Eastward spreading El Ninos matter because the pool of heat over the western Pacific is likely to shift to the east during such events, taking rainfall with it and exacerbating drought conditions over Australia, said lead author Agus Santoso, a senior research associate at UNSW's Climate Change Research Centre.

“The most common theory used to explain these unusual El Ninos was that competing air and ocean feedbacks drove the direction of the warming,” said Dr Santoso.

“But if this was true, La Ninas (the opposite of El Ninos, bringing rain to eastern Australia and drought to the eastern Pacific) would have propagated in the same direction. Observations show they do not.”

As the world gets warmer, the eastern Pacific is expected to warm faster than the west. Since winds respond to pressure differences – with trade winds blowing east to west – those current flows are likely to weaken as conditions across the Pacific become more similar, Dr Santoso said.

“In the future, ...you won't need a strong El Nino to reverse the current,” he said. “If you get weaker trade winds, the current weakens as well.”