'Influence of diurnal primary production on projections of future ocean chemistry extremes'

While Earth System Model projections of surface-ocean ocean chemistry are well constrained for future changes in the mean state and seasonal variations have also begun to be addressed, the impact of climate change and ocean carbon uptake on ocean chemistry extreme events remains uncertain. Indeed, our understanding of present-day ocean chemistry temporal variability and the occurrence of extremes is impeded by a lack of long-term, high-resolution observations. Limited evidence of ocean chemistry variability at volcanic CO₂ seeps suggests that diurnal ocean chemistry variability will dramatically respond to climate change, with consequences for marine organisms. Here we explore multiyear observational time series of 3-hourly surface ocean chemistry taken across multiple ocean basins. Diurnal ocean chemistry variability in the open ocean is shown to be small relative to seasonal variations and predominately driven by variations in sea surface temperature. On the other hand, in the coastal ocean and at coral reefs, diurnal variability in ocean chemistry can be of similar magnitude to seasonal variations and is largely driven by biological activity. These observations are being used to help develop a parameterization of diurnal primary production in a variable stoichiometry ocean biogeochemical model (PISCES-QUOTA). Global simulations were conducted with this modified model under historical and future climate forcing scenarios. Preliminary results indicate that resolving the diurnal cycle of primary production increases the occurrence of ocean chemistry extremes events over the 21^stcentury, exacerbating organism exposure to seawater hypercapnia and calcium carbonate undersaturation.