Open Ocean Currents Research Articles

Reconstruction of the Rheological Parameters in a Sea Ice Model with Viscoplastic Rheology

Abstract We analyzed the feasibility of the reconstruction of the spatially varying rheological parameters through the four-dimensional variational data assimilation of the sea ice velocity, thickness, and concentration into the viscoplastic (VP) sea ice model. The feasibility is assessed via idealized variational data assimilation experiments with synthetic observations configured for a 1-day data assimilation window in a 50 × 40 rectangular basin forced by the open boundaries, winds, and ocean currents and should be viewed as a first step in the developing the similar algorithms which can be applied for the more advanced sea ice models. It is found that “true” spatial variability (∼5.8 kN m−2) of the internal maximum ice strength parameter can be retrieved from observations with reasonable accuracy of 2.3–5.3 kN m−2, when an observation of the sea ice state is available daily in each grid point. Similar relative accuracy was achieved for the reconstruction of the compactness strength parameter α. The yield curve eccentricity e is found to be controlled by the data with less efficiency, but the spatial mean value of e could be still reconstructed with a similar degree of confidence. The accuracy of , α, and e retrievals is found to increase in regions of stronger ice velocity convergence, providing prospects for better processing of the observations collected during the recent MOSAiC experiment. The accuracy of the retrievals strongly depends on the number of the control variables characterizing the rheological parameter fields.

Variability of surface gravity wave field over a realistic cyclonic eddy

Abstract. Recent remote sensing measurements and numerical studies have shown that surface gravity waves interact strongly with small-scale open ocean currents. Through these interactions, the significant wave height, the wave frequency, and the wave direction are modified. In the present paper, we investigate the interactions of surface gravity waves with a large and isolated realistic cyclonic eddy. This eddy is subject to instabilities, leading to the generation of specific features at both the mesoscale and submesoscale ranges. We use the WAVEWATCH III numerical framework to force surface gravity waves in the eddy before and after its destabilization. In the wave simulations the source terms are deactivated, and waves are initialized with different wave intrinsic frequencies. The study of these simulations illustrates how waves respond to the numerous kinds of instabilities in the large cyclonic eddy from a few hundred to a few tens of kilometres. Our findings show that the spatial variability of the wave direction, the mean period, and the significant wave height is very sensitive to the presence of submesoscale structures resulting from the eddy destabilization. The intrinsic frequency of the incident waves is key in the change of the wave direction resulting from the current-induced refraction and in the location, from the boundary where waves are generated, of the maximum values of significant wave height. However, for a given current forcing, the maximum values of the significant wave height are similar regardless of the frequency of the incident waves. In this idealized study it has been shown that the spatial gradients of wave parameters are sharper for simulations forced with the destabilized eddy. Because the signature of currents on waves encodes important information of currents, our findings suggest that the vertical vorticity of the current could be statistically estimated from the significant wave height gradients down to a very fine spatial scale. Furthermore, this paper shows the necessity to include currents in parametric models of sea-state bias; using a coarse-resolution eddy field may severely underestimate the sea-state-induced noise in radar altimeter measurements.

Numerical simulation and dynamical response of a moored hydrokinetic turbine operating in the wake of an upstream turbine for control design

Numerical simulation of a downstream hydrokinetic turbine operating in the wake of an upstream turbine for feedback control design is presented. Wake effects from an upstream turbine are quantified in terms of wake velocity and amplified turbulence levels. These effects are integrated in an in-stream hydrokinetic turbine numerical simulation that utilizes a Blade Element Momentum approach with a dynamic wake inflow model. Simulations are carried out on a fixed turbine model to simulate operation in river or tidal channels with conventional foundations, as well as on a compliantly moored turbine model such as those designed to operate in open ocean currents.

Small‐scale open ocean currents have large effects on wind wave heights

AbstractTidal currents and large‐scale oceanic currents are known to modify ocean wave properties, causing extreme sea states that are a hazard to navigation. Recent advances in the understanding and modeling capability of open ocean currents have revealed the ubiquitous presence of eddies, fronts, and filaments at scales 10–100 km. Based on realistic numerical models, we show that these structures can be the main source of variability in significant wave heights at scales less than 200 km, including important variations down to 10 km. Model results are consistent with wave height variations along satellite altimeter tracks, resolved at scales larger than 50 km. The spectrum of significant wave heights is found to be of the order of times the current spectrum, where is the spatially averaged significant wave height, is the energy‐averaged period, and g is the gravity acceleration. This variability induced by currents has been largely overlooked in spite of its relevance for extreme wave heights and remote sensing.

Understanding past climate variation and environmental change for the future of an iconic landscape – K'gari Fraser Island, Queensland, Australia

The unique combination of landscapes and processes that are present and operate on Fraser Island (K'gari) create a dynamic setting that is capable of recording past environmental events, climate variations and former landscapes. Likewise, its geographic position makes Fraser Island sensitive to those events and processes. Based on optically stimulated luminescence dating, the records archived within the world's largest sand island span a period that has the potential to exceed 750 ka and contain specific records that are of extremely high resolution over the past 40,000 years. This is due to the geographic position of Fraser Island, which lies in the coastal subtropical region of Queensland Australia. Fraser Island is exposed to the open ocean currents of the Coral Sea on the east coast and the waters of Hervey Bay on its western margin and is positioned to receive moisture from the Indo-Australian monsoon, southeast trade winds and experiences occasional tropical and ex-tropical cyclones. We review literature that presents the current level of understanding of sea level change, ecological variation and environmental change on Fraser Island. The previous works illustrate the importance of Fraser Island and may link processes, environments and climates on Fraser Island with global records.

The relationship between the statistics of open ocean currents and the temporal correlations of the wind stress

We study the statistics of wind-driven open ocean currents. Using the Ekman layer model for the integrated currents, we investigate analytically and numerically the relationship between the wind-stress distribution and its temporal correlations and the statistics of the open ocean currents. We found that temporally long-range correlated winds result in currents whose statistics is proportional to the wind-stress statistics. On the other hand, short-range correlated winds lead to Gaussian distributions of the current components, regardless of the stationary distribution of the winds, and therefore to a Rayleigh distribution of the current amplitude, if the wind stress is isotropic. We found that the second moment of the current speed exhibits a maximum as a function of the correlation time of the wind stress for a non-zero Coriolis parameter. The results were validated using an oceanic general circulation model.

Paleoenvironmental changes and depositional history of the Korea (Tsushima) Strait since the LGM

Two deep cores (SSDP 102 and SSDP 105) from the southeastern inner shelves of Korea were examined in order to reconstruct the paleoenvironmental history of the Korea Strait during postglacial sea-level fluctuations. The results of the lithofaces, organic geochemical components, foraminiferal assemblages, and stable isotopes show that the Korea Strait was influenced in different ways by river discharges and open-ocean currents, according to their temporal and spatial variability. The results of core SSDP 105 indicate that the southeastern coast of Korea was probably a shallow coastal environment, such as a shoreface or beach, by about 17.5 cal ka. The area remained a coastal environment that was affected by high energy conditions and partly by freshwater input during the transgressive period of 17.5–8.1 cal ka. The area has changed to a modern-type shelf environment influenced by the inflow of the Tsushima Current and by the high supply of fine-grained sediments derived from the Nakdong River since 8.1 cal ka. Meanwhile, the results of core SSDP 102 taken from the Nakdong River mouth reveal that the river mouth area was most likely a terrestrial or fluvial environment at least before 13.9 cal ka. It then changed to the estuarine (deltaic) environment that was directly affected by the Nakdong River between 13.9 and 7.0 cal ka, and established a modern-type shelf (prodeltaic) environment, which is similar in many respects to the southeastern coast of Korea after temporarily undergoing erosion between 7.0 and 6.1 cal ka.

An OGCM with movable land–sea boundaries

An ocean general circulation model (OGCM) with wetting and drying (WAD) capabilities removes the vertical-wall coastal assumption and allows simultaneous modeling of open-ocean currents and water run-up (and run-down) across movable land–sea boundaries. This paper implements and tests such a WAD scheme for the Princeton Ocean Model (POM) in its most general three-dimensional setting with stratification, bathymetry and forcing. The scheme can be easily exported to other OGCM’s.

Abstract: Integrated Paleoecology and Marine Vertebrate Fauna of the Stone City Formation (Middle Eocene), Brazos River Section, Texas 

ABSTRACT Abundant marine vertebrate faunas, including sharks, rays, teleosts, and reptiles, have been collected from two horizons in the Middle Eocene (Claiborne) Stone City Formation of southeast Texas. Extensive collecting, both surface and bulk, and recent advances, especially in the study of otoliths and foraminifera, provide for additional evaluation of paleoecological parameters. Otolith-based teleosts are dominated by neritic taxa such as sciaenids and ophidiids, while skeletal-based teleosts include catfish, barracuda, mackerel, and gar. Selachians include sand, tiger, and catsharks as well as rays and sawfish. Least common are reptiles, represented by crocodiles and turtles. Vertebrate diversity is 50 taxa, the richest marine fauna yet recorded from the Gulf Coast Middle Eocene. Vertebrates were collected from the top of the Main Glauconite bed and a thin lag zone three m below. The lower fauna (Assemblage A) is rich in tiger and sand sharks, eagle rays, plus estuarine elements, such as catfish, sawfish, and softshell turtle. The mixture of estuarine vertebrates with abundant marine mollusks suggests a possible tidal channel emptying into nearshore marine. The upper fauna (Assemblage B) is dominated by a rich, moderately diverse fish fauna with scarce rays and sharks. An assemblage with sciaenids, ophidiids, and myripristids with abundant, diverse mollusks, especially infaunal pelecypods, suggests a fully marine but nearshore setting of 25 m or less. Microfauna includes 33 benthic foraminifera and 15 ostracode species. The shallow nature of this assemblage, plus the lack of planktonic foraminifera, indicatesan absence of open ocean currents. Macrofaunal and microfaunal paleoecology supports the shallow neritic to estuarine conditions indicated by vertebrate parameters. The Main Glauconite bed was deposited in an open marine bay or lagoon which was separated from ocean currents by some barrier, possibly an offshore marine sandbar complex.

Dynamical forecasting of mesoscale fronts and eddies for acoustical applications

Real-time nowcasts and forecasts of oceanic mesoscale fields (velocity, pressure, temperature, sound speed, etc.) are now feasible and have been initiated in selected regions. Predicting the “internal weather of the sea” is analogous to meterological weather forecasting. Our approach is based on recent progress in phenomenological knowledge and modeling capabilities and is systematic [A. R. Robinson, “Predicting open ocean currents, fronts, and eddies,” in Three-Dimensional Ocean Models of Marine and Estuarine Dynamics, edited by Nihoul and Jamart (Elsevier, Amsterdam, 1987)]; i.e., it combines dynamical model [A. R. Robinson and L. J. Walstad, “The Harvard open ocean model,” J. Appl. Numer. Math 3 (1987)] and observational estimates via data assimilation methods. The prediction problem will be overviewed, results from an ongoing forecast scheme for the Gulf Stream (gulfcasting) will be presented, the coupling of dynamical and acoustical models will be discussed, and preliminary results from a coupled system will be presented.

Shallow-water turbidites: An example from the Mallacoota Beds, Australia

A Late Ordovician turbidite sequence (the Mallacoota Beds) exhibits vertical lithofacies associations that suggest deposition in a “fan-like” environment. However, there are also features indicating occasional, episodic reworking by water currents. Reworking features include both wave-current and unidirectional ripple pavements, sharp-topped turbidites, a difference in current direction between superimposed current rippling and underlying Bouma C division ripples, lensiodal quartzites and occasional HCS beds. Comparisons between the Mallacoota Beds and the depositional products of a variety of open-ocean currents would indicate that storm-related currents were the most likely reworking mechanism. This would indicate that the sequence was deposited within storm wave base. Depositional depths for part of the sequence are estimated at 90?–200 m based on a wave-generated-ripple model. Therefore, these turbidites may have been produced by storm surge with channeling within the turbidites due to the localisation of rip currents during hurricane conditions. A generalised model for the characteristic features of turbidites deposited between fair-weather wave-base and storm wave-base is presented. It is suggested that such turbidites will contain only occasional examples of reworking and that the frequency of such reworking events per stratigraphic interval might be used as a relative measure of water depth throughout a sequence. Moreover, those turbidite sequences that have sharp contacts between the sandstones and the overlying mudstones may have been modified by storms and therefore deposited in shallower water than previously thought.