Executive summary: La Nina is moderate strength right now. Weakening is expected. The weakening of La Nina occurs due to a transient tropical convection oscillation know as the Madden Julian oscillation (MJO) which shuts down trade winds and allows warm subsurface waters to shift eastward from the equatorial subsurface near and west of the Dateline. The second of 4 expected MJO events is occurring now. The next episode is scheduled for the last 1/3 of February and a 4th event around Apr. 1st. La Nina will weaken and end during the MJO sequences described. Neutral ENSO follows. Most forecast models are projecting El Nino any time after mid-year. Forecast confidence is reasonable on neutral ENSO ahead but remains below average confidence on El Nino expectations.
Discussion: During December 2022, leading ENSO climate diagnostics revealed a mix of both weaker and stronger La Nina signals. The conventional measurement of ENSO phase, Nino34 SSTA, was weak-to-moderate intensity at -0.84C in December, slightly warmer than November.
The southern oscillation index (SOI) was a vigorous La Nina-like +1.84 in December after a weak -0.47 value was observed in November. The rare (for the 2020-22 La Nina episode) weak monthly SOI value in November was due to passage through the eastern tropical Pacific Ocean of a Madden Julian oscillation (MJO) episode. MJO was absent from the East Pacific tropics in December explaining the return to robust +SOI. However, during early January, MJO has returned to the tropical East Pacific and vigorous +SOI is forecast to shift to a brief period of negative phase the next 5-10 days.
Multivariate ENSO index (MEI), an index that defines the atmospheric reaction to equatorial Pacific Ocean temperatures, has shown moderate to intense La Nina climate throughout the 2020-22 episode most vigorously during the past 9 months. -MEI has weakened slightly but remains quite intense as the La Nina climate roars on heading into 2023.
The subsurface equatorial Pacific is extremely warm near and west of the Dateline. Some of the warmth shifted east to weaken the previous cool anomaly east of the Dateline in November. The catalyst to this eastward shift of warmer subsurface water was weakening of trade winds caused by the influence of a transient MJO. Another MJO event is shifting through the equatorial East Pacific now and some additional warming in the subsurface East Pacific at the equator is occurring.
The current eastern shift of MJO through the eastern equatorial Pacific is not particularly strong. A flip to daily -SOI values would support a stronger event but SOI may be negative only for a 2-3-day period for this event likely occurring by this weekend. While additional warming of the eastern equatorial subsurface is expected during January, overall support for a continuing La Nina episode remains fairly robust.
Other parts of the ENSO system reveal marginal support from the Pacific decadal oscillation (PDO). The PDO remains in cool phase, which is supportive of La Nina but has likely weakened slightly during the past 2-3 weeks. Cool water inflow via the California Current southwestward into the East Pacific tropics is weak right now suggesting the cool SSTA near the equator could weaken. Cool inflow in the South Pacific via the Humboldt Current has also been weak.
A large area of warm SSTA north of Australia and supportive of a negative Indian Ocean dipole (-IOD) during Q3 into Q4 of 2022 has dissipated. Consequently, the aerial coverage of warm SSTA in the far West Pacific tropics has diminished leading to less equatorial convection which when ample encourages trade winds near the Dateline to sustain La Nina. That pattern has weakened.
Overall, La Nina intensity is projected as moderate and weaker since the 3rd peak of the 2020-22 event which occurred during late Q3/2022. The La Nina climate remains resilient as is stronger than the attendant oceanic counterpart.
Based on a collection of all ENSO phase dynamic and statistical forecast models, the ENSO phase outlook for 2023 is for La Nina weakening over the next several months with neutral phase present for Q2/2023. Dynamic models favor a transition into El Nino during Q3/2023 while statistical models maintain neutral ENSO into early Q4/2023.
The latest NCEP CFS V2 forecast indicates La Nina dissipating by MAR/APR followed by neutral ENSO through northern hemisphere summertime and a weak borderline El Nino for later in the year. The Australia Bureau of Meteorology indicates El Nino potential by June. Their forecast is stronger with the El Nino risk. ECMWF indicates risk of El Nino arriving mid-year.
The Climate Impact Company constructed analog forecast reveals the most likely scenarios are La Nina ending by March with El Nino onset possible by September and lingering through 2024 with weak intensity (Fig. 1).
Fig. 1: The Climate Impact Company constructed analog Nino34 SSTA forecast reveals La Nina ending by March 2023, El Nino beginning by September 2023 and lasting through 2024 with weak intensity.
The key to the forecast is in the effectiveness of transient MJO episodes across the equatorial Pacific during the next several months. One episode is occurring now and is the second in the sequence following the November event. The MJO shifting through the eastern equatorial Pacific is expected to have a slight weakening effect on La Nina. Normally, about 4-6 weeks separate MJO episodes. Consequently, the next MJO episode to pass through the eastern equatorial Pacific and continue the eroding La Nina process is during the last third of February. A 4th MJO episode should follow in early April. La Nina should end after the 3rd MJO episode in early March or early April at the latest.
The La Nina climate has over-achieved and is likely to continue another couple of months after oceanic La Nina has ended. The La Nina climate, based on MEI should last to June.
The second half of next year favors El Nino although forecast confidence is low. Last year, a similar forecast failed. However, there is no historical precedent for La Nina to continue into mid-to-late 2023. Whether El Nino forms or not from a historical point of view has mixed results.
While oceanic El Nino could form as early as June, the El Nino climate may be delayed for several months. Therefore, El Nino climate conditions are likely delayed until the last third of 2023.
El Nino presence would diminish tropical cyclone activity in the North Atlantic basin which in recent years has been historically active. East Pacific tropical activity would increase. West Pacific tropical cyclone activity decreases if an El Nino forms.
El Nino would favor a wetter Southern U.S. regime for SEP-DEC 2023. A mild and dry climate develops over Western Canada and possibly the Northwest U.S. Indonesia, Southeast Asia and parts of Australia shift into a drier climate. Argentina turns wet while Brazil is drier if an El Nino climate forms later next year.