An acoustic propagation experiment was carried out in the Tyrrhenian Sea in October 1985 in which signals from a broadband source were recorded at a range of 5 km with a vertical 62‐m hydrophone array over a period of 5 days. The experiment, named ‘‘NAPOLI 85,’’ was designed to investigate the transfer function of the ocean medium over an acoustic frequency range from 250–2000 Hz as a function of time and position down the vertical array. This paper treats the ocean transfer function in both the time and frequency domains for the lower refracted path. The intensity fluctuations down the array at an acoustic frequency of 1 kHz are presented for the 5 days. Large fluctuations occur simultaneously over the whole extent of the array, and these can be explained by refractive effects in an intrusive layer. There are also weak spatial variations down the array arising from diffractive effects in the layer. The physical mechanisms causing the time variations in the layer are not yet understood.