Description: Litter and plastic pollution in the marine environment is of major concern when considering the health of ocean ecosystems, and have become an important focus of ocean research during recent years. There is still significant uncertainty surrounding the distribution and impact of marine plastic litter on ocean ecosystems, and in particular on the nano- and microplastic fractions that are difficult to observe and may be harmful to marine organisms. Current estimates of ocean plastic concentrations only account for a small fraction of the approximated 8 million tons of plastic litter entering the oceans on an annual basis. Here, we present the distribution of 100–500 μm microplastic particles within the ocean mixed layer, covering a significant fraction of the ocean, in a near-synoptic survey. During The Ocean Race 2017/2018 edition (formerly known as Volvo Ocean Race), two yachts served as ships of opportunity that regularly took samples of microplastics on a regular schedule during their circumnavigation. This effort resulted in information on microplastic distribution along the race track in the ocean’s upper, well-mixed, layer. We found concentrations ranging from 0–349 particles per cubic meter, but with large spatial variability. There was a tendency toward higher concentrations off south-western Europe and in the southwest Pacific, and indications of long-range transport of microplastic with major ocean currents.

Description: The Southern Ocean is among the largest contemporary sinks of atmospheric carbon dioxide on our planet; however, remoteness, harsh weather and other circumstances have led to an undersampling of the ocean basin, compared with its northern hemispheric counterparts. While novel data interpolation methods can in part compensate for such data sparsity, recent studies raised awareness that we have hit a wall of unavoidable uncertainties in air-sea CO2 flux reconstructions. Here, we present results from autonomous observing campaigns using a novel platform to observe remote ocean regions: sailboats. Sailboats are at present a free of charge environmentally friendly platform that recurrently pass remote ocean regions during round-the-globe racing events. During the past 5 years, we collected 〉350 000 measurements of the sea surface partial pressure of CO2 (pCO(2)) around the globe including the Southern Ocean throughout an Antarctic circumnavigation during the Vendee Globe racing event. Our analysis demonstrates that the sailboat tracks pass regions where large uncertainty in the air-sea CO2 flux reconstruction prevails, with regional oversaturation or undersaturation of the sea surface pCO(2). Sailboat races provide an independent cross-calibration platform for autonomous measurement devices, such as Argo floats, ultimately strengthening the entire Southern Ocean observing system.This article is part of a discussion meeting issue 'Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities'.

Description: Enhanced Southern Ocean ventilation in recent decades has been suggested to be a relevant modulator of the observed changes in ocean heat and carbon uptake. This study focuses on the Southern Ocean midlatitude ventilation changes from the 1960s to the 2010s. A global 1/4° configuration of the NEMO–Louvain-la-Neuve sea ice model, version 2 (LIM2), including the inert tracer CFC-12 (a proxy of ocean ventilation) is forced with the CORE, phase II (CORE-II), and JRA-55 driving ocean (JRA55-do) atmospheric reanalyses. Sensitivity experiments, where the variability of wind stress and/or the buoyancy forcing is suppressed on interannual time scales, are used to unravel the mechanisms driving ventilation changes. Ventilation changes are estimated by comparing CFC-12 interior inventories among the different experiments. All simulations suggest a multidecadal fluctuation of Southern Ocean ventilation, with a decrease until the 1980s–90s and a subsequent increase. This evolution is related to the atmospheric forcing and is caused by the (often counteracting) effects of wind stress and buoyancy forcing. Until the 1980s, increased buoyancy gains caused the ventilation decrease, whereas the subsequent ventilation increase was driven by strengthened wind stress causing deeper mixed layers and a stronger meridional overturning circulation. Wind stress emerges as the main driver of ventilation changes, even though buoyancy forcing modulates its trend and decadal variability. The three Southern Ocean basins take up CFC-12 in distinct density intervals but overall respond similarly to the atmospheric forcing. This study suggests that Southern Ocean ventilation is expected to increase as long as the effect of increasing Southern Hemisphere wind stress overwhelms that of increased stratification.