A detailed case study of cyclogenesis over South America during the period 5-9 June 1981 is undertaken. The intensification of the cyclone occurred first in the upper troposphere. The disturbance showed several characteristics found in baroclinic disturbances, such as eastward tJropagation, intensification with westward vertical tilt and barotropic decay. However, the vertical tilt was confined to upper levels only and the surface temperature gradient was low. This suggests ~hat the mechanism of development might be somewhat different than the usual cases of baroclinic :nstability associated with 10w-Ievel vertical inclination. Internal baroclinic instability is suggested as t plausible mechanism of development. Cases of intense cyclogenesis are also studied during the leriod 1979 to 1988 using the Laplacian of the sea-Ievel pressure criterion in the autumn, winter md spring seasons. The case which occurred on 19 September 1984 over Uruguay and southern ~razil is presented; the pressure drop was 12 hPa in 24 hours over Paraguay and South Brazil.Pages: 359-36
