Modeling The Mysterious Tsunami Swirls In The Northeast Coast Of Japan During The March 2011 Tohoku Earthquake

The formation of tsunami swirls near the coast is an obvious oceanographic phenomenon during the occurrence of giant submarine earthquakes and mega-tsunamis. Several tsunami vortices were generated during the Asian tsunami of 2004 and the great Japan tsunami of March 2011 which lasted for several hours.New models of tsunami generation and propagation are hereby proposed and were used to investigate the tsunami inception, propagation and associated formation of swirls in the eastern coast of Japan. The proposed generation model assumes that the tsunami was driven by current oscillations at the seabed induced by the submarine earthquake. The major aim of this study is to develop a tsunami model to simulate the occurrence of tsunami swirls. Specifically, this study attempts to simulate and understand the formation of the mysterious tsunami swirls in the northeast coast of Japan. In addition, this study determines the vulnerability of the Philippines to destructive tsunami waves that originate near Japan. A coarse resolution model was therefore developed in a relatively large area encompassing Japan Sea and the eastern Philippine Sea. On the other hand, a fine-resolution model was implemented in a small area off Sendai coast near the epicenter. The model result was compared with the tsunami record obtained from the National Data Buoy Center with relatively good agreement as far as the height and period of the tsunami are concerned. Furthermore, the fine-resolution model was able to simulate the occurrence of tsunami vortices off Sendai coast with various sizes that lasted for several hours.

Some biological characteristics of the Far Eastern sea cucumber (Apostichopus japonicus) from the Severnaya Bay of the Slavyanka Gulf (Japan Sea)

Some of biological traits of the Far Eastern sea cucumber from the Severnaya Bay were examined in 2016 and 2017. Body length of sea cucumber individuals varied from 3.6 to 23.4 cm. Dermo-muscular bag weight was higher in 2017 (137.5 g). Individual ages were 1–4 years in 2016 and 1–5 years in 2017. Sex ratio was near 1:1 in 2016 and 2017. During the period of the research gonad index was maximal in June – 10.6%. Spawning began in the 2nd decade of June and finished in the 2nd decade of July.

Statistical Characteristics and Composite Environmental Conditions of Explosive Cyclones over the Japan Sea and Kuroshio/Kuroshio Extension

Statistical characteristics and composite synoptic-scale environmental conditions of explosive cyclones (ECs) over the Japan Sea and Kuroshio/Kuroshio Extension are examined and compared using ERA5 atmospheric reanalysis to give a better understanding of their differences. ECs over the Japan Sea frequently occur in late autumn and early winter and those over the Kuroshio/Kuroshio Extension mainly occur in winter and early spring. The maximum deepening rate, minimum central sea level pressure and explosive-developing lifetime of ECs over the Kuroshio/Kuroshio Extension are generally larger, lower and longer, respectively, than those over the Japan Sea. ECs over the Kuroshio/Kuroshio Extension formed over the East China Sea tend to develop more rapidly, and weak and moderate ECs generally begin to develop explosively over the sea to the east of the Japan Islands, while the strong and super ECs over the sea to the south of Japan Islands have longer explosive-developing tracks. Composite analysis shows that synoptic-scale environmental conditions favoring rapid EC development over these two regions are significantly different. ECs over the Japan Sea have stronger baroclinicity and cyclonic vorticity, but weaker water vapor convergence and upper-level jet stream than those over the Kuroshio/Kuroshio Extension. The key factor contributing to the baroclinicity is the cold air intrusion over the Japan Sea and the strong warm current heating over the Kuroshio/Kuroshio Extension. The potential vorticity shows anomalies in upper and low levels for both EC areas and extends further downwards over the Japan Sea.

Nonseasonal Variations in Near-Inertial Kinetic Energy Observed Far Below the Surface Mixed Layer in the Southwestern East Sea (Japan Sea)

Near-inertial internal waves (NIWs) generated by surface wind forcing are intermittently enhanced below and within the surface mixed layer. The NIW kinetic energy below the surface mixed layer varies over intraseasonal, interannual, and decadal timescales; however, these variations remain unexplored, due to a lack of long-term, in situ observations. We present statistical results on the nonseasonal variability of the NIW kinetic energy 400 m below the surface mixed layer in the southwestern East Sea, using moored current measurements from 21 years. We used long time series of the near-inertial band (0.85–1.15 f) kinetic energy to define nine periods of relatively high (period high) and seven periods of relatively low (period low) NIW kinetic energy. The NIW kinetic energy average at period high was about 24 times higher than that at period low and those in specific years (2003, 2012–2013, 2016, and 2020) and decade (2010s) were significantly higher than those in other years and decade (2000s). Composite analysis revealed that negative relative vorticity and strong total strain significantly enhance NIW kinetic energy at 400 m. The relative vorticity was negative (total strain was positively enhanced) during seven (six) out of nine events of period high. NIW trapping in a region of negative relative vorticity and the wave capture process induce nonseasonal variations in NIW kinetic energy below the surface mixed layer. Our study reveals that, over intraseasonal, interannual, and decadal timescales, mesoscale flow fields significantly influence NIWs.

Impact of Shifting Subpolar Front on Phytoplankton Dynamics in the Western Margin of East/Japan Sea

A subpolar front (SPF) generated between the East Korea Warm Current (EKWC) and the North Korea Cold Current (NKCC) in the western margin of the East/Japan Sea has shifted northward in recent decades. This study investigated the biomass and composition of the phytoplankton assemblage in relation to hydrological and biogeochemical features in the shallow shelf and slope off the Korean coast from January to June in 2016 and 2017, to determine the mechanistic effects of SPF on spring–summer phytoplankton bloom dynamics. Monthly average depth-integrated chlorophyll a (Chl a) levels and the contribution of phytoplankton classes revealed bimodal diatom blooms in early spring and summer in the frontal zone. Canonical correspondence analysis showed that the distribution of high Chl a was associated with cold, low-salinity NKCC water in March 2016. No Chl a peak was observed in March 2017 when the warm saline EKWC water mass invaded. These results suggest that the NKCC intrusion acts as a forcing mechanism leading to enhanced phytoplankton biomass in the frontal zone. In contrast, positive correlations of Chl a concentration with water density and nutrient concentrations suggest that summer blooms were fed by the subsurface chlorophyll maximum (SCM) driven by shoaling of the pycnocline and nitracline. Varying water-column stratification determined the thickness of the SCM layer, driving year-to-year variability in the magnitude of diatom blooms. These findings further suggest that seasonal/interannual variability in the timing of algal blooms affects regional trophodynamics and hence could be an important factor in explaining ecosystem changes in this region.

Formation, Thermodynamic Structure, and Airflow of a Japan Sea Polar-Airmass Convergence Zone

The Sea of Japan (SOJ) coast and adjoining orography of central Honshu, Japan receive substantial snowfall each winter. A frequent contributor during cold-air outbreaks (CAOs) is the Japan Sea Polar-Airmass Convergence Zone (JPCZ), which forms downstream of the Korean Highlands, extends southeastward to Honshu, and generates a mesoscale band of precipitation. Mesoscale polar vortices (MPVs) ranging in horizontal scale from tens (i.e., meso-β-scale cyclones) to several hundred kilometers (i.e., “polar lows”) are also common during CAOs and often interact with the JPCZ. Here we use satellite imagery and Weather Research and Forecast model (WRF) simulations to examine the formation, thermodynamic structure, and airflow of a JPCZ that formed in the wake of an MPV during a CAO from 2–7 February 2018. The MPV and its associated warm seclusion and bent-back front developed in a locally warm, convergent, and convective environment over the SOJ near the base of the Korean Peninsula. The nascent JPCZ was structurally continuous with the bent-back front and lengthened as the MPV migrated southeastward. Trajectories illustrate how flow splitting around the Korean Highlands, channeling through low passes and valleys along the Asian coast, and air-sea interactions affect the formation and thermodynamic structure of the JPCZ. Contrasts in airmass origin and thermodynamic modification over the SOJ affect the cross-JPCZ temperature gradient, which reverses in sign along the JPCZ from the Asian coast to Honshu. These results provide new insights into the thermodynamic structure of the JPCZ, which is an important contributor to hazardous weather over Japan.

The Delayed Decrease of Seismicity In The Eastern Margin of The Japan Sea Due To The Megathrust Event In 2011 Along The Japan Trench

In the eastern margin of the Japan Sea, off the west coast of Tohoku district, the seismicity increased right after the M9 megathrust event off the east coast of the Tohoku district on March 11, 2011. Four months later, the seismicity decreased to the half level of that before the M9 event. Such quantitative study was done by the point-process model selection with AIC. The decrease lasted for eight years until an M6.7 event occurred within the area in 2019. When we compare the seismicity change between before and after the M9 event, with the post seismic change of the maximum shear stress obtained by the viscoelastic simulation for a thousand years after the M9 event, we can estimate a loading rate of the shear stress in the area before the M9 as 24 kPa/y. For the term after the M9 event, the rate is a half of it; 12 kPa/y. When we assume the whole dilatation change due to the M9 event had been canceled by the time of the M6.7, the increasing rate of the mean stress after the M9 event is 21 kPa/y at most. When we will be able to use JMA catalog for 2020 or later years, we can obtain the seismicity level after the M6.7 quantitatively, and we will be able to narrow down this estimation.

First Concurrent Measurement of Primary Production in the Yellow Sea, the South Sea of Korea, and the East/Japan Sea, 2018

Dramatic environmental changes have been recently reported in the Yellow Sea (YS), the South Sea of Korea (SS), and the East/Japan Sea (EJS), but little information on the regional primary productions is currently available. Using the 13C-15N tracer method, we measured primary productions in the YS, the SS, and the EJS for the first time in 2018 to understand the current status of marine ecosystems in the three distinct seas. The mean daily primary productions during the observation period ranged from 25.8 to 607.5 mg C m−2 d−1 in the YS, 68.5 to 487.3 mg C m−2 d−1 in the SS, and 106.4 to 490.5 mg C m−2 d−1 in the EJS, respectively. In comparison with previous studies, significantly lower (t-test, p < 0.05) spring and summer productions and consequently lower annual primary productions were observed in this study. Based on PCA analysis, we found that small-sized (pico- and nano-) phytoplankton had strongly negative effects on the primary productions. Their ecological roles should be further investigated in the YS, the SS, and the EJS under warming ocean conditions within small phytoplankton-dominated ecosystems.