Takeyoshi Nagai

Dominant role of eddies and filaments in the offshore transport of carbon and nutrients in the California Current System

The coastal upwelling region of the California Current System (CalCS) is a well-known site of high productivity and lateral export of nutrients and organic matter, yet neither the magnitude nor the governing processes of this offshore transport are well quantified. Here we address this gap using a high-resolution (5 km) coupled physical-biogeochemical numerical simulation (ROMS). The results reveal (i) that the offshore transport is a very substantial component of any material budget in this region, (ii) that it reaches more than 800 km into the offshore domain, and (iii) that this transport is largely controlled by mesoscale processes, involving filaments and westward propagating eddies. The process starts in the nearshore areas, where nutrient and organic matter-rich upwelled waters pushed offshore by Ekman transport are subducted at the sharp lateral density gradients of upwelling fronts and filaments located at ∼25–100 km from the coast. The filaments are very effective in transporting the subducted material further offshore until they form eddies at their tips at about 100–200 km from the shore. The cyclonic eddies tend to trap the cold, nutrient, and organic matter-rich waters of the filaments, whereas the anticyclones formed nearby encapsulate the low nutrient and low organic matter waters around the filament. After their detachment, both types of eddies propagate further in offshore direction, with a speed similar to that of the first baroclinic mode Rossby waves, providing the key mechanism for long-range transport of nitrate and organic matter from the coast deep into the offshore environment.

Spontaneous Generation of Near-Inertial Waves by the Kuroshio Front

AbstractWhile near-inertial waves are known to be generated by atmospheric storms, recent observations in the Kuroshio Front find intense near-inertial internal-wave shear along sloping isopycnals, even during calm weather. Recent literature suggests that spontaneous generation of near-inertial waves by frontal instabilities could represent a major sink for the subinertial quasigeostrophic circulation. An unforced three-dimensional 1-km-resolution model, initialized with the observed cross-Kuroshio structure, is used to explore this mechanism. After several weeks, the model exhibits growth of 10–100-km-scale frontal meanders, accompanied by O(10) mW m−2 spontaneous generation of near-inertial waves associated with readjustment of submesoscale fronts forced out of balance by mesoscale confluent flows. These waves have properties resembling those in the observations. However, they are reabsorbed into the model Kuroshio Front with no more than 15% dissipating or radiating away. Thus, spontaneous generation o...

Seasonal adaptations of Daphnia pulicaria swimming behaviour: the effect of water temperature

Daphnia swimming behaviour is controlled by a variety of external factors, including light, presence of food and predators. Temperature represents a key driver in the dynamics of Daphnia populations, as well as on their motion. In this study, we have investigated the behavioural adaptations of adult Daphnia pulicaria to two different temperatures, representative of the mean winter (3°C) and summer (22°C) temperatures to which these organisms are exposed to in the real environment. Video observations were conducted both in the presence and in the absence of light to investigate possible day/night modifications in the motion strategy. Analyses of mean speed, velocity power spectral density and trajectory fractal dimension point out specific adaptations that allow D. pulicaria to successfully adjust to the changing conditions of the environment. Independently of the light conditions, in cold waters D. pulicaria swim almost vertically with defined motional frequencies, likely to increase the encounter with food items diluted in the fluid. A similar behaviour is displayed by the animals at summertime temperatures in the presence of light; however, in this case the vertical swimming is coupled with the absence of peaks in the power spectra and might be exploited to avoid predators. In contrast, at 22°C in dark conditions D. pulicaria move horizontally with lateral motions to take advantage of possible patches of phytoplankton. This information sheds new light into the complex and dynamic adaptations of D. pulicaria in response to external stimuli.

Multilayer biological structure and mixing in the upper water column of Lake Biwa during summer 2008

We carried out a 24-h station experiment at Lake Biwa (Japan) to measure mixing events and concurrent biological signals using a free-fall microstructure profiler (TurboMAP-L), conventional hydrographic measurement device (F-probe), and the Tracker acoustic profiling system (TAPS). A clearly defined three-layer physical system was observed. Two layers were actively mixed: the surface-mixed layer and the subsurface-mixed layer. Both winds and night-time convection create the surface-mixed layer, and vertical shear due to a counterclockwise gyre maintains turbulence in the subsurface mixing layer. A strongly stratified layer between these two mixing layers is almost turbulence free, so no material flux is expected. A local oxygen maximum layer, a local oxygen minimum layer, and layers of increased chlorophyll and zooplankton abundance are all located in this strongly stratified layer. The data show the intricate influence of physical processes on the structure of biological systems and their combined influence on biogeochemical and trophic transfers in aquatic systems.

Evidence of enhanced double‐diffusive convection below the main stream of the Kuroshio Extension

In this study, a Navis-MicroRider microstructure float and an EM-APEX float were deployed along the Kuroshio Extension Front. The observations deeper than 150 m reveal widespread interleaving thermohaline structures for at least 900 km along the front, presumably generated through mesoscale stirring and near-inertial oscillations. In these interleaving structures, microscale thermal dissipation rates χ are very high O( >10−7 K2s−1), while turbulent kinetic energy dissipation rates ϵ are relatively low O( 10−10−10−9 Wkg−1), with effective thermal diffusivity Kθ of O( 10−3 m2s−1) consistent with the previous parameterizations for double-diffusion, and, Kθ is two orders of magnitude larger than the turbulent eddy diffusivity for density Kρ. The average observed dissipation ratio Γ in salt finger and diffusive convection favorable conditions are 1.2 and 4.0, respectively, and are larger than that for turbulence. Our results suggest that mesoscale subduction/obduction and near-inertial motions could catalyze double-diffusive favorable conditions, and thereby enhancing the diapycnal tracer fluxes below the Kuroshio Extension Front.

A Method to Estimate Three-Dimensional Thermal Structure from Satellite Altimetry Data

Abstract A new empirical method to estimate mesoscale three-dimensional oceanic thermal structures from near-real-time satellite altimetry data is presented. The method uses a two-layer model with a novel set of empirical parameters for stratification. Empirical parameters, which are calculated from altimetry data and in situ temperature and salinity profiles obtained from globally distributed Argo floats, allowed for significant increases in the spatiotemporal mapping of mesoscale temperature distributions. The accuracy of the method was validated by comparing the estimated isothermal depths with in situ temperature data. Three case studies in different regions and seasons showed excellent agreement, with a strong linear correlation between the estimated and observed isothermal depths. The authors present the details of the method, discuss its limitations, and demonstrate the ecological relevance of the method for fisheries management by successfully predicting pelagic fish distributions.

Nutrient interleaving below the mixed layer of the Kuroshio Extension Front

Nitrate interleaving structures were observed below the mixed layer during a cruise to the Kuroshio Extension in October 2009. In this paper, we investigate the formation mechanisms for these vertical nitrate anomalies, which may be an important source of nitrate to the oligotrphoc surface waters south of the Kuroshio Extension Front. We found that nitrate concentrations below the main stream of the Kuroshio Extension were elevated compared to the ambient water of the same density (σ𝜃 = 23.5–25). This appears to be analogous to the “nutrient stream” below the mixed layer, associated with the Gulf Stream. Strong turbulence was observed above the vertical nitrate anomaly, and we found that this can drive a large vertical turbulent nitrate flux >O

Early-winter mixing event associated with baroclinic motions in weakly stratified Lake Biwa

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Author Correction: First Evidence of Coherent Bands of Strong Turbulent Layers Associated with High-Wavenumber Internal-Wave Shear in the Upstream Kuroshio

(1 mmol N m−2 day−1). A realistic, high-resolution (2 km) numerical simulation reproduces the observed Kuroshio nutrient stream and nitrate interleaving structures, with similar lateral and vertical scales. The model results suggest that the nitrate interleaving structures are first generated at the western side of the meander crest on the south side of the Kuroshio Extension, where the southern tip of the mixed layer front is under frontogenesis. Lagrangian analyses reveal that the vertical shear of geostrophic and subinertial ageostrophic flow below the mixed layer tilts the existing along-isopycnal nitrate gradient of the Kuroshio nutrient stream to form nitrate interleaving structures. This study suggests that the multi-scale combination of (i) the lateral stirring of the Kuroshio nutrient stream by developed mixed layer fronts during fall to winter, (ii) the associated tilting of along-isopycnal nitrate gradient of the nutrient stream by subinertial shear, which forms vertical interleaving structures, and (iii) the strong turbulent diffusion above them, may provide a route to supply nutrients to oligotrophic surface waters on the south side of the Kuroshio Extension.

First Evidence of Coherent Bands of Strong Turbulent Layers Associated with High-Wavenumber Internal-Wave Shear in the Upstream Kuroshio

Abstract The annual overturn of lake water (termed holomixis) during winter is essential in maintaining the environment of warm monomictic lakes by transporting heat and organic and inorganic constituents; however, direct observation of wintertime mixing processes is limited. To better understand the detailed physical processes responsible for holomixis, this study investigated a cold low-oxygen water intrusion event on the northeastern slope of weakly stratified Lake Biwa (Japan), observed using mooring systems. The intrusion occurred concurrently with superinertial oscillations of near-bottom currents. The effective Wedderburn number suggests that basin-scale Kelvin waves were excited by northeastward winds (~4 m s−1) prior to the intrusion event. The results of a modal analysis suggest that the intrusion was caused by a combination of Kelvin and Poincaré waves that locally uplifted the cold deep water. The heat budget analyses revealed that a substantial part of the intruded cold water was mixed diapycnally, induced by wind stirring, nighttime convection, and bottom friction. The superinertial currents enhanced the dissipation due to bottom friction by ~30%. The cold water intrusion and subsequent mixing provided local heat flux of 98 W m−2 over 6 days, which was 1.4 times larger than the average surface heat flux over the same period. In addition to previously studied processes, such as surface cooling and gravity currents from the shore, this study indicates that wind-induced baroclinic motions and subsequent diapycnal mixing of a stratified water column contribute to holomixis.