Fig. 1: The southern oscillation index has failed to produce an El Nino-like signature.
By NOAA/CPC definition, El Nino onset arrives when the Nino34 sea surface temperature anomaly (SSTA) zone in the eastern equatorial Pacific warms to +0.5C (or warmer) for a minimum of 3 consecutive months. That level of warmth has arrived as of December 2018. Various government agencies such as NOAA and the Bureau of Meteorology/Australia should officially announce El Nino onset in early January. To this point an El Nino Watch was issued.
The 2015-16 El Nino ranked with the 1982-83 and 1997-98 El Nino episodes as strongest on record. However, there is no sign that El Nino 2019 will be anywhere close to the same intensity.
Ocean/atmosphere diagnostics that identify El Nino strength include the Nino34 SSTA which is marginally warm, subsurface temperatures in the eastern equatorial Pacific Ocean which have been surprisingly very warm, convection patterns in the tropical East Pacific which contribute most significantly to the El Nino climate and the focus of this post, the southern oscillation index (SOI).
The southern oscillation index (SOI) is a comparison of sea level pressure anomalies (SLPA) between the tropical waters of the West Pacific (at Darwin) and East Pacific (at Tahiti). Normally, widespread thunderstorms and low pressure dominate the eastern tropical Pacific during El Nino (while lack of rain and high pressure are evident in the tropical West Pacific). These conditions represent negative phase of the SOI.
During quarter 4 of 2018 the SOI was racing in the opposite direction of El Nino. A La Nina-like SOI appeared in October and strengthened during December. The problem is despite the marginal warming of the equatorial East Pacific the convection patterns that “couple” the ocean warming with the atmospheric climate have not developed (although there are signs during the past few days this pattern is starting to develop).
The global oceans are very warm compared to historical average. Included is both surface and subsurface warmth in the western tropical Pacific where heaviest tropical convection in recent months persists, a very un-El Nino-like signature. Tropical cyclones are present today (TD 36 and “Penny”) on either side of the West Pacific equator – rare for an El Nino episode.
Not surprisingly, El Nino climate (so far) during DEC/JAN/FEB 2018-19 has shown mixed results. The signature wet climate across the southern U.S. has developed and Uruguay has observed one of their wettest months of December on record. But the West Pacific climate is wetter than normal extending toward normally dry Southeast Asia.