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Multi-month forecasts of marine heatwaves and ocean acidification extremes

Abstract

Marine heatwaves and ocean acidification extreme events are periods during which temperature and acidification reach statistically extreme levels (90th percentile), relative to normal variability, potentially endangering ecosystems. As the threats from marine heatwaves and ocean acidification extreme events grow with climate change, there is need for skilful predictions of events months to years in advance. Previous work has demonstrated that climate models can predict marine heatwaves up to 12 months in advance in key regions, but forecasting of ocean acidification extreme events has been difficult due to the complexity of the processes leading to extremes and sparse observations. Here we use the Community Earth System Model Seasonal-to-Multiyear Large Ensemble to make predictions of marine heatwaves and two forms of ocean acidification extreme events, as defined by anomalies in hydrogen ion concentration and aragonite saturation state. We show that the ensemble skilfully predicts marine heatwaves and ocean acidification extreme events as defined by aragonite saturation state up to 1 year in advance. Predictive skill for ocean acidification extremes as defined by hydrogen ion concentration is lower, probably reflecting mismatch between model and observed state. Skill is highest in the eastern Pacific, reflecting the predictable contribution of El Niño/Southern Oscillation to regional variability. A forecast generated in late 2023 during the 2023–2024 El Niño event finds high likelihood for widespread marine heatwaves and ocean acidification extreme events in 2024.

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Fig. 1: Forecast skill for MHW, OAX (Ωa), and OAX ([H+]).
Fig. 2: Dominant modes of tropical Pacific variability in SMYLE FOSI from principal component analysis.
Fig. 3: Gain in SEDI forecast skill from forecasts initialized during ENSO events (El Niño or La Niña) relative to forecasts initialized during neutral ENSO conditions.
Fig. 4: CESM SMYLE forecasts of Niño3.4, MHW and OAX initialized in November 2023.

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Data availability

The CESM Seasonal to Multiyear Large Ensemble and SMYLE FOSI are available at https://doi.org/10.26024/pwma-re41 (ref. 39). OceanSODA-ETHZ can be accessed at https://doi.org/10.25921/m5wx-ja34 (ref. 61).

Code availability

All figures were generated with the open-source software Python. Code used in processing and analysing CESM SMYLE output relative to SMYLE FOSI and OceanSODA-ETHZ can be found at https://doi.org/10.5281/zenodo.12103992 (ref. 71).

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Acknowledgements

S.M. and N.S.L. were supported by the National Science Foundation (OCE 1752724) and the National Oceanic and Atmospheric Administration (NA20OAR4310405). M.P.B. was supported by the National Oceanic and Atmospheric Administration (NA20OAR4310405). S.J.Y. and N.R. acknowledge support from the Regional and Global Model Analysis (RGMA) component of the Earth and Environmental System Modeling Program of the US Department of Energy’s Office of Biological and Environmental Research (BER) under award number DE-SC0022070. W.K. and S.J.Y. acknowledge the support of NOAA Climate Program Office’s CVP programme under grant no. NA20OAR4310408. A.C. was supported by the NOAA Climate Program Office’s MAPP programme, DOE Award no. DE-SC0023228, and NASA Physical Oceanography grant no. 80NSSC21K0556. This work also was supported by the National Center for Atmospheric Research, which is a major facility sponsored by the National Science Foundation (NSF) under Cooperative Agreement no. 1852977. We thank the National Center for Atmospheric Research Earth System Working Group for their development of invaluable software tools used in processing CESM SMYLE: https://github.com/CESM-ESPWG/ESP-Lab. We are grateful for helpful feedback from D. Amaya.

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S.M. and N.S.L. designed the study. S.M. performed the analysis. All authors contributed to interpretation of the results. S.M. and N.S.L. wrote the manuscript with input from all authors.

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Correspondence to Samuel C. Mogen.

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Extended data

Extended Data Fig. 1 Different skill metrics (SEDI, Forecast Accuracy, Brier Skill Score) at lead-time 3.5 months.

Forecast skill calculated by three different metrics at lead-time 3.5 months. SEDI (as in Fig. 1) (row 1; a,b,c), Forecast Accuracy (row 2; d,e,f), and Brier Skill Score (BSS) (row 3: g,h,i) for marine heatwaves (column 1), ocean acidification extremes (Ωa), and ocean acidification extremes ([H+]).

Extended Data Fig. 2 Number, duration, and intensity of extremes in CESM SMYLE and observations.

Number per month (row 1; a,b,c), duration (row 2; d,e,f), and intensity (row 3; g,h,i) of the average extreme event in observations and CESM SMYLE for marine heatwaves (column 1), ocean acidification extremes (Ωa) (column 2), and ocean acidification extremes ([H+]) (column 3) at each location.

Extended Data Fig. 3 Correlation between historically observed and modelled variability and extremes.

Correlation coefficient between historical (column 1) variability of (a) sea surface temperature, (c) Ωa, (e) [H+] and (column 2) extremes (b) marine heatwaves, (d) ocean acidification extremes (Ωa),(f) ocean acidification extremes ([H+]) in SMYLE FOSI and observations (OceanSODA-ETHZ). Higher correlation coefficients indicate more similar historical variability or extreme events.

Extended Data Fig. 4 Model skill (CESM SMYLE relative to observations) and model predictability (CESM SMYLE relative to SMYLE FOSI).

Comparison of (column 1) model skill (CESM SMYLE relative to observations) and (column 2) model predictability (CESM SMYLE relative to SMYLE FOSI) for 20 ensemble members from CESM SMYLE at 1.5 (a-b), 3.5 (c-d), 6.5 (e-f), and 10.5 month lead-time (g-h). Skill scores range from -1 to 1, with SEDI score close to -1 being unskillful, SEDI score of 0 being no better than random forecasts, and SEDI score of 1 being perfect skill.

Extended Data Fig. 5 Decomposition and drivers of extremes in aragonite saturation state.

Decomposition of Ωa to determine drivers of extreme events. (row 1) Decomposition of [CO3]2- into drivers of changes during extremes relative to all times, including effects of (a) temperature, (b) salinity, (c) DIC, (d) Alkalinity. (row 2) Changes to tendency terms of DIC during extremes relative to all times, including: (e) total DIC tendency, (f) circulation tendency, (g) air-sea CO2 flux tendency, and (h) biological tendency.

Extended Data Fig. 6 Example forecast of aragonite saturation state extreme events from the 1999 La Niña.

Forecasts of Ωa initialized during the August 1999 La Niña event. Anomalies (color) and extremes (hatching) in (column 1) from an interpolated observational product (OceanSODA-ETHZ), and (column 2) CESM SMYLE forecasts (a,b) 1.5, (c,d) 3.5, and (e,f) 5.5 months after initialization. Extreme events are defined in observations (below the 10th percentile) and in CESM SMYLE (below the 10th percentile in a minimum of 50% of ensemble members).

Extended Data Fig. 7 Example forecast of aragonite saturation state extremes in the tropical Pacific from 2004-2012.

Example timeseries of Ωa anomalies in the central tropical Pacific (0.5N, 138.5W) for (black) SMYLE FOSI and (grey) two November CESM SMYLE initializations (2006a and 2009; with ensemble spread represented) from 2006-2012. Occurrence of extreme events are indicated for (red lines) SMYLE FOSI and (bar plot) CESM SMYLE (as a percentage of ensemble members).

Extended Data Fig. 8 Magnitude of variability associated with trend (per decade), climatology, interannual variability, and extreme event anomaly for aragonite saturation state.

Relative magnitude of anomalies associated with (row 1; a,b) trend (per decade), (row 2; c,d) seasonal climatology, (row 3; e,f) interannual variability, and (row 4; g,h) mean strength of anomaly to generate extreme event for temperature and Ωa.

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Mogen, S.C., Lovenduski, N.S., Yeager, S.G. et al. Multi-month forecasts of marine heatwaves and ocean acidification extremes. Nat. Geosci. 17, 1261–1267 (2024). https://doi.org/10.1038/s41561-024-01593-0

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