Another key factor last year was the waning of El Niño, a periodic, abnormally warm sea surface current, and the waxing of La Niña, a periodic, abnormally cold sea surface current, both in the Eastern Pacific. El Niño, brought about by a change in prevailing winds on the Pacific side of South America, was originally identified around Christmastime (hence its name, which means "the boy child"); however, every few years a warmer than normal El Niño supplants the cold and weakened Humboldt Current. An abnormally strong El Niño can raise sea surface temperatures by as much as nine degrees. A strong El Niño also lowers the pressure in the Eastern Pacific. Strong El Niño periods also reduce African rainfall and create upper-level winds traveling from west to east that shear the easterly waves headed the other way. None of this makes a tropical storm happy; fewer storms form in stronger El Niño years.