Remote impacts of 2009 and 2015 El Niño on oceanic and biological processes in a marginal sea of the Northwestern Pacific

The significance of long-term teleconnections derived from the anomalous climatic conditions of El Niño has been a highly debated topic, where the remote response of coastal hydrodynamics and marine ecosystems to El Niño conditions is not completely understood. The 14-year long data from a ship-borne acoustic Doppler current profiler was used to examine the El Niño’s impact, in particular, 2009 and 2015 El Niño events, on oceanic and biological processes in coastal regions across the Korea/Tsushima Strait. Here, it was revealed that the summer volume transport could be decreased by 8.7% (from 2.46 ± 0.39 to 2.24 ± 0.26 Sv) due to the anomalous northerly winds in the developing year of El Niño. Furthermore, the fall mean volume backscattering strength could be decreased by 1.8% (from − 97.09 ± 2.14 to − 98.84 ± 2.10 dB) due to the decreased surface solar radiation after the El Niño events. Overall, 2009 and 2015 El Niño events remotely affected volume transport and zooplankton abundance across the Korea/Tsushima Strait through climatic teleconnections.

High-Resolution Vertical Observations of Phytoplankton Groups Derived From an in-situ Fluorometer in the East China Sea and Tsushima Strait

Vertical distribution of phytoplankton composition in the East China Sea (ECS) and Tsushima Strait (TS) was highly variable in the region where the Changjiang River diluted water (CDW), Kuroshio water (KW), and Tsushima water (TW) intersected. An in-situ multiple excitation fluorometer was used to obtain the high-resolution phytoplankton groups data from every meter of the water column. Sharp differences were noted in the distribution of phytoplankton groups in the CDW, KW, and TW. In the CDW, brown algae were generally present ~60% of all depths with exception of subsurface chlorophyll-a maximum (SCM), whereas cyanobacteria (>40%) and green algae plus cryptophytes (>40%) were found above and below the SCM, respectively. In TW, where chlorophyll a (CHL) was lower than in the CDW, brown algae predominated the water column (>60%) and SCM (>80%), except the surface layer where cyanobacteria dominated. In KW, a high fraction of cyanobacteria (>40%) extended up to 40 m, while brown and green algae dominated (>60%) the deeper waters below 40 m at western and eastern stations, respectively. These results can be further related to water property and nutrient concentration of the water masses in each region. This new data show that the in-situ multiple excitation fluorometer can be a powerful tool to estimate high-resolution vertical profiles of phytoplankton groups on a large scale in marine environments.

Long-term trends of oxygen concentration in the waters in bank and shelves of the Southern Japan Sea

While multiple studies have investigated oxygen decrease in Japan Sea Proper Water (JSPW; > 300 m in depth), oxygen variation in continental slope and shelf waters (< 300 m) must also be investigated in order to assess its socioecological impacts. In this study, historical oxygen data in the waters of three continental shelves and a bank of Japan Sea, off-Awashima area (AW), Wakasa Bay (WB), East of Tsushima Straight (ETS), and Yamato Bank (YB), were collected and analyzed to assess temporal variation of oxygen in each region from 1960 to 2000s. Significant decreasing trends of oxygen were detected in the waters below 150 m depth in WB and YB, and below 300 m in AW, in the summer season. In winter, a decreasing trend of oxygen was detected throughout the water column from 300 m to the sea surface in WB and YB. In ETS, a deoxygenation trend was detected throughout the water column from the bottom to the sea surface in the summer season, while no trend was detected in winter. The results suggested that oxygen decreases in AW, WB, and YB were the consequence of the upward propagation of the deoxygenation signal from JSPW, while that of ETS was caused by horizontal propagation of deoxygenation signal from the East China Sea. Assuming that the observed trend will continue in future, it is predicted that part of the water in Tsushima Strait area will reach the general sublethal threshold of oxygen (134 μmol kg−1) by the end of this century.

Preliminary results of integrated oceanographic and geophysical observations in the profile of Peter the Great Bay – Tsushima Strait (the Sea of Japan, October 2019)

Наиболее оперативную информацию о состоянии океана дает проведение комплексных исследований (гравиметрические наблюдения, измерения интенсивности флуоресценции, концентрации метана и хлорофилла-а в приповерхностном слое воды, ртути и газов в атмосферном воздухе) непосредственно во время следования судна. Наши исследования проводились на переходе Владивосток–Нячанг в период с 27 по 29 октября 2019 г. в Японском море в рамках рейса LV88 на НИС «Академик М.А. Лаврентьев». Полученные данные отличаются высокой сходимостью с результатами прошлых экспедиций (например, рейс LV84, 2018 г.). Нами выявлены зоны аномальных концентраций метана (8,3 нмоль/л), углекислого газа (2,3 ppm) и ртути (4 нг/м3), а также отмечены гравитационные аномалии в Корейском проливе. The study of the ocean by complex methods using a flow system and atmospheric air is currently one of the universal methods of obtaining express results (such as gravimetric observations, fluorescence intensity measurements, methane and chlorophyll-a concentrations in near-surface water, mercury and various gases in open air) directly during the vessel’s sailing. The studies were conducted on the Vladivostok – Nha Trang traverse during October 27–29, 2019 in the Sea of Japan as a part of the LV88 voyage aboard the R/V “Academic M.A. Lavrent’yev”. According to the results of the study, abnormal zones of concentrations of methane (8.3 nM/l), carbon dioxide (2.3 ppm) and mercury (4 ng/m3) were identified. We have also noted gravitation anomalies in the Korean Strait.

Influence of regionally warm sea surface on moisture and extreme rainfall in Tsushima Strait during August 2013

&lt;p&gt;The present study investigates short-term (four-day) atmospheric response to regionally warm sea surface in Tsushima Strait for two periods (a sunny period, 19-22 August 2013 and a rainy period, 23-26 August 2013) using ensemble WRF simulations with initial condition altered in the presence and absence of an extremely warm SST core. In this presentation, the author focuses on the influence of regionally warm sea surface on moisture and extreme rainfall. The moisture response is quite different between the sunny and rainy periods. Ensemble averaged distribution of time-mean moisture variation induced by a regionally warm sea surface is well correlated with the SST increase during the sunny period. However, it is not clearly correlated with the SST increase during the rainy period when vapor fluctuated because of frequent rainfall. The high SST enhanced time-mean precipitation in the central area of the warm SST core. In the ensemble experiment, the warm SSTs do not always enhance hourly rainfall because the water-vapor concentrations are decreased by prior rainfall events in some members. In a simulation that well reproduces heavy rainfall at Izuhara located in Tsushima Strait in the presence of the warm SST core, high SSTs induced extreme precipitation (~50 mm/h) in the morning. Water vapor decreased after the morning heavy rainfall. The decreased moisture led to low precipitation in the afternoon. In contrast, a low-SST experiment with the warm-SST core removed shows that water-vapor concentrations were higher after weaker morning rainfall, compared to the high SST experiment with the warm core. Because of the high water-vapor concentrations, low SST led to greater precipitation in the afternoon. Thus, when responses of hourly precipitation to SST are investigated, we must consider the temporal water-vapor variation associated with prior rainfall event.&lt;/p&gt;

Mesoscale Convective Vortex that Causes Tornado-Like Vortices over the Sea: A Potential Risk to Maritime Traffic

Strong gusty winds in a weak maritime extratropical cyclone (EC) over the Tsushima Strait in the southwestern Sea of Japan capsized several fishing boats on 1 September 2015. A C-band Doppler radar recorded a spiral-shaped reflectivity pattern associated with a convective system and a Doppler velocity pattern of a vortex with a diameter of 30 km [meso-β-scale vortex (MBV)] near the location of the wreck. A high-resolution numerical simulation with horizontal grid interval of 50 m successfully reproduced the spiral-shaped precipitation pattern associated with the MBV and tornado-like strong vortices that had a maximum wind speed exceeding 50 m s−1 and repeatedly developed in the MBV. The simulated MBV had a strong cyclonic circulation comparable to a mesocyclone in a supercell storm. Unlike mesocyclones associated with a supercell storm, however, its vorticity was largest near the surface and decreased monotonically with increasing height. The strong vorticity of the MBV near the surface originated from a horizontal shear line in the EC. The tornado-like vortices developed in a region of strong horizontal shear in the western part of the MBV, suggesting that they were caused by a shear instability.