Winds and storms made the South Atlantic impossible to sail. Before 1400 no one had ever crossed it accidentally, let alone on purpose.
Why Couldn't Sailors Cross the South Atlantic Before 1400?All ocean sailors before 1400 relied upon highly predictable seasonal winds and currents often called "trade winds" because when they blew, the traders sailed into or out of harbors. Trade winds and currents were the reason for regular long-distance travel in the Pacific and Indian Oceans and the South China Sea..
Some sailors ( Northern Europeans and Chinese for example) also had a coastal shelf to follow. Leif Ericson could follow an continental shelf part of the way across the northern edge of the North Atlantic, and then use prevailing winds and currents to travel the rest of the way and back. ledge to North America, no such edge connected the tip of Africa with South America's tail, only vicious storms. Crossing the South Atlantic required a different approach to navigation
There were three major weather obstacles in the South Atlantic1. Just west of the Canary Islands (off the coast of Africa), currents would carry ships in one direction--west across the Atlantic. Long periods of calm would sometimes strand ships in this area.
2. Winds and currents in the southern hemisphere flow in the opposite direction from the northern hemisphere. Portuguese sailors had to figure this phenomenon out (now called the Coriolis effect). Therefore in order to avoid fighting these currents, Portuguese sailors would have to sail westward almost all the way across the South Atlantic to Brazil before turning to the east, to the southern tip of Africa. This step was counter-intuitive.
3. Finally in order to sail back across the Atlantic from the bulge of South America to the tip of southern Africa (as Bartholomew Dias finally did in 1487) was difficult because of strong winds, powerful intersecting currents, and storms with winds up to 180 kilometers per hour.