Identifying climatic trends of bright sunshine hours and related driving forces in the central Mediterranean

Abstract

Analysing trends in bright sunshine hours (BSH) is a common area of study in climatology. The current study analyses, for the first time, the monthly and yearly trends of BSH durations over the central Mediterranean region over the 1978-2012 climatic period. The meteorological stations included are; Luqa (Malta), Messina and Trapani (Sicily), Tunis, Jendouba, Kairouan, Gafsa and Gabes (Tunisia), of which data was accessed from the German Weather Service (Deutscher Wetterdienst DWD) from the Maritime Climate Monitoring Centre. The influence of other climatic indices on BSH duration, as measured over the Maltese Islands is also studied. These include; cloud cover, relative humidity, vapour pressure and wind speed for the period of 1978-2010 and aerosol optical thickness for the period 2005-2012. Yearly trends in BSH duration over the central Mediterranean are not similar for all the stations. In fact, Luqa, Messina, Trapani and Tunis show a statistically significant increasing trend in BSH amounts, while Jendouba and Kairouan show a positive trend which is not statistically significant, and Gafsa and Gabes show a negative trend which is not statistically significant. Monthly trends in BSH durations over the eight different stations are statistically similar showing uniformity in the data. Results for the relationship between the different climatic indices and BSH amounts over the Maltese Islands are not similar. Cloud cover, relative humidity and wind speed have a statistically significant negative relationship with BSH durations. Possible explanations include dust uptake affecting the atmospheric transmissivity and therefore the amount of BSH, the cyclonic development in the Mediterranean region affecting cloud formation and wind speed, and the presence of aerosols in the atmosphere acting as cloud condensation nuclei and therefore increasing cloud cover. Using multiple regression analysis, the amount of BSH was mathematically modeled on the basis of its relation with cloud cover, relative humidity, vapour pressure and wind speed, resulting in a prediction model equation which can be used for different purposes such as weather forecasting, harvesting of solar radiation, tourism activities and by the public health sector.