Projecting Chlorophyll-a distributions under climate change using Copula-based inference and SST projections

Sea surface temperature (SST) and chlorophyll-a (Chl-a) are key indicators of marine ecosystem productivity, particularly for small pelagic species that are sensitive to climate-driven environmental changes. This study investigates the coupled dynamics of SST and Chl-a in two ecologically distinct regions, the Alboran Sea (AS) and the North Atlantic Moroccan Ocean (NAMO), to better understand their response under future climate scenarios. Historical satellite observations from MODIS-Aqua and projections from six Coupled Model Intercomparison Project Phase VI (CMIP6) General Circulation Models (GCMs) are analyzed under two Shared Socioeconomic Pathways (SSP2-4.5 and SSP5-8.5). Multivariate bias correction (MBCp) is performed to correct systematic biases in the model outputs. While GCMs effectively capture SST trends, they show significant limitations in simulating Chl-a variability. To address this issue, we introduce a conditional copula-based inference framework that links SST and Chl-a distributions based on their joint probabilistic behavior. In addition, marginal distributions are identified using goodness-of-fit tests, AIC and BIC. The copula families have been selected based on AIC, taking into account regional and seasonal variability. Conditional simulations from fitted copulas, informed by future SST projections, are used to predict Chl-a levels under climate change. However, the method is initially validated during the historical period using historical SST models, confirming the robustness of the approach. Results highlight a pronounced divergence between the optimistic and the pessimistic scenarios, suggesting a consistent reduction of the most productive phases that sustain higher trophic levels. This decline in productivity indicates that the far future ocean will not merely be a warmer version of the present system but a biogeochemically altered and less resilient ocean, characterized by lower productivity and reduced variability. Regionally, the NAMO region is projected to undergo a gradual yet persistent weakening. In contrast, the AS region is projected to face two contrasting futures, partial resilience under an optimistic scenario or a potential catastrophic ecological transition under a pessimistic scenario.