Near-surface thermal structure and surface diurnal warming in the Adriatic Sea using satellite and drifter data

An extensive set of in situ temperature data collected by surface drifters is combined with satellite-derived sea surface temperature images to study the difference between the pseudo-bulk and bulk temperatures (ΔTpb–b) in the Adriatic Sea for the period 21 September 2002–31 December 2003. The variations of this temperature difference are described as a function of local wind speed and incoming solar radiation provided by a local area atmospheric model. The daily sea surface temperature variability is also assessed by computing the temperature difference between the daily maximal and minimal values (ΔTday–night). The data show that the smaller the wind speed and the larger the solar radiation, the larger ΔTpb–b. The temperature difference reached the highest value (∼5 °C) on a hot day (more than 600 W/m2) of May 2003 in weak wind condition (around 3 m/s). For strong winds (speed > 6 m/s) the dependence on both the wind and solar radiations vanishes as the temperature difference approaches zero because the near-surface water becomes thermally homogenous due to the wind-induced vertical mixing. Strong diurnal warming of the sea surface, as derived by the pseudo-bulk estimates, and a strong near-surface stratification were found during the spring/summer season. Monthly mean statistics show that the diurnal cycle of the pseudo-bulk and bulk temperature starts to become significant already in February and March. Subsequently (from April to August) both the diurnal warming and the stratification are maximal (monthly means of ΔTday–night ∼1–2 °C and of ΔTpb–b ∼0.5 °C ), while in fall and early winter the ΔTpb–b values are quite small (monthly means near 0 °C) and the ΔTday–night monthly means are bounded by 0.5–1.5 °C. Maximal amplitudes of the diurnal cycle can exceed 4 °C (mostly in spring–summer) for both the pseudo-bulk and bulk temperatures. However, the monthly means of ΔTday–night is generally twice as large for the pseudo-bulk estimates (∼2 °C) with respect to the bulk layer (∼1 °C). The diurnal warming of the sea surface, as derived by the pseudo-bulk temperature, occurs at about 14:30 local time, that is more than 2 h after the maximal sun elevation and an hour earlier than the bulk temperature maximum at 20–40 cm depth.