Synoptic flow patterns leading to the generation of north‐west African depressions

• Published in: International Journal of Climatology
• Main Author: Prezerakos, N. G.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1990
• Subjects: North Atlantic
• Online Access: http://dx.doi.org/10.1002/joc.3370100105; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1002%2Fjoc.3370100105; https://rmets.onlinelibrary.wiley.com/doi/pdf/10.1002/joc.3370100105

Abstract This paper reports a study and analysis of the synoptic evolution of the atmospheric flow patterns that cause the formation of north‐west African depressions. The study uses mean 500 and 850 hPa heights and their anomalies, associated with the subcategories into which the north‐west African depressions have been classified. The time period under consideration covers the consecutive years from 1970 to 1974. To study better the synoptic evolution and the separation of existing centres of action for each subcategory, charts of the mean 500 and 850 hPa heights and temperatures for 72, 48, and 24 h before the first appearance of a depression have been prepared. For the mean 500 hPa height field and for each subcategory, anomalies based on differences from the long‐term (1950–1973) average have been calculated (at the 0·01 statistical level of significance) for each grid point of the ‘anomaly’ fields. From the study as a whole it can be deduced that the synoptic flow patterns, which cause the generation of north‐west African depressions, are unstable baroclinic waves, at the synoptic scale, initiated in the middle troposphere over geographical regions far from the Atlas Mountains. These waves can be seen on the 500 hPa charts at least 72 h before the first appearance of the depressions on the mean sea‐level (MSL) charts for north‐west Africa. It also seems that the intensive low‐level tropospheric baroclinicity, which is most pronounced along the north‐west African coast, plays an important role in the initiation and self‐maintenance of the surface lows. Finally, the key atmospheric circulation system that causes these baroclinic waves to form must be a large‐scale ridge, often of a blocking type, located in the North Atlantic Ocean or north‐west Europe.