Discussion: The multivariate ENSO index (MEI) for JUN/JUL 2018 was +0.076 indicating presence of neutral ENSO and ranking 32 for the 1950-2018 climatology where 1 = strongest La Nina and 69 = strongest El Nino. MEI was approaching an El Nino category in APR/MAY and MAY/JUN ranking 45 and 43 respectively. The JUN/JUL trend is clearly away from an anticipated El Nino onset as forecast by many dynamic and statistical models. What’s the problem?
A leading diagnostic foreshadowing a potential phase change in the El Nino southern oscillation is the equatorial Pacific subsurface ocean temperature and trend. Last APR/MAY anomalous warm water in the subsurface of the west and central equatorial Pacific Ocean emerged and shifted east of the Dateline. In JUN/JUL the subsurface warmth reached the northwest coast of South America. The sequence described is a classic contribution of the subsurface equatorial Pacific to initializing onset of an El Nino episode.
But…as identified by MEI, a climate diagnostic that identifies the reaction of the atmosphere to the equatorial sea surface temperature anomaly (SSTA) pattern, the air environment is not reacting to the ocean warming. Specifically, trade winds have not died off to allow the subsurface warming to emerge at surface level. Southern oscillation index (SOI) which measures the sea level pressure (SLP) patterns across the tropical Pacific Ocean has not shifted to the negative El Nino phase.
So does this set of circumstances mean El Nino will not develop? No. But, the ENSO outlook for later 2018 into early 2019 is much more questionable now. Reacting to short-term changes as a long-term forecast has been a mistake regarding ENSO the past 5 years or so.