Waves interactions during the 1984 African monsoon onset
Flore Mounier, CNRS, Paris, France; and S. Janicot and G. N. Kiladis
Intraseasonal variability of the West and Central African monsoon is analyzed as a case study of the onset of the 1984 monsoon. Using temporal and spectral filtering techniques, and composite analyses, an EOF reconstructed convection index over West Africa is produced. The dominant waves and modes for this selected case are convectively coupled Kelvin waves and a 15 day periodicity we call the Guinean Quasi-Biweekly Mode of convection (GQBM).
The convectively coupled Kelvin waves originate from the Pacific sector, interact with deep convection of the ITCZ over the Atlantic and West and Central Africa, then weaken over East Africa and the Indian ocean. The convective and dynamical patterns identified over the Atlantic and Africa show some resemblance to the theoretical equatorially trapped Kelvin wave solution on an equatorial beta-plane. Most of the flow is in the zonal direction as predicted by theory and there is a tendency for the dynamical fields to be symmetric about the equator, even though the ITCZ is concentrated well north of the equator at the full development of the African monsoon. The 15 day periodicity intra-seasonal mode of convection in the African monsoon during northern summer identified as the GQBM is mostly characterized by a quasi-stationary zonal dipole of convection whose dimension is larger than the West African monsoon domain, with two poles centered respectively along the Guinean coast and between 30°W-60°W in the equatorial Atlantic. The GQBM dipole pattern appears to be controlled by an equatorial atmospheric dynamics through a Kelvin wave type pattern propagating eastward between the two poles and also by land surface processes over Africa, inducing combined fluctuations in surface temperatures, surface pressures and low-level zonal winds off the coast. When convection is the suppressed over West and Central Africa, the lower cloud cover induces higher net shortwave flux at the surface which increases surface temperatures and lowers surface pressures. This induces an east-west pressure gradient both at the latitude of the ITCZ (10°N) and of the Saharan heat low (20°N) leading to an increase of the westerly moisture advection inland. The arrival from the Atlantic of the positive pressure pole front amplifies the low-levels westerly wind component and the moisture advection inland leading to high convective activity over West and Central Africa. Then opposite phase of the dipole develops. When the effect of the Kelvin wave propagation is removed by filtering, the stationary character of the GQBM is highlighted.
The impact of the GQBM combined with a Kelvin wave train is also illustrated during monsoon onset in 1984 as it effects convection, rainfall and on the life cycle and track of individual westward propagating convective systems.
Extended Abstract (764K)
Supplementary URL: http://www.lodyc.jussieu.fr/~fmlod/ams4b5.ppt
Session 4B, The African Monsoon
Tuesday, 16 January 2007, 1:30 PM-3:00 PM, 214C
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