Atmospheric synoptic conditions associated with the initiation of north‐west African depressions

Abstract

This study presents a discussion of the main characteristics, and at the same time gives a detailed classification, of the north-west African depressions within the framework of the close relationship between the 500-hPa flow and the origin, development, and movement of surface depressions.The classification relies mainly on the similarities and differences present in the 500-hPa flow patterns when each north-west African depression first appears on the surface synoptic chart. For a detailed discussion and, especially for a record of the synoptic conditions that are favourable for the formation of north-west African depressions, mean 500-hPa height and anomaly charts and mean temperature charts, for the years 1970-1974, have been drawn for each subcategory into which the 79 cases fall.For each of these subcategories, mean charts have also been drawn for 850-hPa height and temperature, mean sea-level (MSL) pressure, and MSL pressure anomalies. The classification resulted in the north-west African depression being divided in five main categories A, B, C , D, and ST. Category A is divided into four subcategories, the depressions of which appear only during spring. Categories B and C are divided into two subcategories, B1, B2 and C1, C2, and these too occur only in spring, except for subcategory C2, which also appears in autumn. Category D is also divided into two subcategories, which appear only during winter. Category ST (stationary depressions) includes only four cases out of 79 north-west African depressions. These are shallow, warm-cored atmospheric circulation systems having n o correspondence with the 500-hPa flow and which appear in late spring and early autumn.The study showed that the north-west African depressions d o not exhibit significant differences from other extratropical depressions. Their origin, development, and movement largely follow (except for category ST) Sutcliffe's and Petterssen's traditional theories of development.

Comprehensive analysis of the climate variability in the eastern Mediterranean. Part I: map‐pattern classification

“…Southsoutheasterly flow occurs along Italy but the weather conditions in the major part of the eastern Mediterranean seem rather calm. This mode indicates cyclogenesis over North Africa originating from the lee of the Atlas Mountains (Prezerakos et al, 1990), where Alpert et al (1990 a,b) state that major cyclonic activity takes place as spring progresses. Spring C2 (Figure 11) depicts a trough deriving from an enhanced low centre over northeastern Europe, which is orientated towards the study area bringing the continental polar air from a north-northwestern direction.…”

“…In the first pattern (winter C1; Figure 9), the main feature is the low-pressure system positioned over the central Mediterranean. This pattern is presumably related to discernible areas of cyclogenesis, such as the Gulf of Genoa (Prezerakos et al, 1990;Maheras et al, 2001). The conjunction of the latter with the eastern European High skirting around the 25°E longitude causes moist meridional maritime airflow from the Mediterranean to intrude into the Balkan Peninsula.…”

“…PC1 (16.8% of the explained variance) shows a centre of activity located southwest of the study region. The negative mode of this pattern is probably related to a centre of below normal pressure over North Africa, implying the presence of the North Africa low-pressure systems, known as Saharan depressions (Prezerakos et al, 1990;. Such circulation favours the transportation of desert-sand into European countries, which later precipitates out generating the 'redrain' effect (Prezerakos, 1997;Ozsoy and Saydam, 2001).…”

Dust emission and transport associated with a Saharan depression: February 2007 case

“…Previous studies [e.g., Elfandy, 1940;Pedgley, 1972;Tantawy, 1964;Alpert and Ziv, 1989;Alpert et al, 1990;Prezerakos et al, 1990;Dayan et al, 1991;Barkan et al, 2005;Horvath et al, 2006] have pointed out the role of at least four mechanisms for the occurrence of these depressions over North Africa: (1) Large-scale weak baroclinity, (2) vigorous boundary layer baroclinity (due to the strong meridional temperature gradient along the northern African coast), (3) subtropical jet stream related circulations, and (4) lee effects of the mountains.…”

Climatic features of atmospheric stability in the Mediterranean region (1948–2006): spatial modes, inter‐monthly and inter‐annual variability

“…Such conditions favour the formation of the so-called Saharan depressions (see, e.g. Prezerakos et al, 1990). The seasonal distribution shows that the pattern of cluster 10 prevails during the whole year except summer, while the highest frequency occurs in October (Figure 11(d)).…”