Multi annual variability and climatic signal analysis of sunshine duration at a large urban area of Mediterranean (Athens)

• The annual and seasonal sunshine duration in Athens was analysed from 1897 to 2011.
• Sunshine duration until mid 20th century is strongly related to cloudiness.
• Sunshine duration after mid 20th century agrees with emissions variations.
• Brightening after 1980s is stronger under clear sky conditions and in spring.
• Singular spectrum analysis revealed significant oscillation at ∼3.0 yrs.

This study analyses the interannual variability of sunshine duration (SDU) at the urban area of Athens from 1897 to 2011. Observations of total cloud cover (TCC) are also used for a better interpretation of SDU variations. The annual SDU in Athens has increased by +8% (+19 h/decade) over the past century, mainly due to increase in the summer and spring SDU, however, distinct sub periods with decreasing and increasing trends are also discerned. SDU in Athens has undergone an abrupt increase during 1940s with early 1950s being the brightest period of the record. For long periods the course of SDU mirrors TCC, indicating a strong negative correlation between the two variables, nevertheless during the last three decades, both variables reveal trends of the same sign (more evident in spring). Under all-sky conditions, annual SDU decreased by approximately 7% from 1950s to 1980s and increased by 3% thereafter. Under clear sky conditions, the increase of SDU after 1980s is larger, amounting to 9%. Singular spectrum analysis and Continuous Wavelet Transform indicated significant non-linear trends of SDU and an intermittent oscillation, centered at 2.9–3.0 yrs.