From the analysis of 16 months of transionospheric propagation data taken near Arecibo it has been found that there are associations between nighttime enhancements and quasi-periodic fluctuations in total electron content and the occurrence of amplitude scintillations. A postmidnight peak is found in the occurrence of electron content increases, maximum wave activity, and largest scintillation index, with some evidence to suggest a second premidnight peak in winter and reduced probability of the features being found at equinox. An analysis of the large-scale variations of the total content data for three stations has shown that the postsunset minimum in electron content propagates at high speed toward the northeast, while the premidnight minimum advances toward the south with the main postmidnight maximum having northward component of motion. The results are discussed in terms of the known dynamical features of the ionosphere at Arecibo and in particular the effects of the neutral wind on the bottpmside ionosphere. It appears that conditions conducive to the production of nighttime enhancements in total electron content may also allow the penetration of medium scale gravity waves through the lower thermosphere with an unstable breakdown process giving rise to the development of scintillation-producing irregularities.