Seasonality in carbon flux attenuation explains spatial variability in transfer efficiency

Each year, the biological carbon pump is responsible for converting carbon dioxide into millions of tonnes of organic carbon, and for transferring a fraction of it to the deep ocean, where it can remain for hundreds of years. The efficiency of this surface-to-depth carbon transfer varies geographically, and is a key determinant of the atmosphere-ocean carbon dioxide balance. Traditionally, the attention has been focused on explaining perceived geographical variation in an attempt to understand it, an approach that has led to conflicting results. Here we use a combination of observations and modelling to show that the spatial variability in transfer efficiency can instead be due to seasonal variability in carbon flux attenuation. We also show that seasonality can explain the contrast between known global estimates of transfer efficiency, due to differences in the date and duration of sampling, as well as the methodologies used to derive the estimates. Our results suggest caution in the mechanistic interpretation of annual-mean patterns in transfer efficiency and demonstrates that seasonally and spatially-resolved datasets are required to generate accurate evaluations of the biological carbon pump.