Discussion: Weak La Nina remains in-place across the eastern equatorial Pacific. The Nino SSTA regions each are consistent with a cool bias most strongly indicated in the far eastern equatorial Pacific (Nino3) while the Nino4 region near and just east of the Dateline stays in neutral phase (Fig. 1).
Fig. 1: The 12-week Nino SSTA observations indicate a steady weak La Nina episode with cool bias in the far eastern equatorial Pacific. Nino4 represents the equatorial Pacific near and just east of the Dateline while Nino34 and Nino3 represent the east-central equatorial Pacific. Nino12 is off the northwest coast of South America. La Nina 2017-18 cool waters did not reach the Dateline as with La Nina’s in the past.
During La Nina the southern oscillation index is usually in the positive phase. Note that since La Nina evolution late last year SOI has been in the marginal La Nina phase except for a brief El Nino-like character during early January (Fig. 2). The early January El Nino-like character was attributed to a strong Madden Julian oscillation (MJO) episode. The MJO was so intense that a contribution to stratospheric warming over the North Pole region occurred which in-turn lead to an arctic outbreak into the U.S. During this event atmospheric angular momentum was at a record low (index) leading to an amplified middle latitude trough/ridge pattern in both hemispheres. Clearly, La Nina 2017-18 has been weak and possibly the weakest on record (based on multivariate ENSO index).
Fig. 2: Daily southern oscillation index observations back to January 2016 according to the Bureau of Meteorology/Australia.
Further demonstrating the weak nature of La Nina 2017-18 is the brief and limited cool anomalies in the upper ocean east of the Dateline during the last quarter of 2017 which initiated La Nina but have now depleted during early 2018 (Fig. 3). The subsurface details for January 2018 indicate a Kelvin Wave, a warm subsurface anomaly born in the West Pacific late last year shifting east past the Dateline in early January (Fig. 3). During the month of January the Kelvin Wave shifted east and weakened but was strong enough to erode the subsurface cool in the equatorial East Pacific required to sustain La Nina (Fig. 4). A new area of warmth in the subsurface equatorial Pacific is emerging.
The subsurface events of the past 1-2 months indicate an attempt at weakening La Nina was attempted. Weak La Nina hangs on. While a new Kelvin Wave regenerates in the West Pacific a shoaling process is likely to regenerate in the eastern equatorial Pacific causing the up-welling regime to return to sustain weak La Nina through the remainder of quarter 1 of 2018.
Fig. 3: Upper ocean heat east of the Dateline is weakening rapidly.
Fig. 4: During January a central subsurface Pacific Kelvin Wave shifted east and weakened while eroding the cool waters of the eastern equatorial Pacific needed to sustain La Nina.
The ENSO forecast is uncertain somewhat based on the (subsurface) dynamics just described. To depict the uncertainty 3 forecast models are reviewed each projecting their global SSTA regimes for JUN/JUL/AUG 2018. The lead model (NMME) indicates neutral ENSO emerges for mid-2018 combined with a weak cool Pacific decadal oscillation and robust warm Atlantic multi-decadal oscillation (Fig. 5).
The NCEP CFS V2, a popular model due to its daily updates indicates La Nina continues during northern hemisphere meteorological summer (Fig. 6). The model indicates a weak PDO regime but is consistent with a warm AMO pattern.
The Canadian model projects an El Nino for JUN/JUL/AUG 2018 (Fig. 7). The ECMWF outlook trends in that direction. The CMC model also indicates a +PDO and +AMO regime.
Fig. 5: The NMME global SSTA forecast for JUN/JUL/AUG 2018.
Fig. 6: The CFS V2 global SSTA forecast for JUN/JUL/AUG 2018.
Fig. 7: The CMC1 global SSTA forecast for JUN/JUL/AUG 2018.
Summary/Conclusion: La Nina 2017-18 is possibly the weakest La Nina episode on record. A recent transient subsurface Kelvin Wave eroded the cool waters required to sustain La Nina. However, weak La Nina is likely to continue as the effects of the Kelvin Wave fade. A new Kelvin Wave forms in the subsurface West Pacific during the next few weeks and is likely to shift east by early meteorological spring and if sufficiently strong would dissipate La Nina. If so, neutral ENSO follows. The extended-range forecast is uncertain as models vary from sustained La Nina to a possible El Nino onset. Tracking the dynamics of the subsurface over the next 2-3 months is the best diagnostic to determine ENSO phase into mid-2018.