NOAA dropped their odds of an El Niño event forming this winter from 67% in their October outlook to 58% in their November outlook
, but a surge of warm water over the equatorial Eastern Pacific over the past week could signal the onset of El Niño. The departure of sea surface temperature (SST) from average in the equatorial Pacific region 120°W - 170°W, 5°S - 5°N (called the Niño 3.4 region) crossed the +0.5°C from average threshold in mid-October, and as of November 10, these temperatures were +0.8°C from average--the greatest weekly anomaly since late August 2012.
By definition, an El Niño episode occurs when SSTs are at least +0.5°C from average in this region for five consecutive months, with each month representing a 3-month average. Furthermore, ocean currents along the Equator flowing from east to west have weakened significantly over the past week, as apparent from plots made using NOAA's Ocean Surface Current Analyses - Real time
web site. This sort of weakening typically happens at the onset of an El Niño event.Figure 1.
Departure of sea surface temperature (SST) from average in the equatorial Pacific region 120°W - 170°W, 5°S - 5°N (called the Niño 3.4 region.) By definition, an El Niño episode occurs when SSTs are at least +0.5°C from average (above the red line) in this region for five consecutive months, with each month representing a 3-month average. Niño 3.4 temperatures crossed the +0.5°C from average threshold in mid-October, and as of November 10, were +0.8°C from average--the greatest weekly anomaly since late August 2012.
Image credit: NOAA Climate Prediction Center
.Why we may not get an El Niño episode
While the progression towards El Niño over the past week may seem to be compelling evidence that El Niño is imminent, we've been fooled before. One of the strongest sub-surface waves of warm water (a Kelvin wave) ever recorded in the Eastern Pacific pushed eastwards earlier this year, giving rise to speculation back in May that a strong El Niño event might be on the way. However, the westerly winds needed to push this warm water to the east towards South America never got strong enough, and progress towards El Niño conditions faltered. A similar sequence of events unfolded in September 2012.
Forecasting El Niño is hard, and has been made more difficult in recent years since the character of El Niño itself seems to be changing on decadal (10-year) or longer timescales. Emily Becker, a researcher at the NOAA’s Climate Prediction Center (CPC), provides some insight as to why NOAA dropped their El Niño odds in NOAA's latest ENSO Blog post
(ENSO is the acronym for “El Niño - Southern Oscillation”, which is the more scientifically rigorous term for what I typically refer to as “El Niño.") One key reason she gives: "We’re still not seeing much of an atmospheric response to the surface warming, so there is now concern that if El Niño conditions are achieved, they won’t persist for the five overlapping seasons required for this to be called an El Niño event."
An El Niño event typically brings cooler and wetter than average winter weather to the Southern U.S.