The high-resolution wind vectors observed by the space-based scatterometer QuikSCAT, from 1999 to 2002, show that the double intertropical convergence zones (ITCZ) exist in the Atlantic and the eastern Pacific oceans for most of the annual cycle, and are far more extensive than previously recognized. For most of the time, the southern ITCZ is weaker than the northern one. The stronger ITCZ occurs when the northerly trade winds meet the southerly trade winds over warm water, resulting in deep convection. The weaker ITCZ over cooler water is caused by the deceleration of the surface winds as they approach the cold upwelling water near the equator. Decreases in vertical mixing and increases in vertical wind shear in the atmospheric boundary layer are suggested to be the causes of the deceleration of the trade winds as they move from warmer to colder water.