The West African westerly jet (WAWJ) is a low-level (~ 925 hPa) feature of the summer climatology that transports moisture from the eastern Atlantic onto the African continent at 8-11°N. Here the dynamics of the jet's formation and its role in Sahel precipitation variations are examined. Horizontal momentum budgets analysis shows that the jet forms when a region of westerly acceleration is generated by the superposition of the Atlantic Intertropical Convergence Zone (ITCZ) and the westward extension of the continental thermal low, which is associated with the formation of an offshore low related to seasonal sea surface temperature (SST) warming at 6°-18°N along the coast. Variations of the westerly jet are significantly positively correlated to precipitation variations over the Sahel (10°-20°N, 18°W-30°E) on both interannual and decadal time scales. In wet periods of 1958-1971 and 1988-2009 (dry period of 1972-1987), enhanced (decreased) westerly moisture fluxes associated with a strong (weak) jet increase (decrease) the low-level moisture content over the Sahel, decreasing (enhancing) the stability of the atmosphere. While variations of the jet are closely associated with variations of the Atlantic marine ITCZ between 20°W and 30°W, regional model simulations suggest that decadal SST variations in the eastern Atlantic do not force the observed decadal variations in the jet. Climate response over North America to a hypothetical shutdown of the Atlantic meridional overturning circulation in the context of global warming is investigated using a regional climate model. The model predicates precipitation decreases in most of the United States and Mexico from April to September, except over the eastern U.S. where rainfall increases in April, May, June, and September. Moisture budgets analysis shows that precipitation variations over the eastern and western U.S and Mexico are mainly due to changes in moisture convergence associated with large-scale circulation changes, e.g., the westward extension of the North Atlantic subtropical high and the formation of an anomalous high over the eastern Pacific and the Gulf of Mexico. Variations in land surface temperature are dominated by greenhouse gas warming, which is magnified by local hydrological changes in the summer. More extreme warm temperatures and dry spells occur.