Colacino, M. (Istituto di Fisica dell'Atmosfera, CNR - Roma Italy)
Conte, M. (same affiliation)
Piervitali, E. (same affiliation)
During the warm season (from June to September) in large areas of the Mediterranean basin, the air temperature sometimes increases up to several degrees above the normal value. These hot spells can be sudden and very intense, but of short duration, 3-5 days, or more gradual and less intense, but of long duration, that is 10 days or more. These phenomena have been defined as heat waves by KARAPIPERIS an MARIOLOPOULOS (1956).
The heat waves produce very warm and dry environmental conditions, which are characterised, among other things, by a complete drought: as a consequence, an increase of the aridity of the soil, and in some cases also the mechanisms of desertification, could be favoured in some zones of the Mediterranean region. In addition, dryness and high temperature can feed the forest fires, that are very serious disasters for the territory and the local climate, having many negative impacts on the economy of the burnt lands. Also several losses of human lives are observed during the occurrences of such waves.
In this work a research is presented which analyses all the heat waves occurred in the central Mediterranean in the period 1950-1995. Sixtyseven events have been found out: 34 short lasting and 33 long lasting. Among other things the following features are stressed in the present paper:
i - The short lasting phenomena are due to downward vertical motions related to northward displacements of the Subtropical Jet Stream and the connected areas of upper-air convergence. The long lasting cases are associated to a particular meteorological pattern, named "Omega", which produces persistent horizontal southerlies. ii - The number of days of heat waves presents an increasing trend and an oscillation of about 22 years, strictly correlated with similar behaviour of the anticyclonic persistence in the central-western part of the Mediterranean basin. iii - The extension of the damages due to forest fires is connected with the number of heat waves. Also a simple statistical analysis is performed from which a question arises: are the heat waves extreme phenomena? Actually the values of the associated meteorological parameters are very severe, but the mean time during summer in which the waves are operating is about 10% of the whole summertime; this is an appreciable percentage. The conclusion could be that the heat waves are both extreme phenomena and typical features of the Mediterranean climate.