Kuroshio Meanders in the East China Sea (original) (raw)
The PI attended three ONR workshops to discuss and help define the future integrated observational program for "Quantifying, Predicting and Exploiting Uncertainty (QPEU), i.e., to help formulate an experiment to study the Kuroshio intrusion, nonlinear internal waves (NLIWs), internal tides, inertial waves, and turbulence mixing resulting from the Kuroshio-topography interaction. Using historical CTD data collected by the National Center for Ocean Research (NCOR) between 1985 and 2002, the PI computed the fluctuations of sound speed in different regions along the Kuroshio path and across the continental shelf and slope. Preliminary analysis concludes that strong sound speed anomalies are induced by NLIWs, internal tides, and processes associated with the Kuroshio interaction with the continental slope and shelf. Such sound speed anomalies have the temporal and spatial scales and characteristics associated with the corresponding oceanic processes. 15. SUBJECT TERMS 16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF 18. NUMBER 19a. NAME OF RESPONSIBLE PERSON a. REPORT b.
Variations of Kuroshio Intrusion and Internal Waves at Southern East China Sea
The PI attended three ONR workshops to discuss and help define the future integrated observational program for "Quantifying, Predicting and Exploiting Uncertainty (QPEU), i.e., to help formulate an experiment to study the Kuroshio intrusion, nonlinear internal waves (NLIWs), internal tides, inertial waves, and turbulence mixing resulting from the Kuroshio-topography interaction. Using historical CTD data collected by the National Center for Ocean Research (NCOR) between 1985 and 2002, the PI computed the fluctuations of sound speed in different regions along the Kuroshio path and across the continental shelf and slope. Preliminary analysis concludes that strong sound speed anomalies are induced by NLIWs, internal tides, and processes associated with the Kuroshio interaction with the continental slope and shelf. Such sound speed anomalies have the temporal and spatial scales and characteristics associated with the corresponding oceanic processes. 15. SUBJECT TERMS 16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF 18. NUMBER 19a. NAME OF RESPONSIBLE PERSON a. REPORT b.
Interannual Modulation of Kuroshio in the East China Sea Over the Past Three Decades
Frontiers in Marine Science
Previous studies have suggested that westward-migrating mesoscale eddies are a dominant factor that modulate the interannual Kuroshio intensity in the East China Sea (ECS), indicating a close positive correlation between them. According to the extended record of altimetry-based sea level anomalies (SLAs) until 2020, however, the interannual variation of the Kuroshio intensity no longer has a strong positive correlation with eddy activity in the subtropical countercurrent (STCC) region since the early 2000s. Our observational analyses showed that the Kuroshio intensity in the ECS can be modulated by the combined effect of westward-migrating mesoscale eddies and westward-propagating oceanic planetary waves from the east. Until the early 2000s, the interannual variability of Kuroshio was mainly affected by eddy migration from the STCC region, associated with oceanic instability driven by large-scale wind patterns over the western North Pacific. Since then, oceanic planetary waves propa...
Progress in Oceanography, 2020
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Observations on the propagation, growth, and predictability of Gulf Stream meanders
Journal of Geophysical Research, 1985
During a 3-year period, Gulf Stream positions determined by satellite infrared imagery on a grid northeast of Cape Hatteras have been correlated against "inlet" path parameters (displacement from the mean position, angle, and curvature) monitored by an array of inverted echo sounders (IES). By crossspectral calculations between these measurements, we determined the downstream coherences, phase speeds, and spatial growth rates. The downstream path predictability from these inlet parameters was tested in a multiple input linear response model. The most energetic meanders, with periods of 33-50 days, remained highly coherent for 300 km downstream, roughly the dominant wavelength, with marginal coherence again at 500-575 km. Phase speeds are frequency dependent and decrease with distance downstream. For the 50-(33) day meanders, phase speeds decreased from 10 (17) km/d at the inlet to 6 (8) km/d at a distance downstream of about 550 km. The average growth rate (spatial e-folding wave number) over the entire region is 1.3 x 10 -3 km-•. Within 300 km of the inlet, the multiple coherence accounts for 55-65% of the total path-displacement variance, with displacement and angle being the best individual predictors in the first and last halves of this region, respectively. 7143
Journal of Geophysical Research, 1996
A set of numerical simulations is used to investigate the Pacific Ocean circulation north of 20øS, with emphasis on the Kuroshio/Oyashio current system. The primitive equation models used for these simulations have a free surface and realistic geometry that includes the deep marginal seas such as the Sea of Japan. Most of the simulations have 1/8 ø resolution for each variable but range from 1/2 ø, 1.5-layer reduced gravity to 1/16 ø , six layer with realistic bottom topography. These are used to investigate the dynamics of the Kuroshio/Oyashio current system and to identify the processes that contribute most to the realism of the simulations. This is done by model-data comparisons, by using the modularity of layered ocean models to include/exclude certain dynamical processes, by varying the model geometry and bottom topography, and by varying model parameters such as horizontal grid resolution, layer structure, and eddy viscosity. In comparison with observational data the simulations show that the barotropic mode, at least one internal mode, nonlinearity, high "horizontal" resolution (1/8 ø or finer), the regional bottom topography, and the wind forcing are critical for realistic simulations. The first four are important for baroclinic instability (eddy-mean energetics actually show mixed barotropic-baroclinic instability), the wind curl pattern for the formation and basic placement of the current system, and the bottom topography for the distribution of the instability and for influences on the pathways of the mean flow. Both the Hellerman and Rosenstein (1983) (HR) monthly wind stress climatology and 1000-mbar winds from the European Centre for Medium-Range Weather Forecasts (ECMWF) have been used to drive the model. East of about 150øE, they give a mean latitude for the Kuroshio Extension that differs by about 3 ø, approximately 34øN for HR, 37øN for ECMWF, and 35øN observed. The subarctic front is the northern boundary of the subtropical gyre. It is associated with the annual and April-September mean zero wind stress curl lines (which are similar), while the Kuroshio Extension is associated with wintertime zero wind stress curl. This means that part of the flow from the Kuroshio must pass north of the Kuroshio Extension and connect with the Oyashio and subarctic front. Realistic routes for this connection are flow through the Sea of Japan, a nonlinear route separated from the east coast of Japan, and bifurcation of the Kuroshio at the Shatsky Rise. In addition, the sixlayer simulations show a 3-Sv meridional overturning cell with southward surface flow and northward return flow centered near 400 m depth. Baroclinic instability plays a critical role in coupling the shallow and abyssal layer circulations and in allowing the bottom topography to strongly influence the shallow circulation. By this means the Izu Ridge and Trench and seamounts upstream and downstream of these have profound influence on (1) the mean path of the Kuroshio and its mean meanders south and east of Japan and (2) on separating the northward flow connecting the Kuroshio and the Oyashio/subarctic front from the east coast of Japan. Without the topographic influence the models show an unrealistic northward current along the east coast of Japan. In essence, the topography regulates the location and strength of the baroclinic instability. The baroclinic instability gives eddy-driven deep mean flows that follow the f/h contours (where f is the Coriolis parameter and h is the depth of the water column) of the bottom topography. These abyssal currents then strongly influence the pathway for subtropical gyre flow north of the Kuroshio Extension and steer the mean meanders in the Kuroshio south and east of Japan. This is corroborated by current meter data from the Kuroshio Extension Regional Experiment (World Ocean Circulation Experiment line PCM 7). The meander path south Paper number 95JC01674. 0148-0227/96/95 J C-01674 $05.00 941 942 HURLBURT ET AL.' DYNAMICS OF THE KUROSHIO/OYASHIO CURRENT SYSTEM of Japan depends on the occurrence of baroclinic instability west of the Izu Ridge; otherwise, a straight path occurs. The pathway shows little sensitivity to the Tokara Strait transport over the range simulated (36-72 Sv in yearly means). However, interannual increases in wind forcing or Tokara Strait transport give rise to a predominant meander path, while decreases yield a predominant straight path. Resolution of 1/8 ø in an ocean model is comparable to the 2.5 ø resolution used in atmospheric forecast models in the early 1980s based on the first internal mode Rossby radius of deformation. Model comparisons at 1/8 ø and 1/16 ø resolution and comparisons with current meter data and Geosat altimeter data show that 1/16 ø resolution is needed for adequate eastward penetration of the high eddy kinetic energy associated with the Kuroshio Extension. 1. Introduction The dynamics of the Kuroshio/Oyashio current system are investigated by using several variations of an eddy-resolving numerical ocean model covering the Pacific Ocean north of 20øS and by model-data comparisons. The horizontal grid resolution of the principal simulations is 1/8 ø for each variable (1/8 ø in latitude by 45/256 ø in longitude), and the vertical structure ranges from 1.5-layer reduced gravity to six layers with realistic bottom topography. In addition, it was possible to extend one of the 1/8 ø simulations at 1/16 ø resolution and two different eddy viscosities. Simulations at 1/4 ø resolution and comparisons of 1/8 ø and 1/16 ø subtropical gyre simulations are also used. The subtropical gyre model treats that gyre in isolation by incorporating solid boundaries at 15øN, 48øN, 121øE, and 140øW. The basic model has a free surface, and all versions of the Pacific model include the deep marginal seas such as the Bering Sea, the Sea of Okhotsk, the Sea of Japan, the South China Sea, the Sulu Sea, and the Indonesian archipelago. The subtropical gyre model includes the Sea of Japan. These seas can play a significant role in the main basin dynamics. The Sea of Japan plays a role in the Kuroshio/Oyashio dynamics, which is discussed in the paper. The modularity of layered ocean models can be used to include or exclude certain features or dynamical processes. For example, a 1.5-layer reduced gravity model can allow barotropic instability but excludes baroclinic instability. This approach and comparisons with observations form the paradigm used to identify the essential model features and dynamical processes for realistic simulations. Section 2 covers the model design, and in section 3, simulations of the basic upper ocean features of the equatorial and North Pacific are discussed. Kuroshio/Oyashio current system dynamics are discussed in section 4. Topics include (1) the development of the basic current system including the roles of the wind curl, nonlinearity, bottom topography, and the Sea of Japan; (2) the essential model features and dynamics required for realistic simulation of the Kuroshio Extension; (3) how the Kuroshio feeds part of its transport north of the Kuroshio Extension and into the Oyashio/subarctic frontal region; and (4) the combined effects of baroclinic instability and bottom topography on the meandering of the Kuroshio/Kuroshio Extension mean path and in generating local maxima in variability. The dynamics of the meander path versus straight path for the Kuroshio south of Japan has been a topic of considerable debate (e.g., see Yoon and Yasuda [1987], who include a substantial review of the earlier literature, and Yamagata and Umatani [1989]). Section 5 discusses the eastward penetration of the Kuroshio Extension including model-data comparisons from the surface to 4000 m depth and the effects of model resolution (1/8 ø versus 1/16ø). Previous investigations of this topic for the Kuroshio and/or the Gulf Stream include those by Schmitz and Holland [1982, 1986], Holland and Schmitz [1985], Thompson and Schmitz [1989], Hogan et al. [1992], Marshall and Marshall [1992], and Schmitz and Thompson [1993]. Section 6 contains the summary and conclusions. Hurlburt et al. [1992] discuss an initial 1/8 ø six-layer Pacific simulation with realistic bottom topography. Hogan et al. [1992] include model-Geosat comparisons for a 1/8 ø two-layer version of the model. Jacobs et al. [this issue] and Mitchell et al. [this issue] investigate additional aspects of the dynamics and provide extensive comparisons between Geosat altimeter measurements and more recent simulations by improved versions of the 1/8 ø six-layer Pacific model run 1981-1993. These simulations are also used in model-data comparisons in section 5. 2. The Model The model is a primitive equation layered formulation where the model equations have been vertically integrated through each layer. It is a descendent of the semi-implicit free-surface model of Hurlburt and Thompson [1980] but with expanded capability [Wallcraft, 1991]. The equations for the n layer finite depth, hydrodynamic model are for layers k = looon OVa Ot --+ (V. Va + Va' V)va + fcx fVa = -ha E Gt,V(hl-Hi) + max (0, to0va+, /=1
Roles of Mesoscale Eddies in the Kuroshio Paths
Journal of Physical Oceanography, 2004
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,...
Journal of Geophysical Research, 2002
The Affiliated Surveys of the Kuroshio off Ashizuri-Misaki (ASUKA) project is a collaboration between scientists in Japan and the United States to improve our understanding of the structure and dynamics of the Kuroshio off the coast of southwest Japan. In this project an array of nine inverted echo sounders and nine current meter moorings were deployed for 2 years (1993-1995) along a line extending 1000 km from Shikoku. Numerous hydrographic profiles along this section were used to determine the regional gravest empirical modes (GEMs), which relate vertical profiles of temperature and specific volume anomaly to vertical acoustic echo time. These GEMs were used in conjunction with the inverted echo sounder data to calculate 2-year time series of entire sections of relative geostrophic velocity and temperature. Current meter data provided absolute velocity references for the geostrophic velocity sections. In addition, a time series of Kuroshio position was determined from the GEM temperature sections. It shows three big (>80 km displacement) and three little offshore meanders of the Kuroshio occurring in February, May/June, and October of both years. This suggests a 4-month periodicity, possibly phase-locked to the annual cycle. The meanders may be caused by mesoscale eddies arriving from the east. The observed meanders were preceded by low Kuroshio volume transport and were followed by high Kuroshio volume transport. The Kuroshio volume and temperature transports were highly variable, with respective means of 65 ± 4 Sv and 3.7 ± 0.2 PW and standard deviations of 20 Sv and 0.9 PW.