Weak La Nina Could Regain Some Strength in April as Subsurface Cools

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Fig. 1-2: The daily Nino12 SSTA observations and 14-day change.

Discussion: During the first half of March the Nino12 SSTA region off the northwest coast of South America warmed significantly. Warming was due to the arrival of a subsurface Kelvin Wave. The warming also signaled validation of some forecast models which forecast warming in this region although not until April followed by possible western expansion of that warmth heading toward mid-year. The implication was an ending of La Nina and possible trend toward El Nino.

However, changes have occurred. During March, the Madden Julian oscillation (MJO) was strong and caused anomalous convection in the far western equatorial Pacific region. The rising air currents associated with the convection is replaced from the east by increasing trade winds which has up-welled cooler subsurface waters and caused the central/east-central equatorial Pacific to cool restrengthening La Nina. More recently, trade winds are starting to regenerate in the Nino12 SSTA region and waters have cooled substantially during the past 2 weeks (Fig. 1-2).

As always, the subsurface equatorial Pacific offers clues as to where ENSO phase is going, at least in the short-term. During the past week, the eastern subsurface equatorial Pacific Ocean has cooled sharply (Fig. 3). Given persistent trade winds, the cool subsurface waters are likely to reach surface level and sustain (or strengthen) La Nina into April. Remnants of the (warm) Kelvin Wave in the Nino12 SSTA region are likely to weaken.

Fig. 3: Subsurface equatorial Pacific Ocean temperature anomalies.

The next Kelvin Wave is forming in the subsurface equatorial West Pacific. Part of this feature has extended eastward to near the Dateline. If the next Kelvin Wave traverses the East Pacific the timing for reaching to northwest coast of South America is toward June 1st. Until that time (at least) La Nina continues. The catalyst to shift a Kelvin Wave across the equatorial Pacific is an eastward pules of MJO-style convection which is not indicated through the next 2-3 weeks.

In summary, La Nina continues as implied by the re-cooling upper ocean heat pattern in the eastern equatorial Pacific Ocean. The weekly Nino SSTA are steady and indicate a weak La Nina signature (Fig. 4). Some strengthening of La Nina is possible during April.

Fig. 4: The 12-week SSTA pattern of the Nino SSTA regions.