SST diurnal variability in the North Sea and the Baltic Sea

Abstract Five years of sea surface temperature (SST) retrievals from a geostationary platform are utilised to identify and characterise diurnal warming in the North and Baltic Seas. Observations from the Spinning Enhanced Visible Infrared Imager (SEVIRI) on board the Meteosat Second Generation satellite are proven valuable for quantifying the daily SST cycle in shallow and coastal waters at latitudes 48–60 N. Satellite minus in situ observations from moored buoys show biases ranging from − 0.3 K to 0.1 K. A thorough analysis is carried out to obtain the optimal reference field used for calculating the daytime temperature anomalies. The best night-time reference field candidate is demonstrated to be an average of 3 days of satellite observations from local midnight to 0300. When compared against night-time in situ measurements from various locations, mean biases are found in the order of ± 0.1 K and standard deviation (σ) between 0.4 K and 0.9 K. The 5 year record with daytime temperature anomalies is used to derive robust statistical description of duration, spatial extent, proximity to coast and water depth of the diurnal warming events. Seasonal and inter-annual variations in the diurnal warming are also quantified. Daytime anomalies exceeding 2 K are identified during the spring and summer months of every year, peaking at 1500 LT. Events with daily anomalies exceeding 5 K are observed. Areas where diurnal variability is often observed coincide with areas of frequently observed low winds and turbid waters (high Kd(490) values).