The trade winds are the prevailing pattern of easterly surface winds found in the tropics, within the lower portion of the Earth's atmosphere. They occur in the lower section of the troposphere near the Earth's equator. The trade winds blow predominantly from the northeast in the Northern Hemisphere. They blow from the southeast in the Southern Hemisphere. They are stronger during the winter and when the Arctic oscillation is in its warm phase. Trade winds have been used by captains of sailing ships to cross the world's oceans for centuries. They enabled European empire expansion into the Americas. They also allowed trade routes to become established across the Atlantic and Pacific oceans.