Journal of Oceanography最新文献

Abstract

Euphausiids are a vital component of global marine micronekton. To reveal the primary environmental factors influencing euphausiid distribution patterns in the previously overlooked low-latitude ecosystems, we investigated a large-scale community structure of euphausiids covering the North Pacific subtropical gyre (NPSG) and low-latitude eastern Indian (EI), and South Pacific Oceans (SP). A total of 41 euphausiid species from six genera were identified. Integrated primary production (PP) correlated significantly with the euphausiid abundance and species diversity and displayed the most critical influence on the variations in euphausiid community structure in low latitudes. Dissolved oxygen (DO) was the second significant environmental driver. Due to the distinct distribution patterns of euphausiid species in response to different PP and DO levels, the low-latitude euphausiid assemblages were mainly distinguished into subtropical and tropical communities. The subtropical euphausiid community associated with lower PP and higher DO demonstrated significantly lower euphausiid abundance but higher diversity than the tropical community. Euphausia brevis, E. mutica, and Stylocheiron abbreviatum, which characterize the subtropical community, appear to depend less on the phytoplanktonic prey. Euphausia diomedeae and Hansarsia gracilis, typifying the tropical population, showed stronger hypoxia tolerance. Additionally, each community was further divided into three subgroups under the influence of surface PP (subtropical: marginal NPSG, central NPSG, and austral EI-SP subgroups; tropical: the Bay of Bengal, equatorial EI, and equatorial EI-SP subgroups). These results suggested that food resource is the most important in shaping euphausiids’ community structure in an oligotrophic ecosystem with subtle hydrography gradients.

Marine heatwaves (MHWs) are oceanic conditions characterized by extremely high sea surface temperature (SST) anomalies that last for several days to years. Because MHWs have devastating effects on marine ecosystems and significant impacts on fisheries, understanding future MHWs is important for adapting to upcoming climate changes. In this study, we examined future changes in MHWs in the northwestern Pacific Ocean (18–53ºN, 117ºE–170ºW) under two CO₂ emission scenarios using a high-resolution ensemble (four members for each scenario) simulation product using a high-resolution ocean model that satisfactorily resolves the Kuroshio, Kuroshio Extension, and SST fronts. Following global warming, MHWs based on a threshold in the historical period (1981–2005) will increase and intensify (i.e., occur with higher SST anomalies than before). In the historical period, the annual MHW days ranged from 20 to 34 days. Annual MHW days increase to 63–313 days (188 days–all year round) depending on the region under the high CO₂ mitigation (emission) scenario at the end of the twenty-first century of 2076–2100. Furthermore, we investigated the spatial details of future MHWs. Future MHWs reflect the magnitude of SST variability in addition to that of sea surface warming in the twenty-first century; future MHWs are less frequent and more intense in the subtropical–subarctic frontal zone with large SST variability than in other regions.

We utilized data from the numerical wave model WAVEWATCH-III and examined the spatial variability of waves considering 30 nearshore locations in the eastern Arabian Sea. The wave parameters from the model compare well with the buoy data (correlation coefficient ~ 0.98 and bias ~ 0.17 m). During monsoon, wave heights in the central-eastern Arabian Sea are higher than those in the southern and northern parts due to the influence of the Findlater jet and intermediate-period waves are dominating the entire area. The significant wave height is less than 1.5 m in non-monsoon and reaches 5 m in July. Variation in wave height between two nearby locations is highest in the northeastern Arabian Sea along the Gujarat coast. For a distance of 388 km from central Kerala to Karnataka, there is no significant spatial variability in wave height. Eastern Arabian Sea experiences a higher peak period in the non-monsoon due to reduction in the local wind speed. The integral period does not show significant spatial variability similar to wave height. The maximum (minimum) wave heights were found in 2013 (2015) and the variations are linked to the monsoon intensity.

The sea-surface roughness or drag coefficient is ascribed to the effect of various components of ocean waves. Many studies have been focused on the investigation of the dependence of drag coefficient on sea states that are usually denoted by wave age. However, no universally accepted relationship has been obtained up to now and the results are significantly scattered or even contradicted. We reviewed the parameterizations of sea-surface roughness as a function of wave age, and found that the phase speed at spectral peak c_p is an important parameter to characterize the drag coefficient. For the same wave age, drag coefficient increases with increasing c_p. Contrary to the traditional concept, the older waves with greater c_p possesses higher sea-surface roughness for the same wind speed because more wave components participate the air–sea interaction and intensify the wind stress. With the buoy meansurements and the theory of equilibrium range of wind waves, we estimated fricition velocity and proposed that the frequency bandwidth and spectral width of the wave spectrum are more suitable parameters than the traditional wind speed and wave age to be used to parameterize drag coefficient. This study provides a new way to estimate wind stress through the reliable spectra of ocean waves.

Aluminium (Al), manganese (Mn), iron (Fe), cobalt (Co), and lead (Pb) are trace metals that exhibit significant scavenging tendencies in the oceans. This study presents the full-depth distributions of the dissolved (d) and labile particulate (lp) fractions of these five elements in the subarctic Pacific Ocean, obtained during the GEOTRACES Japan KH-17-3 cruise. Along the 145° W meridional transect, the d and lp species of Al, Mn, Fe, and Co reflected fluvial supply from Alaska and benthic input from the continental shelf. We estimated that the boundary-scavenging zone has a width of approximately 250 km off Alaska. Along the 47° N zonal transect (GEOTRACES GP02 Line), we found input of trace metals from the Okhotsk and Bering Seas in the west, contrasting to the limited input of trace metals due to boundary scavenging in the east. The hydrothermal activity of the Juan de Fuca Ridge influenced the distribution of deep-water trace metals at the easternmost station, CL-21. Temporal change in the vertical profiles of dPb in the middle of the subarctic gyre highlighted a decline in anthropogenic Pb emissions from 2005 to 2017. Temporal change of the vertical profiles of Al, Mn, and Fe at 47° N, 160° E from 2011 to 2017 indicates the influence of the Great East Japan Earthquake. In particular, lp trace metals were brought by tsunami and ocean circulation in 2011, and decreased over time through scavenging.

Abstract

Vortex pairs formed at a strait outlet by tidal flow effectively induce water exchange and material transport in coastal areas. However, the effects of complex bottom topography and density stratification remain unclear. Here, we investigated the development and propagation of vortex pairs in the Naruto Strait, which has complex topography. Satellite observations indicated that the vortex pairs formed on the northern side of the strait continue to move away from the strait after the reversal of tidal flow, shifting their propagation from northward to westward. Numerical experiments revealed that: (1) changes in depth affect the propagation speed and overall size of vortex pairs; (2) density stratification reduces the effects of depth changes; (3) coastline geometry affects the propagation direction of vortex pairs. Furthermore, experiments with idealized topography showed that in a region where depth increases with vortex-pair propagation, the jet decelerates, and the vortex pair shrinks in size. Conversely, in a region where depth decreases, the jet widens, and the vortex pair expands. The changes in jet flow speed can be attributed to flow continuity and depth change, as the latter alters the cross-sectional area. Meanwhile, the changes in vortex pair size and jet width can be explained by vortex propagation on a slope due to potential vorticity conservation. These effects of topography and density stratification may also be significant in other coastal areas and potentially influence the Strouhal number threshold below which vortex pairs leave an outlet.

Estuarine processes regulate the transport of dissolved black carbon (DBC) and associated contaminants to the ocean. However, there is limited understanding of the geochemical behavior of DBC in estuaries. In this study, DBC in the Pearl River Estuary (PRE) and the northern shelf of the South China Sea were examined using the benzene polycarboxylic acid (BPCA) method. DBC, bulk dissolved organic carbon (DOC), and chromophoric dissolved organic matter (CDOM) exhibited distinct behaviors during their transport from the PRE to the sea. DOC and CDOM decreased during the initial mixing of river water and seawater but increased at the lower PRE. In contrast, high aromaticity DBC inputs were observed throughout the PRE, likely originating from local terrestrial sources, such as runoff from nearby islands, as indicated by the high R_H/L values (i.e., the ratio of BPCA containing 5 and 6 carboxyl groups to that containing 3 and 4 carboxyl groups; 2.03–2.30). In the Pearl River-plume zone (salinity < 33.0), DOC, CDOM, and DBC showed quasi-conservative behaviors against salinity, indicating that their geochemical behaviors were primarily governed by physical mixing between plume water and seawater. Using a flux model, it was estimated that the discharge of riverine DBC from the Pearl River Delta ranged from 11.2 to 16.3 Gg year⁻¹, representing an important source of bio-resistant DOC to the northern South China Sea.