Predicting the El Nino of 2023/24 and its climate impacts over North America

The CESM2 Seasonal-to-Multiyear Large Ensemble (SMYLE) prediction system exhibits good skill at predicting tropical Pacific sea surface temperature (SST) evolution up to a year in advance based on hindcasts spanning 1970-2019. A real-time SMYLE forecast initialized on August 1st 2023 predicted a major El Niño event for December 2023 – February 2024 (DJF23/24), with an ensemble-mean Niño 3.4 SST anomaly exceeding +2°C. A subsequent SMYLE forecast initialized on November 1st 2023 predicted a slightly weaker event that has verified well against emerging observations. SMYLE skill at predicting climate impacts over land is much lower than that for tropical SST, likely due in part to the growth with leadtime of coupled model bias. To minimize the effect of model SST error on forecasted climate impacts, monthly SMYLE SST anomaly fields are rescaled based on hindcast performance and added to observed SST climatology. We then use the post-processed SMYLE SST forecasts as a lower boundary condition for a set of CAM6 AMIP simulations to address the predictability of El Nino’s climate impacts over North America. Specifically, we conduct a 50-member AMIP ensemble from Nov 2023 – June 2024 using ensemble-mean bias-corrected SST anomalies from the SMYLE predictions initialized on Nov 1, 2023. The AMIP ensemble shows a high chance for wetter conditions over the southern tier of the US and Alaska, accompanied by drier conditions over British Columbia, the Pacific Northwest and parts of the northeastern US during DJF23/24. A counterfactual AMIP ensemble further reveals that these precipitation impacts result from a complex interplay between atmospheric teleconnections driven by El Nino and those driven by long-term SST trends. In particular, the observed strengthening of the zonal SST gradient across the tropical Pacific induces a high-pressure response over the North Pacific, partially counteracting the El Nino-driven deepening of the Aleutian Low. These experiments underscore the modulating influence of long-term SST trends on predicted climate impacts from the 2023/24 El Nino.