Relating Agulhas leakage to the Agulhas Current retroflection location

The relation between the Agulhas Current retroflection location and the magnitude of Agulhas leakage, the transport of water from the Indian to the Atlantic Ocean, is investigated in a high-resolution numerical ocean model. Sudden eastward retreats of the Agulhas Current retroflection loop are linearly related to the shedding of Agulhas rings, where larger retreats generate larger rings. Using numerical Lagrangian floats a 37 year time series of the magnitude of Agulhas leakage in the model is constructed. The time series exhibits large amounts of variability, both on weekly and annual time scales. A linear relation is found between the magnitude of Agulhas leakage and the location of the Agulhas Current retroflection, both binned to three month averages. In the relation, a more westward location of the Agulhas Current retroflection corresponds to an increased transport from the Indian Ocean to the Atlantic Ocean. When this relation is used in a linear regression and applied to almost 20 years of altimetry data, it yields a best estimate of the mean magnitude of Agulhas leakage of 13.2 Sv. The early retroflection of 2000, when Agulhas leakage was probably halved, can be identified using the regression

Modelling size distributions of marine plastics under the influence of continuous cascading fragmentation

Field studies have shown that plastic fragments make up the majority of plastic pollution in the oceans in terms of abundance. How quickly environmental plastics fragment is not well understood, however. Here, we study this fragmentation process by considering a model which captures continuous fragmentation of particles over time in a cascading fashion. With this cascading fragmentation model we simulate particle size distributions (PSDs), specifying the abundance or mass of particles for different size classes. The fragmentation model is coupled to an environmental box model, simulating the distributions of plastic particles in the ocean, coastal waters, and on the beach. We compare the modelled PSDs to available observations, and use the results to illustrate the effect of size-selective processes such as vertical mixing in the water column and resuspension of particles from the beach into coastal waters. The model quantifies the role of fragmentation on the marine plastic mass budget: while fragmentation is a major source of (secondary) plastic particles in terms of abundance, it seems to have a minor effect on the total mass of particles larger than 0.1 mm. Future comparison to observed PSD data allow us to understand size-selective plastic transport in the environment, and potentially inform us on plastic longevity

Iron sources and pathways into the Pacific Equatorial Undercurrent

Using a novel observationally constrained Lagrangian iron model forced by outputs from an eddy-resolving biogeochemical ocean model, we examine the sensitivity of the Equatorial Undercurrent (EUC) iron distribution to EUC source region iron concentrations. We find that elevated iron concentrations derived from New Guinea Coastal Undercurrent (NGCU) alone is insufficient to explain the high concentrations observed in the EUC. In addition, due to the spread in transit times, interannual NGCU iron pulses are scavenged, diluted, or eroded, before reaching the eastern equatorial Pacific. With an additional iron source from the nearby New Ireland Coastal Undercurrent, EUC iron concentrations become consistent with observations. Furthermore, as both the New Guinea and New Ireland Coastal Undercurrents strengthen during El Niño, increased iron input into the EUC can enhance the iron supply into the eastern equatorial Pacific. Notably, during the 1997/1998 El Niño, this causes a simulated 30% iron increase at a 13 month lag

Global Modeled Sinking Characteristics of Biofouled Microplastic

Microplastic debris ending up at the sea surface has become a known major environmental issue. However, how microplastic particles move and when they sink in the ocean remains largely unknown. Here, we model microplastic subject to biofouling (algal growth on a substrate) to estimate sinking timescales and the time to reach the depth where particles stop sinking. We combine NEMO‐MEDUSA 2.0 output, that represents hydrodynamic and biological properties of seawater, with a particle‐tracking framework. Different sizes and densities of particles (for different types of plastic) are simulated, showing that the global distribution of sinking timescales is largely size‐dependent as opposed to density‐dependent. The smallest particles we simulate (0.1 μm) start sinking almost immediately around the globe and their trajectories take the longest time to reach their first sinking depth (relative to larger particles). In oligotrophic subtropical gyres with low algal concentrations, particles between 1 mm and 10 μm do not sink within the 90‐day simulation time. This suggests that in addition to the comparatively well‐known physical processes, biological processes might also contribute to the accumulation of floating plastic (of 1 mm–10 μm) in subtropical gyres. Particles of 1 μm in the gyres start sinking largely due to vertical advection, whereas in the equatorial Pacific they are more dependent on biofouling. The qualitative impacts of seasonality on sinking timescales are small, however, localised sooner sinking due to spring algal blooms is seen. This study maps processes that affect the sinking of virtual microplastic globally, which could ultimately impact the ocean plastic budget

Water Mass Transports and Pathways in the North Brazil- Equatorial Undercurrent Retroflection

The equatorial retroflection of the North Brazil Current (NBC) into the Equatorial Undercurrent (EUC) and its posterior tropical recirculation is a major regulator for the returning limb of the Atlantic Meridional Overturning Circulation. Indeed, most surface and thermocline NBC waters retroflect at the equator all the way into the central and eastern Atlantic Ocean, before they recirculate back through the tropics to the western boundary. Here, we use cruise data in the western equatorial Atlantic during April 2010 and reanalysis time series for the equatorial and tropical waters in both hemispheres in order to explore the recirculation pathways and transport variability. During the 1998–2016 period, the annual-mean EUC transports 15.1 ± 1.3 Sv at 32°W, with 2.8 ± 0.4 Sv from the North Atlantic and 11.4 ± 1.3 Sv from the South Atlantic. At 32°W most of the total EUC transport comes from the western boundary retroflection south of 3°N (7.2 ± 0.9 Sv), a substantial fraction retroflects north of 3°N (5.6 ± 0.4 Sv), and the remaining flow (2.3 Sv) joins through the interior basin. The South Atlantic subtropical waters feed the EUC at all thermocline depths while the North Atlantic and South Atlantic tropical waters do so at the surface and upper-thermocline levels. The EUC transport at 32°W has a pronounced seasonality, with spring and fall maxima and a range of 8.8 Sv. The 18 yr of reanalysis data shows a weak yet significant correlation with an Atlantic Niño index, and also suggests an enhanced contribution from the South Atlantic tropical waters during 2008–2016 as compared with 1997–2007En prensa3,17

Global mass of buoyant marine plastics dominated by large long-lived debris

The fate of plastics that enter the ocean is a longstanding puzzle. Recent estimates of the oceanic input of plastic are one to two orders of magnitude larger than the amount measured floating at the surface. This discrepancy could be due to overestimation of input estimates, processes removing plastic from the surface ocean or fragmentation and degradation. Here we present a 3D global marine mass budget of buoyant plastics that resolves this discrepancy. We assimilate observational data from different marine reservoirs, including coastlines, the ocean surface, and the deep ocean, into a numerical model, considering particle sizes of 0.1–1,600.0 mm. We find that larger plastics (>25 mm) contribute to more than 95% of the initially buoyant marine plastic mass: 3,100 out of 3,200 kilotonnes for the year 2020. Our model estimates an ocean plastic input of about 500 kilotonnes per year, less than previous estimates. Together, our estimated total amount and annual input of buoyant marine plastic litter suggest there is no missing sink of marine plastic pollution. The results support higher residence times of plastics in the marine environment compared with previous model studies, in line with observational evidence. Long-lived plastic pollution in the world’s oceans, which our model suggests is continuing to increase, could negatively impact ecosystems without countermeasures and prevention strategies

Optimising fisheries management in relation to tuna catches in the western central Pacific Ocean: A review of research priorities and opportunities

Some of the most important development goals for the countries and territories of the Western and Central Pacific Ocean (WCPO) involve the sustainable management of their fisheries in light of environmental, economic and social uncertainties. The responses of fish populations to variability in the marine environment have implications for decision making processes associated with resource management. There is still considerable uncertainty in estimating the responses of tuna populations to short-to-medium-term variability and longer-term change in the oceanic environment. A workshop was organised to examine how advances in oceanography, fisheries science and fisheries economics could be applied to the tuna fisheries of the WCPO and in doing so identify research priorities to improve understanding relevant to progressing management. Research priorities identified included: (i) improved parameterisation of end to end ecosystem model components, processes and feedbacks through expanded biological observations and incorporation of higher resolution climate models; (ii) development of seasonal and inter-annual forecasting tools enabling management responses to short-term variability in tuna distributions and abundances; (iii) improved understanding of the population dynamics of and the energy transfer efficiencies between food web components; (iv) assessment of the optimal value of access rights and overall fishery value under multiple scenarios of tuna distribution and abundance and influences on decision making by fisheries managers and fleets and (v) development of management strategy evaluation frameworks for utilisation in the implementing and testing of fishery management procedures and to help prioritise research directions and investment. Issues discussed and research priorities identified during the workshop have synergies with other internationally managed fisheries and therefore are applicable to many other fisheries

A comparison of Eulerian and Lagrangian methods for vertical particle transport in the water column

A common task in oceanography is to model the vertical movement of particles such as microplastics, nanoparticles, mineral particles, gas bubbles, oil droplets, fish eggs, plankton, or algae. In some cases, the distribution of the vertical rise or settling velocities of the particles in question can span a wide range, covering several orders of magnitude, often due to a broad particle size distribution or differences in density. This requires numerical methods that are able to adequately resolve a wide and possibly multi-modal velocity distribution. Lagrangian particle methods are commonly used for these applications. A strength of such methods is that each particle can have its own rise or settling speed, which makes it easy to achieve a good representation of a continuous distribution of speeds. An alternative approach is to use Eulerian methods, where the partial differential equations describing the transport problem are solved directly with numerical methods. In Eulerian methods, different rise or settling speeds must be represented as discrete classes, and in practice, only a limited number of classes can be included. Here, we consider three different examples of applications for a water column model: positively buoyant fish eggs, a mixture of positively and negatively buoyant microplastics, and positively buoyant oil droplets being entrained by waves. For each of the three cases, we formulate a model for the vertical transport based on the advection–diffusion equation with suitable boundary conditions and, in one case, a reaction term. We give a detailed description of an Eulerian and a Lagrangian implementation of these models, and we demonstrate that they give equivalent results for selected example cases. We also pay special attention to the convergence of the model results with an increasing number of classes in the Eulerian scheme and with the number of particles in the Lagrangian scheme. For the Lagrangian scheme, we see the 1/Np convergence, as expected for a Monte Carlo method, while for the Eulerian implementation, we see a second-order (1/Nk2) convergence with the number of classes.</p

Assessing the accuracy of satellite derived ocean currents by comparing observed and virtual buoys in the Greater Agulhas Region

In this study, we assess the accuracy of a combined geostrophic and Ekman current product (GlobCurrent) that estimates ocean currents at 15 m depth, by coupling it to a synthetic particle tracking tool and comparing the virtual trajectories to those of surface drifting buoys drogued at 15 m in the Greater Agulhas Current Region. The velocities from a total of 1041 drifters are compared and evaluated to the synthetic particle-derived velocities for the period 1993–2015. On average the GlobCurrent underestimates the velocity in the Greater Agulhas Current by approximately 27%. The underestimation ranges from 4 to 64% in different regions, with the smallest error found in the Agulhas retroflection region, and the highest in the Benguela Upwelling System. Furthermore, we compare the time taken for the separation between the virtual and real drifters to reach 35 km. The mean separation time was found to be 78 h, with the shortest time (35 h) found in the Agulhas Current and the longest time (116 h) located in the Agulhas Return Current. Deploying 10,000 virtual drifters in a 1° × 1° box within the southern Agulhas Current shows a convergence of trajectories towards the core of the current, while higher divergence is evident in the Agulhas retroflection. To evaluate the utility of this synthetic particle tracking tool coupled with GlobCurrent in open ocean search and rescue operations, two test cases are examined: (1) a capsized catamaran spotted south of Cape Recife and recovered 5 days later south of Cape Agulhas; and (2) a drifter trajectory in the same region. The comparison suggests that the GlobCurrent forced synthetic particle tracking tool is not appropriate for predicting the trajectory of a capsized catamaran that does not have the same drift characteristics as a surface drifting buoy drogued to 15 m