Impact of ocean-forcing temporal resolution on biogeochemical dynamics and forecasting in the Mediterranean Sea

Abstract. The temporal forcing frequency of ocean dynamic fields used to drive transport–biogeochemical models in off-line coupling configurations can critically affect model performance, especially when mesoscale and high-frequency biogeochemical variability are resolved. This study demonstrates the benefits of using high-temporal-frequency ocean dynamics to force biogeochemical simulations within the Mediterranean Sea forecasting system of the EU Copernicus Marine Service. The OGSTM–BFM transport–biogeochemical model was forced with 6-hourly and 24-hourly averaged outputs from the Mediterranean Forecasting System (MedFS, based on NEMO–WW3-OceanVar) with particular attention to the role of vertical transport processes in regulating primary productivity. Validation against BGC-Argo and satellite observations revealed that the 6-hourly forcing improved the timing of phytoplankton blooms across several Mediterranean subbasins, as well as the spatial distribution of chlorophyll and nutrient concentrations—most notably in the western Mediterranean and Ionian Sea during winter. Enhanced sub-daily advection in winter and intensified mixing in summer were identified as the dominant processes driving these biogeochemical responses, leading to better representation of winter surface blooms and a broader deep chlorophyll maximum layer in summer. Although basin-scale primary productivity patterns remain broadly consistent, local productivity intensity varies in response to the altered vertical transport regime.