Fig. 1-2: The 12-week Nino SSTA regions indicate a cool reversal last week into La Nina qualifiers (left). Multivariate ENSO index indicates weak La Nina has been present for 2 months (right).
Trade winds increased across the eastern equatorial Pacific Ocean last week. As a result, the Nino SSTA regions cooled significantly. Last week the Nino34 region cooled to -1.2C which is the coolest value of this run toward La Nina (Fig. 1). The atmospheric reaction to the cooler SSTA trend of the past few months is measured by multivariate ENSO index (MEI). The MEI has transitioned into weak La Nina as of September (Fig. 2). Subsurface temperatures in the eastern equatorial Pacific are much cooler than normal (Fig. 3). If trade winds remain stronger than normal the La Nina pattern will become stronger. Formal announcement of La Nina by NOAA is expected in December although as indicated by MEI a La Nina episode has likely been present since September.
Fig. 3: Subsurface cool waters across the equatorial East Pacific are plentiful ready to fuel La Nina onset.