of the monsoon flow across the Guinean coast.