scholarly journals Effects of Koshu Seamount on the Development of Baroclinic Instability Leading to the Kuroshio Large Meander

2017 ◽  
Vol 47 (10) ◽  
pp. 2563-2576 ◽  
Author(s):  
Yuki Tanaka ◽  
Toshiyuki Hibiya
Keyword(s):  
Lower Layer ◽  
Bottom Topography ◽  
Baroclinic Instability ◽  
Northern Slope ◽  
Large Meander ◽  
Resonant Coupling ◽  
Small Meander ◽  
South Of Japan ◽  
Rapid Amplification ◽  
The Kuroshio

AbstractThe Kuroshio south of Japan shows bimodal path fluctuations between the large meander (LM) path and the nonlarge meander (NLM) path. The transition from the NLM path to the LM path is triggered by a small meander generated off southwestern Japan. The small meander first propagates eastward (downstream) along the Kuroshio and then rapidly amplifies over Koshu Seamount, located about 200 km south of Japan, leading to the formation of the LM path of the Kuroshio. Although Koshu Seamount is essential for the rapid amplification of the small meander, the underlying physical mechanism is not fully understood. In this study, the role of Koshu Seamount is revisited using a two-layer quasi-geostrophic model that takes into account the effects of bottom topography. Numerical experiments show that the transition from the NLM path to the LM path can be successfully reproduced only when bottom topography mimicking Koshu Seamount is incorporated. In this case, the upper-layer meander trough is rapidly amplified together with a lower-layer anticyclone by baroclinic instability during their passage over the northern slope of Koshu Seamount. A linear stability analysis shows that baroclinic instability over a seamount is caused by resonant coupling between the upper-layer Rossby wave in the eastward background flow and the lower-layer seamount-trapped wave during their eastward propagation over the northern slope of the seamount. The spatial scale and structure of this baroclinically unstable mode are close to those of the numerically reproduced small meander in its early amplification stage over the seamount.

scholarly journals The Effect of Koshu Seamount on the Formation of the Kuroshio Large Meander South of Japan

2011 ◽  
Vol 41 (9) ◽  
pp. 1624-1629 ◽  
Author(s):  
Takahiro Endoh ◽  
Hiroyuki Tsujino ◽  
Toshiyuki Hibiya
Keyword(s):  
Stability Analysis ◽  
Numerical Model ◽  
Water Depth ◽  
Baroclinic Instability ◽  
Model Output ◽  
Northern Slope ◽  
Large Meander ◽  
Kuroshio Large Meander ◽  
South Of Japan ◽  
The Kuroshio

Abstract Using an inflow–outflow numerical model, the authors demonstrate that the existence of Koshu Seamount, located about 200 km to the south of Cape Shiono-misaki, is essential in creating the large meander (LM) of the Kuroshio. When Koshu Seamount is completely smoothed out, the meander trough propagates away without being amplified to form the LM. In contrast, nearly the same LM as in the case with full topography is formed when the foot of Koshu Seamount remains without being smoothed out and also when the foot of Koshu Seamount is filled in so that the upper part of Koshu Seamount remains. A linear stability analysis applied to the model output shows that the Kuroshio becomes baroclinically most unstable when the water depth decreases offshoreward. The authors therefore conclude that the enhancement of baroclinic instability over the northern slope of Koshu Seamount is a prerequisite to the formation of the LM.

scholarly journals An Objective Method for Probabilistic Forecasting of Multimodal Kuroshio States using Ensemble Simulation and Machine Learning

2020 ◽  
Vol 50 (11) ◽  
pp. 3189-3204
Author(s):  
Kunihiro Aoki ◽  
Yasumasa Miyazawa ◽  
Tsutomu Hihara ◽  
Toru Miyama
Keyword(s):  
Machine Learning ◽  
Baroclinic Instability ◽  
Mixture Distribution ◽  
Gaussian Mixture ◽  
Distribution Model ◽  
Probabilistic Forecasting ◽  
Large Meander ◽  
Mixture Distribution Model ◽  
The Kuroshio ◽  
Kuroshio Region

AbstractThis paper presents a method for detecting the ensemble means, spreads, and occurrence probabilities for each of the multiple Kuroshio states. This is accomplished by classifying the forecasts of the ensemble members with a Gaussian mixture distribution model, a machine learning method. Ensemble simulations with 80 members are conducted to reproduce possible occurrences of the multiple Kuroshio states, targeting the large meander event in 2017. To test its performance, first, the method is applied for the southernmost latitude, a conventional index that represents meander intensity. The results show that the Kuroshio initially taking the nearshore nonlarge meander state bifurcates into the large meander and offshore nonlarge meander states, which occur with similar probabilities. Both developments are accompanied by positive potential energy extraction rates, consistent with baroclinic instability. As a more objective approach, the method is then applied for the dominant modes derived from empirical orthogonal function (EOF) analysis of the sea surface height field in the entire Kuroshio region. Importantly, almost identical results can be achieved. In particular, the bimodality between the large meander and nonlarge meander is shown to appear on the axis of the first EOF mode. From a mathematical perspective, this mode can be interpreted as the singular vector which grows most rapidly following the time-evolution operator. Finally, the multimodality of the Kuroshio is reinterpreted as a phase transition phenomenon where the nearshore nonlarge meander constitutes the basic state.

scholarly journals Subtropical Mode Water Variability in a Climatologically Forced Model in the Northwestern Pacific Ocean

2012 ◽  
Vol 42 (1) ◽  
pp. 126-140 ◽  
Author(s):  
Elizabeth M. Douglass ◽  
Steven R. Jayne ◽  
Synte Peacock ◽  
Frank O. Bryan ◽  
Mathew E. Maltrud
Keyword(s):  
Pacific Ocean ◽  
Mixed Layer Depth ◽  
Northwestern Pacific ◽  
Northwestern Pacific Ocean ◽  
Large Meander ◽  
The West ◽  
Mode Water ◽  
Subtropical Mode Water ◽  
South Of Japan ◽  
The Kuroshio

Abstract A climatologically forced high-resolution model is used to examine variability of subtropical mode water (STMW) in the northwestern Pacific Ocean. Despite the use of annually repeating atmospheric forcing, significant interannual to decadal variability is evident in the volume, temperature, and age of STMW formed in the region. This long time-scale variability is intrinsic to the ocean. The formation and characteristics of STMW are comparable to those observed in nature. STMW is found to be cooler, denser, and shallower in the east than in the west, but time variations in these properties are generally correlated across the full water mass. Formation is found to occur south of the Kuroshio Extension, and after formation STMW is advected westward, as shown by the transport streamfunction. The ideal age and chlorofluorocarbon tracers are used to analyze the life cycle of STMW. Over the full model run, the average age of STMW is found to be 4.1 yr, but there is strong geographical variation in this, from an average age of 3.0 yr in the east to 4.9 yr in the west. This is further evidence that STMW is formed in the east and travels to the west. This is qualitatively confirmed through simulated dye experiments known as transit-time distributions. Changes in STMW formation are correlated with a large meander in the path of the Kuroshio south of Japan. In the model, the large meander inhibits STMW formation just south of Japan, but the export of water with low potential vorticity leads to formation of STMW in the east and an overall increase in volume. This is correlated with an increase in the outcrop area of STMW. Mixed layer depth, on the other hand, is found to be uncorrelated with the volume of STMW.

Bottom pressure change associated with the 2004–2005 large meander of the Kuroshio south of Japan

2018 ◽  
Vol 68 (7) ◽  
pp. 847-865 ◽  
Author(s):  
Akira Nagano ◽  
Takuya Hasegawa ◽  
Hiroyuki Matsumoto ◽  
Keisuke Ariyoshi
Keyword(s):  
Pressure Change ◽  
Bottom Pressure ◽  
Large Meander ◽  
South Of Japan ◽  
The Kuroshio

scholarly journals New Perspectives on the Generation and Maintenance of the Kuroshio Large Meander

2019 ◽  
Vol 49 (8) ◽  
pp. 2095-2113 ◽  
Author(s):  
Yang Yang ◽  
X. San Liang
Keyword(s):  
Baroclinic Instability ◽  
Mesoscale Eddy ◽  
Low Frequency ◽  
Analysis Tool ◽  
Mean Flow ◽  
Large Meander ◽  
Kuroshio Large Meander ◽  
The Mean ◽  
The Kuroshio ◽  
Canonical Transfer

AbstractThe internal dynamical processes underlying the Kuroshio large meander are investigated using a recently developed analysis tool, multiscale window transform (MWT), and the MWT-based canonical transfer theory. Oceanic fields are reconstructed on a low-frequency mean flow window, a mesoscale eddy window, and a high-frequency synoptic window with reference to the three typical path states south of Japan, that is, the typical large meander (tLM), nearshore non-large meander (nNLM), and offshore non-large meander (oNLM) path states. The interactions between the scale windows are quantitatively evaluated in terms of canonical transfer, which bears a Lie bracket form and conserves energy in the space of scale. In general, baroclinic (barotropic) instability is strengthened (weakened) during the tLM state. For the first time we found a spatially coherent inverse cascade of kinetic energy (KE) from the synoptic eddies to the slowly varying mean flow; it occupies the whole large meander region but exists only in the tLM state. By the time-varying multiscale energetics, a typical large meander is preceded by a strong influx of mesoscale eddy energy from upstream with a cyclonic eddy, which subsequently triggers a strong inverse KE cascade from the mesoscale window to the mean flow window to build up the KE reservoir for the meander. Synoptic frontal eddies are episodically intensified due to the baroclinic instability of the meander, but they immediately feed back to the mean flow window through inverse KE cascade. These results highlight the important role played by inverse KE cascades in generating and maintaining the Kuroshio large meander.

Characteristics of an atypical large-meander path of the Kuroshio current south of Japan formed in September 2017

2018 ◽  
Vol 40 (4) ◽  
pp. 525-539 ◽  
Author(s):  
Akira Nagano ◽  
Yusuke Yamashita ◽  
Takuya Hasegawa ◽  
Keisuke Ariyoshi ◽  
Hiroyuki Matsumoto ◽  
...  
Keyword(s):  
Kuroshio Current ◽  
Large Meander ◽  
South Of Japan ◽  
The Kuroshio
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