A Simple Identification Scheme for Upper-Level Troughs and Its Application to Winter Precipitation Variability in Northwest Africa

Abstract

A simple algorithm for the identification of upper-level trough axes is introduced. In contrast to the assumption of a basically circular geometry in many comparable routines found in the literature, the presented identification scheme is based on the east?west gradient in the 500-hPa geopotential height field, accounting for the meridionally elongated troughs often found in the subtropics. On the basis of the NCEP?NCAR reanalysis data from 1958 to 1998, climatological aspects are discussed for the area 22.5°?47.5°N, 38.5°W?18.5°E. For the period from 1977 to 1998, 12-hourly precipitation reports from 37 stations in northwestern Africa are used to investigate the influence of the identified troughs on winter precipitation. It is demonstrated that more than 90% of the precipitation in this region can be assigned to nearby trough axes. A comparison of rainy periods with the climatology reveals a significant enhancement of the occurrence of trough axes in a range of 10°?15° longitude close to the region considered. While in northwestern Morocco most precipitation falls in connection with the convective instability and dynamical lifting close to and ahead of the trough axes, the orography modifies this relation in the areas along the Mediterranean coast and south of the Atlas Mountains. In the former region, a considerable part of the precipitation falls to the west of the trough axis, where the associated northerly flow is directed against the Tell Atlas. A similar relation is derived for the latter region, where precipitation appears to occur primarily east of the trough axis in the flow that is directed against the southern side of the Atlas range. Here, a particularly strong relation is found between the rainfall and the relatively rare occurrence of upper-level troughs. It is suggested that the consideration of upper-level features like the ones identified by the presented algorithm should be an integrative part of investigations of precipitation variability in the equatorward part of the midlatitudes and the subtropics.