Deep ocean warming-induced El Niño changes

The Earth's unprecedented warming rate due to human activities necessitates understanding climate irreversibility. Even with CO2 emission reductions, the heat absorbed by the deep ocean will gradually release, influencing climate patterns. This study focuses on the impact of this irreversible deep ocean warming on the El Niño-Southern Oscillation (ENSO), a major climate variability affecting global weather and climate. Previous research has explored ENSO changes under greenhouse warming, but the role of irreversible deep ocean warming remains largely uninvestigated. This paper aims to address this gap by examining how deep ocean warming, even after CO2 stabilization, modifies ENSO characteristics, providing insights into the long-term effects of anthropogenic climate change.

Existing literature extensively studies ENSO changes under greenhouse warming, focusing on intensity, asymmetry, duration, spatial patterns, and teleconnections. Studies show that greenhouse warming leads to more Central Pacific (CP) El Niño events and more extreme Eastern Pacific (EP) El Niño events. The increase in SST variability over the eastern Pacific and the southward shift of the ITCZ have also been noted. However, the direct role of deep ocean warming in driving these changes during the climate recovery process is largely unexplored. This study directly investigates the link between deep ocean warming and El Niño changes by examining changes in ENSO during the period of climate restoration after CO2 levels return to present-day conditions.

The study utilizes the Community Earth System Model (CESM) Version 1.2 for its simulations. Two primary experiments were conducted: a ramp-up and ramp-down experiment simulating CO2 increase and subsequent decrease to present-day levels, and initial warming experiments (IW EXPs). The ramp-up and ramp-down experiment modeled the gradual increase and decrease of atmospheric CO2 concentration, mimicking a scenario of increased emissions followed by net-zero emission policies. The recovery period after returning to present-day CO2 levels was then analyzed. Three IW EXPs involved adding horizontally uniform vertical profiles of ocean temperature and salinity anomalies below 700m, below 100m and to the entire ocean depth respectively, to the initial ocean conditions. The responses in terms of SST, precipitation, wind patterns, ENSO amplitude (Niño3 and Niño4 indices), and ENSO characteristics were analyzed. The study employed a 1-standard deviation threshold of the December-January-February (DJF) mean Niño3 sea surface temperature anomaly (SSTA) to define El Niño events, distinguishing between CP and EP El Niño based on the longitude of the SSTA peak. Linear regression analysis was performed to assess the ocean current response to zonal wind stress and the precipitation and wind response to Niño3 SSTA.

The results show that even when CO2 levels return to present-day values, the tropical mean state exhibits significant differences compared to the current climate. An El Niño-like warming pattern emerges in the spatial pattern of SST difference, along with a southward shift of the ITCZ. This southward shift is attributed to faster cooling in the Northern Hemisphere compared to the Southern Hemisphere during climate recovery. The deep ocean warming acts as a significant driver of this El Niño-like warming and the southward shift of the ITCZ, as confirmed by the IW EXPs. The IW EXPs show a strong positive correlation (0.95 for SST, 0.97 for precipitation) between the SST and precipitation patterns in the restoring period and the patterns generated when initial ocean warming is added below 700m. The amplitude of the Niño3 SSTA increases significantly in all IW EXPs, indicating enhanced SST variability over the eastern Pacific. The analysis of El Niño events shows a shift toward more frequent EP El Niño events and a decrease in CP El Niño events during the restoring period and in the IW EXPs. The occurrence of convective extreme El Niño events is projected to increase by 40-80%. Further analysis reveals that deep ocean warming strengthens the surface-layer feedback in the eastern Pacific, leading to enhanced zonal advective feedback and an eastward shift of the ENSO feedback system. This eastward shift, characterized by an eastward extension of the precipitation and wind response to Niño3 SSTA, contributes to the increased frequency of EP El Niño events.

The findings demonstrate that deep ocean warming significantly alters the characteristics of ENSO, even after achieving net-zero carbon emissions. The observed El Niño-like warming and the resultant changes in ENSO feedback mechanisms are primarily driven by the delayed release of heat from the deep ocean. This impacts decadal predictability and requires a reassessment of future ENSO projections. The discrepancy between model predictions and observations of El Niño-like conditions might be partly explained by the potential underestimation of deep ocean heat accumulation in models. The study's results underscore the need to consider deep ocean warming in future climate modeling and ENSO projections, given its influence on tropical mean state and ENSO characteristics.

This study reveals the critical influence of deep ocean warming on ENSO characteristics. Even under net-zero emission scenarios, the continued release of heat from the deep ocean will likely lead to more frequent and intense EP El Niño events. This highlights the long-term consequences of anthropogenic warming and the need to incorporate deep ocean warming processes into future climate projections and decadal prediction systems. Further research should focus on refining climate models to accurately represent deep ocean heat storage and release processes to improve future ENSO predictions.

The study relies on a single climate model (CESM), limiting the generalizability of the findings. Further research using multiple models and exploring different CO2 emission scenarios is crucial to confirm the robustness of these results. The idealized nature of the initial warming experiments, using uniform temperature anomalies, may not perfectly capture the complexity of real-world ocean warming patterns. Future studies should investigate the impact of more realistic ocean warming patterns on ENSO.