Variability of static stability over West Africa during northern summer 1979–2005

Abstract

This work investigates six static stability measures, σ, over West Africa. Using National Center for Environmental Prediction (NCEP) and National Center for Atmospheric Research (NCAR) reanalysis in summer (June to September: JJAS) for the period 1979–2005, the mean vertical distribution of the stability measures for different climatic zones over West Africa were computed at different tropospheric layers between 1000 and 70hPa. Five of the six stability measures considered here, show a decrease in the mean troposphere by a factor of about three and seven from 925hPa to 300hPa. This contributes to identify the mean features of the vertical structure of the West Africa troposphere circulation. An Empirical Orthogonal Function (EOF) analysis reveals that the JJAS stability measures variability is almost entirely controlled by one entirely positive EOF. The first 2 EOFs represent more than 90% of the total variance and are sufficient to describe the spatial variability of the summer mean stability measure. The relationship of the stability measure field to the convection, represented by Outgoing Longwave Radiation (OLR) field, was investigated by means of a Canonical Correlation Analysis (CCA). The correlations are stronger in areas where the first and second CCA pair of σ and OLR have correlations of 72% and 65% respectively. The first CCA pair mostly determine the arid and semi-arid area whereas the second is related to the Guinea coast and wet equatorial region.