The impact of sampling density on surface ocean pCO2 trends

Climate-driven change in the global ocean carbon sink needs to be distinguished from effects of climate variability. Utilizing gyre-scale global biomes and updated ocean carbon datasets, pCO₂ trends over decadal to multi-decadal timescales are presented. Since the beginning of the observational record in the 1980s, surface ocean pCO₂ has exhibited significant decadal variability with substantial global coherence. On multi-decadal time frames, most biomes display pCO₂ trends that are slightly shallower than trends in atmospheric pCO₂, which is consistent with the ocean’s long-term response to carbon accumulation in the atmosphere and with supply of waters with low anthropogenic carbon from the deep ocean. Particularly on decadal timescales, global climate models predict smaller amplitude variability in surface ocean carbon. It is important to understand if models have smaller variability or the larger observed estimates are attributable to sparse sampling. In this presentation, we report on our analysis of the impact of sampling density of surface ocean pCO₂ on estimates of decadal timescale trends using a range of ocean hindcast models sampled as the observations and with full coverage. For both the observations and the models, we apply an empirical decomposition of biogeochemical and temperature influences on pCO₂ to evaluate drivers of change in the CO₂ sink. Finally, we discuss the degree to which internal variability can be distinguished from externally forced signals.