Variability of the Intertropical Front (ITF) and Rainfall over the West African Sudan–Sahel Zone

• Published in: Journal of climate 2010-07, Vol.23 (14), p.3984-4004
• Main Authors: Lélé, M. Issa, Lamb, Peter J.
• Format: Article
• Language: English
• Subjects: Atmospheric circulation; Climate change; Contingency; Correlation coefficients; Daily temperatures; Datasets; Digitization; Drought; Earth, ocean, space; Exact sciences and technology; External geophysics; Humidity; Hydrologic data; Latitude; Malaria; Marine; Meningitis; Mesoscale phenomena; Meteorology; Moisture effects; Monsoon rainfall; Monsoons; Monthly; Monthly rainfall; Precipitation; Rain; Rainfall; Rainfall anomalies; Rainfall data; Rainfall forecasting; Rainfall patterns; Rainfall variability; Rainy season; Rainy seasons; Sahel; Seasonal rainfall; Seasons; Secular variations; SPECIAL U.S.–China Symposium on Meteorology COLLECTION; Time series; Tropical climates; Variability; Weather; Wet season; Wind
• ISSN: 0894-8755; 1520-0442
• DOI: 10.1175/2010JCLI3277.1

The Intertropical Front (ITF) is a fundamental feature of the atmospheric circulation over West Africa. It separates the wedge of warm moist southwesterly monsoon flow off the tropical Atlantic from much hotter and very dry northeasterly wind from the Sahara Desert. Here, the daily temperature, humidity, and rainfall data for 1974–2003 are analyzed to document the northward advance and southward retreat of the ITF between boreal spring and autumn, and assess its role in Sudan–Sahel (10°–20°N) rainfall patterns. Using largely dekadal (10 day) and monthly resolutions, analyses are performed for the 30-yr-average seasonal time scale and sets of extreme years, with a major focus on concurrent monthly ITF–rainfall relations. The seasonal rainfall predictive potential of the early season ITF latitude is also investigated, as is the secular variation of ITF latitude–weather system–rainfall associations during 1974–2003. The northward advance of the ITF across the Sudan–Sahel from April to early August is relatively slow, averaging 0.8° latitude dekad−1(8.8 km day−1). The southward ITF retreat between mid-August and mid-November is almost twice as fast, averaging 1.4° latitude dekad−1(15.5 km day−1). Coupled with the ITF advance, the monsoon rainbelt migrates northward and intensifies. However, its northern boundary (1 mm day−1monthly average isohyet) lags 100–250 km south of the ITF, while the most useful rainfall for society (>3 or 4 mm day−1monthly average) generally occurs more than 400 km south of the ITF. There, the monsoon wedge is thickest and the horizontal velocity and moisture convergence are maximized in a regional ITCZ. The rapid ITF retreat during September–October is preceded by a similar rainbelt displacement. During both ITF advance and retreat, rainfall over the Sudan–Sahel region is positively related to the ITF’s latitude. The association is strongest during the early (April–June) and late (October) rainy season months (linear correlation,r= +0.74 to +0.81), when the ITF is located to the south and rainfall is low. It is weaker during the July–September rainy season core when the ITF is farthest north (r= +0.50 to +0.58). This concurrent rainfall dependence on ITF latitude is established further by contingency analyses for the 30-yr study period and by investigation of several extremely dry and wet individual seasons. The April ITF latitude anomaly is amoderately consistent indicator of the subsequent ITF latitude and associated rainfall anomal