Marine heatwaves and marine cold spells are extreme ocean temperature events that can extend over vast oceanic regions and persist from weeks to months. Beyond their thermal signature, these events are often associated with compound biogeochemical perturbations, including deoxygenation, changes in nutrient supply, altered light availability, and modifications of phytoplankton biomass and productivity. Marine heatwaves, in particular, are becoming an increasingly prominent threat under climate change, with cascading impacts across marine food webs and major ecological and socio-economic consequences. However, most existing diagnostics remain focused on the surface ocean, whereas ecosystem impacts depend strongly on the four-dimensional structure of these events, including their vertical extent, temporal evolution, and coupling with subsurface biogeochemical anomalies.

Here, we develop a 4D framework to characterize major marine heatwaves and cold spells in the global ocean by combining satellite observations, BGC-Argo measurements, and gridded physical and biogeochemical products. This approach provides a depth-resolved, multi-variable description of ocean extremes, linking their surface expression to their subsurface structure and temporal evolution. Events are characterized throughout the water column using temperature, oxygen, nitrate, and phytoplankton biomass proxies. For each event, we extract standardized metrics describing its duration, intensity, horizontal and vertical extent, volume, and the co-occurrence of physical and biogeochemical stressors. This framework allows us to move beyond a purely thermal definition of extremes and to categorize events according to their compound 4D environmental signatures. As an output, it leads to a global 4D atlas of major warm and cold ocean extremes, providing a comparative, depth-resolved inventory of their physical and biogeochemical signatures.

We apply this framework to several emblematic case studies, including the North Pacific Blob, recent North Atlantic extremes, and El Niño-related anomalies, to compare the mechanistic drivers and ecosystem-relevant signatures of both warm and cold events. This comparative analysis assesses how contrasting physical pathways, including enhanced stratification, nutrient resupply, light exposure, and oxygen anomalies, shape the biogeochemical expression of ocean temperature extremes. Preliminary results show that marine heatwaves and cold spells can generate distinct, and sometimes regionally opposite, ecological pathways, suggesting that phytoplankton responses cannot be inferred from surface temperature anomalies alone. Finally, we provide insights into broader ecosystem implications, including changes in depth-resolved net primary production and potential shifts in habitat suitability and ecological niches for large pelagic fish species.