The latest February ENSO data is now available for February from NOAA, and it confirms that a second successive La Niña event began in the central Pacific (fig 2) in October 2017. The reason why it takes so long to confirm the fact is that the rules dictate that there have to be at least five concurrent months with three-month running average SST anomalies of -0.5°C or lower.
The latest advisory from NOAA suggests that this La Niña might not be particularly a long-lived event:
“La Niña is loosening its grip on the tropics. Sea surface temperatures in the central and eastern tropical Pacific remain cooler than average, but the typical La Niña wind and rainfall anomalies have faded. Conditions are likely (55% chance) to return to neutral by the March-May season“