Headline: Warm Kelvin Wave No. 2 of 2018 Eroding La Nina
Discussion: An interesting development has evolved in the eastern equatorial Pacific. During the past 2-3 weeks surface pressure has increased in this zone as identified by a suddenly persistent positive phase of the southern oscillation index (+SOI). The higher surface pressure is also related to stronger trade winds causing the eastern equatorial Pacific (Nino34, Nino12) to turn cooler the past 2-3 weeks (Fig. 1).
This occurrence takes place while the deeper subsurface is warming dramatically due to an eastward traveling Kelvin Wave (Fig. 2). The warm Kelvin Wave is slipping beneath the remaining cool water in the upper ocean of the eastern equatorial Pacific where La Nina lingers. Similar to a Kelvin Wave occurring earlier this year, the east surge of warmth from the West to East Pacific in the subsurface equatorial region is riding beneath the cool water just-beneath the surface where La Nina lingers (Fig. 3).
The subsurface cool waters required to sustain La Nina linger into April but should fade by May. Therefore the complete dissipation of La Nina is expected by later northern hemisphere spring.
The coolest forecast model of Nino34 SSTA in 2018 has been the NCEP CFS V2. However, note the latest ensembles are trending Nino34 warmer causing the operational forecast to also shift warmer for approaching northern hemisphere summer (Fig. 4).
Conclusion: There is some disagreement on whether La Nina has dissipated. In this situation use the multivariate ENSO index (MEI) to identify ENSO phase. Currently, MEI is weak negative which indicates the global climate is in a weak La Nina status.
Fig. 1: 12-week Nino SSTA observations indicate a steady weak La Nina.
Fig. 2: NOAA subsurface equatorial Pacific Ocean temperature anomaly analysis identifies a Kelvin Wave moving east of the Dateline. Note that most recently the Kelvin Wave is slipping beneath shallow cool water sustaining weak La Nina.
Fig. 3: Analysis of the influence of 2 Kelvin Waves on the East Pacific upper ocean heat.
Fig. 4: The NCEP CFS V2 Nino34 SSTA forecast. The blue lines represent the latest set of ensemble forecasts which are warmer.