West African monsoon index

925-hPa wind speed minus 200-hPa zonal wind, averaged 3–13°N, 20°W–20°E (Fontaine et al. 1995, as used by Ndiaye et al. 2009); falls back to 850 hPa if 925 is absent. — Fontaine, Janicot & Moron 1995, J. Climate 8, 1503–1510