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1.
Norihisa Imasato Shinzou Fujio Qian Zhang Toshiyuki Awaji Kazunori Akitomo 《Journal of Oceanography》1994,50(2):119-139
We have investigated the three-dimensional Lagrangian motion of water particles related with tidal exchange between two basins with a constant depth connected through a narrow strait and the effects of density stratification on the exchange processes by tracking a number of the labeled particles. Tide-induced transient eddies (TITEs), which are similar to those in two-dimensional basin, are generated behind the headlands. Upwelling appears around the center of the eddy and sinking around the boundary. When the basins are filled with homogeneous water, a pair of vortices are produced in the vertical cross section of the strait due to bottom stress, with upwellings along the side walls of the strait and sinking in the center of the strait. These circulations form the horizontally convergent field in the cross-strait direction in the upper layers while the horizontal divergence takes place in the bottom layer. These vertical water-motions produce the three-dimensional distribution of velocity shear and phase lag of the tidal current around the strait, and the Lagrangian drifts of water particles become large. As a result, water exchange through the strait is greatly enhanced: The water exchange rate reaches 94.1% which is much larger than that obtained in the vertically integrated two-dimensional model. When the basins are stratified, the stable stratification suppresses the vertical motion so that a pair of vertical vortices are confined in the lower layers. This leads to a decrease in the exchange rate, down to 88.6%. Our numerical results show that the three-dimensional structure of tidal currents should be taken into account in tidal exchange through a narrow strait. 相似文献
2.
State Estimation of the North Pacific Ocean by a Four-Dimensional Variational Data Assimilation Experiment 总被引：1，自引：0，他引：1
Toshiyuki Awaji Shuhei Masuda Yoichi Ishikawa Nozomi Sugiura Takahiro Toyoda Tomohiro Nakamura 《Journal of Oceanography》2003,59(6):931-943
A four-dimensional variational data assimilation system has been applied to an experiment to describe the dynamic state of
the North Pacific Ocean. A synthesis of available observational records and a sophisticated ocean general circulation model
produces a dynamically consistent dataset, which, in contrast to the nudging approach, provides realistic features of the
seasonally-varying ocean circulation with no artificial sources/sinks for temperature and salinity fields. This new dataset
enables us to estimate heat and water mass transports in addition to the qualification of water mass formation and movement
processes. A sensitivity experiment on our assimilation system reveals that the origin of the North Pacific Intermediate Water
can be traced back to the Sea of Okhotsk and the Bering Sea in the subarctic region and to the subtropical Kuroshio region
further south. These results demonstrate that our data assimilation system is a very powerful tool for the identification
and characterization of ocean variabilities and for our understanding of the dynamic state of ocean circulation.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
3.
Tomohiro Nakamura Takahiro Toyoda Yoichi Ishikawa Toshiyuki Awaji 《Journal of Oceanography》2010,66(1):41-60
A steady quasi-geostrophic 2.5-layer model, forced by both Ekman pumping and a mass source/sink situated at the western boundary
has been constructed to investigate the effect of diapycnal transport due to convection in the Okhotsk Sea and tidal mixing
at the Kuril Straits on the intermediate layer in the North Pacific. The model illustrates a combined effect of the wind-driven
and mass-driven circulations. First, net mass input induces a “barotropic” mode inter-gyre flow along the western boundary
through the dynamical influence of Kelvin waves. This flow creates characteristic curves (geostrophic contours) that facilitate
inter-gyre communication through the western boundary layer from the location of the mass source to the subtropical gyre.
Due to the effect of wind-driven circulation, the offshore part turns eastward into the interior, encircles the outer rim
of the region (which would otherwise be the pool region in the absence of mass input), and then encounters the western boundary.
Eventually, the water fed into the lower layer flows mostly along this path and later flows away to the equatorial region.
Conversely, in the upper layer, water is fed from the equator to the subtropics, and to the subpolar interior region through
the western boundary current. The water then circulates along the outer rim and is absorbed into the mass sink. The model
is controlled mainly by three nondimensional parameters: (1) the ratio of net mass input rate to the maximum Sverdrup transport
(Q/T
Sv
max
), which affects the inter-gyre communication by altering the paths of geostrophic contours, (2) the ratio of a mass input
rate into the lower layer to that in total (Q
2/Q), which controls the vertical structure of the inter-gyre flow, and (3) the measure of the wind forcing effect relative to
the β effect, which determines the horizontal extent of the area influenced by the mass input. The other parameter regimes with
respect to Q/T
Sv
max
and Q
2/Q are also presented. 相似文献
4.
5.
Tomohiro Nakamura Takahiro Toyoda Yoichi Ishikawa Toshiyuki Awaji 《Journal of Oceanography》2004,60(2):411-423
A numerical study using a 3-D nonhydrostatic model has been applied to baroclinic processes generated by the K
1 tidal flow in and around the Kuril Straits. The result shows that large-amplitude unsteady lee waves are generated and cause
intense diapycnal mixing all along the Kuril Island Chain to levels of a maximum diapycnal diffusivity exceeding 103 cm2s−1. Significant water transformation by the vigorous mixing in shallow regions produces the distinct density and potential vorticity
(PV) fronts along the Island Chain. The pinched-off eddies that arise and move away from the fronts have the ability to transport
a large amount of mixed water (∼14 Sv) to the offshore regions, roughly half being directed to the North Pacific. These features
are consistent with recent satellite imagery and in-situ observations, suggesting that diapycnal mixing within the vicinity
of the Kuril Islands has a greater impact than was previously supposed on the Okhotsk Sea and the North Pacific. To examine
this influence of tidal processes at the Kurils on circulations in the neighboring two basins, another numerical experiment
was conducted using an ocean general circulation model with inclusion of tidal mixing along the islands, which gives a better
representation of the Okhotsk Sea Mode Water than in the case without the tidal mixing. This is mainly attributed to the added
effect of a significant upward salt flux into the surface layer due to tidal mixing in the Kuril Straits, which is subsequently
transported to the interior region of the Okhotsk Sea. With a saline flux into the surface layer, cooling in winter in the
northern part of the Okhotsk Sea can produce heavier water and thus enhance subduction, which is capable of reproducing a
realistic Okhotsk Sea Mode Water. The associated low PV flux from the Kuril Straits to the open North Pacific excites the
2nd baroclinic-mode Kelvin and Rossby waves in addition to the 1st mode. Interestingly, the meridional overturning in the
North Pacific is strengthened as a result of the dynamical adjustment caused by these waves, leading to a more realistic reproduction
of the North Pacific Intermediate Water (NPIW) than in the case without tidal mixing. Accordingly, the joint effect of tidally-induced
transport and transformation dominating in the Kuril Straits and subsequent eddy-transport is considered to play an important
role in the ventilation of both the Okhotsk Sea and the North Pacific Ocean.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
6.
Kazunori Akitomo Kiyoshi Tanaka Toshiyuki Awaji Norihisa Imasato 《Journal of Oceanography》1995,51(2):171-185
We have investigated the fundamental processes of deep convection in a lake at high latitudes triggered by wind during spring or autumn and the associated deep water formation, executing vertically two-dimensional numerical experiments with a nonhydrostatic model. The water column in which a relatively cold mixed layer overlies a relatively warm layer becomes unstable, when the Ekman convergence on the shore due to along-shore wind deepens the mixed layer below the compensation depth, where water densities in both layers becomes equal to each other because of the thermobaric effect. At the onset of deep convection, the critical Rayleigh number agrees with that predicted by the linear theory. The onset time of deep convection is inversely proportional to the magnitude of wind stress. On the other hand, the onset time is minimal when water temperature in the mixed layer
m
is 3.1°C because a change of
m
has two effects oppositely acting on the stability of the water column. After the first onset, deep convection occurs intermittently for a few days. The sinking of the mixed layer water occurs in a thermal-like shape, and its amount is 4184% of the time-integrated Ekman transport when
m
3°C while it decreases to less than 10% for
m
lower than 1.5°C. The present process can explain 30% of the amount of deep water renewal which is expected from the observation in Lake Baikal. 相似文献
7.
Yoichi Ishikawa Toshiyuki Awaji Nobumasa Komori Takahiro Toyoda 《Journal of Oceanography》2004,60(2):293-301
A sensitivity experiment has been performed by assimilating altimetric data into a 1.5-layer primitive equation model as a
first attempt to examine the impact of initialization on forecasts of the Kuroshio path variability south of Japan. By exploiting
the advantage of an adjoint model, our approach clearly shows that a small meander off Shikoku Island has a large impact on
the prediction of meander growth in the Kuroshio region. Further, the strengthening of the Kuroshio current and its recirculation
clearly becomes an important factor in the development of the meander. These results demonstrate the effectiveness of our
assimilation approach in identifying efficient initialization schemes on numerical forecasting of the Kuroshio south of Japan
and should help in the construction of an effective observing system for improving the forecasting.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
8.
In order to better estimate meso-scale variabilities in the energetic Kuroshio Extension (KE) region, simultaneous assimilation of drifter-derived velocity data, together with satellite and in situ hydrographic data, is attempted by using a high-resolution 4-dimensional variational data assimilation (4D-VAR) system. Our experimental results, both with and without assimilation of drifter data (Exp. Drf and Exp. Ref, respectively) for the period during Aug–Oct 2005, show that the reproduced fields in Exp. Drf better reflect the observed meso-scale features such as the KE meandering jet and associated eddies. The adjoint sensitivity analysis indicates that our 4D-VAR system has the ability to provide a more realistic timeseries of the meandering jet structures that play a key role in the intergyre exchange between the subtropical and subarctic gyres in the North Pacific. In addition, the observed information from the surface drifters works to improve the subsurface structure. These results illustrate the advantage of our 4D-VAR simultaneous assimilation with the addition of drifter-derived surface velocity information. 相似文献
9.
10.
A Simple Method for Precise Determination of 23 Trace Elements in Granitic Rocks by ICP-MS after Lithium Tetraborate Fusion 总被引：1，自引：0，他引：1
Abstract. An improved alkali fusion method followed by HF-HNO3 -HC1O4 treatment is performed for simultaneous determination of 23 trace elements (Sr, Y, Zr, Nb, Ba, Hf, Ta, Th, U, and REE) by ICP-MS in rock reference materials: basaltic rocks (JB-2, JB-3) and granitic rocks (JG-la, JG-2, JG-3). Our improved method offers several advantages including: (1) suppression of whitish precipitates probably composed of insoluble fluorides by addition of HCIO4, (2) simple and reliable preparation procedure, (3) instrument calibration which enables straightforward simultaneous multi-elemental analysis, and (4) the very low background levels by using pure lithium tetraborate flux. We obtained the analytical results with a reproducibility of mostly <2 % (1) for the basaltic rocks and <7 % for the granitic rocks. The higher relative standard deviation (RSD) values for granitic rocks may be attributed to sample heterogeneity of coarse-grained granitic rocks. The analytical results of the granitic rocks demonstrate that Zr and Hf abundances are consistent with the compiled values and that REE concentrations agree well with recently published data, suggesting that the Li2 B4 O7 fusion method applied in the present study is suitable for the analysis of the granitic rocks. 相似文献