Leftover warm water in Pacific Ocean fuelled massive El Nino

Warm water left over in the Pacific Ocean from 2014 stacked the deck for a monster El Nino to occur last year, making it one of the strongest on record, a new study has found.
Easterly winds in the tropical Pacific Ocean stalled a potential El Nino in 2014 and left a swath of warm water, researchers said.
The presence of that warm water led to the massive El Nino to occur in 2015, they said.
El Nino and La Nina are the warm and cool phases of a recurring climate pattern across the tropical Pacific Ocean called the El Nino-Southern Oscillation (ENSO).

The warm and cool phases shift back and forth every two to seven years, and each phase triggers predictable disruptions in temperature, wind and rain across the globe.
During El Nino events, water temperatures at the sea surface are higher than normal. Low-level surface winds, which normally blow east to west along the equator, or easterly winds, start blowing the other direction, west to east, or westerly.

In 2014, strong westerly winds near the equator in the western and central Pacific Ocean created a buzz among scientists - they saw the winds as a sign of a large El Nino event to come in the winter of 2014, said Aaron Levine, from the US National Oceanic and Atmospheric Administration (NOAA).

However, El Nino did not form the way scientists expected it to - sea surface temperatures in the eastern Pacific never warmed enough to truly be called an El Nino.
However, last year, episodes of very strong westerly wind bursts occurred and became more frequent throughout summer.
Following a pattern set by previous large El Ninos, 2015 to 2016 became one of the three strongest El Ninos on record, along with 1982 to 1983 and 1997 to 1998, Levine said.
In the study, researchers examined changes in sea surface and sub-surface temperatures, winds, and volumes of warm water in the Pacific Ocean from 2014 to 2016.

They also used a mathematical model to analyse how these factors were related.
"As an El Nino develops and matures into its peak phase, (warm water) gets discharged out of the equatorial regions to the polar regions," Levine said.
In 2014, easterly winds prevented that warm water from being transported poleward. The warm water stuck around through the winter and was available as a reservoir of heat that could be tapped into the following year.
"Once we started to get some additional westerly winds - unusually strong westerly winds that occurred in the spring and summer of 2015 - an El Nino developed," he said.

Researchers found a similar event occurred in 1990, when easterly winds counteracted a budding El Nino, and leftover warm water fuelled El Nino conditions in 1991 to 1992.
The study was published in the journal Geophysical Research Letters.