Inter‐annual and inter‐decadal variability modes of the Athens total cloud cover. (13th July 2018) - Kalimeris, Anastasios
Abstract : The historical observations of the total cloud cover (TCC) index since 1900 from the National Observatory of Athens archives were used here, in order to investigate the inter‐annual variability of TCC. Multi‐spectral decomposition analysis was applied in order to resolve statistically significant modes of TCC variability out of the estimated stochastic variations. An increase of the Athens TCC (A‐TCC) was ascertained during the observational period (1900–2012). The observed A‐TCC variability was found to be composed by "a dominant nonlinear trend" that represents the centennial increment of the cloud cover, and "a small number of sub‐decadal oscillatory modes, " particularly in the winter series. Additionally, the evolutionary history of the A‐TCC variability involves a significant regime shift in about 1960 that was followed by an increase on the mean TCC level and an enhancement of the positive TCC trends apart from winter, in which the trend was faded out. The influence of global and hemispheric climatic variability patterns on the observed A‐TCC changes was explored in the timescale domain along with the impacts of the Mediterranean evaporation field. Results showed that the A‐TCC inter‐decadal variability is strongly coupled with the Atlantic Multidecadal Oscillation (AMO) and the Mediterranean evaporation during all seasons, except winter, in which the atmospheric dynamic drives decadal and sub‐decadal TCC variations, primarily modulated by the North Atlantic Oscillation (NAO). During the hot season, the AMO and the Mediterranean Sea thermodynamic processes become major regulators of the A‐TCC variability, particularly on the inter‐decadal scales, but the tropospheric North Atlantic activity still affects the A‐TCC through the summer NAO (SNAO) pattern. In this framework, different epochs were discerned in the relation of the A‐TCC with the Mediterranean evaporation field, which seem to be associated with the prevalence either of sea thermodynamic processes or of the atmospheric dynamics. Abstract : The time evolution of the Athens total cloud cover (A‐TCC) during the period 1900–2012 reveals an abrupt increment of the mean TCC level at about 1960, a significant positive long‐scale trend, and a small number of oscillatory modes on the sub‐decadal scales. The observed A‐TCC variability is strongly correlated with the Atlantic Multidecadal Oscillation (AMO) fluctuations in all seasons apart from winter, in which the North Atlantic Oscillation (NAO) prevails as the main modulator of the A‐TCC variations.