Xinyu Guo

The Kuroshio Onshore Intrusion along the Shelf Break of the East China Sea: The Origin of the Tsushima Warm Current

Abstract A 1/18° nested ocean model is used to determine locations, volume transports, and temporal variations of Kuroshio onshore fluxes across the shelf break of the East China Sea (ECS). The Kuroshio onshore flux shows strong seasonality: maximum (∼3 Sv; 1 Sv ≡ 106 m3 s−1) in autumn and minimum (<0.5 Sv) in summer. Another short-term (∼17 days) variation due to Kuroshio meanders introduces large fluctuations in the onshore fluxes but its seasonal average almost vanishes. The Kuroshio onshore fluxes have two major sources, Kuroshio intrusion northeast of Taiwan and Kuroshio separation southwest of Kyushu; the former provides larger onshore flux than the latter. Therefore, in addition to the waters from the Taiwan Strait and the Kuroshio separation region southwest of Kyushu, the water due to the Kuroshio intrusion northeast of Taiwan is also a major source of the Tsushima Warm Current. A vorticity equation is used to separate the contribution of surface Ekman transport to the Kuroshio onshore fluxes in ...

A Triply Nested Ocean Model for Simulating the Kuroshio—Roles of Horizontal Resolution on JEBAR

Abstract A triply nested ocean general circulation model was used to examine how the model horizontal resolution influences the Kuroshio in the East China Sea (ECS) and the sea level variability. As the model resolution increases from 1/2° to 1/18° the path, current intensity, and vertical structure of the model Kuroshio and the variability of sea level become closer to observations. In general, the higher-resolution model improves the baroclinic as well as barotropic component of the Kuroshio and thus reproduces more realistic density and current fields. This improvement, in addition to better representation of topography, results in better reproduction of the interaction between baroclinicity and bottom topography, that is, JEBAR (joint effect of baroclinicity and bottom relief) in a high-resolution model. Modeling the Kuroshio in the ECS provides an ideal example of such improvement. In particular, the Kuroshio veering phenomenon at (30°N, 129°E) southwest of Kyushu is discussed, together with the seas...

Three-Dimensional Structure of Tidal Current in the East China Sea and the Yellow Sea

A three-dimensional tidal current model is developed and applied to the East China Sea (ECS), the Yellow Sea and the Bohai Sea. The model well reproduces the major four tides, namely M2, S2, K1 and O1 tides, and their currents. The horizontal distributions of the major four tidal currents are the same as those calculated by the horizontal two-dimensional models. With its high resolutions in the horizontal (12.5 km) and the vertical (20 layers), the model is used to investigate the vertical distributions of tidal current. Four vertical eddy viscosity models are used in the numerical experiments. As the tidal current becomes strong, its vertical shear becomes large and its vertical profile becomes sensitive to the vertical eddy viscosity. As a conclusion, the HU (a) model (Davieset al., 1997), which relates the vertical eddy viscosity to the water depth and depth mean velocity, gives the closest results to the observed data. The reproduction of the amphidromic point of M2 tide in Liaodong Bay is discussed and it is concluded that it depends on the bottom friction stress. The model reproduces a unique vertical profile of tidal current in the Yellow Sea, which is also found in the observed data. The reason for the reproduction of such a unique profile in the model is investigated.

Roles of Mesoscale Eddies in the Kuroshio Paths

A high-resolution ocean general circulation model is developed to simulate connections between the Kuroshio path variations and mesoscale eddy activities as realistically as possible. The climatological mean of the modeled Kuroshio takes a nearshore nonlarge meander path. It is found that the model is capable of simulating two types of nonlarge meander state and a possible version of the large meander state. The offshore nonlarge meander is generated through interaction between the Kuroshio and an anticyclonic eddy. The large meander occurs just after significant intensification of the anticyclonic Kuroshio recirculation; successive intrusion of anticyclonic eddies from the upstream region is responsible for this process. Those anticyclonic eddies are advected by the Kuroshio from the region northeast of Luzon Island and increase the upstream Kuroshio volume transport on an interannual time scale. The cyclonic eddies propagating from the Kuroshio Extension region, on the other hand, weaken the Kuroshio meander after the merger. The Kuroshio path variations south of Japan thus seem to be closely related to eddy activities in the subtropical gyre system.

Rapid water exchange between the lagoon and the open ocean at Majuro Atoll due to wind, waves, and tide

Current measurements and conductivity-temperature-depth surveys of the lagoon and ocean at Majuro Atoll, the Republic of the Marshall Islands, were made from January 10 to 24, 1997. A vertically integrated tidal current model reproduced qualitatively well tidal ellipses calculated from the observed current measurements by Fourier transform. A three-dimensional, robust diagnostic residual current model explained the major features observed in the current measurements averaged over the dominant tidal cycles. We used the diagnostic model to examine the effects of wind stress, radiation stress, density gradients, and tidal stress on the exchange of water between the lagoon and the ocean. Wind effectively mixes the lagoon water in approximately 2 weeks. Tidal flushing appears to be restricted to a small area near the main channels connecting the lagoon to the ocean. Cross-reef-flat currents induced by radiation stress and flowing to the deep channels in the center of the northern boundary form the dominant mechanism for exchange between the lagoon and the open ocean, causing water to exchange completely with the ocean in about 15 days. Computer-generated particles tracked through the lagoon showed that radiation stress is also the main forcing mechanism for particle export from the lagoon. However, the coupling of tidal exchange through the Calalin Channel and wind-stress-induced mixing in the lagoon could also provide a significant export mechanism, particularly for particles originating uniformly inside the lagoon.

Oceanic fronts and jets around Japan: a review

This article reviews progress in our understanding of oceanic fronts around Japan and their roles in air–sea interaction. Fronts associated with the Kuroshio and its extension, fronts within the area of the Kuroshio-Oyashio confluence, and the subtropical fronts are described with particular emphasis on their structure, variability, and role in air–sea interaction. The discussion also extends to the fronts in the coastal and marginal seas, the Seto Inland Sea and Japan Sea. Studies on oceanic fronts have progressed significantly during the past decade, but many of these studies focus on processes at individual fronts and do not provide a comprehensive view. Hence, one of the goals of this article is to review the oceanic fronts around Japan by describing the processes based on common metrics. These metrics focus primarily on surface properties to obtain insights into air–sea interactions that occur along oceanic fronts. The basic characteristics derived for each front (i.e., metrics) are then presented as a table. We envision that many of the coupled ocean-atmosphere global circulation models in the coming decade will represent oceanic fronts reasonably well, and it is hoped that this review along with the table of metrics will provide a useful benchmark for evaluating these models.

Interannual variations of Kuroshio transport in the East China Sea and its relation to the Pacific Decadal Oscillation and mesoscale eddies

Results of a data-assimilative ocean model (JCOPE2) from 1993 to 2012 were used to examine the correlation between the Pacific Decadal Oscillation (PDO) index and interannual variations of the Kuroshio transport in the East China Sea (ECS) and the influences of mesoscale eddies on this correlation. In a period from 1993 to 2002, the Kuroshio transport estimated from the JCOPE2 reanalysis has a positive correlation with the PDO index. This well-known correlation became weak or even disappeared when the analysis period was extended from 1993–2002 to 1993–2012. This occurs because the variation range of the PDO index became small during enhanced mesoscale eddy activity southeast of Taiwan in years after 2002. The eddies caused a larger variation in the Kuroshio transport in the years after 2002 than before 2002, and therefore, changed the correlation between the PDO index and Kuroshio transport in the ECS. The influence of mesoscale eddies on the Kuroshio transport has strong regional dependence: the Kuroshio transport from the area east of Taiwan to the midway along the shelf break in the East China Sea depends mainly on eddies arriving from southeast of Taiwan, while transport from the midway along the shelf break to the Tokara Strait depends mainly on the eddies arriving from northeast of Okinawa Island. The combination of PDO-related signals and eddy-related signals determines the interannual variations of the Kuroshio transport in the ECS and sufficient attention must be paid to the spatial dependence of the Kuroshio transport in the ECS on eddies.

Seasonal variation of residual current in Tokyo Bay, Japan —diagnostic numerical experiments—

The residual currents in Tokyo Bay during four seasons are calculated diagnostically from the observed water temperature, salinity and wind data collected by Unokiet al. (1980). The calculated residual currents, verified by the observed ones, show an obvious seasonal variable character. During spring, a clear anticlockwise circulation develops in the head region of the bay and a strong southwestward current flows in the upper layer along the eastern coast from the central part to the mouth of the bay. During summer, the anticlockwise circulation in the head region is maintained but the southwestward current along the eastern coast becomes weak. During autumn, the preceding anticlockwise circulation disappears but a clockwise circulation develops in the central part of the bay. During winter, the calculated residual current is similar to that during autumn. As a conclusion, the seasonal variation of residual current in Tokyo Bay can be attributed to the variation of the strength of two eddies. The first one is the anticlockwise circulation in the head region of the bay, which develops in spring and summer and disappears in autumn and winter. The second one is the clockwise circulation in the central part of the bay, which develops in autumn and winter, decreases in spring and nearly disappears in summer.

High Resolution Kuroshio Forecast System: Description and its Applications

We have developed a forecast system for the Kuroshio large meander with a high horizontal resolution (approximately 10 km). Using the system, we succeeded in predicting the path transitions of the Kuroshio from the nearshore nonlarge meander path to the offshore nonlarge meander path in 2003, and from the nearshore nonlarge meander path to the typical large meander path in 2004 as well as the occurrence of its triggering small meander south of Kyushu Island. We have also been developing a higher resolution forecast model for coastal oceans and bays south of Japan, where physical and biological states of the ocean are much affected by the path variation of the Kuroshio. This model, although the development is still under way, represents tides and tidal currents in two bays south of Japan in a realistic way.

M2 baroclinic tide variability modulated by the ocean circulation south of Japan

We analyze a concurrent simulation result of the ocean circulation and tidal currents using a data-assimilative ocean general circulation model covering the Western North Pacific with horizontal resolution of 1/36° to investigate possible interactions between them. Four sites of active M2 internal tide variability in open ocean (hot spots), such as Tokara Strait, Izu Ridge, Luzon Strait, and Ogasawara Ridge, are detected from both the satellite observation and the simulation. Energy cycle analysis of the simulated M2 baroclinic tide indicates two types of the hot spots: dissipation (Tokara Strait and Izu Ridge) and radiation (Luzon Strait and Ogasawara Ridge) dominant sites. Energy conversion from barotropic to baroclinic M2 tides at the hot spots is modulated considerably by the lower-frequency changes in the density field. Modulation at the two spots (Tokara Strait and Izu Ridge) is affected by the Kuroshio path variation together with the seasonal variation of the shallow thermocline. At the other two sites, influence from changes in the relatively deep stratification through the Kuroshio intrusion into South China Sea (Luzon Strat) and mesoscale eddy activity (Ogasawara Ridge) is dominant in the modulation.