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A number of studies have illustrated the utility of environmental DNA (eDNA) for detecting marine vertebrates. However, little is known about the fate and transport of eDNA in the ocean, thus limiting the ability to interpret eDNA measurements. In the present study, we explore how fate and transport processes affect oceanic eDNA in Monterey Bay, California, USA (MB). Regional ocean modeling predictions of advection and mixing are used for an approximately 10,000 km2 area in and around MB to simulate the transport of eDNA. These predictions along with realistic settling rates and first-order decay rate constants are applied as inputs into a particle tracking model to investigate the displacement and spread of eDNA from its release location. We found that eDNA can be transported on the order of tens of kilometers in a few days and that horizontal advection, decay, and settling have greater impacts on the displacement of eDNA in the ocean than mixing. The eDNA particle tracking model was applied to identify possible origin locations of eDNA measured in MB using a quantitative PCR assay for Northern anchovy (Engraulis mordax). We found that eDNA likely originated from within 40 km and south of the sampling site if it had been shed approximately 4 days prior to sampling.

Aim Identify climate change impacts on spawning and settlement of a tropical herbivore (Tripneustes), optimal habitat of a temperate kelp (Ecklonia) and implications for these species regions of interaction under future climate. Location Along eastern Australia and into the Tasman Sea including Lord Howe Island (LHI). Time period A contemporary scenario (2006–2015) and future “business as usual” RCP 8.5 climate change scenario (2090–2100). Major taxa studied The tropical sea urchin, Tripneustes gratilla, and the temperate kelp, Ecklonia radiata. Methods We combined mechanistic models to create a predictive map of Ecklonia and Tripneustes distributions, and their future potential to co‐occur. We use 3D velocity and temperature fields produced with a state‐of‐the‐art configuration of the Ocean Forecasting Model version 3 that simulates the contemporary oceanic environment and projects it under an RCP 8.5 climate change scenario. We map the contemporary and future Ecklonia's realized and fundamental thermal niche; and simulate Tripneustes larval dispersal under both climate scenarios. Results Based on the thermal affinity of kelp and increases in projected temperatures, we predict a poleward range contraction of ~530 km by 2100 for kelp on Australia's east coast. Climate‐driven changes in dispersal of Tripneustes lead to its range expansion into Bass Strait and poleward, ~340–650 km further into Ecklonia's habitat range inducing new areas of co‐occurrence in the future. We find warming decreases spawning and settlement of Tripneustes in the tropics by 43%, and causes significant connectivity changes for LHI with future reliance on self‐recruitment. Major conclusions We predict novel regions of co‐occurrence between a temperate Ecklonia and tropical Tripneustes species which may lead to greater loss of kelp. Our results provide a new modelling approach for predicting species range shifts that is transferable to other marine ecosystems; it considers species response to abiotic change, predicts spatial spread and anticipates new regions for species interactions.

Mesoscale eddies are abundant in the global oceans and known to affect oceanic and atmospheric conditions. Understanding their cumulative impact on the air‐sea carbon dioxide (CO2) flux may have significant implications for the ocean carbon sink. Observations and Lagrangian tracking were used to estimate the air‐sea CO2 flux of 67 long lived (>1 year) mesoscale eddies in the South Atlantic Ocean over a 16 year period. Both anticyclonic eddies originating from the Agulhas retroflection and cyclonic eddies originating from the Benguela upwelling act as net CO2 sinks over their lifetimes. Anticyclonic eddies displayed an exponential decrease in the net CO2 sink, whereas cyclonic eddies showed a linear increase. Combined, these eddies significantly enhanced the CO2 sink into the South Atlantic Ocean by 0.08 ± 0.04%. The studied eddies constitute a fraction of global eddies, and eddy activity is increasing; therefore, explicitly resolving eddies appears critical when assessing the ocean carbon sink. Plain Language Summary Ocean mesoscale eddies can form when part of a main current becomes separated or through internal ocean instabilities which form circular rotating currents that propagate across the oceans. These eddies last from weeks to years and can modify the ocean properties of the water captured within them, which in turn affects the net exchange of carbon between this water and the atmosphere. Little is known about how these eddies modify the absorption of carbon across the global ocean, collectively referred to as the ocean carbon sink, despite them being ubiquitous features of the global oceans. Using in situ and satellite‐based observations, we show that eddies in the South Atlantic Ocean enhance the absorption of carbon from the atmosphere, thus modifying the ocean to be a stronger net sink of carbon. These results are important as they quantify how much eddies contribute to the absorption of carbon from the atmosphere to the ocean, and highlight the need to include eddies when assessing ocean carbon budgets. Key Points Satellite and in situ observations with Lagrangian tracking were used to estimate the cumulative CO2 flux of long lived mesoscale eddies Drivers of the pCO2(sw) variability significantly changed over the anticyclonic eddies lifetime but did not in cyclonic eddies Both anticyclonic and cyclonic eddies enhance the CO2 sink into the South Atlantic Ocean

Extratropical cyclones (ETCs) in East Asia are automatically detected and tracked by applying a Lagrangian tracking algorithm to the 850-hPa relative vorticity field. The ETC statistics, which are derived from ERA-Interim reanalysis dataset from 1979 to 2017, show that East Asian ETCs primarily form over Mongolia, East China, and the Kuroshio Current region with a maximum frequency of six to seven cyclones per month. Both Mongolia and East China ETCs are initiated on the leeward side of the mountains. While Mongolia ETCs downstream of the Altai–Sayan Mountains develop slowly, East China ETCs downstream of the Tibetan plateau develop rapidly as they travel across the warm ocean. Both of them show a maximum frequency and intensity in spring rather than in winter. In contrast, oceanic ETCs across the Kuroshio Current and the Kuroshio–Oyashio Extension, where sea surface temperature gradient is sharp, reach a maximum frequency in winter although their intensity is still maximum in spring. On the decadal timescale, both ETC frequency and intensity exhibit insignificant trends. Exceptions are springtime East China and summertime Mongolia ETCs whose frequencies have slightly decreased since 1979. This declining trend is consistent with the enhanced static stability in the region.

Vertical mixing is often regarded as the Achilles' heel of ocean models. In particular, few models include a comprehensive and energy‐constrained parameterization of mixing by internal ocean tides. Here, we present an energy‐conserving mixing scheme which accounts for the local breaking of high‐mode internal tides and the distant dissipation of low‐mode internal tides. The scheme relies on four static two‐dimensional maps of internal tide dissipation, constructed using mode‐by‐mode Lagrangian tracking of energy beams from sources to sinks. Each map is associated with a distinct dissipative process and a corresponding vertical structure. Applied to an observational climatology of stratification, the scheme produces a global three‐dimensional map of dissipation which compares well with available microstructure observations and with upper‐ocean finestructure mixing estimates. This relative agreement, both in magnitude and spatial structure across ocean basins, suggests that internal tides underpin most of observed dissipation in the ocean interior at the global scale. The proposed parameterization is therefore expected to improve understanding, mapping, and modeling of ocean mixing. Plain Language Summary When tidal ocean currents flow over bumpy seafloor, they generate internal tidal waves. Internal waves are the subsurface analog of surface waves that break on beaches. Like surface waves, internal tidal waves often become unstable and break into turbulence. This turbulence is a primary cause of mixing between stacked ocean layers—a key process regulating ocean currents and biology and a key ingredient of computer models of the global ocean. In this article, a three‐dimensional global map of mixing induced by internal tidal waves is presented. This map incorporates a large variety of energy pathways from the generation of tidal waves to turbulence, accounting for the conservation of energy. The map is compared to available observations of turbulence across the globe and found to reproduce with good fidelity the main patterns identified in observations. This relatively good agreement suggests that internal tidal waves are the main source of turbulence in the subsurface ocean and implies that the map may serve a range of applications. In particular, the three‐dimensional map provides an efficient and realistic means to represent mixing by internal tidal waves in global ocean models. Key Points A global three‐dimensional map of mixing induced by internal tides is presented The map can serve as a comprehensive and energy‐constrained tidal mixing parameterization in global ocean models The map compares well to available microstructure and upper‐ocean finestructure mixing estimates

Yao, J. and Tao, J., 2018. An Investigation of the Mixing and Exchange Characteristics in Tidal Channels of Radial Sand Ridges in the South Yellow Sea, China. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 136–140. Coconut Creek (Florida), ISSN 0749-0208. A three-dimensional hydrodynamic model coupled with a Lagrangian particle tracking model was applied in the radial sand ridges (RSR) area of the South Yellow Sea to investigate the flow and mass exchange characteristics between different channels. The Xiyang (XY), Chenjiawucao (CJWC), Kushuiyang (KSY) and Huangshayang (HSY) channels were chosen to represent the channels in all directions. The results showed that during the neap tides at both the surface and bottom locations, the particles were limited in their respective channels characterized by similar reciprocating trajectories. During the spring tides, the particles moved along reciprocating straight lines or in a clockwise spiral. The vertical circulation and water exchange in the tidal channels were significant, coinciding with the movement patterns of the particles. The hydrodynamic characteristics of the RSR are responsible for the significant differences of the transport characteristics in different channels. The XY Channel is controlled by the reciprocating flow, while others are dominated by varying degrees of rotary flow. The special geomorphology also plays an important role. The outcomes of this study may provide theoretical support for environmental management regarding the RSR area.

Sub‐Antarctic Mode Waters (SAMWs) form to the north of the Antarctic Circumpolar Current in the Indo‐Pacific Ocean, whence they ventilate the ocean's lower pycnocline and play an important role in the climate system. With a backward Lagrangian particle‐tracking experiment in a data‐assimilative model of the Southern Ocean (B‐SOSE), we address the long‐standing question of the extent to which SAMWs originate from densification of southward‐flowing subtropical waters versus lightening of northward‐flowing Antarctic waters sourced by Circumpolar Deep Water (CDW) upwelling. Our analysis evidences the co‐occurrence of both sources in all SAMW formation areas, and strong inter‐basin contrasts in their relative contributions. Subtropical waters are the main precursor of Indian Ocean SAMWs (70%–75% of particles) but contribute a smaller amount (<${< } $ 40% of particles) to Pacific SAMWs, which are mainly sourced from the upwelled CDW. By tracking property changes along particle trajectories, we show that SAMW formation from northern and southern sources involves contrasting drivers: subtropical source waters are cooled and densified by surface heat fluxes, and freshened by ocean mixing. Southern source waters are warmed and lightened by surface heat and freshwater fluxes, and they are made either saltier by mixing in the case of Indian SAMWs, or fresher by surface fluxes in the case of Pacific SAMWs. Our results underscore the distinct climatic impact of Indian and Pacific SAMWs formation, involving net release of atmospheric heat and uptake of atmospheric freshwater, respectively; a role that is conferred by the relative contributions of subtropical and Antarctic sources to their formation. Plain Language Summary The formation of Sub‐Antarctic Mode Waters (SAMWs) in the sub‐Antarctic Zone of the southern Indo‐Pacific Ocean is crucial for the transfer of large amounts of heat and carbon dioxide to the ocean interior, thereby mitigating climate change. Despite their importance, there are contrasting views regarding the origins and history of these water masses prior to subduction, which determine the waters' heat and carbon sequestration capacity. Using a state‐of‐the‐art ocean model constrained by observations, we identify and track the sources of SAMWs and quantify their heat exchange with the atmosphere. Our results confirm the co‐existence of two sources of SAMWs: warm, shallow subtropical waters and cold, deep Antarctic waters. The formation of SAMW from contrasting sources has distinct impacts on the climate system. On their path to SAMW formation, subtropical waters release heat into the atmosphere, whilst Antarctic waters absorb heat. Furthermore, subtropical and Antarctic sources dominate SAMW formation in the Indian and Pacific Oceans, respectively, highlighting the contrasting nature of the two main pools of SAMW. Our results shed new light on the intricate nature of SAMWs, helping to predict and understand their role in slowing down future climate change. Key Points Sub‐Antarctic Mode Waters originate from both southward‐flowing subtropical thermocline waters and northward‐flowing Circumpolar Deep Water Sub‐Antarctic Mode Water formation is dominated by subtropical sources in the Indian Ocean versus Antarctic sources in the Pacific Ocean Northern and southern sources undergo contrasting surface heat and freshwater fluxes and interior mixing to become Sub‐Antarctic Mode Waters

The oceanward surface transport of particles, including marine litter, from the northwestern African upwelling zone is influenced by multiple interacting physical processes. This study applies the OceanParcels Lagrangian framework to investigate the mechanisms that may contribute to oceanward surface transport in this region, motivated by the hypothesis that the northwestern African upwelling system could represent a potential source of marine litter in the vicinity of the Canary Islands. The simulations suggest that the coastal jet stream and its detachment, upwelling filaments, and Stokes drift play key roles in shaping particle trajectories. In particular, coastal jet detachment appears to organize surface transport into narrow, oceanward-oriented particle corridors, while upwelling filaments may provide additional offshore export pathways. Stokes drift introduces a predominantly southward deflection that can reduce or modulate oceanward advection and enhance alongshore transport. These results provide a process-based, model-derived first assessment of previously understudied oceanward transport corridors in the NW African upwelling system. They are consistent with the hypothesis that this region may contribute to surface tracer transport toward the Canary Islands. However, caution is required when extrapolating these findings to marine debris, as windage is not included and may significantly alter transport pathways. Continued investigation, including observational validation and improved surface forcing representations, will help further constrain the mechanisms shaping particle transport in the NW African upwelling system.

Modeled geospatial Lagrangian trajectories are widely used in Earth Science, including in oceanography, atmospheric science and marine biology. The typically large size of these data sets makes them arduous to analyze, and their underlying pathways challenging to identify. Here, we show that we can use a machine learning unsupervised k‐means++ clustering method combined with expert aggregation of clusters to identify the pathways of the Labrador Current from a large set of modeled Lagrangian trajectories. The presented method requires simple pre‐processing of the data, including a Cartesian correction on longitudes and a principal component analysis reduction. The clustering is performed in a kernelized space and uses a larger number of clusters than the number of expected pathways. To identify the main pathways, similar clusters are grouped into pathway categories by experts in the circulation of the region of interest. We find that the Labrador Current mainly follows a westward‐flowing and an eastward retroflecting pathway (20% and 50% of the flow, respectively) that compensate each other through time in a see‐saw behavior. These pathways experience a strong variability (representing through time 4%–42% and 24%–73% of the flow, respectively). Two thirds of the retroflection occurs at the tip of the Grand Banks, and one quarter at Flemish Cap. The westward pathway is mostly fed by the on‐shelf branch of the Labrador Current, and the eastward pathway by the shelf‐break branch. Among the pathways of secondary importance, we identify a previously unreported one that feeds the subtropics across the Gulf Stream. Plain Language Summary Lagrangian trajectories, in which parcels of a fluid or objects are tracked as they move, are widely used in Earth Science, including in oceanography, atmospheric science and marine biology. They typically come in very large data sets containing chaotic trajectories, from which it is difficult to identify the main pathways of the flow. Here, we use a machine learning based algorithm, more specifically an unsupervised clustering algorithm, to identify the main pathways of the Labrador Current in the North Atlantic based on a large set of Lagrangian trajectories obtained from an ocean model. This study shows the power of such a method to help analyze this type of data, and provides a detailed description of the method so it can be used by a broad community on various applications. We find that, when it reached the Grand Banks of Newfoundland, most of the Labrador Current flows either westward toward the Slope Sea or eastward toward the North Atlantic Ocean, in a see‐saw behavior. We also identify a previously unknown minor pathway that brings Labrador Current waters south of the Gulf Stream front. Key Points Unsupervised clustering followed by expert aggregation can identify the main pathways in geospatial Lagrangian trajectories The clusters provide information on the properties, origin and time‐scale of the pathways At the tip of the Grand Banks, the Labrador Current breaks into an east‐west see‐saw system

In recent years, CFD has proven to be a very useful asset to help with predicting complex flows in a wide range of situations, including multiphase and gas-particle flows. On this track, numerical modelling of particle-laden flows in multistage turbomachinery has become an important step in helping to analyse the behaviour of a discrete phase in gas turbines. Furthermore, unsteady effects due, for example, to rotor–stator interaction may have an effect on trajectories and capture efficiencies of the discrete phase. Unfortunately, computational times for transient simulations can be exceedingly high, especially if a discrete-phase needs also to be simulated. For this reason, this work reports a new method for the efficient and accurate simulation of particle-laden flows in gas turbine engines components. The Harmonic Balance Method is exploited to gain orders of magnitude speedup exploiting the idea that once the flow field has been embedded in the spectral basis, it can be reconstructed at any desired time. In this way, not only can the computational time needed to reach convergence of the flow field be dramatically reduced, but there is also no need to keep simulating the flow field during particle tracking. On the contrary, the continuous phase field can be retrieved at any desired time through flow reconstruction. This technique is conceptually simple, but, to the authors’ knowledge, has never been applied so far in particle-laden flow simulations and represents a novelty in the field. First, the implementation of the method is described, and details are given on how phase-lagged boundary conditions can be applied to flow and particles to further speed up the calculation. Then, some relevant case studies are presented to highlight the performance of the method.