Why Does La Nina End Q1/2023 and El Nino Form Later in 2023?

Fig. 1: ECMWF global SSTA forecast for February 2023.

Discussion: Last week, La Nina remained moderately strong as the Nino34 SSTA was slightly warmer than the previous week at -0.9C while strong daily positive southern oscillation index (+SOI) continues, a sign that La Nina climate is well intact. In the subsurface, waters east of the Dateline stay cool to maintain La Nina. However, as the 3^rd year of La Nina continues, Climate Impact Company is forecasting El Nino to form later in 2023. By early next year, the very warm SSTA pattern enhanced by the negative Indian Ocean Dipole (-IOD) in the far eastern equatorial Indian Ocean will fade. Right now, the super warm waters north and northwest of Australia are fueling convection which includes rising air currents and that rising air is replaced by enhanced trade winds from the east. The trade winds up-well cool water in the central/eastern equatorial Pacific Ocean to maintain La Nina. However, that ENSO accelerator weakens and is gone by FEB/MAR next year (Fig. 1). Consequently, La Nina should also end.

What follows? The 2020-22 La Nina is the 4^th (since 1950). Based on history, La Nina weakens to neutral ENSO for Q1/Q2 of 2023 and a 2-in-3 risk of El Nino development is likely after mid-2023 (Fig. 2). Note that the NCEP CFS V2 Nino34 SSTA forecast is trending toward El Nino middle 2023 (Fig. 3)

Conclusion: Once the super warm SSTA in the far eastern equatorial Indian Ocean/western Pacific Ocean recedes, the mechanism driving La Nina should ease. The catalyst to this change is dissipation of the -IOD pattern in January. ENSO has natural character to oscillate from one phase to the opposite phase and that character should be in-place for later 2023.

Fig. 2: Nino34 SSTA regimes for lengthy (3+ years) La Nina episodes La Nina episodes.

Fig. 3: NCEP CFS V2 Nino34 SSTA forecast indicates El Nino evolution later in 2023.