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

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 hight 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 hight 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 no correspondence with the 500‐hPa flow and which appear in late spring and early autumn. The study showed that the north‐west African depressions do 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.