We assess the Southern Ocean CO2 uptake (1985-2018) using data sets gathered in the REgional Carbon Cycle Assessment and Processes Project phase 2 (RECCAP2). 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 PgCyr-1) and pCO2-observation-based products (0.73±0.07 PgCyr-1). This sink is only half that reported by RECCAP1. 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 PgCyr-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 towards better process understanding of drivers of the carbon cycle.

The oceans are acidifying in response to the oceanic uptake of anthropogenic CO2 from the atmosphere, yet the global-scale progression of this acidification has been poorly documented so far by observations. Here, we fill this gap and use an observation-based product, OceanSODA-ETHZ, to determine the trends and drivers of the surface ocean aragonite saturation state (Ωar) and pH over the last four decades (1982-2021). In the global mean, Ωar and pH declined at rates of -0.071 ± 0.001 decade-1 and -0.0170 ± 0.0001 decade-1, respectively. These trends are driven primarily by the increase in the surface ocean concentration of dissolved inorganic carbon (DIC) in response to the uptake of anthropogenic CO2 but moderated by changes in natural DIC. Surface warming enhances the decrease in pH, accounting for ∼15% of the global trend. Substantial ENSO-driven interannual variability is superimposed on these trends, with Ωar showing greater variability than pH.

Measurements of the surface ocean fugacity of carbon dioxide (fCO2) provide an important constraint on the global ocean carbon sink, yet the gap filling products developed so far to cope with the sparse observations are relatively coarse (1°x1° by 1 month). Here, we overcome this limitation by using the newly developed surface Ocean Carbon dioxide Neural Network (OceanCarbNN) method to estimate surface ocean fCO2 and the associated air sea CO2 fluxes (FCO2) at a resolution of 8-daily by 0.25°x0.25° (8D) over the period 1982 through 2022. The method reconstructs fCO2 with accuracy like that of low-resolution methods (~19 µatm) but improves it in the coastal ocean. Although global ocean CO2 uptake differs little, the 8D product captures 15\% more variance in FCO2. Most of this increase comes from the better-represented subseasonal scale variability, which is largely driven by the better resolved variability of the winds, but also contributed to by the better resolved fCO2. The high-resolution fCO2 is also able to capture the signal of short-lived regional events such as coastal upwelling and hurricanes. For example, the 8D product reveals that fCO2 was at least 25 µatm lower in the wake of Hurricane Maria (2017), the result of a complex interplay between the decrease in temperature, the entrainment of carbon-rich waters, and an increase in primary production. By providing new insights into the role of higher frequency variations of the ocean carbon sink and the underlying processes, the 8D product fills an important gap.

We use a statistical emulation technique to construct synthetic ensembles of global and regional sea-air carbon dioxide (CO2) flux from four observation-based products over 1985-2014. Much like ensembles of Earth system models that are constructed by perturbing their initial conditions, our synthetic ensemble members exhibit different phasing of internal variability and a common externally forced signal. Our synthetic ensembles illustrate an important role for internal variability in the temporal evolution of global and regional CO2 flux and produce a wide range of possible trends over 1990-1999 and 2000-2009. We assume a specific externally forced signal and calculate the likelihood of the observed trend given the distribution of synthetic trends during these two periods. Over the decade 1990-1999, three of the four observation-based products exhibit small negative trends in globally integrated sea-air CO2 flux (i.e., enhanced ocean CO2 absorption with time) that are highly probable (44-72% chance of occurrence) in their respective synthetic trend distributions. Over the decade 2000-2009, however, three of the four products show large negative trends in globally integrated sea-air CO2 flux that are somewhat improbable (17-19% chance of occurrence). Our synthetic ensembles suggest that the largest observation-based positive trends in global and Southern Ocean CO2 flux over 1990-1999 and the largest negative trends over 2000-2009 are somewhat improbable (<30% chance of occurrence). Our approach provides a new understanding of the role of internal and external processes in driving sea-air CO2 flux variability.

The Blob was a marine heat wave in the Northeast Pacific from 2013 to 2016. While the upper ocean temperature in the Blob has been well described, the impacts on marine biogeochemistry have not been fully studied. Here, we characterize and develop understanding of Eastern North Pacific upper ocean biogeochemical properties during the Winter of 2013-14 using in situ observations, an observation-based product, and reconstructions from a collection of ocean models. We find that the Blob is associated with significant upper ocean biogeochemical anomalies: a 5% increase in aragonite saturation state (temporary reprieve of ocean acidification) and a 3% decrease in oxygen concentration (enhanced deoxygenation). Anomalous advection and mixing drives the aragonite saturation anomaly, while anomalous heating and air-sea gas exchange drive the oxygen anomaly. Marine heatwaves do not necessarily serve as an analogue for future change as they may enhance or mitigate long-term trends.