The 6-9-Day Wave and the Spatial Distribution of Air Temperature in Northern Africa in Summer: Inter - Annual Variability

The composite method is used to analyse 6-9 day waves and air temperature over North Africa during the summer of 1981-1990. Based on the daily outputs of the NCEP/NCAR reanalysis, the results of this study demonstrate that a large peak in the 6-9 days band (at 7.2 days) in the power spectra of the zonal wind component at 700 hPa during summer 1981 has been found. The waves were clearly visible in the filtered (around 7.2 days) and even in the unfiltered series of the zonal wind component. The structure of the 6-9 day waves shows two opposing circulation vortices on both sides of latitude 12.5N, with the maximum (minimum) temperature anomaly appearing around 15N - 25N above the equator. In addition, these vortices are associated with anomalies in the meridional component of the northerly (southerly) wind, which are respectively connected to the maximum (minimum) temperature anomalies. The spatial configuration of the waves also shows strong temperature modulations associated with the disturbed meridional component of the wind, mainly north of 20°N, and for summers when these waves are very active, there is an opposition of signs between the temperature anomalies to the north-east and to the north-west in the areas of the Libyan and Azores highs. Due to the strong influences of the Libyan and Azores highs, a distribution of opposite surface level pressure (SLP) values is also observed. On the contrary, when these waves were not active, this opposition of signs in the composite of the air temperature anomalies did not exist and the isopleths tended to be paralleled to the parallels of latitude, and the filtered SLP anomalies had the same sing in the two anticyclones areas. Thus, the appearance and/or activity of the 6 to 9 day waves could be linked to the existence of a phase opposition between these two anticyclones. This study reinforces our understanding of the involvement of atmospheric dynamic and thermodynamic processes in the variation and evolution of waves in North Africa.