Simulation of West African monsoon using the RegCM3. Part I: Model validation and interannual variability

Summary The West African monsoon oscillates each year with remarkable regularity but the interannual variability associated with the monsoon is not fully understood although much progress has been made in recent years. This study examines and evaluates the mean state and the interannual variability of the West African climate as simulated by the International Centre for Theoretical Physics (ICTP) Regional Climate Model version 3 (RegCM3) over the period 1979 through 1990 using the National Center for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) reanalysis data as lateral boundary conditions. Our analysis shows that the averaged rainfall over the region is well represented by the model and demonstrates considerable skill in reproducing the extreme rainfall regimes. There is however a tendency to overestimate rainfall amounts along the Guinean coast, particularly around mountainous areas, and to underestimate it over the Soudano-Sahel. The increased rainfall along the coast is due to an enhanced low-level convergence of the moist southwesterly winds along the coast leading to a reduction of the moisture content in the atmosphere. The decrease over the Soudano-Sahel could be associated with the weakening of the land–sea temperature gradient and hence the decrease in the low level southerly flows. The spatial and temporal variations in temperature are well captured by the model except for slightly cold bias over the coastal region due to an overestimation of precipitation.     

Instability of squall-inducing waves, global sea-surface temperature anomalies and climate change in tropical north Africa

Summary An inviscid form of the hydrodynamical equations is solved with enhanced horizontal shear, which is a synoptic feature consistent with stronger African Easterly Jet (AEJ) in Sahelian dry years, for unstable waves generated along the boundary between the two tropospheric air masses in tropical north Africa (i.e. the moist south-westerlies and the dry north-easterlies). Using a two-layer model of the atmosphere in order to correctly simulate the tropospheric synoptic situation in the sub-region, results show that the mode of the waves which is known to be fundamental to the development of West African squall lines is more unstable in dry years. This instability is found to be most-pronounced when the surface of discontinuity between the south-westerlies and the north-easterlies is at 700 mb level. Further, it is shown that in Sahelian dry years, the zone of these unstable waves shifts slightly southwards. This shift causes a deficit in rainfall in West African isohyet bands north of latitude 12°. The persistence of this deficit is linked with the continuous warming, in July, August and September of the 18-year period 1969–1986, of the three oceans (Indian, Pacific and South Atlantic) whose sea-surface temperature (SST) anomalies influence rainfall in tropical north Africa. It is shown that anytime these oceans warm up anomalously, the strength of the AEJ is enhanced leading to the climate-change process of: SST anomaly, increased AEJ strength, southward shift of the zone of squall-inducing waves and consequent reduction in total annual rainfall north of latitude 12° in tropical north Africa.With 5 Figures     

Wave propagation and amplification along the surface of discontinuity between air masses of different atmospheric static stabilities: A case study of West African tropospheric air masses

Summary A linearised inviscid form of the hydrodynamical equations is solved in shear, with the aid of a two-layer model of the atmosphere, for waves generated along the boundary between the air masses. Results show that some modes, generated by assuming different atmospheric static stabilities for the two layers of air, propagate and amplify with time; the static stability has been given a novel definition in units of height.A case study, in pressure co-ordinates, of typical tropospheric air masses in West Africa reveals that the mode with the largest growth rate of 1.6 per hour propagates in an East-West (E-W) direction along the surface of discontinuity between the (monsoon) southwesterlies and the (dry) northeasterlies with a phase speed of 7.7 m s^–1, a period of 3.6 hours and a wavelength of 100 km. Similar study inz co-ordinate reveals a mode with a growth rate of 1.8 per hour in an E-W direction, with a phase speed of 8.0 m s^–1, a period of 3.5 hours and a wavelength of 100 km.With 2 Figures     

Initiation of West African squall lines

Summary It is proposed that squall lines are initiated through the growth (with time) of wave-like perturbations along the surface of discontinuity between the monsoon southwesterlies and the dry northeasterlies in West Africa. Out of the many possible modes of different growth rates, the modes with the largest amplification could block the 650-mb mid-tropospheric jet which, because it is cold, progressively sinks as it traverses West Africa from east to west. The distortion created by the blockage forces up parcels of convectively-unstable southwesterlies resulting in precipitation. Precipitation falls or partly evaporates into the underlying jet the latent heat of vaporisation being supplied by the jet. The jet, now cooler, sinks. While sinking, it can reach the surface of the earth and, due to the strong convergence created, a gust front is formed. The front, as a result of ascending southwesterlies, constitutes an area of vigorous convective activity which triggers off a self-regenerative mechanism of condensation, evaporation and sinking. This hypothesis, with others, is able to explain the predominance of highlands as source regions of squall lines, the close association between the propagation speed of squall lines and that of the mid-tropospheric jet, the observed overturning of the atmosphere after the passage of squalls and the possible effects of insolation and African easterly waves on the initiation process.With 2 Figures     

SST-induced climate change in tropical North Africa: The intermediary role of lower tropospheric oscillations

Summary  The growth rates of amplifying mid-tropospheric perturbations in tropical North Africa is known to reduce with increased vertical shear in the troposphere. This phenomenon leads to a reduction in the frequency of generation of squall lines – the main rain-producing mechanism in tropical North Africa – because squalls are initiated by amplifying modes of African Easterly Waves (AEW). Ultimately, therefore, tropical North Africa experiences a shortfall, with respect to long-term averages, in annual rainfall. Weakening of AEW intensity is shown to be linked with the warming up to the sea-surface temperatures (SST) of the South Atlantic, Pacific and Indian Oceans. These findings are consistent with the observed reduction in the incidence of intense hurricanes along the entire westem Atlantic in Sahelian dry years. It is shown that the frequency of occurrence of Atlantic tropical storms and hurricanes is unaffected by the dryness or otherwise in the Sahel, but the paths of the storms are determined by the zonal exit point, from the African continental land mass to the Atlantic, of West African disturbance lines. These results have applications, and implications, in the level of preparedness for the economic impacts of Atlantic storms and hurricanes. Received June 8, 1996 Revised June 8, 2000