Accurately predicting total sea-level including tides and storm surges is key to protecting and managing our coastal environment. However, dynamically forecasting sea level extremes is ...computationally expensive. Here a novel alternative based on ensembles of artificial neural networks independently trained at over 600 tide gauges around the world, is used to predict the total sea-level based on tidal harmonics and atmospheric conditions at each site. The results show globally-consistent high skill of the neural networks (NNs) to capture the sea variability at gauges around the globe. While the main atmosphere-driven dynamics can be captured with multivariate linear regressions, atmospheric-driven intensification, tide-surge and tide-tide non-linearities in complex coastal environments are only predicted with the NNs. In addition, the non-linear NN approach provides a simple and consistent framework to assess the uncertainty through a probabilistic forecast. These new and cheap methods are relatively easy to setup and could be a valuable tool combined with more expensive dynamical model in order to improve local resilience.
The turbulent mixing in thin ocean surface boundary layers (OSBL), which occupy the upper 100 m or so of the ocean, control the exchange of heat and trace gases between the atmosphere and ocean. Here ...we show that current parameterizations of this turbulent mixing lead to systematic and substantial errors in the depth of the OSBL in global climate models, which then leads to biases in sea surface temperature. One reason, we argue, is that current parameterizations are missing key surface‐wave processes that force Langmuir turbulence that deepens the OSBL more rapidly than steady wind forcing. Scaling arguments are presented to identify two dimensionless parameters that measure the importance of wave forcing against wind forcing, and against buoyancy forcing. A global perspective on the occurrence of wave‐forced turbulence is developed using re‐analysis data to compute these parameters globally. The diagnostic study developed here suggests that turbulent energy available for mixing the OSBL is under‐estimated without forcing by surface waves. Wave‐forcing and hence Langmuir turbulence could be important over wide areas of the ocean and in all seasons in the Southern Ocean. We conclude that surface‐wave‐forced Langmuir turbulence is an important process in the OSBL that requires parameterization.
Key Points
Climate models have biases in the depth of the ocean surface boundary layer
Langmuir turbulence is a key process mixing the ocean surface boundary layer
Langmuir turbulence deepens the layer more quickly than wind‐forced turbulence
Lagrangian ocean analysis: Fundamentals and practices van Sebille, Erik; Griffies, Stephen M.; Abernathey, Ryan ...
Ocean modelling (Oxford),
January 2018, 2018-01-00, 2018-01, 2018-01-01, 2018, Letnik:
121, Številka:
C
Journal Article
Recenzirano
Odprti dostop
•Lagrangian ocean analysis is a powerful way to analyse the output of ocean circulation models.•We present a review of the Kinematic framework, available tools, and applications of Lagrangian ocean ...analysis.•While there are unresolved questions, the framework is robust enough to be used widely in ocean modelling.
Lagrangian analysis is a powerful way to analyse the output of ocean circulation models and other ocean velocity data such as from altimetry. In the Lagrangian approach, large sets of virtual particles are integrated within the three-dimensional, time-evolving velocity fields. Over several decades, a variety of tools and methods for this purpose have emerged. Here, we review the state of the art in the field of Lagrangian analysis of ocean velocity data, starting from a fundamental kinematic framework and with a focus on large-scale open ocean applications. Beyond the use of explicit velocity fields, we consider the influence of unresolved physics and dynamics on particle trajectories. We comprehensively list and discuss the tools currently available for tracking virtual particles. We then showcase some of the innovative applications of trajectory data, and conclude with some open questions and an outlook. The overall goal of this review paper is to reconcile some of the different techniques and methods in Lagrangian ocean analysis, while recognising the rich diversity of codes that have and continue to emerge, and the challenges of the coming age of petascale computing.
Highly connected networks generally improve resilience in complex systems. We present a novel application of this paradigm and investigated the potential for anthropogenic structures in the ocean to ...enhance connectivity of a protected species threatened by human pressures and climate change. Biophysical dispersal models of a protected coral species simulated potential connectivity between oil and gas installations across the North Sea but also metapopulation outcomes for naturally occurring corals downstream. Network analyses illustrated how just a single generation of virtual larvae released from these installations could create a highly connected anthropogenic system, with larvae becoming competent to settle over a range of natural deep-sea, shelf and fjord coral ecosystems including a marine protected area. These results provide the first study showing that a system of anthropogenic structures can have international conservation significance by creating ecologically connected networks and by acting as stepping stones for cross-border interconnection to natural populations.
ABSTRACT
Coastal trapped waves (CTWs) carry the ocean’s response to changes in forcing along boundaries and are important mechanisms in the context of coastal sea level and the meridional overturning ...circulation. Motivated by the western boundary response to high-latitude and open-ocean variability, we use a linear, barotropic model to investigate how the latitude dependence of the Coriolis parameter (
β
effect), bottom topography, and bottom friction modify the evolution of western boundary CTWs and sea level. For annual and longer period waves, the boundary response is characterized by modified shelf waves and a new class of leaky slope waves that propagate alongshore, typically at an order slower than shelf waves, and radiate short Rossby waves into the interior. Energy is not only transmitted equatorward along the slope, but also eastward into the interior, leading to the dissipation of energy locally and offshore. The
β
effect and friction result in shelf and slope waves that decay alongshore in the direction of the equator, decreasing the extent to which high-latitude variability affects lower latitudes and increasing the penetration of open-ocean variability onto the shelf—narrower continental shelves and larger friction coefficients increase this penetration. The theory is compared with observations of sea level along the North American east coast and qualitatively reproduces the southward displacement and amplitude attenuation of coastal sea level relative to the open ocean. The implications are that the
β
effect, topography, and friction are important in determining where along the coast sea level variability hot spots occur.
The influence of surface waves and an applied wind stress is studied in an ensemble of large eddy simulations to investigate the nature of deeply penetrating jets into an unstratified mixed layer. ...The influence of a steady monochromatic surface wave propagating parallel to the wind direction is parameterized using the wave‐filtered Craik‐Leibovich equations. Tracer trajectories and instantaneous downwelling velocities reveal classic counterrotating Langmuir rolls. The associated downwelling jets penetrate to depths in excess of the wave's Stokes depth scale, δs. Qualitative evidence suggests the depth of the jets is controlled by the Ekman depth scale. Analysis of turbulent kinetic energy (tke) budgets reveals a dynamical distinction between Langmuir turbulence and shear‐driven turbulence. In the former, tke production is dominated by Stokes shear and a vertical flux term transports tke to a depth where it is dissipated. In the latter, tke production is from the mean shear and is locally balanced by dissipation. We define the turbulent Langmuir number Lat = (v*/Us)0.5 (v* is the ocean's friction velocity and Us is the surface Stokes drift velocity) and a turbulent anisotropy coefficient Rt = /( + ). The transition between shear‐driven and Langmuir turbulence is investigated by varying external wave parameters δs and Lat and by diagnosing Rt and the Eulerian mean and Stokes shears. When either Lat or δs are sufficiently small the Stokes shear dominates the mean shear and the flow is preconditioned to Langmuir turbulence and the associated deeply penetrating jets.
The ecological role of offshore man-made infrastructure is of growing international interest. By 2030, globally more than 7500 oil and gas platforms could be removed, many of which now host mature ...hard substrate ecosystems formed by sessile benthic species including sponges, corals and mussels. We investigated the spatiotemporal scales of generalised species dispersal and connectivity among oil and gas structures in the North Sea using strategically designed 3D advective passive particle tracking experiments forced by high resolution (1.8 km, hourly) velocity fields including tide-, density- and wind-driven currents. Trajectories from 2 seasonal releases during mixed winter (February) and stratified summer (July) conditions of 2010 were analysed for a variety of pelagic larval durations (PLDs) spanning 2 to 28 d. Particles dispersed on average 32 km away from their origins after just 5 d, 67 km after 15 d, and 109 km after 28 d, with considerable spatial variability and limited seasonal variations. Short (2 d) PLDs generated highly connected networks over smaller spatial scales, while longer PLDs (28 d) generated less fragmented networks covering a much larger area but with fewer connections. Tidally driven dispersal was isolated using a new method based on the harmonic analysis of the velocity fields: the resulting maximum linear dispersal distances varied from ~4 km in the northern North Sea to ~8 km in the southern North Sea. The present study provides baseline spatiotemporal scales of dispersal and connectivity patterns and optimized relocatable methods to assess connectivity in tidally active shelf seas.
Individual specialisations in behaviour are predicted to arise where divergence benefits fitness. Such specialisations are more likely in heterogeneous environments where there is both greater ...ecological opportunity and competition‐driven frequency dependent selection.
Such an effect could explain observed differences in rates of individual specialisation in habitat selection, as it offers individuals an opportunity to select for habitat types that maximise resource gain while minimising competition; however, this mechanism has not been tested before.
Here, we use habitat selection functions to quantify individual specialisations while foraging by black‐legged kittiwakes Rissa tridactyla, a marine top predator, at 15 colonies around the United Kingdom and Ireland, along a gradient of environmental heterogeneity.
We find support for the hypothesis that individual specialisations in habitat selection while foraging are more prevalent in heterogeneous environments. This trend was significant across multiple dynamic habitat variables that change over short time‐scales and did not arise through site fidelity, which highlights the importance of environmental processes in facilitating behavioural adaptation by predators.
Individual differences may drive evolutionary processes, and therefore these results suggest that there is broad scope for the degree of environmental heterogeneity to determine current and future population, species and community dynamics.
Individuals specialise on habitats, rather than specific sites, in more heterogeneous environments.
This study investigates the pressure–strain tensor (
$\unicodeSTIX{x1D72B}$
) in Langmuir turbulence. The pressure–strain tensor is determined from large-eddy simulations (LES), and is partitioned ...into components associated with the mean current shear (rapid), the Stokes shear and the turbulent–turbulent (slow) interactions. The rapid component can be parameterized using existing closure models, although the coefficients in the closure models are particular to Langmuir turbulence. A closure model for the Stokes component is proposed, and it is shown to agree with results from the LES. The slow component of
$\unicodeSTIX{x1D72B}$
does not agree with existing ‘return-to-isotropy’ closure models for five of the six components of the Reynolds stress tensor, and a new closure model is proposed that accounts for these deviations which vary systematically with Langmuir number,
$La_{t}$
, and depth. The implications of these results for second- and first-order closures of Langmuir turbulence are discussed.
Huthnance Estuarine Coastal Mar. Sci. 1973, 1, 89-99 is reviewed, whereby an oscillating tide over bathymetric features induces a mean flow generally along isobaths. The effect is a superposition of ...Coriolis and frictional processes. These are discussed with the intention of elucidating the processes for a more general readership. Induced velocities of order several
are expected around the UK shelf seas. The effect is dynamically significant over bathymetric scales of order a few kilometres and has previously been of most interest to dynamicists studying processes on this scale. However, with the increase in computing power, appropriate scales can be simulated in shelf-wide regional models and in next generation operational models. It is demonstrated that this small-scale effect is likely to be important for shelf-wide regional models and that a spatial resolution of at least 1.8 km is recommended for shelf sea simulations.
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