Analysis of long-term variation of the annual number of warmer and colder days using Mahalanobis distance metrics — A case study for Athens

Abstract

According to contemporary observations and analyses an increase in the number of warm days is evident, associated to global warming and the air temperature increase. At the same time, many questions regarding the variation of the yearly number of warm (and cold) day anomalies remain unanswered.In the present work, we used a 99-year long time series of daily maximum air temperatures from the historical, climatic station in Athens, Greece, to produce data sets of sequences of the yearly number of days with anomalies significantly larger (warmer)/lower (colder), with respect to the long-term mean anomalies for the same days. The main goal was to introduce and test the appropriateness of our approach, in assessing changes of the local climate system during last century. This approach is based on the Mahalanobis distance metrics of the variability of the annual number of warmer and colder days derived from the long period data sets of the daily max air temperatures.We show that in Athens the features of the local climate system, assessed by the variability of warmer and colder days, have significantly changed during the observation period.In particular, essential changes are revealed in the temporal correlations of the variability of the annual number of warmer and colder days, during different sub-periods in the past century. Greater changes are found in the last two to three decades, though similar or even stronger changes have also occurred in the past.