An Updated Parametrization of Algorithms to Retrieve the Diffuse Attenuation of Light in the Ocean from Remote Sensing and its Impact on Estimates of Net Primary Productivity

Abstract

We recently found a significant bias while validating frequently used ocean color algorithms retrieving the spectral diffuse attenuation coefficient (Kd(λ)) and the attenuation coefficient for photosynthetically available radiation (Kd(PAR)) [1]. Here we compute new coefficients for existing algorithms for Kd(λ), so as to remove the observed bias at Kd(490), and evaluate the impact on global and regional estimates of net primary production (NPP) using two different primary production models. The new parametrization results in improved retrieval of Kd(490) by both the empirical and the semi-analytical algorithms. Match-ups between BGC-Argo floats and Remote Sensing Reflectance (Rrs) for six different satellite sensors no longer have a small- value bias, and show a reduced RMSE and error ratio. The new coefficients are validated using measurements not included in the training dataset and are found to perform significantly better at a different wavelength (412nm) than the one used for the new parametrization (490nm) and perform reasonably well in Case-2 waters. Since the new coefficients presented here were developed with a dataset encompassing a larger proportion of the ocean’s variability, they are better suited to compute Kd(λ) in regions that were not present in the original algorithm’s dataset and are therefore appropriate for global Kd(λ) estimation. Using the new Kd parameterization results in a global increase of NPP of ≈ 37% in both models used, mostly driven by the previous overestimation of Kd(λ) (underestimation of light penetration) in the clear, subtropical gyres. Subtropical gyres show the largest increase (79%) in the VGPM model, with the presence of a strong seasonal cycle in the difference. High Kd(λ) areas are less affected by the new parameterization (no increase in NPP in VGPM, ≈ 20% in CbPMv2). Although the subtropical gyres are not very productive regions of the ocean, their large surface area and the magnitude of the bias in Kd(λ) between the old and new parameterization causes the observed significant difference in global NPP estimates. Our results suggest that the oceanic carbon uptake is larger than previously thought, which will be most relevant to the oceanic carbon dioxide budget once humanity slows the increase of atmospheric CO2.