3D Ocean Model Research Articles

Deciphering the Ages of Saline Water in the Baltic Sea by Anthropogenic Radiotracers

AbstractThe slow water renewal endows the Baltic Sea a strong retention of pollutants/nutrients. Constraining water age is a practical way to depict the transport timescales for water masses and accompanying soluble substances. Although the water ages in the Baltic Sea have been resolved by 3D ocean models 20 year ago, the simulated results have not been verified. In this work, we exploited two anthropogenic radionuclides (129I and 236U) to constrain the ages of inflowing North Sea saline waters into the Baltic Sea by using the Tracer Age and Transit Time Distribution approaches. Our results indicate that the Baltic Sea has a highly stratified structure with a more diffusive bottom circulation and a more advective surface circulation. The circulation timescale for the saline water in the Baltic Sea was estimated to be >30 years from the bottom of Arkona Basin to the surface of central Baltic Sea. This work demonstrates the power of anthropogenic radiotracers in investigating circulation timescale and mixing processes in the Baltic Sea and provides the first observation‐based proof for the multi‐decadal retention of (radioactive) pollutants within the Baltic Sea.

Multivariate Data Assimilation at a Partially Mixed Estuary

Abstract In 2013 and 2014, multiple field excursions of varying scope were concentrated on the Columbia River, a highly energetic, partially mixed estuary. These experiments included surface drifter and synthetic aperture radar (SAR) measurements during the ONR RIVET-II experiment, and a novel animal tracking effort that samples oceanographic data by employing cormorants tagged with biologging devices. In the present work, several different data types from these experiments were combined in order to test an iterative, ensemble-based inversion methodology at the mouth of the Columbia River (MCR). Results show that, despite inherent limitations of observation and model accuracy, it is possible to detect dynamically relevant bathymetric features such as large shoals and channels while originating from a linear, featureless prior bathymetry in a partially mixed estuary by inverting surface current and gravity wave observations with a 3D hydrostatic ocean model. Bathymetry estimation skill depends on two factors: location (i.e., differing estimation quality inside versus outside the MCR) and observation type (e.g., surface currents versus significant wave height). Despite not being inverted directly, temperature and salinity outputs in the hydrodynamic model improved agreement with observations after bathymetry inversion.

Modelling Antarctic ice shelf basal melt patterns using the one-layer Antarctic model for dynamical downscaling of ice–ocean exchanges (LADDIE v1.0)

Abstract. A major source of uncertainty in future sea level projections is the ocean-driven basal melt of Antarctic ice shelves. While ice sheet models require a kilometre-scale resolution to realistically resolve ice shelf stability and grounding line migration, global or regional 3D ocean models are computationally too expensive to produce basal melt forcing fields at this resolution on long timescales. To bridge this resolution gap, we introduce the 2D numerical model LADDIE (one-layer Antarctic model for dynamical downscaling of ice–ocean exchanges), which allows for the computationally efficient modelling of detailed basal melt fields. The model is open source and can be applied easily to different geometries or different ocean forcings. The aim of this study is threefold: to introduce the model to the community, to demonstrate its application and performance in two use cases, and to describe and interpret new basal melt patterns simulated by this model. The two use cases are the small Crosson–Dotson Ice Shelf in the warm Amundsen Sea region and the large Filchner–Ronne Ice Shelf in the cold Weddell Sea. At ice-shelf-wide scales, LADDIE reproduces observed patterns of basal melting and freezing in warm and cold environments without the need to re-tune parameters for individual ice shelves. At scales of 0.5–5 km, which are typically unresolved by 3D ocean models and poorly constrained by observations, LADDIE produces plausible basal melt patterns. Most significantly, the simulated basal melt patterns are physically consistent with the applied ice shelf topography. These patterns are governed by the topographic steering and Coriolis deflection of meltwater flows, two processes that are poorly represented in basal melt parameterisations. The kilometre-scale melt patterns simulated by LADDIE include enhanced melt rates in grounding zones and basal channels and enhanced melt or freezing in shear margins. As these regions are critical for ice shelf stability, we conclude that LADDIE can provide detailed basal melt patterns at the essential resolution that ice sheet models require. The physical consistency between the applied geometry and the simulated basal melt fields indicates that LADDIE can play a valuable role in the development of coupled ice–ocean modelling.

A Numerical Study on the Role of Mesoscale Cold-Core Eddies in Modulating the Upper-Ocean Responses to Typhoon Trami (2018)

Abstract With two groups of numerical experiments with and without the cold-core eddy (CCE), the impacts of the CCE on the upper-ocean responses to Typhoon Trami (2018) were investigated using a coupled atmosphere–ocean model. It is commonly accepted that the CCE promotes the sea surface cooling (SSC) primally through the enhanced vertical mixing, while the contributions from the wind-driven advection and the near-inertial advection to the differences in the sea surface temperature (dSST) were underestimated. This study found that the presence of CCE contributed to the stronger along-track cold advection, which dominated the increase in the SSC near the radius of maximum wind (RMW) to the right of Trami’s track, and the stronger cross-track warm advection was acting to prevent the cooling induced by the vertical mixing. During the relaxation stage, the stronger near-inertial advection within the CCE accounted largely for the amplification and the redistribution of the dSST. As for the dynamic responses, the enhanced upwelling and downwelling within the CCE explained the larger cooling and warming in the subsurface temperature oscillations. The wind-driven acceleration of the currents in the mixing layer was larger during the typhoon–eddy interaction so that the CCE became an efficient mixer, thus contributing to the rapid surfacing of the cold water and the ensuing stronger wind-driven advection. These results highlight the importance of the advection processes in the modulating effect of the CCE. Therefore, 3D ocean models are needed to incorporate the mesoscale features of the oceanic eddies for realistically reproducing the upper-ocean responses to tropical cyclones (TCs).

Hydrodynamic effects of large-scale suspended mussel farms: Field observations and numerical simulations

The hydrodynamic effects of the largest suspended mussel farms in the East China Sea near Gouqi Island, was investigated using a high-resolution 3D ocean model and field observations. To capture the 3D farm effects on hydrodynamics, an additional depth dependent momentum sink term was introduced in the model. The model results compared well with the field observations. The present model and observational results indicate that the presence of farms reduces the flow by more than 79%, 55%, and 34% in the upper, middle, and bottom layers at the farm center, respectively. According to the harmonic analysis of predicted current, mussel farms reduce the magnitude of the semidiurnal tidal current and also alter the magnitude and direction of the diurnal tidal current. The blockage by the farm weakens the Eulerian residual tidal current within the farm in the NE-SW direction, while strengthens that at the edge of the farm in the SE-NW direction. Cross sections, Sec1 and Sec2 are perpendicular to these two major residual currents and intercept with the center of the farm from SE to NW and from NE to SW respectively. The farm effect on the total water flux over a month through the Sec2 displays a semi-lunar periodic oscillation and is one order of magnitude smaller than that at Sec1. An asymmetry tidal current was observed in the farm north of Gouqi Island. The field observation of vertical profiles of current suggests that the thickness of surface canopy boundary layer can reach 5 m upstream from the farm during flood tide, increases gradually downstream up to 10 m under the cumulative influences of the farm. And a wake zone was observed downstream from the farm during flood tide. Better understanding of farm-induced hydrodynamic effects provides insight into how to optimize farm layouts based on local hydrodynamics, to maximize farm productivity and minimize environmental impacts.

Applications of generalized vertical coordinates in ocean circulation models

Two types of generalized vertical coordinates are introduced to reduce model errors in calculating the baroclinic pressure gradient (BPG), advection term and horizontal mixing in tracer equations in a three-dimensional (3D) ocean circulation model. These two types of vertical coordinates are defined as the generalized terrain-following coordinate (GTC) and generalized piecewise terrain-following coordinate (GPTC). The main feature of the GTC is the use of a series of constant-depth levels in the water column above the sea bottom. The GPTC differs from the GTC in which the depth differences between two adjacent grids are reduced, especially in the bottom layer over steep topography. Performances of these two vertical coordinate systems are assessed first in two types of idealized numerical experiments with a tall seamount and continuously varying bottom slope respectively. The model results in the tall seamount experiment demonstrate that both the GTC and GPTC perform well in reducing the model errors in the BPG over the steep topography, and greatly alleviate the spurious diapycnal mixing in terrain-following coordinate models. In the continuously varying bottom slope experiment, both the GTC and GPTC perform better than the z-level coordinate in simulating the 3D currents. The performances of the GTC and GPTC are further assessed by examining prognostic simulations of 3D currents and hydrography in the Yellow and East China Seas with realistic external forcing and bottom topography. The model results in the third experiment demonstrate that the GPTC restrains the overestimated upward climbing of the bottom cold water in comparison with other coordinates. The GPTC also performs better than the GTC and the conventional sigma-layer coordinate in simulating the shoreward intrusion of relatively high-salinity subsurface waters, intermediate low-salinity waters and the Yellow Sea bottom cold waters. By comparison, the GTC simulates the Kuroshio, the Slope Counter-Current and their seasonal variability significantly better than other coordinates.

A sensitivity analysis of a regression model of ocean temperature

AbstractThere has been much recent interest in developing data-driven models for weather and climate predictions. However, there are open questions regarding their generalizability and robustness, highlighting a need to better understand how they make their predictions. In particular, it is important to understand whether data-driven models learn the underlying physics of the system against which they are trained, or simply identify statistical patterns without any clear link to the underlying physics. In this paper, we describe a sensitivity analysis of a regression-based model of ocean temperature, trained against simulations from a 3D ocean model setup in a very simple configuration. We show that the regressor heavily bases its forecasts on, and is dependent on, variables known to be key to the physics such as currents and density. By contrast, the regressor does not make heavy use of inputs such as location, which have limited direct physical impacts. The model requires nonlinear interactions between inputs in order to show any meaningful skill—in line with the highly nonlinear dynamics of the ocean. Further analysis interprets the ways certain variables are used by the regression model. We see that information about the vertical profile of the water column reduces errors in regions of convective activity, and information about the currents reduces errors in regions dominated by advective processes. Our results demonstrate that even a simple regression model is capable of learning much of the physics of the system being modeled. We expect that a similar sensitivity analysis could be usefully applied to more complex ocean configurations.

Changes in Arctic Stratification and Mixed Layer Depth Cycle: A Modeling Analysis

AbstractClimate change is especially strong in the region of the Arctic Ocean, and will have an important impact on its thermo‐haline structure. We analyze the results of a hindcast simulation of a new 3D ocean model of the Arctic and North Atlantic oceans for the period 1970–2019. We compared the time period 1970–1999 with the time period 2010–2019. The comparison showed that there is a decrease of stratification between the two periods over most of the shallow Arctic shelf seas and in the core of the Transpolar Ice Drift. Fresh water inputs to the ocean surface decline, and inputs of momentum to the ocean increase, which can explain the decrease in stratification. The comparison also showed that the mixed layer becomes deeper during winter, in response to the weakened stratification owing to increased vertical mixing. The comparison of summer mixed layer depths between the two time periods follows a deepening pattern that is less evident. Regional exceptions include the Nansen Basin and the part of the Canadian Basin bordering the Canadian Archipelago, where the mixed layer shoals. Trends of freshwater fluxes imply that the changes of haline stratification in these regions are also influenced by other processes, for example, horizontal advection of fresh water, increased mixing and changes in the underlaying water masses. Runoff increase toward the Arctic Ocean can locally decrease but also increase salinity, and has an impact on stratification which can be explained by coastal dynamics. The results emphasize the non‐linear nature of Arctic Ocean dynamics.

Sediment Dispersion Modeling of a Non-Orthogonal Boundary Fitted Technique: Mangrove Reclamation to Protect Cliff Erosion in Bintuni Bay, Indonesia

Muin, M., 2021. Sediment dispersion modeling of a non-orthogonal boundary fitted technique: Mangrove reclamation to protect cliff erosion in Bintuni Bay, Indonesia. In: Lee, J.L.; Suh, K.-S.; Lee, B.; Shin, S., and Lee, J. (eds.), Crisis and Integrated Management for Coastal and Marine Safety. Journal of Coastal Research, Special Issue No. 114, pp. 544–548. Coconut Creek (Florida), ISSN 0749-0208. Bintuni Bay is a semi-closed bay located in West Papua Province, Indonesia. The facility of Tangguh LNG Project was faced serious problems due to erosion along the cliff. It was decided that reclamation is the best solution. Mangrove trees are planted in the reclamation area to reduce the wave. The reclamation was designed until one meter below highest water spring to allow seawater to flow in and out through the inlet/outlet. This paper presents and discusses simulations for several options of reclamation layout, inlet/outlet, during and after reclamation. MuSed3D, the 3D ocean hydrodynamics and sediment transport model using non-orthogonal boundary fitted technique, was employed to conduct the simulation.

Analysis of workload required for removal of drifting pumice after a volcanic disaster as an aspect of a port business continuity plan: A case study of Kagoshima Port, Japan

For most major ports in Japan, a business continuity plan (Port-BCP) has been developed to cope with potential functional disorder in port operations caused by natural disasters. The objectives of a Port-BCP are to maintain the minimum vital activities, such as transportation of emergency supplies, under critical conditions, and to enable resumption of normal operations within a reasonable period. However, Japan's existing Port-BCP plans focus only on earthquake and/or tsunami disasters and do not consider volcanic events. This study assessed the impact of pumice fallout in the Kagoshima Port area following a supposed large-scale eruption of the adjacent volcano, Sakurajima. To estimate the potential volume and mass of pumice deposited in Kagoshima Bay, numerical simulations were conducted using a tephra transport model with varying wind parameters. Based on the results, the duration of the work required to remove the deposited pumice from the port area and navigation channels was estimated. Then, analysis of tidal circulations under surface wind stress was conducted using a 3D ocean model to clarify the characteristics of the currents in Kagoshima Bay. On the basis of the calculations of both pumice fallout and tidal currents, the surface drift of the floating pumice was analyzed. The total mass of floating pumice, as the object for removal, is presumed to decrease with time. Therefore, the temporal variation of the total mass of floating pumice was investigated by considering several physical mechanisms.

Spatial variability in rates of net primary production (NPP) and onset of the spring bloom in Greenland shelf waters

Greenland extends from 60° to 83° N, with 80% of its land mass covered by the Greenland Ice Sheet (GrIS). This extensive latitudinal gradient is associated with concomitant environmental gradients that impact the biogeochemical properties of its coastal waters. Although the Greenland fjords have been the subject of intense study, less is known of the productivity in the continental shelves, regions that support local fisheries and influence fjord oceanography. This study provides a large-scale overview of annual net primary production rates (NPP) and their spatial variability in 7 regional shelves around Greenland, over the last decade (2008–2017), with special emphasis on spring bloom initiation. NPP is estimated by two independent approaches already established as best for estimating Arctic productivity: a physically-biologically coupled, regional 3D ocean model (SINMOD) and a spectrally-resolved, light-photosynthesis model of primary production (UQUAR-Takuvik model) that is applied to satellite observations of phytoplankton chlorophyll a, which is derived from ocean color remote sensing (OCRS). Both OCRS and SINMOD provide similar estimates of the timing and rates of productivity in Greenlandic waters, when compared with scarce field estimates. Bloom initiation shows a strong south-north gradient, beginning in April in the southern regions and late June in the Arctic Ocean. OCRS-modeled NPP highlights the effect of sea ice presence on bloom initiation; this method depicts the start of the bloom consistently later, by up to 13 days on average, than SINMOD-modeled NPP estimates. In contrast, numerical modeling is able to detect early phytoplankton growth in Greenland shelves, particularly underneath seasonal sea ice. Rates of annual NPP show a strong south-north gradient, with higher NPP rates observed in the North Atlantic water-influenced southern regions, with up to a factor of 3 decrease in NPP towards the north, in the western Eurasian Basin of the Arctic Ocean. Annual NPP varies from 78.3 ± 12.3 g C m−2 yr−1 and 80.3 ± 18.7 g C m−2 yr−1 in the southern regions to 24.7 ± 3.9 g C m−2 yr−1 in the Arctic Ocean. In each region, sea ice distribution and timing of formation and retreat affect location and timing of seasonal productivity with earlier and higher NPP offshore, moving inshore towards the summer. An average 55% to 75% of the annual production is estimated to be exported to depth, higher in Arctic and sub-Arctic regions, suggesting Greenland shelf waters have a potential for high carbon export to depth and relatively less carbon associated with the microbial food web.

Simulating major storm surge events in a complex coastal region

Climate change and increased coastal urbanization are causing low-lying coastlines to become increasingly susceptible to the threat of extreme water levels and coastal flooding. Robust decision-making on adaptation in the coastal zone, based on reliable ocean-modelling tools, is therefore crucially contingent on accurate assessments of current and future storm surge hazards. This accuracy relies considerably on the quality of the wind forcing used in the ocean models. In this paper, we use a high-resolution, regional 3D ocean model (HBM) covering the North Sea and Baltic Sea to simulate extreme water levels during three extreme storm surge events with different dynamics and patterns, in order to assess their impacts along Denmark’s coastlines, which are of varying levels of complexity. We demonstrate that the model is able to reproduce the observed extreme high-water levels accurately, indicating that the system is well suited for producing simulations of present and future projections of extreme storm surges with high resulting impacts and damage potentials. Additionally, we quantify the level at which acknowledged deficiencies in the otherwise most suitable atmospheric forcing data set influence the results of the storm surge simulations. We found that reducing the temporal resolution of the forcing data – that is, replacing two out of every six time stamps with linearly interpolated values – is preferable to using the original forcing data set when recurring noise is present in these time stamps. As a result, for given storm surge events, and depending on the stage reached in the storm’s evolution, mean absolute errors can be reduced by 4.5cm. This emphasizes the importance of considering such model fluctuations when coupling high-resolution atmosphere and ocean models.

Implementation and assessment of a flux limiter based wetting and drying scheme in NEMO

Wetting and drying processes in shallow water systems by surges, tides and seiches have important societal, physical and biological impacts. Operational 3D regional models are of sufficient resolution, O(1 km), that the processes of wetting and drying need to be included. Here we describe a flux limiter based approach that allows a numerical ocean model with a flux formulation of tracer advection to wet and dry. The flux limiter approach limits the outflow from a cell whose depth is below a critical value defined by the user. The limiter can be a step function or a smooth function of the water depth, the latter increases model stability and avoids rapid alternation between dry and wet states on long slopes as the critical depth is approached. Furthermore, the user may proportionally limit the baroclinic fluxes as a cell transitions from wet to dry over the course of the large baroclinic time step. The simplicity of the flux limiter approach lends itself to its application within existing numerical models without significant intrusion into the code base. In this paper the vertical coordinate is restricted to pure terrain following coordinates with no z-like steps.We explore the scheme’s effectiveness, sensitivities and limitations within the 3D NEMO ocean model by assessing it using test cases of increasing complexity. It is shown to perform well in classic channel test cases and 2D parabolic test cases with analytic solutions. All test cases with initially uniform temperature and salinity preserve this initial condition throughout the wetting and drying phases. The scheme’s performance against analytical 1D dam break experiments is explored and used to interpret its performance against laboratory measurements of a 2D dam break. It is also shown to run stably for a realistic 3D regional domain of the North West European shelf and to improve some aspects of the model’s performance against tide gauges.

Temporal variability in the Holocene marine radiocarbon reservoir effect for the Tropical and South Pacific

Understanding the marine radiocarbon reservoir effect (i.e., marine radiocarbon reservoir age (R) and/or correction (ΔR)) is important for the construction of robust radiocarbon chronologies for marine archives for various research areas including archaeology, palaeoecology, paleoceanography, Quaternary research and climate change studies. In this study, we examined temporal ΔR variability for the South China Sea (SCS) and the Great Barrier Reef (GBR) during the past ∼8.1 ka based on 14C analysis of 230Th-dated corals. Results show large ΔR variations of ∼410 yr and ∼490 yr for the SCS and the northern GBR for ∼5.5–8.1 ka and ∼5.5–7 ka, respectively, and a smaller ΔR variability of ∼200 yr for the SCS for ∼2–3.5 ka. Our data, together with those previously published for the Tropical and South Pacific, indicate that variability in ocean upwelling bringing old subsurface waters to the surface and/or changes in the sources (or 14C level) of the upwelled waters are responsible for seeing large ΔR variations in coastal areas along the eastern Pacific and the Tropical East Pacific (TEP) of several hundred to a thousand years mostly during the Early to Middle Holocene. ΔR variations in the central and western Pacific of several and a couple of hundred years during the Middle and Late Holocene, respectively, might be due to variability in Pacific-wide ocean circulation associated with climatic changes controlling the spread of upwelled waters from the TEP to the west. This mechanism together with local/regional effects, such as changes in ocean upwelling in the SCS resulting from East Asian monsoon variability and changes in upwelling and/or horizontal advection in the northern GBR associated with variability in the El Niño Southern Oscillation, might be responsible for the observed ΔR variations in these areas. The results of our study also indicate the need for regional marine radiocarbon calibration curves for improved radiocarbon dating of marine samples as the observed Holocene ΔR values for the Tropical and South Pacific are not fully reproduced by a recent modelling work using a 3D ocean model, which takes into account climate change effects. Ocean circulation changes were included in the model for the period of 11.5–50 cal kyr BP but possibly not considered or not well represented for the Holocene, which might explain the differences between the observed and modelled ΔR values.

Improvements in turbulence model realizability for enhanced stability of ocean forecast and its importance for downstream components

The numerical stability of ocean circulation models is of high significance in operational forecasting. A substantial improvement in numerical stability of the 3D-ocean model HBM could be achieved by the implementation of new realizability criteria in the turbulence closure scheme. Realizability criteria which were already well documented for closure functions without double diffusion were therefore extended to those using double diffusion. A purely technical validation method called ε-test which is suitable for the detection of numerical stability problems is presented, and the effect of the development in turbulence model is demonstrated under severe weather conditions during extreme storm events. Evaluation of statistics of longer simulations indicate that instabilities appeared only locally and temporary; nevertheless, a significant impact on drift products relying on the current forecasts could be demonstrated, which underlines the importance of realizability in turbulence closure schemes in comprehensive operational model systems including ocean circulation and downstream drift components.

Dynamics of water exchange and salt flux in the Macuse Estuary, central Mozambique, southern Africa

Studies of water-particle flow dynamics in shallow estuarine systems show that tidal currents control water exchange, salt flux and residence time. We used the 3D Estuary, Lake and Coastal Ocean Model (ELCOM) to estimate the dynamics of tidal currents, salt flux and residence time in the Macuse Estuary, central Mozambique. The model was calibrated using data acquired from field-data measurements obtained in 2014 with an acoustic Doppler current profiler and conductivity-temperature-depth (CTD) casts and a tide-gauge. The water flow dynamics that were tracked indicated that tidal currents of 100 cm s−1 were dissipated by friction caused by bathymetric morphology during both the spring and neap tides. The water dissipation was translated into a phase-difference delay of ∼15 min between the catchment zone and the outlet during flood and ebb tides. The study showed that tidal currents and river discharges controlled the salt flux (2.07 × 105 g kg−1 m3 s−1) and the variation in residence time from hours to 40 days. The water ages were mostly driven by U-velocity tidal currents, bathymetric gradients and seasonal river discharges of ∼500 m3 s−1. River discharges seasonally affected salinity changes between 15 and 30, and changes in the concentration of suspended sediments of ∼300 mg l−1. In addition, it was shown that ELCOM was able to track the water-particle dynamics well, indicating the model’s suitability. These results contribute to our understanding of the effect of water-exchange dynamics on residence time and salt flux in shallow coastal inlet systems.

Observed Inter annual Variability of Upwelling Characteristics during 2016 2017 A Study using Princeton Ocean Model

Oceanographic observations carried out during 2016 and 2017 onboard INS Sagardhwani in the Southeastern Arabian sea are used to study the inter-annual variability of the upwelling. In 2016, the strong upwelling signatures are noticed in the observations (SST < 27°C and strong up-slopping of isotherms) as well as in the satellite derived sea level anomaly data. Whereas in 2017 the low sea level in June (-2 cm) are weakened during the mid of July (+3 cm) along the southern track (8 °N and 9 °N). This decrease in the strength in 2017 can be attributed to two major reasons. One is the presence of an anti-cyclonic eddy along the coast (8.5 °N, 76.5 °E) weakens the upwelling processes and second is the weak northerly component of the wind compared to 2016. In addition, Lakshadweep low is less prominent and situated towards the southern side (around 7°N) of its usual region of occurrence in 2017. The inter-annual variability of upwelling during July 2016 and 2017 is investigated using the 3D ocean model Princeton Ocean Model. Experiments with model in different combinations of forcing reveals that the alongshore wind component is the major parameter influencing the upwelling characteristics during these periods.

Weakly Coupled Ocean–Atmosphere Data Assimilation in the ECMWF NWP System

Numerical weather prediction models are including an increasing number of components of the Earth system. In particular, every forecast now issued by the European Centre for Medium-Range Weather Forecasts (ECMWF) runs with a 3D ocean model and a sea ice model below the atmosphere. Initialisation of different components using different methods and on different timescales can lead to inconsistencies when they are combined in the full system. Historically, the methods for initialising the ocean and the atmosphere have been typically developed separately. This paper describes an approach for combining the existing ocean and atmospheric analyses into what we categorise as a weakly coupled assimilation scheme. Here, we show the performance improvements achieved for the atmosphere by having a weakly coupled ocean–atmosphere assimilation system compared with an uncoupled system. Using numerical weather prediction diagnostics, we show that forecast errors are decreased compared with forecasts initialised from an uncoupled analysis. Further, a detailed investigation into spatial coverage of sea ice concentration in the Baltic Sea shows that a much more realistic structure is obtained by the weakly coupled analysis. By introducing the weakly coupled ocean–atmosphere analysis, the ocean analysis becomes a critical part of the numerical weather prediction system and provides a platform from which to build ever stronger forms of analysis coupling.

A global scavenging and circulation ocean model of thorium-230 and protactinium-231 with improved particle dynamics (NEMO–ProThorP 0.1)

Abstract. In this paper we set forth a 3-D ocean model of the radioactive trace isotopes 230Th and 231Pa. The interest arises from the fact that these isotopes are extensively used for investigating particle transport in the ocean and reconstructing past ocean circulation. The tracers are reversibly scavenged by biogenic and lithogenic particles. Our simulations of 230Th and 231Pa are based on the NEMO–PISCES ocean biogeochemistry general circulation model, which includes biogenic particles, namely small and big particulate organic carbon, calcium carbonate and biogenic silica. Small and big lithogenic particles from dust deposition are included in our model as well. Their distributions generally compare well with the small and big lithogenic particle concentrations from recent observations from the GEOTRACES programme, except for boundary nepheloid layers for which, as of today, there are no non-trivial prognostic models available on a global scale. Our simulations reproduce 230Th and 231Pa dissolved concentrations: they compare well with recent GEOTRACES observations in many parts of the ocean. Particulate 230Th and 231Pa concentrations are significantly improved compared to previous studies, but they are still too low because of missing particles from nepheloid layers. Our simulation reproduces the main characteristics of the 231Pa∕230Th ratio observed in the sediments and supports a moderate affinity of 231Pa to biogenic silica as suggested by recent observations relative to 230Th. Future model development may further improve understanding, especially when this will include a more complete representation of all particles, including different size classes, manganese hydroxides and nepheloid layers. This can be done based on our model as its source code is readily available.

Modeling dispersal and spatial connectivity of macro-invertebrates in Danish waters: An agent-based approach

Marine organisms with planktonic life stages are subjected to extensive transport that results from the interactions between ocean currents and their behavioral responses to environmental changes in the course of their life. Questions remain on the identification of key drivers of dispersal and connectivity in marine populations as they can have multiple uses in the conservation and management of marine ecosystems. Here we investigate whether the open Kattegat, at the entrance to Baltic Sea, is the main source of recruitment to the benthos in associated estuaries and coastal sites through export of planktonic invertebrate larvae. We couple a 3D hydrodynamic ocean model (MIKE3FM) to an agent-based model and simulate the dispersal of macro-invertebrate populations in Danish waters. We use characteristic dispersal traits of the larval community (pelagic larval duration, spawning season, and settling behavior) and simulate dispersal processes within the muddy bottom habitats to derive recruitment rates and potential donor populations leading to population connectivity patterns on each site, one bay and two Danish fjords. We then use our recruitment results in the bay to compare them with field data on species diversity in the same area. A total of 48 different combinations of pelagic larval durations and spawning seasons of macro-invertebrates are simulated in two years 2004 and 2010. From these results, we conclude that the central and southern parts of the Danish waters are identified as important spawning grounds whereas the Kattegat does not seem to be the main provider of larvae into the selected sites. The model also predicts higher abundance and recruitment rates of macro-invertebrate larvae in 2010 compared to 2004. These results are supported by comparable species distribution data collected in the study area. Our results show the importance of an integrated modeling tool combining ocean circulation and biological traits to obtain a detailed description of dispersal and connectivity of macro-invertebrate community in the area, which can provide a more accurate baseline to manage marine biodiversity.