Ocean Biogeochemical Predictions - Initialization and Sources of Predictability

Predictions of ocean biogeochemistry, such as primary productivity and CO₂ uptake, would be a helpful tool for understanding the changing marine environment and global climate. Successful attempts in predicting ocean biogeochemistry at regional scales with initializations of ocean physics only, has raised the question of what potential there is for biogeochemical predictions on larger spatial scales, and the improvement that could be gained by assimilating biogeochemical data to ameliorate uncertainties the initial conditions. Here we perform a suite of idealized twin experiments with the aim to i) investigate the role of biogeochemical tracer's initial conditions on their predictability, and to ii) to understand the physical limits for interannual to decadal predictions of ocean carbon uptake and export production.

Our results suggest that initialization of biogeochemistry through e.g. assimilation do not significantly improve interannual to decadal predictions, which we relate to the strong control ocean physics exerts on the biogeochemical variability on these time scales. The predictability of ocean CO₂ uptake generally agrees well with the predictability of the mixed layer depth (MLD), suggesting that the predictable signal comes from the exchange of DIC with deep-waters. The longest predictability is found in high latitudes, as for other physical properties such as sea surface temperature and salinity, but the predictability of the MLD and CO₂ uptake is in general shorter than these properties due to its higher sensitivity to the wind. The predictability of the annual mean export production is, on the contrary, nearly non-existing at high latitudes, despite a strong predictive skill for the annual mean nutrient concentrations in these regions. This is related to the low predictability of the physics in the summer mixed layer, which is heavily influenced by the chaotic atmosphere.