North and South America were joined at Panama around 3-4 million years ago. When this happened the waters of the Pacific and Atlantic Ocean were no longer able to mix freely. The easterly winds in the north and south which would carry the water vapor from the Pacific to the Atlantic are stopped by the Rockies and Andes while the westerly winds from the Caribbean are still able to carry water vapor from the Atlantic to the Pacific. This causes the Atlantic Ocean to be more salty than the Pacific Ocean by about 1 part per 1000. While this doesn’t sound like a lot it is enough to create the great ocean current carrying water from the Caribbean north to the north Atlantic then south and around Africa to the Indian Ocean and then into the Pacific and then in a similar route back through the Indian and back into the Atlantic Ocean.

(The Great Courses – The Physics of History)