Near bottom currents and their relation to the transport in the Kuroshio Extension (original) (raw)
Related papers
Evidence of Bottom-Trapped Currents in the Kuroshio Extension Region
Journal of Physical Oceanography, 2012
As part of the Kuroshio Extension System Study, observations from five current meter moorings reveal that the abyssal currents are weakly bottom intensified. In the framework of linear quasigeostrophic flow, the best fitted vertical trapping depths range from 8 to 15 km in the absence of steep topography, but one mooring near an isolated seamount exhibited vertical trapping that was more pronounced and energetic with a vertical trapping depth of 5 km. The ratios of current speeds and geostrophic pressure streamfunctions at the sea surface compared to the bottom are 88% in the absence of steep topography, 63% near an isolated seamount, and overall on average 83% of their value at a reference depth of 5300 m. It is hypothesized that weakly depth-dependent eddies impinging upon topographic features introduce to the flow the horizontal length scales of the topography, and these smaller lateral scales are subject to bottom intensification.
Structure and Variability of the Kuroshio Current in Tokara Strait *
Journal of Physical Oceanography, 2000
Four years of mooring array measurements in Tokara Strait, south of Kyushu, Japan, from 1992 to 1996 are used to analyze the structure and temporal variability of the Kuroshio Current. The mean Kuroshio current in Tokara Strait shows a nearly permanent subsurface double-core structure, possibly due to topographic blockage effects.
Variability of Northeastward Current Southeast of Northern Ryukyu Islands
Journal of Oceanography, 2004
To better understand the mechanism underlying the variation of the Kuroshio south of central Japan, we have examined the variability of current structure in its upstream region, southeast of Amami-Ohshima Island in the northern Ryukyu Islands. By combined use of ship-mounted Acoustic Doppler Current Profiler (ADCP) and the TOPEX/POSEIDON satellite altimeter data on Path 214, the sea surface absolute geostrophic currents were estimated every ten days from January 1998 to July 2002. The 4.5-year mean surface current was found to flow northeastward north of 26.8°N with a maximum speed of 14 cm s−1 over the shelf slope at 3000 m depth. The moored current-meter observations at three or four mooring stations from Dec. 1998 to Oct. 2002 suggested the existence of a northeastward undercurrent with a maximum core velocity of 23 cm s−1 at 600 m depth over the shelf slope at 1600 m depth. The mean volume transport in the top 1500 m between 27.9°N and 26.7°N is estimated to be 16 × 106 m3s−1 northeastward, including the subsurface core current related component of 4 × 106 m3s−1.
The calculation of Kuroshio current structure in the East China Sea—early summer 1986
Progress in Oceanography, 1988
g)n the basis of hydrographic data and moored current meter records obtained during an early summer cruise (May 20-June 23) of 1986, a three dimensional diagnostic calculation of the circulation is performed in the survey area, which covers the East China Sea continental shelf, Okinawa Trough and an area east of the Ryukyu Island. The Kuroshio Current condition and structure in the East China Sea, its branches and their interrelationship as well as the eddies around the Kuroshio, are discussed. When the Kuroshio entered the area northeast of Taiwan, there were two branches. The main branch flowed northeastward along the continental slope and the other branch was at the eastern part of the Okinawa Trough. The main axis of the Kuroshio followed the continental slope above the 300 m level, but moved gradually eastward to the Okinawa Trough below the 300 m level.
Deep Currents in the Northwest Pacific Off Japan During KERE
1994
Pubbe repoftqI burdenl ot M11 c ofstm Woinssion~ f is eslirridd lo average 1 hour per response. kag ft tons Wc reiwingffmt riuctior. sarachirig exwistig dea bources. gaftweweg da mauwibiegw*todeftiwedsd.a wedoornipl. aedrevie" fwcsI ",04kiomwIbn. Sendoononstepwdwerighsbuadmeanovow rpadoI~cl~sanoItedonewlon. eckdng auost~mn for reedJce Uew burdeci. to Weabeeon Hoadquuawrs Services. Obeectraf lt eImkonnabon Operatious and Rapouta. 1215 Joilluucu Dom~ Higway. Sude 1204. Ailingon, VA 22202-43l, end Jo 3w Oftof kce i Mnaeeewu and udget. Paeework Reducion Project (0704-018). Washington, DC 20503W,_____________ 1. Agency Use Only (Lvea Mvbank). 2. Report Dafte. 3. Report Type and Dates Covered. I February 4, 1994 Final 4. Title and Subtitle. 5. Funding Numbers.
Journal of Physical Oceanography, 2006
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 o...
Hydrographic structure in association with deep boundary current in the north of the Shikoku Basin
Journal of Oceanography, 1995
High quality CTD data were collected in the north of the Shikoku Basin where an abyssal boundary current has been observed through direct current measurements. Analyses of hydrographic data showed: 1. Colder and saltier water (heavier water) compared to surrounding waters is found above the continental shelf-toe and the eastern flank of the Kyushu-Palau Ridge where the existence of the abyssal boundary current has been expected. The heavier water has a horizontal extent of about 50 km. 2. The heavier water has the vertical scale of 2000 m from the sea bottom, and is associated with a thermal wind shear which enhances a component of the flow toward a direction looking the Nankai Trough (a trough located along the northern end of the Shikoku Basin) to the left in the abyss. The assumed "level of no motion" at about 2500 m depth gives the geostrophically estimated current in a good agreement with the directly measured current. A volume transport associated with the colder and higher salinity water is estimated to be about 2 Sv off Cape Shiono-misaki which may include a recirculation above the Nankai Trough. This is about twice of the transport estimated in the interior of the Shikoku Basin through a vorticity balance between the stretching term and latitudinal variation of the planetary vorticity.
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