The Nino34 SSTA is the most common measure of eastern equatorial Pacific Ocean temperature to determine (and forecast) ENSO phase. During the climate change era of the past 2-3 decades other factors have produced a tendency to alert known climatology of ENSO events. There are many examples best represented by the tendency of middle latitude SSTA regimes to also contribute to climate. These regimes are measured by Pacific decadal oscillation (PDO) and Atlantic multi-decadal oscillation (AMO).
Due to the increased factors affecting global climate (and ENSO influence on climate) a different measure of ENSO is more highly considered. Multivariate ENSO index developed by Klaus Wolter at NOAA measures the reaction of the atmosphere to the ENSO regime over a 2-month period. Note that while Nino34 SSTA is shifting into La Nina (cool) territory the MEI has shifted to neutral phase during the past 1-2 months.
What does this tell us? First, La Nina 2017-18 will be very weak. Second, the standard La Nina climate for northern hemisphere winter/southern hemisphere summer is likely not reliable and other factors such as PDO and AMO (and other factors) may have larger influences on climate.
Another possible reason for the weaker MEI signature (versus Nino34) is the near record warm global oceans which is preventing La Nina from gathering organization. Similarly, last May when Nino34 SSTA was marginal El Nino the MEI was in bold El Nino intensity also a signature of the global oceanic warmth on the ENSO regime (and atmospheric reaction).