High-resolution Ocean Circulation Model Research Articles

Upper-ocean thermal variability controlled by ocean dynamics in the Kuroshio-Oyashio Extension region

To understand the upper-ocean thermal variability in the Kuroshio−Oyashio Extension (KOE) region, the upper 400 m heat budget in the western North Pacific is analyzed for the 1981 − 2013 period using outputs from a high resolution (1/12°) ocean general circulation model. Winter heat storage rate on interannual to decadal time scales is mainly determined by oceanic heat advection rather than by net air-sea heat flux. The role of heat advection becomes particularly prominent and widely spread over the entire western North Pacific after the 1990 regime shift in association with the reduced variability of surface heat flux caused by weakened SST variability. The net heat flux acts to dampen temperature anomalies caused by the ocean dynamics. The ocean dynamics causing the upper-ocean heat storage rate is principally associated with the meridional shift of the Oyashio Extension front, which is significantly correlated with both the West Pacific and Pacific-North America teleconnection patterns.

Fluctuations in population fecundity drive variation in demographic connectivity and metapopulation dynamics.

Demographic connectivity is vital to sustaining metapopulations yet often changes dramatically through time due to variation in the production and dispersal of offspring. However, the relative importance of variation in fecundity and dispersal in determining the connectivity and dynamics of metapopulations is poorly understood due to the paucity of comprehensive spatio-temporal data on these processes for most species. We quantified connectivity in metapopulations of a marine foundation species (giant kelp Macrocystis pyrifera) across 11 years and approximately 900 km of coastline by estimating population fecundity with satellite imagery and propagule dispersal using a high-resolution ocean circulation model. By varying the temporal complexity of different connectivity measures and comparing their ability to explain observed extinction-colonization dynamics, we discovered that fluctuations in population fecundity, rather than fluctuations in dispersal, are the dominant driver of variation in connectivity and contribute substantially to metapopulation recovery and persistence. Thus, for species with high variability in reproductive output and modest variability in dispersal (most plants, many animals), connectivity measures ignoring fluctuations in fecundity may overestimate connectivity and likelihoods of persistence, limiting their value for understanding and conserving metapopulations. However, we demonstrate how connectivity measures can be simplified while retaining utility, validating a practical solution for data-limited systems.

Numerical modeling of intrinsically and extrinsically forced seasonal circulation in the China Seas: A kinematic study

AbstractWe developed a new three‐dimensional, high‐resolution ocean circulation model for the entire China Seas (CS) region. The model considered the linked physics associated with the western boundary current, monsoonal wind, and tidal forcings, and topography in both the CS and the adjacent oceans. From this well‐validated model, we derived new insights into the three‐dimensional seasonal circulation of the CS in response to the intrinsic forcing of monsoonal winds and extrinsic forcing of flow exchange with adjacent oceans through the straits and over the slope around the periphery of the CS. Besides the East Asian monsoon forcing, we found that the extrinsic forcings interact coherently with each other and with the interior circulation to jointly shape the CS circulation. Specifically, we revealed rotating layered circulation in the CS. The circulation in the South China Sea has a vertical cyclonic‐anticyclonic‐cyclonic pattern in the upper‐middle‐lower layers, which we relate to the inflow‐outflow‐inflow transport in those layers in the Luzon Strait. The circulation in the East China Sea (ECS) is characterized by a vertically variable cyclonically rotating flow, and the circulation in the Yellow Sea (YS) is represented by a cyclonic movement in the upper layer and an anticyclonic movement in the lower layer. We attribute the cross‐shelf variation of the along‐shelf current to the ECS circulation pattern, while the vertically variable intrusive current at the central trough, together with the seasonally varied west and east coastal currents, shape the two‐layer circulation in the YS.

Tidal forcing, energetics, and mixing near the Yermak Plateau

Abstract. The Yermak Plateau (YP), located northwest of Svalbard in Fram Strait, is the final passage for the inflow of warm Atlantic Water into the Arctic Ocean. The region is characterized by the largest barotropic tidal velocities in the Arctic Ocean. Internal response to the tidal flow over this topographic feature locally contributes to mixing that removes heat from the Atlantic Water. Here, we investigate the tidal forcing, barotropic-to-baroclinic energy conversion rates, and dissipation rates in the region using observations of oceanic currents, hydrography, and microstructure collected on the southern flanks of the plateau in summer 2007, together with results from a global high-resolution ocean circulation and tide model simulation. The energetics (depth-integrated conversion rates, baroclinic energy fluxes and dissipation rates) show large spatial variability over the plateau and are dominated by the luni-solar diurnal (K1) and the principal lunar semidiurnal (M2) constituents. The volume-integrated conversion rate over the region enclosing the topographic feature is approximately 1 GW and accounts for about 50% of the M2 and approximately all of the K1 conversion in a larger domain covering the entire Fram Strait extended to the North Pole. Despite the substantial energy conversion, internal tides are trapped along the topography, implying large local dissipation rates. An approximate local conversion–dissipation balance is found over shallows and also in the deep part of the sloping flanks. The baroclinic energy radiated away from the upper slope is dissipated over the deeper isobaths. From the microstructure observations, we inferred lower and upper bounds on the total dissipation rate of about 0.5 and 1.1 GW, respectively, where about 0.4–0.6 GW can be attributed to the contribution of hot spots of energetic turbulence. The domain-integrated dissipation from the model is close to the upper bound of the observed dissipation, and implies that almost the entire dissipation in the region can be attributed to the dissipation of baroclinic tidal energy.

Predicting connectivity of green turtles at Palmyra Atoll, central Pacific: a focus on mtDNA and dispersal modelling

Population connectivity and spatial distribution are fundamentally related to ecology, evolution and behaviour. Here, we combined powerful genetic analysis with simulations of particle dispersal in a high-resolution ocean circulation model to investigate the distribution of green turtles foraging at the remote Palmyra Atoll National Wildlife Refuge, central Pacific. We analysed mitochondrial sequences from turtles (n = 349) collected there over 5 years (2008-2012). Genetic analysis assigned natal origins almost exclusively (approx. 97%) to the West Central and South Central Pacific combined Regional Management Units. Further, our modelling results indicated that turtles could potentially drift from rookeries to Palmyra Atoll via surface currents along a near-Equatorial swathe traversing the Pacific. Comparing findings from genetics and modelling highlighted the complex impacts of ocean currents and behaviour on natal origins. Although the Palmyra feeding ground was highly differentiated genetically from others in the Indo-Pacific, there was no significant differentiation among years, sexes or stage-classes at the Refuge. Understanding the distribution of this foraging population advances knowledge of green turtles and contributes to effective conservation planning for this threatened species.

Seasonal variation of the M 2 tide

The seasonal cycle of the main lunar tidal constituent M 2 is studied globally by an analysis of a high-resolution ocean circulation and tide model (STORMTIDE) simulation, of 19 years of satellite altimeter data, and of multiyear tide-gauge records. The barotropic seasonal tidal variability is dominant in coastal and polar regions with relative changes of the tidal amplitude of 5–10 %. A comparison with the observations shows that the ocean circulation and tide model captures the seasonal pattern of the M 2 tide reasonably well. There are two main processes leading to the seasonal variability in the barotropic tide: First, seasonal changes in stratification on the continental shelf affect the vertical profile of eddy viscosity and, in turn, the vertical current profile. Second, the frictional effect between sea-ice and the surface ocean layer leads to seasonally varying tidal transport. We estimate from the model simulation that the M 2 tidal energy dissipation at the sea surface varies seasonally in the Arctic (ocean regions north of 60°N) between 2 and 34 GW, whereas in the Southern Ocean, it varies between 0.5 and 2 GW. The M 2 internal tide is mainly affected by stratification, and the induced modified phase speed of the internal waves leads to amplitude differences in the surface tide signal of 0.005–0.0150 m. The seasonal signals of the M 2 surface tide are large compared to the accuracy demands of satellite altimetry and gravity observations and emphasize the importance to consider seasonal tidal variability in the correction processes of satellite data.

On the space- and time-dependence of barotropic-to-baroclinic tidal energy conversion

Barotropic to baroclinic tidal conversion rates and baroclinic tidal energy fluxes are derived from a high resolution ocean circulation and tide model (STORMTIDE) simulation. Two semi-diurnal (M2 and S2) and two diurnal (K1 and O1) tidal constituents are considered in the present study. The model resolves mesoscale eddies and internal tides and it is forced by a climatological forcing. It is estimated that 1.7TW of tidal energy is converted from barotropic tides into baroclinic motions. About 32% (50%) of this energy conversion occurs in shallow waters with ocean depths shallower than 1000m (2000m). The spatial patterns of tidal energy conversion and the global net conversion are consistent with previous analytical and numerical model studies and it shows that most of the energy is converted in the West Pacific. An important result from the present study is that about 30% (0.11TW) of diurnal tidal energy is converted poleward of the critical latitudes, and presumably transfered locally to turbulent mixing processes with a high efficiency. Further, a modulation function is derived from the full lunisolar tidal potential described by ephemerides, in order to allow the internal tide generation to be modulated on monthly to bi-decadal timescales. This function reproduces the modulation of the tidal constituents in real-time. The coupling of this function with the spatial conversion rates depicts the regional and temporal dependence of tidal energy conversion. In some regions the nodal cycles of semi-diurnal and diurnal tides interfere destructively (e.g. North Atlantic and South Pacific) and in others the strong diurnal nodal cycle is dominant and modulates the tidal energy conversion by 10–20% (North West Pacific and Southern Ocean). In sight of climate bi-decadal variability this approach shows that the consideration of tidal mixing parameterizations in climate models should acknowledge the spatial and temporal dependence of internal tide generation.

Modelling larval fish navigation: the way forward

Abstract Recent advances in high-resolution ocean circulation models, coupled with a greater understanding of larval behaviour, have increased the sophistication of individual-based, biophysical models used to study the dispersal of larvae in the sea. Fish larvae, in particular, have the ability to swim directionally and increasingly fast during ontogeny, indicating that they may not only disperse, but also migrate using environmental signals. How and when larvae use local and large-scale cues remains a mystery. Including three-dimensional swimming schemes into biophysical models is becoming essential to address these questions. Here, we highlight state-of-the-art modelling of vertical and horizontal migrations of fish larvae, as well as current challenges in moving towards more realistic larval movements in response to cues. Improved understanding of causes for orientation will provide insight into the evolutionary drivers of dispersal strategies for fish and marine organisms in general.

Particle-tracking simulation for the drift/diffusion of spilled oils in the Sea of Okhotsk with a three-dimensional, high-resolution model

To conduct the simulation of oil spills in the Sea of Okhotsk, we developed a three-dimensional, high-resolution ocean circulation model. The model particularly improved the reproducibility of velocity field during the strong stratification period. Particle-tracking experiments with the effects of evaporation and biodegradation were performed using the combined data of daily ocean currents from the present model and the hourly diurnal tidal currents from the tidal model. The results are shown by the relative concentration of the particles averaged over the 8 years of 1998–2005 based on the ensemble forecast idea. For the case of particles released from the Sakhalin II oil field, the particles deployed in September–January are carried southward by the East Sakhalin Current, finally arriving at the Hokkaido coast, after 60–90 days. The particles deployed in March–August are diffused offshore by the synoptic wind drift, and hardly transported to regions south of Sakhalin. For the case of particles released from the region off Prigorodnoye, the oil export terminal, after the diffusion by the synoptic wind drift, a part of them are carried offshore of Hokkaido by the Soya Warm Current. The particles released in November–April flow out to the Japan Sea through the Soya Strait, mainly by the synoptic wind drift and secondly by the diffusion due to strong tidal currents around the Soya Strait. By considering the effects of evaporation and biodegradation, the relative concentration of the particles is considerably decreased before arriving at the Hokkaido coast, particularly in the case of drift from the Sakhalin II oil field.

Origin of the Tsushima Warm Current in a high resolution ocean circulation model

Park, Y.-G., Yeh, S.-W., Hwang, J. and Kim, T., 2013, East China Sea circulation from a high resolution ocean circulation model.Using the results from a high-resolution global ocean circulation model, the present study describes the circulation over the East China Sea and the origin of the Tsushima Warm Current. We focus on the interaction between the Taiwan Warm Current, the Tsushima Warm Current, and the Kuroshio intrusion north of Taiwan and east of Kyushu by comparing mass fluxes. Although the modeled transport of the Tsushima Warm Current is weaker than observed values by about 30 %, that of the Taiwan Warm Current is within the known range. The transport of the latter is large enough to supply the former. Thus, in this model, the net intrusion of the Kuroshio towards the shelf between Taiwan and Kyushu would not make a significant contribution to the Tsushima Warm Current. Conversely, the flows northeast of Taiwan and west of Kyushu show a strong presence of branch currents toward the inner shelf from the Kuroshio. To resolve this apparent contradiction, we estimated mass budgets for the areas around Taiwan and west of Kyushu. The Taiwan–Tsushima Current System and the Kuroshio are located very closely to each other along the shelf break, and if they are not differentiated properly using high-resolution data, one could overestimate the strength of the branch currents or the onshore intrusion of the Kuroshio. Considering the similarity between the apparent branch currents from this model and the previously reported results, the branch currents, especially west of Kyushu, may not be as strong as previously suggested.

Effects of ocean thermal energy conversion systems on near and far field seawater properties—A case study for Hawaii

In light of renewed interest and efforts in the development of ocean thermal energy conversion (OTEC) systems with a vision to provide mankind with a long-lasting energy resource, the potential environmental impacts of this technology should be considered from the perspective of sustainability. As an important step toward such a goal, we examine effects of OTEC effluent discharge on the physical aspects of the ocean environment near a Hawaiian Island in the North Pacific Ocean. We use modeling tools comprised of a mixing model that predicts the near field dilution of effluent plumes and a high-resolution ocean circulation model that simulates the dispersion of effluent in the far field. Numerical experiments are conducted to explore factors that influence effluent dispersal. We find that OTEC thermal resource is favorable and stable at the chosen location for the time period experimented. For a given OTEC design, the effluent discharge settles at a depth sufficiently far from the depths of discharge or intakes, and becomes dispersed quickly away from the site by highly variable ocean currents. Changes in ocean stratification and flow field are negligible for one OTEC device but are notable when multiple OTEC devices are present. These changes do not persist beyond 2 weeks after switching off OTEC activity.

고해상도 해양순환모형을 이용한 씨프린스호 유류유출 사고 수치실험

This study investigates the effects of tide, wind and oceanic currents on oil spill dispersions through a series of numerical floats tracking experiments on the Sea Prince oil spill incident occurred in 1995 using a 3-dimensional high resolution ocean circulation model. For that, a total of four experimental cases (experiment with tide, wind and oceanic currents, experiment with tide and oceanic currents, experiment with wind and oceanic currents, and experiment with tide and wind) were compared. It could be seen that results from experiment involving all external forces showed better agreement with the observed pattern of oil slick movement than other cases. The oceanic currents acted to drive floats to move to the western channel of the Korea straits and wind accelerated the eastward movement of floats in the early stage of the incident. Tidal currents played significant role in the horizontal dispersion of floats.

Simulating transoceanic migrations of young loggerhead sea turtles: merging magnetic navigation behavior with an ocean circulation model

Young loggerhead sea turtles (Caretta caretta) from eastern Florida, USA, undertake a transoceanic migration in which they gradually circle the Sargasso Sea before returning to the North American coast. Loggerheads possess a 'magnetic map' in which regional magnetic fields elicit changes in swimming direction along the migratory pathway. In some geographic areas, however, ocean currents move more rapidly than young turtles can swim. Thus, the degree to which turtles can control their migratory movements has remained unclear. In this study, the movements of young turtles were simulated within a high-resolution ocean circulation model using several different behavioral scenarios, including one in which turtles drifted passively and others in which turtles swam briefly in accordance with experimentally derived data on magnetic navigation. Results revealed that small amounts of oriented swimming in response to regional magnetic fields profoundly affected migratory routes and endpoints. Turtles that engaged in directed swimming for as little as 1-3 h per day were 43-187% more likely than passive drifters to reach the Azores, a productive foraging area frequented by Florida loggerheads. They were also more likely to remain within warm-water currents favorable for growth and survival, avoid areas on the perimeter of the migratory route where predation risk and thermal conditions pose threats, and successfully return to the open-sea migratory route if carried into coastal areas. These findings imply that even weakly swimming marine animals may be able to exert strong effects on their migratory trajectories and open-sea distributions through simple navigation responses and minimal swimming.

On the Role of Eddies and Surface Forcing in the Heat Transport and Overturning Circulation in Marginal Seas

Abstract The factors that determine the heat transport and overturning circulation in marginal seas subject to wind forcing and heat loss to the atmosphere are explored using a combination of a high-resolution ocean circulation model and a simple conceptual model. The study is motivated by the exchange between the subpolar North Atlantic Ocean and the Nordic Seas, a region that is of central importance to the oceanic thermohaline circulation. It is shown that mesoscale eddies formed in the marginal sea play a major role in determining the mean meridional heat transport and meridional overturning circulation across the sill. The balance between the oceanic eddy heat flux and atmospheric cooling, as characterized by a nondimensional number, is shown to be the primary factor in determining the properties of the exchange. Results from a series of eddy-resolving primitive equation model calculations for the meridional heat transport, overturning circulation, density of convective waters, and density of exported waters compare well with predictions from the conceptual model over a wide range of parameter space. Scaling and model results indicate that wind effects are small and the mean exchange is primarily buoyancy forced. These results imply that one must accurately resolve or parameterize eddy fluxes in order to properly represent the mean exchange between the North Atlantic and the Nordic Seas, and thus between the Nordic Seas and the atmosphere, in climate models.

Characterization of ocean surface current properties from single site HF/VHF radar

Surface current mapping from HF/VHF coastal radars traditionally requires at least two distant sites. Vector velocities are estimated by combining the radial velocity components measured by the radars. In many circumstances (e.g., failures, interferences, logistics constraints), such a combination is not possible by lack of data from one station. Two methods are evaluated to get information on surface circulation from a single site radar: the Vectorial Reconstruction Method (VRM) for current vector mapping and the Vortex Identification Method (VIM) for detecting eddy-like structures. The VRM assumes a non-divergent horizontal surface current, and the VIM analyzes radial velocities and their radial and orthoradial gradients. These two methods are tested on modeled and measured data sets in the Northwestern Mediterranean Sea where both high-resolution ocean circulation model and radar campaigns are available. The VRM performance is strongly limited by the divergence-free hypothesis which was not satisfied in our real data. The VIM succeeded in detection of vortex in the Gulf of Lions and from an operating single site radar located on the Provence coasts in summer.

Modeling seasonal circulation, upwelling and tidal mixing in the Arafura and Timor Seas

The extensive shallow tropical seas off northern Australia, encompassing the Arafura and Timor Seas, have been identified as one of the most pristine marine environments on the planet. However, the remoteness and the absence of major industrial development that has contributed to this status have the additional consequence that relatively little is known about these systems. This study is the first to model oceanographic conditions across the tidally dominated Arafura and Timor Seas, and their seasonal variability. The results are based on a high-resolution (0.05°) ocean circulation model forced by realistic winds, waves and tides. The main focus of the study is on physical processes that influence the distributions of sediments and primary productivity across the system. Regions of high bottom stress and tidal mixing have been identified, including a large offshore area around Van Diemen Rise (Timor Sea). Lagrangian particle tracks have revealed a seasonal overturning cell that stretches across the Gulf of Carpentaria (Arafura Sea) with upwelling and downwelling on either side of the Gulf. The presence of coastal upwelling and downwelling is shown to provide a dynamically consistent explanation for the persistent turbid boundary layer observed around the shallow coastal waters of the Gulf.

Control of primary production in the Arctic by nutrients and light: insights from a high resolution ocean general circulation model

Abstract. Until recently, the Arctic Basin was generally considered to be a low productivity area and was afforded little attention in global- or even basin-scale ecosystem modelling studies. Due to anthropogenic climate change however, the sea ice cover of the Arctic Ocean is undergoing an unexpectedly fast retreat, exposing increasingly large areas of the basin to sunlight. As indicated by existing Arctic phenomena such as ice-edge blooms, this decline in sea-ice is liable to encourage pronounced growth of phytoplankton in summer and poses pressing questions concerning the future of Arctic ecosystems. It thus provides a strong impetus to modelling of this region. The Arctic Ocean is an area where plankton productivity is heavily influenced by physical factors. As these factors are strongly responding to climate change, we analyse here the results from simulations of the 1/4° resolution global ocean NEMO (Nucleus for European Modelling of the Ocean) model coupled with the MEDUSA (Model for Ecosystem Dynamics, carbon Utilisation, Sequestration and Acidification) biogeochemical model, with a particular focus on the Arctic basin. Simulated productivity is consistent with the limited observations for the Arctic, with significant production occurring both under the sea-ice and at the thermocline, locations that are difficult to sample in the field. Results also indicate that a substantial fraction of the variability in Arctic primary production can be explained by two key physical factors: (i) the maximum penetration of winter mixing, which determines the amount of nutrients available for summer primary production, and (ii) short-wave radiation at the ocean surface, which controls the magnitude of phytoplankton blooms. A strong empirical correlation was found in the model output between primary production and these two factors, highlighting the importance of physical processes in the Arctic Ocean.

Ocean data assimilation: a case for ensemble optimal interpolation

The Bluelink forecast and reanalysis system is comprised of a high resolution ocean general circulation model and an ensemble optimal interpolation (EnOI) system. The Bluelink system has been integrated for a series of 15-year reanalyses and is run operationally at the Bureau of Meteorology to produce short-range ocean forecasts. This system has performed robustly, demonstrating reliable skill that is comparable to other ocean forecast systems around the world. One of the key components of Bluelink is the EnOI system. This system has proved to be a valuable and flexible tool for both operational and research applications. Drawing on Bluelink outcomes and a series of experiments with simple models, a case for using EnOI for ocean data assimilation is made. This includes a discussion of the benefits of EnOI as well as its known limitations. EnOI is robust, flexible, portable and inexpensive, and is not burdened with the technical difficulties that some other methods carry. EnOI is readily applied for coupled data assimilation and may be an appropriate choice for coupled forecast systems.

Anthropogenic carbon dioxide transport in the Southern Ocean driven by Ekman flow

The Southern Ocean, with its large surface area and vigorous overturning circulation, is potentially a substantial sink of anthropogenic CO(2) (refs 1-4). Despite its importance, the mechanism and pathways of anthropogenic CO(2) uptake and transport are poorly understood. Regulation of the Southern Ocean carbon sink by the wind-driven Ekman flow, mesoscale eddies and their interaction is under debate. Here we use a high-resolution ocean circulation and carbon cycle model to address the mechanisms controlling the Southern Ocean sink of anthropogenic CO(2). The focus of our study is on the intra-annual variability in anthropogenic CO(2) over a two-year time period. We show that the pattern of carbon uptake is correlated with the oceanic vertical exchange. Zonally integrated carbon uptake peaks at the Antarctic polar front. The carbon is then advected away from the uptake regions by the circulation of the Southern Ocean, which is controlled by the interplay among Ekman flow, ocean eddies and subduction of water masses. Although lateral carbon fluxes are locally dominated by the imprint of mesoscale eddies, the Ekman transport is the primary mechanism for the zonally integrated, cross-frontal transport of anthropogenic CO(2). Intra-annual variability of the cross-frontal transport is dominated by the Ekman flow with little compensation from eddies. A budget analysis in the density coordinate highlights the importance of wind-driven transport across the polar front and subduction at the subtropical front. Our results suggest intimate connections between oceanic carbon uptake and climate variability through the temporal variability of Ekman transport.

High-resolution synthetic monitoring by a 4-dimensional variational data assimilation system in the northwestern North Pacific

A data assimilation system is applied to the integrated monitoring of oceanic states in the northwestern North Pacific by combining a high resolution ocean general circulation model with an adjoint method. A comparison of assimilation results with observations shows that the system is better able to represent synoptic features of ocean circulation than do models or data alone. Furthermore, meso-scale features associated with frontal structures and eddies, which are often seen in the Kuroshio and Oyashio extension regions and the Sea of Japan, are better defined in the assimilation results. These features suggest that our 4D-VAR high-resolution data assimilation system is capable of providing time series data which satisfy the model physics and fit the observations, and hence the ocean state derived from our system has greater information content than that obtained from earlier methods.