Nocturnal temperature structure in the mesopause region over the Arecibo Observatory (18.35°N, 66.75°W):: Seasonal variations

[1] We present the mean seasonal climatology of the nocturnal temperature structure in the mesopause region ( 80 - 105 km) above the Arecibo Observatory, Puerto Rico ( 18.35 degrees N, 66.75 degrees W) from 106 nights of potassium Doppler lidar observations between December 2003 and September 2006. This first complete range-resolved mesopause climatology for a tropical latitude exhibits several unique features. Compared to higher latitude sites, mesospheric temperature inversion layers in the nocturnal means are much weaker at Arecibo. Seasonally large inversions occur in summer but are almost non-existent during the rest of the year. The Arecibo climatology shows a three-level mesopause: a high altitude in summer (similar to 100 km), a medium altitude in late autumn and winter (similar to 96 km), and a low altitude in early spring (similar to 91 km). The mesopause is cold in the solstices (similar to 171 K in summer, similar to 176 K in winter) and warm around equinoxes, particularly late autumn when it is near 195 K, while the spring mesopause temperature is close to 185 K. The lower thermosphere around 100 km at Arecibo shows a decreasing temperature from spring to summer when it reaches its coldest temperature, which is contrary to the increasing temperature observed at all midlatitude locations. Semiannual variations in the seasonal temperature have amplitudes as large as the annual variations through most of the MLT altitude range at Arecibo. These observed seasonal variations appear to be associated with the semi-annual oscillation, a predominantly tropical phenomenon. This report provides one of the very few observations of the semi-annual oscillation in lower thermosphere temperature.