The role of meso-scale eddies in mixed layer deepening and mode water formation in the western North Pacific

Distributions of mixed layer depths around the centers of anti-cyclonic and cyclonic eddies in the North Pacific Ocean were composited by using satellite-derived sea surface height anomaly data and Argo profiling float data. The composite distributions showed that in late winter, deeper mixed layers were more (less) frequently observed inside the cores of the anti-cyclonic (cyclonic) eddies than outside. This relationship was the clearest in the region of 140°E–160°W and 35°N–40°N, where the temperature and salinity of the deep mixed layers were similar to those of the lighter variety of central mode water (L-CMW). A simple one-dimensional bulk mixed layer model showed that both strong sea-surface heat and momentum fluxes and weak preexisting stratification contributed to formation of the deep mixed layer. These conditions were associated with the anti-cyclonic eddies, suggesting that these eddies are important in the formation of mode waters, particularly L-CMW.     

Quantitative micro‐X‐ray fluorescence scanning spectroscopy of wet sediment based on the X‐ray absorption and emission theories: Its application to freshwater lake sedimentary sequences

Micro‐X‐ray fluorescence scanning spectroscopy of marine and lake sedimentary sequences can provide detailed palaeoenvironmental records through element intensity proxy data. However, problems with the effects of interstitial pore water on the micro‐X‐ray fluorescence intensities have been pointed out. This is because the X‐ray fluorescence intensities are measured directly at the surfaces of split wet sediment core samples. This study developed a new method for correcting X‐ray fluorescence data to compensate for the effects of pore water using a scanning X‐ray analytical microscope. This involved simultaneous use of micro‐X‐ray fluorescence scanning spectroscopy and an X‐ray transmission detector. To evaluate the interstitial pore water content from the X‐ray transmission intensities, a fine‐grained sediment core retrieved from Lake Baikal (VER99‐G12) was used to prepare resin‐embedded samples with smooth surfaces and uniform thickness. Simple linear regression between the linear absorption coefficients of the samples and their porosity, based on the Lambert–Beer law, enabled calculation of the interstitial pore spaces and their resin content with high reproducibility. The X‐ray fluorescence intensities of resin‐embedded samples were reduced compared with those of dry sediment samples because of: (i) the X‐ray fluorescence absorption of resin within sediment; and (ii) the sediment dilution effects by resin. An improved micro‐X‐ray fluorescence correction equation based on X‐ray fluorescence emission theory considers the instrument's sensitivity to each element, which provides a reasonable explanation of these two effects. The resin‐corrected X‐ray fluorescence intensity was then successfully converted to elemental concentrations using simple linear regression between the data from micro‐X‐ray fluorescence scanning spectroscopy and from the conventional analyzer. In particular, the calculated concentration of SiO₂ over the depth of the core, reflecting diatom/biogenic silica concentration, was significantly changed by the calibrations, from a progressively decreasing trend to an increasing trend towards the top of the core.     

Importance of Reference Dataset Improvements for Argo Delayed-Mode Quality Control

For the Argo Project, monitoring the global upper ocean by a large number of profiling floats, maintaining the quality of salinity data is critical; the goal for measurement accuracy is ±0.01. Experiments using the method of Wong et al. (2003), the standard delayed-mode quality control (dQC) for the Project, show that its performance depends critically on the reference datasets used. This study concludes that the method is useful for Argo and has sufficient potential to achieve the goal for salinity measurement in the North Pacific, when suitable reference datasets are prepared. Considering the Wong et al. (2003) algorithms, we suggest that reference datasets with the following characteristics will be most suitable for Argo dQC: They should be basically derived from the most extensive datasets, such as the latest World Ocean Database; in regions with denser observations, datasets with carefully quality controls should be used; in the regions with subsurface temperature inversions, such as the subarctic North Pacific, the profiles used for the reference must extend below the deepest temperature maximum to prepare proper salinities for the deep layer reference.     

Observational Evidence of Dissipative Small Scale Processes: Geotail Spacecraft Observation and Simulation of Electrostatic Solitary Waves

In recent spacecraft observations, coherent microscale structures such as electrostatic solitary waves are observed in various regions of the magnetosphere. The Geotail spacecraft observation has shown that these solitary waves are associated with high energy non-thermal electrons flowing along the magnetic field. The solitary structures are generated as a result of a long time evolution of coherent nonlinear trapping of electrons as found in bump-on-tail, bi-stream and Buneman instabilities. It is noted that these solitary waves can be generated at distant regions far away from the spacecraft locations, because these trapped electrons, or electron holes, are drifting much faster than the local thermal plasmas. Some of the solitary waves are accompanied by perpendicular electric fields indicating that two-or three-dimensional potential structures are passing by the spacecraft. Depending on the local plasma parameters, these multi-dimensional solitary structures couple with perpendicular modes such as electrostatic whistler modes and lower-hybrid modes. In a long time evolution, these perpendicular modes are dissipated via self-organization of small solitary potentials, leading to formation of one-dimensional potential troughs as observed in the deep magnetotail. The above dissipative small-scale processes are reproduced in particle simulations, and they can be used for diagnostics of electron dynamics from spacecraft observation of multi-dimensional solitary waves in various regions of the magnetosphere. This revised version was published online in July 2006 with corrections to the Cover Date.     

Reflections on post-pandemic university teaching,the corresponding digitalisation of education and the lecture attendance crisis

This short commentary discusses effective university teaching in the context of the pandemic, the corresponding digitalisation of tertiary education, and the recent lecture attendance crisis. By critically reflecting on my own experience as a university educator and as a student in a teacher education course, I suggest that the attendance crisis presents an opportunity to explore effective teaching in a rapidly changing context. To improve our teaching and learning, we can reflect on what students and teachers have gone through and seek to understand who our students are.     

Fifty years of the 137°E repeat hydrographic section in the western North Pacific Ocean

The 137°E repeat hydrographic section of the Japan Meteorological Agency across the western North Pacific was initiated in 1967 as part of the Cooperative Study of the Kuroshio and Adjacent Regions and has been continued biannually in winter and summer. The publicly available data from the section have been widely used to reveal seasonal to decadal variations and long-term changes of currents and water masses, biogeochemical and biological properties, and marine pollutants in relation to climate variability such as the El Niño-Southern Oscillation and the Pacific Decadal Oscillation. In commemoration of the 50th anniversary in 2016, this review summarizes the history and scientific achievements of the 137°E section during 1967–2016. Through the publication of more than 100 papers over this 50-year span, with the frequency and significance of the publication increasing in time, the 137°E section has demonstrated its importance for future investigations of physical–biogeochemical–biological interactions on various spatiotemporal scales, and thereby its utility in enhancing process understanding to aid projections of the impact of future climate change on ocean resources and ecosystems over the twenty-first century.     

Current Structure and Behavior of the River Plume in Suo-Nada

The behavior of a river plume in Suo-Nada, Japan, has been studied using a primitive equation numerical model, the Princeton Ocean Model. Special attention has been paid to the current structure and behavior of the anticyclonic eddy (bulge) induced by high freshwater inflow changing on a timescale of one week. First, the freshwater is supplied from a river to a rectangular basin with a simple topography. When the river discharge subsides after reaching its peak value, the bulge propagates upstream (i.e., opposite to the direction of the Kelvin wave propagation). Next, the freshwater is supplied from eight major rivers to the basin with realistic topography. The less saline water mass in the southern part of Suo-Nada propagates to the west (i.e., upstream) after the river discharge subsides. This is consistent with an observed phenomenon, viz., that the less saline water mass appears in the western part of Suo-Nada, suggesting that the upstream propagation of the bulge is possible in the real ocean. Finally, the cause of the upstream propagation is considered. Onshore currents appear in the bottom layer beneath the bulge, propagating upstream. They produce an anticyclonic barotropic eddy due to the conservation of potential vorticity. The current component associated with the eddy crosses normally to the isohaline in the upper layer, and therefore transports the bulge upstream. No other current component (such as surface current velocity minus vertically-averaged value) is responsible for the upstream propagation of the bulge. This revised version was published online in July 2006 with corrections to the Cover Date.     

Ecosystem shift resulting from loss of eelgrass and other submerged aquatic vegetation in two estuarine lagoons, Lake Nakaumi and Lake Shinji, Japan

Zostera marina L. was intensively harvested until the early 1950s in Lake Nakaumi, a eutrophic estuarine lagoon. We have estimated the amount of nitrogen (N) and phosphorus (P) removed from the lagoon through Z. marina harvesting. Lake Nakaumi lies in Tottori and Shimane prefectures, and the annual harvest of Z. marina in the late 1940s in Tottori was recorded as at least 56,250 t wet weight. The nutrient content of 56,250 t of Z. marina was calculated to be 61.9 t of N and 12.9 t of P, which is equivalent to 5.3% and 11%, respectively, of present annual nutrient loads to the lake. The nutrients formerly used by Z. marina were likely used by phytoplankton after the Z. marina started to decline in the mid-1950s at Lake Nakaumi. This shift in the chief primary producer, from benthic macrophytes to phytoplankton, caused a subsequent shift in secondary producers. Benthic fish and crustacean populations decreased and the non-commercial filter-feeding bivalve, Musculus senhausia, increased in Lake Nakaumi after the decline of seagrass beds. This affected the local economy, inducing not only eutrophication but also the collapse of local fisheries. On the other hand, at adjacent Lake Shinji, loss of submerged aquatic vegetation induced an increase of the commercial filter-feeding bivalve, Corbicula japonica, which doubled the fishery yield in the lake.     

Physical oceanographic conditions around the S1 mooring site

We describe physical oceanographic conditions around the S1 biogeochemical mooring site (30°N, 145°E) between February 2010 and July 2013. At the S1 mooring site, there is a clear seasonal variability of the mixed layer depth, wind forcing as well as horizontal kinetic energy in a near-inertial band. Interannual variability of the winter mixed layer was observed. The winter mixed layer depth was shallower in early 2010 and became deeper afterwards. Several mesoscale eddies and typhoons passed by the S1 mooring sites every year. Based on observed events, we suggest that those physical processes possibly affected biogeochemical properties around the S1 mooring site.