The diurnal cycle of the West African monsoon circulation

Abstract

SUMMARYUsing numerical model analyses, it is shown that there is a coherent diurnal cycle of the West African monsoon winds. As has been observed in previous studies of arid and semi-arid areas, the winds are at their weakest in the afternoon when the convective boundary layer (CBL) is deep, and intensify overnight when the boundary-layer turbulence is much weaker. This diurnal cycle is maximized in the northern part of the monsoon layer, where the meridional pressure gradient and the diurnal cycle of the CBL are both strong.The diurnal cycle can also be resolved in surface and upper-air data, which show how the nocturnal meridional circulation acts to stratify the lower part of the monsoon layer. In contrast, mixing in the daytime CBL acts to maintain the baroclinicity, as has been observed in laboratory flows. This pattern has implications for the efficiency of the monsoon circulation in the continental water budget, as well as in mixing of trace gases and aerosols between the surface layer and the free troposphere. Vertical mixing occurs by day, while meridional advection, with isentropic upgliding and downgliding, is most efficient at night.Finally, high-resolution observations from the JET2000 experiment are used to show that there is mesoscale structure in the diurnally varying monsoon circulation. In the nocturnal flows, local circulations have been observed and appear to represent a response to recent deep convective events. In contrast, the daytime CBL properties at these scales have been shown in a previous study to map closely onto patterns of soil moisture, with horizontal advection playing a weaker role.

A modelling case‐study of soil moisture–atmosphere coupling

“…The wind differences are stronger at the northern border of the humidity differences, where they have a southerly component, than at the southern border. The direction of the wind differences is in accordance with the local changes of monsoon circulations found by Parker et al (2005) in observed fields as a response to cool anomalies associated to the previous passage of convective systems. To the north of a cool anomaly the large-scale gradients are enhanced, whereas to the south of it these gradients are weakened.…”

“…In the afternoon, the turbulent mixing within the PBL causes weak winds, whereas during the night winds intensify, leading to a northward extension of the southwesterly monsoon flow and to the formation of a low-level jet. This diurnal cycle can also interact with surface-induced circulations, as proposed by Parker et al (2005). Another feature of the West African monsoon that can interact with the land surface are the African Easterly Waves, as discussed by Taylor and Clark (2001).…”

“…Interaction of the response with the diurnal cycle of the monsoon flow An interesting aspect of the response to the soil moisture perturbation is its interaction with the diurnal changes of the monsoon flow. The diurnal cycle of the monsoon circulation, as explained by Parker et al (2005), produces weak winds in the afternoon over the Sahel, due to turbulent mixing of momentum in the vertical. After sunset, the low-level monsoon winds intensify and advance to the north.…”

“…An important feature of the West African monsoon is the diurnal cycle of the monsoon winds (Parker et al, 2005). In the afternoon, the turbulent mixing within the PBL causes weak winds, whereas during the night winds intensify, leading to a northward extension of the southwesterly monsoon flow and to the formation of a low-level jet.…”

Motivation, rationale and key results from the GERBILS Saharan dust measurement campaign

“…The diurnal cycle is likely to be influenced owing to LW effects acting throughout the day, while SW impacts only act during daylight (e.g. Parker et al, 2005). Milton et al (2008) modelled the overall impact of mineral dust during a Saharan dust event and found that dust exerts a net atmospheric warming and a surface cooling which increases the static stability of the atmosphere.…”