Marine monitoring by SCUBA divers reveals new aspects of the temperature variability inside the Gulf of Naples (Tyrrhenian Sea)

This study presents results of long-term monitoring activities carried out by SCUBA divers inside the Gulf of Naples, in the context of an ongoing citizen science project. In the last five years, systematic high-frequency measurements of the sea temperature have been performed at three different depths over the Santa Croce Bank, a shallow area in the inner shelf of the gulf that is a hotspot of biodiversity. Analysis of the temperature archive resulting from the monitoring has revealed elements of variability that could strongly impact on the local ecosystems. The main purpose of this note, however, is to stress that some of these elements are also of considerable oceanographic interest. For example, all summer measurements display strong diurnal temperature oscillations, with sizes of several degrees Celsius, in the thermocline area and below, which cannot be explained by the diurnal modulation of the solar radiation. We have found similar oscillations in the outputs of a recently developed high-resolution, tide-including model for the forecast of the Mediterranean Sea circulation; they are fairly localized across-shore, but appear to extend to the whole inner shelf of the gulf. Analysis of observed data of temperature and wind, and of the model outputs, indicate that the oscillations are due to baroclinic dynamics driven by the barotropic forcing exerted by the diurnal summer breeze over the thermocline, in the inner portion of the shelf. However, the specific underlying dynamical mechanisms, which could also involve tidal effects, remain to be identified. Another interesting element emerging from the measurements is an abrupt decrease of the surface temperature at the end of September 2018, followed by a rapid destruction of the summer stratification and a premature homogenization of the water column. We shall briefly discuss this event, in relation with the complex atmospheric dynamics that took place in that period.