Evolution of causal relationships under climate change: controls of Net Primary Productivity in the North Altantic Subpolar Gyre

Abstract. Understanding how climate change affects marine primary productivity requires examining the evolving causal relationships between physical and biogeochemical processes. We applied the PCMCI+ causal discovery algorithm to investigate how the mechanisms controlling Net Primary Productivity (NPP) in the North Atlantic Subpolar Gyre evolve under different climate scenarios across five Earth System Models. Using 100-year sliding windows, we compare causal relationships in future scenarios against pre-industrial conditions, focusing on the roles of mixed layer nutrients, vertical mixing and horizontal transport. Our analysis reveals three main categories of relationship evolution: the emergence of links, the disappearance of links, and changes in link strengths. For example, while the link between stratification and NPP emerges under climate change in CanESM5-CanOE, it strengthens in CMCC-ESM2 and remains stable with moderate to high intensities in other models. At the end of the 21st century, the spread between models regarding the effect of stratification on NPP is reduced compared to pre- industrial conditions, suggesting a reduction in inter-model uncertainty. However, the transport and vertical mixing controls on the supply of nutrients to the mixed layer exhibit a more diverse evolution among the ESMs studied. The CMCC-ESM2 model has a strengthening of the relationships between winter vertical mixing and nutrients, while IPSL-CM6A-LR and CanESM5CanOE show weakening of these relationships. Furthermore, the evolution of the link between nutrient supply to the mixed layer for NPP exhibits a large variability between models. These divergent pathways reveal that the dynamics of nutrients has uncertain evolution between models. Lastly, model-specific dynamics are also observed, such as the strengthening of the link between horizontal transport and the nutrient content of the mixed layer in IPSL-CM6A-LR. Together with the decreasing strength of the vertical mixing/nutrients link, this suggests the presence of compensation mechanisms and a shift from vertical mixing dominance to enhanced horizontal transport control over the course of the scenario. These findings offer mechanistic insights into the dynamics of ESMs, specifically in the evolving relationships between physical and biogeochemical processes that shape the projections of NPP and nutrients. The causality-based approach identifies mechanisms that traditional analyses miss, offering a novel framework for model intercomparison and understanding ecosystem responses to climate change.