Journal of Oceanography最新文献

This study provides the first estimation of sea ice-melt amount in the Sea of Okhotsk based on spring hydrographic data accumulated for nearly a hundred years. Just after sea ice melts completely, a low-salinity layer appears on the ocean surface, overlying the layer of Winter Water at the freezing point. The integration of the salinity decrease from Winter Water should correspond to the total ice-melt amount. We developed an algorithm to extract the profiles that clearly show the salinity deficit and converted the salinity deficit to the ice-melt amount from all available data. The climatological map shows that ice-melt amount decreases toward the ice edge and exhibits large values around the northern Sakhalin Island, reflecting the ice thickness distribution. In the southern area (south of 48°N), where sea ice is transported from the north, the average ice-melt amount is estimated to be ~ 71 cm in thickness. It is clearly shown that the ice-melt amount has decreased by ~ 30% in the southern area since the 1990s. These changes possibly affect the regional climate through the decreased latent heat of sea ice and potentially affect biological production through weakened stratification caused by decreased ice melt. We also suggested that ice-melt amount did not show a significant trend during the 1930s–1970s, implying that our methodology could extract information on sea ice before the era of satellite observations.

During August 2017, the Kuroshio began to follow its large meander (LM) path, and this was 12 years after the last LM event. Such LM events lead to the formation of an inshore cyclonic eddy (ICE) within the Kuroshio inshore region between the Kuroshio and the southern coast of Japan. We analyzed data from repeated seasonal surveys along a transect that followed 138°E, and from additional surveys, satellite observations, Argo floats, and a high-resolution reanalysis dataset to describe the temporal variation of the LM and ICE between 2017 and 2021. Cross sections from the repeated surveys highlighted the variations in the water mass structures. The time series of the indicators of the ICE intensity such as the Kuroshio volume transport and the planetary contribution of potential vorticity (Q) in the ICE revealed a maintenance trend in their intensity during the observation period. High-Q water was distributed in the ICE during the LM period compared to the non-LM period, and showed a clear seasonality within the shallower layers, suggesting the injection of Q via the advection from the upstream region in summer. Additional surveys captured an eddy that became detached from the ICE during summer 2020. This detached eddy had a Q value greater than the ICE and velocity similar to the Kuroshio, suggesting that the main stream had become temporarily separated. The structures and volume transport of warm water intrusions into the inshore region were also examined, and centrifugal instability was considered to be one of their generation mechanism.

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.