Frequency and trajectories of the Mediterranean cyclones and their influence on precipitation in Bulgaria during the months of September-April for the period 2010-2020

The Mediterranean region is known for the formation of an area of low atmospheric pressure during the cold half of the year and the formation of cyclones. Creation of cyclones in this area is influenced by both cold air advections along the cold fronts of the Atlantic cyclones, the East European Maximum, and high pressure ridges coming from the Azores Maximum located to the west. The formed cyclones over the waters of the Mediterranean Sea have an impact on a vast territory covering southern Europe, the Middle East and the northern parts of Africa, which defines the zone of Mediterranean climates. Their frequency, mode, trajectories, speed of movement and duration of retention over a given territory determine to varying degrees the intra-annual rainfall regime, as well as the individual economic activities related to them.

For the purposes of this article, data from synoptic maps at a height of 500 hPa were used, as well as average monthly precipitation data for 10 meteorological stations - four located in Northern Bulgaria (Vidin, Pleven, Ruse, Varna) and six in Southern Bulgaria: Burgas , Sliven, Plovdiv, Sofia, Kyustendil and Sandanski. They are collected from NIMH monthly newsletters.

Pisarski's (1955) classification scheme was used as the basis for the types of trajectories of Mediterranean cyclones. The monthly, seasonal and annual total number of Mediterranean cyclones is calculated. The main areas of cyclogenesis and their percentage ratios in relation to the total number of formed cyclones have been established. For each trajectory, the shares of cyclone formation areas relative to the total number of Mediterranean cyclones formed were calculated. The monthly and seasonal distribution of cyclones passing along the different trajectories, as well as average, minimum and maximum duration, are presented.

In order to analyze the influence of Mediterranean cyclones on the precipitation in Bulgaria, the correlation coefficient (r) between the precipitation amounts for the 10 selected stations and the frequency of occurrence of the formed cyclones was used.