The impact that resolving the internal tide in shelf sea models makes to the dynamics and variability of phytoplankton growth with the subsurface chlorophyll maximum

Shelf seas, while only occupying less than 10% of the global ocean, are highly biologically productive regions. They account for as much as 30% of all oceanic primary production and play an important role in the global carbon cycle. During a typical seasonal cycle primary productivity during the summer is maintained within the Subsurface Chlorophyll maximum (SCM) where phytoplankton are adapted to survive in low light and nutrient conditions. Internal tides, generated at the shelf break that propagate across the continental shelf can lift phytoplankton cells towards higher light levels and can enhance nutrient fluxes at the base of the SCM through mixing. The internal tide, however, is not typically resolved in most regional scale coupled hydrodynamic-biological models, which limits their ability to simulate SCM dynamics and variability. Here we present high resolution observations of the SCM in the Celtic Sea on the NW European Shelf, made from a vertically profiling Wirewalker and compare them to output from two models: one that is able to resolve the internal tide (1.5 km resolution NEMO-ERSEM), and one that is not (7 km resolution NEMO-ERSEM). We find phytoplankton growth inferred from the Wirewalker data exhibited strong peaks at semi-diurnal periodicity consistent with the regional internal tide. The dominant periodicity in behaviour and variability of primary productivity in the internal tide resolving (high resolution) model was also semi-diurnal. The low resolution model, however, was dominated by 24-hour cycles. We further found that production during the summer months in the internal tide resolving model compares more favourably to observed levels of summer production on the NW European Shelf. We conclude that using models that have sufficient resolution to resolve the internal tide should be a key consideration for biological-physical shelf sea studies.