排序方式: 共有24条查询结果,搜索用时 15 毫秒
11.
Norihisa Usui Tsuyoshi Wakamatsu Yusuke Tanaka Nariaki Hirose Takahiro Toyoda Shiro Nishikawa Yosuke Fujii Yasushi Takatsuki Hiromichi Igarashi Haruka Nishikawa Yoichi Ishikawa Tsurane Kuragano Masafumi Kamachi 《Journal of Oceanography》2017,73(2):205-233
We produced a four-dimensional variational ocean re-analysis for the Western North Pacific over 30 years (FORA-WNP30). It is the first-ever dataset covering the western North Pacific over 3 decades at eddy-resolving resolution. The four-dimensional variational analysis scheme version of the Meteorological Research Institute Multivariate Ocean Variational Estimation system (MOVE-4DVAR) is employed to conduct a long-term reanalysis experiment during 1982–2012. After evaluating the basic performance of FORA-WNP30, the interannual to decadal variability is analyzed. Overall, FORA-WNP30 reproduces basic features in the western North Pacific well. One of outstanding features in FORA-WNP30 is that anomalous events such as the Kuroshio large meander and anomalous intrusion of the Oyashio in the 1980s, when there were no altimeter data, are successfully reproduced. FORA-WNP30 is therefore a valuable dataset for a variety of oceanographic research topics and potentially for related fields such as climate study, meteorology and fisheries. 相似文献
12.
含有开关参数化物理过程的模式变分资料同化的非光滑优化方法:几个理论问题 总被引:7,自引:0,他引:7
时间、空间上离散的模式如果含有带开关的参数化物理过程,某些对应的变分同化问题可以看成为非光滑优化问题。本系统地讨论了与此相关的几个理论问题。首先,利用次梯度这个非光滑优化中的基本概念可以对通常的伴随方程的解进行清楚的解释和定义。利用一个带对流调整的多层扩散模式,演示了通常的伴随方程的解在奇异点处不是由代价函数的Gateaux导数构成,而是代价函数的一个次梯度。其次,在非光滑优化的框架下对现有的解决这类问题的方法进行了评述。这些方法包括:(1)利用光滑优化方法和通常的伴随模式,并且讨论了这个方法的收敛条件;(2)正则化方法。这个方法是把模式中不光滑的项用光滑的项近似,以把问题变为光滑优化问题。(3)次梯度方法。这个方法利用通常的伴随模式来计算一个次梯度,是收敛的算法,不过收敛速度很慢。(4)非光滑优化中的Bundle方法。这个方法比次梯度方法收敛得快,不过需要利用所有的次梯度信息。由于计算所有的次梯度十分困难,现有的Bundle方法大都进行了一些改变,仅需要用户能够计算出一个次梯度。不过这个改变是无奈的,多个次梯度的信息可以提高Bundle方法的效率。本最主要的工作是提出了集值伴随方程的概念。集值伴随模式可以在奇异点处计算出所有支撑次梯度,因此利用集值伴随方程有可能提出基于Bundle方法的更好算法。 相似文献
13.
Atsuhiko Isobe Masafumi Kamachi Yukio Masumoto Hiroshi Uchida Tsurane Kuragano 《Journal of Oceanography》2004,60(2):321-328
The seasonal variation of the Kuroshio transport south of Japan has been investigated using the results of an assimilation
model. Annual and semiannual variations of the transport and dynamic depth anomaly are reconstructed by CEOF (complex orthogonal
empirical function) analysis. In the basin west of the Izu-Ogasawara Ridge, the annual component of the variation propagates
westward with the phase speed of the long Rossby wave associated with the first baroclinic mode. The variation also shows
a similar tendency to that reproduced in a wind-driven, two-layer model with a ridge. This suggests that the annual variation
revealed in the assimilation model is associated with the baroclinic first mode of motion excited above the Izu-Ogasawara
Ridge. Furthermore, it is found that both the semiannual component and the annual component are important members determining
the seasonal variation of the Kuroshio transport south of Japan. The semiannual component is revealed as a double gyre pattern
in the basin west of the Izu-Ogasawara Ridge.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
14.
Toshiya?NakanoEmail author Ikuo?Kaneko Masahiro?Endoh Masafumi?Kamachi 《Journal of Oceanography》2005,61(4):681-697
We investigated the variation of the North Pacific Intermediate Water (NPIW) distribution in the western North Pacific, focusing on the intermediate salinity minimum (S < 34.2) core observed along the meridional hydrographic sections including the 137°E repeat section by the Japan Meteorological Agency. This core is a cross-section of a low salinity tongue extending westward along the recirculation in the subtropical gyre. The core size shows remarkable variabilities in interannual and decadal time scales. The salinity change in the density layer during the period of core expansion (shrinking) represents the spatial salinity change in the tongue toward the west (east). Thus, we conclude that the core size variation is associated with the zonal wobble of the tongue having thicker distribution to the east, rather than temporal changes of the water mass itself. The core size at 137°E is well correlated with the meridional gradient of the depth in the isopycnal surface at the salinity minimum representing the recirculation intensity, suggesting a relation with the intensity of the subtropical gyre. A significant lag-correlation between the gradient and the wind forcing over the North Pacific suggests that the first mode baroclinic Rossby waves excited in the central North Pacific propagated westward to change the intensity of the recirculation in interannual time scales. In decadal time scales, it is found that the wind stress curl and heat flux fields in the North Pacific precede the recirculation by about 11 years. 相似文献
15.
Yosuke?FujiiEmail author Shiro?Ishizaki Masafumi?Kamachi 《Journal of Oceanography》2005,61(4):655-662
Two kinds of nonlinear constraints, not previously studied in oceanography, have been adopted with the Preconditioned Optimizing Utility for Large-dimensional analyses (POpULar) in a three-dimensional oceanic variational analysis in the equatorial Pacific. One is the constraint for the variational Quality Control (QC) procedure and the other is used to avoid density and temperature inversions. Estimation of the large heat content anomaly in the upper ocean related to El Nino and La Nina phenomena is improved with the variational QC. For example, it prevents unusual but correct observation data on the thermocline deepening in the 1997/98 El Nino from being ignored. As a result, it improves the temperature field estimation in the eastern equatorial Pacific. The constraint for avoiding inversions prevents the low salinity layer at the surface and the barrier layer in the eastern equatorial Pacific in the El Nino period from being destroyed by the convective adjustment procedure performed after minimizing the cost function. Incorporating nonlinear constraints in variational analyses is thus a strong candidate for increasing the accuracy of analysis. 相似文献
16.
Operational Data Assimilation System for the Kuroshio South of Japan: Reanalysis and Validation 总被引:3,自引:5,他引:3
Masafumi Kamachi Tsurane Kuragano Hiroshi Ichikawa Hirohiko Nakamura Ayako Nishina Atsuhiko Isobe Daisuke Ambe Masazumi Arai Noriaki Gohda Satoshi Sugimoto Kumi Yoshita Toshiyuki Sakurai Francesco Uboldi 《Journal of Oceanography》2004,60(2):303-312
We describe an operational ocean data assimilation system for the Kuroshio and its validation using a nine-year reanalysis
(historical run from 1993 to 2001) dataset of upper-ocean state estimation in the North Pacific. The horizontal structure
of volume transport of the Ryukyu Current System (RCS) is shown from the reanalysis: The RCS is connected to the flow of the
subtropical gyre, and its volume transport gradually increases from south-east of Okinawa (5–10 Sv) to the east of Amami-Ohshima
Island (20 Sv). Comparing the reanalysis with independent observations on the southeast slope of the Amami-Ohshima Island
indicates that the root mean square differences (RMSDs) are 0.076 (0.037) m/s in the period of December 1998 to November 1999
(November 1999 to November 2000) respectively. The reanalysis field has a bias (3.1 Sv) of the volume transport of the RCS
and the RMSD (3.5 Sv) which is larger than the observed variability (2.81 Sv). Surface velocity and the Kuroshio axis south
of Japan are also examined. Comparison of the reanalysis and ADCP data gave maximum RMSD of 0.749 (0.271) m/s in the strong
(weak) current regions, respectively. The annual mean value of the axis error is 19 km in 1998. The RMSD of the error is at
most 50 km, in 294 cases in the observation period, which is smaller than the observed root mean square variability of the
axis (64 km).
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
17.
Short-Range Prediction Experiments with Operational Data Assimilation System for the Kuroshio South of Japan 总被引:3,自引:3,他引:3
Masafumi Kamachi Tsurane Kuragano Satoshi Sugimoto Kumi Yoshita Toshiyuki Sakurai Toshiya Nakano Norihisa Usui Francesco Uboldi 《Journal of Oceanography》2004,60(2):269-282
The short-range (one month) variability of the Kuroshio path was predicted in 84 experiments (90-day predictions) using a
model in an operational data assimilation system based on data from 1993 to 1999. The predictions started from an initial
condition or members of a set of initial conditions, obtained in a reanalysis experiment. The predictions represent the transition
from straight to meander of the Kuroshio path, and the results have been analyzed according to previously proposed mechanisms
of the transition with eddy propagation and interaction acting as a trigger of the meander and self-sustained oscillation.
The reanalysis shows that the meander evolves due to eddy activity. Simulation (no assimilation) shows no meander state, even
with the same atmospheric forcing as the prediction. It is suggested therefore that the initial condition contains information
on the meander and the system can represent the evolution. Mean (standard deviation) values of the axis error for all 84 cases
are 13, 17, and 20 (10, 10, and 12) km, in 138.5°E, in the 30-, 60-, and 90-day predictions respectively. The observed mean
deviation from seasonal variation is 30 km. The predictive limit of the system is thus about 80 days. The time scale of the
limit depends on which stage in the transition is adopted as the initial condition. The gradual decrease of the amplitude
in a stage from meander to straight paths is also predicted. The predictive limit is about 20 days, which is shorter than
the prediction of the opposite transition.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
18.
Horizontal and meridional volume transports on timescales from intra-seasonal to interannual in the North Pacific subarctic
region were investigated using a reanalysis dataset for 1993–2001 that was constructed from an assimilation of the TOPEX altimeter
and in situ data into an eddy-permitting North Pacific ocean general circulation model. The barotropic flow is excited along east of
the Emperor Seamounts by the western intensification dynamics. The volume transport of this flow compensates for that across
the interior region east of the Seamounts below the summit depth of the Seamounts. The Oyashio, which is also considered as
a compensation flow for the transport in the whole interior region, includes baroclinic as well as barotropic components.
Baroclinic transports in the whole interior region exceed those in the western boundary region in the upper (200–1000 m) and
lower (2000–5000 m) layers, and the total transport is northward (southward) in the upper (lower) layer. These excesses of
the baroclinic transport are balanced by a vertical transport of the meridional overturn. The meridional overturn has a complementary
relation to the basin-scale baroclinic circulation in the North Pacific subactic region.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
19.
This short survey presents several research and operational developments of ocean data assimilation in the tropical Pacific,
primarily for climate-scale phenomena. Aspects of theoretical error estimations, diagnostics and practical reduced space techniques
are also briefly reported.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献