ALBERT

Description: This contribution to the RECCAP2 (REgional Carbon Cycle Assessment and Processes) assessment analyzes the processes that determine the global ocean carbon sink, and its trends and variability over the period 1985-2018, using a combination of models and observation-based products. The mean sea-air CO2 flux from 1985 to 2018 is -1.6 +/- 0.2 PgC yr(-1) based on an ensemble of reconstructions of the history of sea surface pCO(2) (pCO(2) products). Models indicate that the dominant component of this flux is the net oceanic uptake of anthropogenic CO2, which is estimated at -2.1 +/- 0.3 PgC yr(-1) by an ensemble of ocean biogeochemical models, and -2.4 +/- 0.1 PgC yr(-1) by two ocean circulation inverse models. The ocean also degasses about 0.65 +/- 0.3 PgC yr(-1) of terrestrially derived CO2, but this process is not fully resolved by any of the models used here. From 2001 to 2018, the pCO2 products reconstruct a trend in the ocean carbon sink of -0.61 +/- 0.12 PgC yr(-1) decade(-1), while biogeochemical models and inverse models diagnose an anthropogenic CO2-driven trend of -0.34 +/- 0.06 and -0.41 +/- 0.03 PgC yr(-1) decade(-1), respectively. This implies a climate-forced acceleration of the ocean carbon sink in recent decades, but there are still large uncertainties on the magnitude and cause of this trend. The interannual to decadal variability of the global carbon sink is mainly driven by climate variability, with the climate-driven variability exceeding the CO2-forced variability by 2-3 times. These results suggest that anthropogenic CO2 dominates the ocean CO2 sink, while climate-driven variability is potentially large but highly uncertain and not consistently captured across different methods.

Description: Oceanic mesoscale eddies constitute ephemeral hotspots for marine life and are pivotal for the lateral transport of nutrients and organic matter. Here, we use a high-resolution coupled physical-biogeochemical model to study the processes sustaining biological production and export in long-living cyclonic (CE) and anticyclonic (AE) eddies of the northern Canary Upwelling System (CanUS). We track the eddies for 18 months as they propagate offshore, and study their composite properties in time in a Lagrangian manner. Our model shows that long-living CEs sustain their production with the nitrogen that they initially trap in the nearshore nutrient-rich waters and keep isolated in their cores. The vertical input of nitrate from below tends to be comparatively small, and is mostly driven by mixing. In contrast, AEs tend to start with low nutrient concentrations in their core as they do not trap coastal waters, but have elevated concentrations at their periphery. In AEs, stirring is responsible for both the building up of the positive nitrate anomaly at depth and the enhanced lateral input of organic nitrogen in the near-surface. Compared to CEs, the input of nitrate into the euphotic zone by vertical mixing is substantially more important. Though regenerated production dominates in both types of eddies, new production is higher than the regional average in CE cores and at the rim of AEs, partially compensating for the intense losses due to sinking. Both cyclonic trapping and transport and anticyclonic stirring shape the regional pattern of organic matter and nutrients in the northern CanUS.

Description: We assess the Southern Ocean CO2 uptake (1985–2018) using data sets gathered in the REgional Carbon Cycle Assessment and Processes Project Phase 2. The Southern Ocean acted as a sink for CO2 with close agreement between simulation results from global ocean biogeochemistry models (GOBMs, 0.75 ± 0.28 PgC yr−1) and pCO2-observation-based products (0.73 ± 0.07 PgC yr−1). This sink is only half that reported by RECCAP1 for the same region and timeframe. The present-day net uptake is to first order a response to rising atmospheric CO2, driving large amounts of anthropogenic CO2 (Cant) into the ocean, thereby overcompensating the loss of natural CO2 to the atmosphere. An apparent knowledge gap is the increase of the sink since 2000, with pCO2-products suggesting a growth that is more than twice as strong and uncertain as that of GOBMs (0.26 ± 0.06 and 0.11 ± 0.03 Pg C yr−1 decade−1, respectively). This is despite nearly identical pCO2 trends in GOBMs and pCO2-products when both products are compared only at the locations where pCO2 was measured. Seasonal analyses revealed agreement in driving processes in winter with uncertainty in the magnitude of outgassing, whereas discrepancies are more fundamental in summer, when GOBMs exhibit difficulties in simulating the effects of the non-thermal processes of biology and mixing/circulation. Ocean interior accumulation of Cant points to an underestimate of Cant uptake and storage in GOBMs. Future work needs to link surface fluxes and interior ocean transport, build long overdue systematic observation networks and push toward better process understanding of drivers of the carbon cycle. Key Points: - Ocean models and machine learning estimates agree on the mean Southern Ocean CO2 sink, but the trend since 2000 differs by a factor of two - REgional Carbon Cycle Assessment and Processes Project Phase 2 estimates a 50% smaller Southern Ocean CO2 sink for the same region and timeframe as RECCAP1 - Large model spread in summer and winter indicates that sustained efforts are required to understand driving processes in all seasons