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1.
The sea surface dynamic topography (the sea surface height relative to the geoid; hereafter abbreviated SSDT) can be divided into the temporal mean SSDT and the fluctuation SSDT around the mean. We use the optimal interpolation method to reduce the satellite radial orbit error and estimate the fluctuation SSDT southeast of Japan from Seasat altimetry data during the 17-day near-repeat mission. The fluctuation SSDT is further combined with the mean geopotential anomalies estimated from hydrographic data during the Seasat mission in order to give the approximated total SSDT, called here the composite SSDT (the approximated mean plus fluctuation SSDT's). The fluctuation SSDT is in accord with the low-frequency sea-level fluctuation recorded at tide gauge stations in the Japanese islands. The composite SSDT describes thoroughly variations of the location of the Kuroshio axis south of Japan determined on the basis of the GEK (Geomagnetic Electro-Kinematograph) surface velocities and the horizontal temperature distribution. The composite SSDT also agrees with oceanic variations east of Japan found in the temperature distribution at the depth of 200 m. These results confirm that the SSDT derived from altimetry data can provide fairly precise synoptic views of low-frequency oceanic phenomena. 相似文献
2.
3.
Optimal interpolation method is applied to Geosat altimetry data both to remove orbit error and to separate temporal mean sea surface dynamic topography (SSDT) from temporal fluctuations around the mean. Loss of long-wavelength oceanic signals at orbit error reduction procedure is smaller in this method than that in conventional collinear methods, but the areal average height over the study domain is still removed as the orbit error. The fluctuation SSDT is quantitatively evaluated by sea level data from tide gauge stations at Japanese islands. The correlation coefficient of the two sea-level variations is 0.83 when the loss of the areal average is compensated by the seasonal variation of the areal average height determined from the climatological monthly-mean SSDT. In addition, the improvement of the geoid model by combined use of Seasat altimetry data and hydrographic data is validated through the estimated temporal mean SSDT. In a local area where hydrographic data contemporary with the Seasat mission exist, the geoid model has been significantly improved so that the absolute SSDT can be determined from combination of the altimetry data and geoid model; the absolute SSDT describes the onset event of a quasi-stationary large meander of the Kuroshio south of Japan very well. Outside this local area, however, errors of several tens of centimeters still remain in the improved geoid model. 相似文献
4.
Surface velocities determined from trajectory of a drifting buoy from March through November 1987 are compared with surface geostrophic velocities determined from sea surface dynamic topography (SSDT) obtained from altimetry data with the aid of long-term hydrographic observation data. In general, these velocities show similar temporal variations in both zonal and meridional components, except in a period when obvious error is found in the altimetric SSDT field. When the buoy was trapped by several mid-ocean meso-scale eddies, the comparison is especially good. Systematic discrepancy is found, however, when the buoy was in the Kuroshio region, because of using both temporally and spatially smoothed mean SSDT estimated from hydrographic data; instead, surface geostrophic velocities determined from the altimetric SSDT referred to the improved geoid model result in better comparison. 相似文献
5.
Koji Kakinoki Shiro Imawaki Hiroshi Uchida Hirohiko Nakamura Kaoru Ichikawa Shin-Ichiro Umatani Ayako Nishina Hiroshi Ichikawa Mark Wimbush 《Journal of Oceanography》2008,64(3):373-384
Two inverted echo sounders were maintained on coastal and offshore sides of the Kuroshio south of Japan from October 1993
to July 2004. Applying the gravest empirical mode method, we obtained a time series of geostrophic transport. Estimated transports
generally agree well with geostrophic transports estimated from hydrography. Their agreement with the hydrographic transports
is better than that of transports estimated from satellite altimetry data. The geostrophic transport is expressed as the surface
transport per unit depth multiplied by the equivalent depth. The geostrophic transport varies mostly with the surface transport
and fractionally with the equivalent depth. Seasonal variation of the geostrophic transport has a minimum in March and a maximum
in September, with a range of about one fifth of the total transport. 相似文献
6.
Motohiko Kashima Shiro Imawaki Shin-Ichiro Umatani Hiroshi Uchida Yuji Hashibe Hiroshi Ichikawa Masao Fukasawa 《Journal of Oceanography》2003,59(3):291-301
Theoretically, the geostrophic approximation holds for the low-frequency flow field, but no detailed examination has been
done on how well the estimated geostrophic velocity corresponds with the observed velocity. Intensive surveys were carried
out during 1993–1995 in the Kuroshio and its recirculation regions south of Shikoku, Japan, including repeated hydrographic
surveys and direct current measurements at nominal depths of 700, 1500 and 3000 m. For these depth intervals, vertical differences
of estimated geostrophic velocity are compared with those of observed velocity. For the intermediate layer (between 700 and
1500 m depths), the slope of the regression line is 0.99, correlation coefficient is 0.98, and the root-mean-square of difference
from geostrophic balance is 2.8 cm/s which is close to the estimated error of 2.1 cm/s. For the deep layer (between 1500 and
3000 m depths), the corresponding values are 0.82, 0.93, 1.2 cm/s and 2.0 cm/s, respectively. The results indicate that the
estimated geostrophic velocity compares well with the observed velocity in these regions.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
7.
The bottom layer in Osaka Bay was occupied in August from 1980 to 1995 by a water mass of relatively low temperature and rich
in nutrients in comparison to previous and following decades. The relationship between Osaka Bay bottom temperature and Kuroshio
axis location south of Kii Peninsula has been investigated using the dataset archived by Osaka Research Institute of Environment
of Agriculture and Fisheries and axis-location data provided by Marine Information Research Center. The correlation between
bottom temperature in the bay and Kuroshio-axis distance from Cape Shionomisaki indicates that the bottom temperature in the
bay decreases when the Kuroshio axis is a long distance from the cape, and vice versa. To investigate why the temperature
distribution depends on the axis location, composite temperature maps are depicted using summer temperature data from 1970
to 2005 archived in the Japan Oceanographic Data Center (JODC) after dividing all data into two groups with different axis
locations. These temperature maps and temperature-salinity plots using the same JODC data suggest a scenario: cold water in
the Kuroshio intermediate layer is first upwelled on the eastern side of the Kii Peninsula and thereafter moves westward as
a coastal boundary current in the Kelvin wave sense of the Northern Hemisphere when the Kuroshio axis is located around 74-km
distance from Cape Shionomisaki. This scenario is validated using internal Froude number maps depicted using the JODC-archived
hydrographic data and geostrophic current fields. In addition, the reanalysis daily data provided by Japan Coastal Ocean Predictability
Experiment are used for the validation. 相似文献
8.
Kaoru Ichikawa 《Journal of Oceanography》2001,57(1):55-68
Temporal variations of the Kuroshio volume transport in the Tokara Strait and at the ASUKA line are decomposed by phase-propagating Complex EOF modes of high-resolution sea surface dynamic topography (SSDT) field during the first tandem period of TOPEX/POSEIDON and ERS-1 (from October 1992 to December 1993). Both variations are dominated by a mode with nearly semi-annual cycle, which indicates a series of interactions between the Kuroshio and meso-scale eddies. Namely, northern part of a westward-propagating meso-scale eddy at 23°N is captured into the southern side of the Kuroshio at the south of Okinawa, then it moves downstream along the Kuroshio path passing the Tokara Strait, and reaches to the ASUKA line where it merges with another eddy propagating from the east at 30°N. The variation at the ASUKA line is, however, less dominated by this mode; instead, it includes the SSDT variations in the south of Shikoku and the east of Kyushu which would be directly affected by eddies from the east without passing the Tokara Strait. On the other hand, the same analysis for movements of the Kuroshio axis in the Strait indicates that they are governed by short-term variations locally confined to the Kuroshio in the East China Sea without being induced by meso-scale eddies. This results, however, seem to depend strongly on a time scale of interest. It is suggested that the long-term movements of the Kuroshio axis in the Strait would demonstrate coincidence with SSDT variation in the south of Japan. 相似文献
9.
Masaki Kawabe 《Journal of Oceanography》1980,36(4):227-235
The sea level difference between Naze and Nishinoomote and sea level anomalies (the residuals after removal of seasonal variations) around the Nansei Islands were examined in relation to the large meander in the Kuroshio south of central Japan. They are indices of surface velocity and geostrophic transport of the Kuroshio in the Tokara Strait and in the East China Sea, respectively. All of them were large during the meandering period, and each of them reached a maximum before or after the generation of the large meander in 1975. Thus the surface velocity and the geostrophic transport of the Kuroshio in the Tokara Strait and the East China Sea were large during the meandering period. The sea level difference between Naze and Nishinoomote (or Makurazaki) shows that the surface velocity and geostrophic transport in the Tokara Strait were significantly larger during the extinction stage in 1963 and during the generation stage in 1975 and were correlated with the position of the Kuroshio east of Kyûshû in 1974 and 1975 before the generation of the large meander.The surface velocity of the Kuroshio southeast of Yakushima (E-line) based on dynamic calculation referred to 1,000 db was weak during the meandering period, and was out of phase with the variation of surface velocity in the Tokara Strait monitored by tide gauge data. The analysis of GEK and hydrographic data shows that southwestward flow existed below 600 m in the slope region on the E-line and weakened during the meandering period. Thus, the out-of-phase variation in surface velocity mentioned above seems to be partly explained by the variation in velocity on the reference level at the E-line. 相似文献
10.
由于卫星高度计数据分辨率高、观测范围广的特点，我们使用该数据开展了黑潮流的研究。在之前的研究中，卫星绝对地转流都被用于对黑潮流域的表层流场的时空变化特征进行研究，并采用了一些探测方法提取了黑潮流轴和流路。然而，海面绝对地转流是由绝对动力地形估计得到，应该被当做实际流场的地转分量，在实际应用中并不能代表真实流场。在本研究中，建立了气候态绝对地转流与网格平均的漂流浮标流场间的数学校验关系，以此对卫星绝对地转流场进行修正，即便这两种数据的性质存在些许偏差。因此，基于主成分探测法，修正后的卫星绝对地转流被用于探测黑潮流轴和流路。对比结果表明，由修正后的卫星地转流场探测得到的黑潮流轴和流路均要好于地转流和表层流估计结果。修正后的地转流有助于开展更加准确的黑潮流轴和流路的逐日探测。 相似文献
11.
Estimating the Kuroshio Axis South of Japan Using Combination of Satellite Altimetry and Drifting Buoys 总被引：3，自引：4，他引：3
Daisuke Ambe Shiro Imawaki Hiroshi Uchida Kaoru Ichikawa 《Journal of Oceanography》2004,60(2):375-382
By tracking the locally strongest part of the sea-surface velocity field, which was obtained by integrating data of satellite
altimeters and surface drifting buoys, we extracted the Kuroshio axis south of Japan every 10 days from October 1992 to December
2000. The obtained axes clearly express the effect of the bottom topography; three modes were observed when the Kuroshio ran
over the Izu Ridge. The axis was very stable to the south of ‘Tosa-bae,’ off the Kii Channel. Mean current speed at the Kuroshio
axis gradually increased from 0.65 m/s south of Kyushu to 1.45 m/s off Enshu-nada.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
12.
A method has been developed to monitor the surface velocity field by combining repeated acoustic Doppler current profiler
(ADCP) observations and satellite altimetry data. The geostrophic velocity anomaly is calculated from the sea surface height
anomaly field estimated from the altimetry data by an optimal interpolation. It has been confirmed that this accurately observes
the smoothed velocity anomaly field when the interpolation scales are set according to the spatio-temporal sampling pattern
of the altimeter used. The velocity anomaly obtained from the altimetry data is subtracted from the repeated ADCP observations
to estimate temporal mean velocity along the ship tracks. Regularly sampled, nine-year time series of surface velocity can
then be obtained by adding the computed mean velocity and the altimetry anomaly components. This clearly illustrates surface
velocity fluctuations such as the movement of the Kuroshio axis due to its meandering and an increase of the interannual variability
of the Subtropical Countercurrent toward its downstream region.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
13.
Comparison of Kuroshio surface velocities derived from satellite altimeter and drifting buoy data 总被引：2，自引：0，他引：2
Sea-surface geostrophic velocities for the Kuroshio region calculated from TOPEX/POSEIDON altimetry data together within situ oceanographic data are compared with surface velocities derived from drifting buoy trajectories. The geostrophic velocities
agree well with the observed velocities, suggesting that the Kuroshio surface layer is essentially in geostrophic balance,
within measurement error. The comparison is improved a little when the centrifugal acceleration is taken into account. The
observed velocities are divided into the temporal mean and fluctuation components, and the partitioning of velocities between
these two components is examined. For the Kuroshio region, most of the fluctuation components of the velocities derived from
drifting buoys are found to be positive. This result suggests that Eulerian mean velocities for the Kuroshio region estimated
from drifting buoy data tend to be larger than actual means, due to the buoy’s tendency to sample preferentially in the high-velocity
Kuroshio. 相似文献
14.
Velocity structures and transports of the Kuroshio and the Ryukyu current during fall of 2000 estimated by an inverse technique 总被引：7，自引：0，他引：7
An inverse calculation using hydrographic section data collected from October to December 2000 yields velocity structure and
transports of the Kuroshio in the Okinawa Trough region of the East China Sea (ECS) and south of central Japan, and of the
Ryukyu Current (RC) southeast of the Ryukyu Islands. The results show the Kuroshio flowing from the ECS, through the Tokara
Strait (TK), with a subsurface maximum velocity of 89 cm s−1 at 460 dbar. In a section (TI) southeast of Kyushu, a subsurface maximum velocity of 92 cm s−1 at 250 dbar is found. The results also show the RC flowing over the continental slope from the region southeast of Okinawa
(OS) to the region east of Amami-Ohshima (AE) with a subsurface maximum velocity of 67 cm s−1 at 400 dbar, before joining the Kuroshio southeast of Kyushu (TI). The volume transport around the subsurface velocity maximum
southeast of Kyushu (TI) balances well with the sum of those in TK and AE. The temperature-salinity relationships found around
these velocity cores are very similar, indicating that the same water mass is involved. These results help demonstrate the
joining of the RC with the Kuroshio southeast of Kyushu. The net volume transport of the Kuroshio south of central Japan is
estimated to be 64∼79 Sv (1 Sv ≡ 106 m3s−1), of which 27 Sv are supplied by the Kuroshio from the ECS and 13 Sv are supplied by the RC from OS. The balance (about 24∼39
Sv) is presumably supplied by the Kuroshio recirculation south of Shikoku, Japan. 相似文献
15.
Hiroshi Kuroda Manabu Shimizu Yuuichi Hirota Hideki Akiyama 《Journal of Oceanography》2007,63(5):849-862
Through analysis of monthly in situ hydrographic, tide gauge, altimetry and Kuroshio axis data for the years 1993–2001, the intraannual variability of sea level
around Tosa Bay, Japan, with periods of 2–12 months is examined together with the intraannual variability of the Kuroshio
south of the bay. It is shown that the intraannual variation of steric height on the slope in Tosa Bay can account for that
of sea level at the coast around the bay as well as on this slope. It is found that the steric height (or sea level) variation
on the slope in this bay is mainly controlled by the subsurface thermal variation correlated with the Kuroshio variation off
Cape Ashizuri, the western edge of Tosa Bay. That is, when the nearshore Kuroshio velocity south of the cape is intensified
[weakened] concurrently with the northward [southward] displacement of the current axis, temperature in an entire water column
decreases [increases] simultaneously, mainly due to the upward [downward] displacement of isotherms, coincident with that
of the main thermocline. It follows that the steric height (or sea level) decreases [increases]. 相似文献
16.
Hiroshi Kuroda Manabu Shimizu Yuuichi Hirota Daisuke Ambe Hideki Akiyama 《Journal of Oceanography》2008,64(1):81-91
In order to specify a vertical thermal structure related to surface current variation on the continental slope in Tosa Bay,
Japan, we analyzed monthly regular hydrographic measurements in the years 1991–2004. Subsurface temperature below 200 m on
the slope was found to vary synchronously with the vertical displacement of the main thermocline around 200 m. It is shown
that the vertical-averaged temperature below 200 m is significantly correlated with an along-isobath/southwestward surface
current velocity on the slope. This correlation indicates that when a strong (weak) southwestward surface current is observed,
temperature below 200 m decreases (increases) simultaneously, that is, isotherms below the 200 m are displaced upward (downward)
together with the main thermocline. Moreover, when the strong southwestward flow is detected, across-isobath isotherms around
200 m slope upward toward the offshore direction. Furthermore, it is suggested that as the Kuroshio axis moves offshore south
of the bay, the southwestward flow tends to be weakened by the combined effect of other Kuroshio parameters such as transport
and stream width as well as the Kuroshio axis position. As a result, it is inferred that the correlation between the surface
current and subsurface temperature can be interpreted in terms of the formation and decay of an anticlockwise circulation
interacting with a cold eddy. 相似文献
17.
Kazuyuki Otsuka 《Journal of Oceanography》1985,41(6):441-451
Characteristics of the Kuroshio in the vicinity of the Izu Islands are analyzed by using hydrographic observations collected from 1938 to 1980 mostly by the Hydrographic Department, Maritime Safety Agency, Japan and by the Japan Meteorological Agency.It is shown that flow characteristics change as the Kuroshio approaches the Izu Ridge. 1) The position of the 10°C isotherm at the 500 m level, which is, an index of the position of the temperature front or of the current axis of the Kuroshio, is highly variable near the ridge. 2) The horizontal temperature gradient across the Kuroshio estimated from the positions of the 8 and 12°C isotherms across the Kuroshio decreases considerably near the Izu Ridge. 3) The horizontal distance between the 15°C isotherm at the 200 m level and 10°C isotherm at the 500 m level becomes larger over the ridge. The 15°C isotherm at the 200 m level is an index of the current axis at the sea surface. This implies that the inclination of the Kuroshio axis becomes larger over the ridge. 相似文献
18.
Hirohiko Nakamura Ayako Nishina Kotaro Tabata Masataka Higashi Akimasa Habano Toru Yamashiro 《Journal of Oceanography》2012,68(2):321-336
A time series of surface geostrophic velocity is developed using satellite altimetry data during 1992–2010 for a track across
the Kuroshio southeast of Kyushu, Japan. The temporal mean geostrophic velocity is estimated by combining the along-track
sea level anomaly and shipboard ADCP data. This approximately 6-km resolution dataset is successful in representing the Kuroshio
cross-current structure and temporal variation of the Kuroshio current-axis position during 2000–2010. The authors use this
dataset to examine the winter Kuroshio path destabilization phenomenon. Its seasonal features are characterized as follows:
the velocity shear on the inshore side of the Kuroshio becomes stronger and the Kuroshio path state becomes unstable from
the summer to winter. This evidence is consistent with the hypothetical mechanism governing the destabilization phenomenon
discussed in a previous study. Furthermore, the interannual amplitude modulation of the seasonality is examined in relation
to interannual variations in the winter northerly wind over the northern Okinawa Trough and the Pacific Decadal Oscillation
(PDO) index. The destabilization phenomenon appears 15 times in the period 2000–2010. Ten cases are related to local wind
effects, and 7 of these are also connected with the PDO index. This is probably because the winter northerly wind over the
northern Okinawa Trough is regulated by the PDO signal in interannual time-scales. Only 4 cases are related to the PDO index,
but their driving mechanism remains uncertain. 相似文献
19.
Uncertainty Reduction of Estimated Geostrophic Volume Transports with Altimeter Observations East of Taiwan 总被引：1，自引：0，他引：1
The common geostrophic estimation of ocean current velocity uses only water temperature and conductivity profiles. The geostrophic
volume transport of a western boundary current, like the Taiwan Current (Kuroshio east of Taiwan), between the coast and its
eastern boundary can be easily estimated based on hydrographic survey data. But the eastern boundary of the Taiwan Current
is very uncertain due to extremely variable hydrographic conditions. This uncertainty is strongly correlated with the propagating
mesoscale eddies originating from the interior of the western North Pacific Ocean. The uncertainty of estimated transport
can be greatly reduced if eddy distribution is considered when determining the integration boundaries with the assistance
of satellite altimeter measurements. Eight hydrographic surveys east of Taiwan between November 1992 and June 1996 are demonstrated
in this study. The average geostrophic transport of the Taiwan Current with a reference set to 1000 dbar at 22°N between the
east coast of Taiwan and 124°E is 22.9 ±14.2 Sv and changes to 22.1 ± 8.3 Sv, the uncertainty of which is nearly halved after
taking account of the eddy distribution. The estimation uncertainty is insensitive to vertical displacements of the reference
level within the depth range between 800 and 2000 dbar.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
20.
Sea surface temperature (SST) has been measured in the south of Japan using a thermometer set up in the ferry boat to investigate the characteristics of the warm water intrudes into the coastal areas from the Kuroshio. Time series analysis was applied to the SST data with satellite images and hydrographic observation data from April 1987 to September 1989. The results indicate that the warm Kuroshio water intruded into the coastal areas on the Enshu-nada and the Kumano-nada Seas intermittently with periods of about 50 and 20 days associated with the fluctuation of the Kuroshio path and the Kuroshio frontal disturbance respectively. The intrusion with a 50-day period was dominant when the Kuroshio took a stationary small meander path (B- and C-types). The warm water spread to the west at 20 cm s–1, and was estimated to have a depth of 150 m at least and supply enough heat to make up the loss due to the evaporation in the coastal area. During the straight path of the Kuroshio, it was detected that the warm water intruded into coastal areas only with a 20-day period. The warm water that intrudes with a period of 20 days spreads to the west at 25 cm s–1 in a small scale. 相似文献