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991.
To examine whether the regime shift in 1998 that has been variously reported to have occurred in the oceanographic conditions
of the central and eastern North Pacific also occurred in the Oyashio region, western North Pacific, we compared data over
the period 1990–2003. Oceanographic conditions were compared before 1997 with those after 1998, using the A-line dataset (1990–2003) obtained by the oceanographic surveys of the Hokkaido National Fisheries Research Institute, Fisheries
Research Agency (HNFRI/FRA). Seasonal changes of the monthly-mean SST (as temperature in the surface layer) show a significant
increase in spring after 1998. After 1998, the mean concentration of chlorophyll a at the surface was higher in spring than that before 1997. This was more remarkable in the main current of the Oyashio. These
changes suggest that the spring phytoplankton bloom in the Oyashio region after 1998 was larger in magnitude and initiated
earlier. Consumption of nutrients during the spring bloom and standing stock of netplankton also shows a distinct difference
between the time period before 1997 and after 1998. These results support the occurrence of the regime shift around 1998 in
the Oyashio region. The changes of hydrographical conditions accompanying with the 1998 regime shift are discussed. The hydrographic
mechanism of enhancement of primary productivity during the spring phytoplankton bloom was not fully clarified, though. Results
in this study may support the usefulness of the A-line dataset for analysis of long-term variability in the western subarctic Pacific. 相似文献
992.
Organic Matter Derived from a Seagrass Meadow: Origin, Properties, and Quality of Particles 总被引:1,自引:0,他引:1
Abstract. An investigation of the properties of particles found in wrack beds and suspended in the water body of a Posidonia oceanica system led to the following conclusion: except for the period of the leaf fall and at times of strong water movement (e. g. winter storms), the majority of the particles in the water show no similarity with seagrass particles. This is based on the comparison of C: N ratios and SEM inspection. Information on the magnitude of DOC fluxes in this system led to the formulation of the phase shift hypothesis from DOM to POM. Because wrack particles were shown to have relatively fast settling rates, it is expected that processes like adsorption onto inorganic particles, micellation and aggregate formation have more importance in regulating particle density in the water body than the resuspension of directly fractionated Posidonia debris. The implication of these findings are discussed and the importance of the Mediterranean seagrass P. oceanica as an energy source supporting secondary production in adjacent ecosystems is questioned. 相似文献
993.
Numerical solutions for the hydroelastic problems of bodies are studied directly in the time domain using Neumann–Kelvin formulation. In the hydrodynamic part of problem, the exact initial boundary value problem is linearized using the free stream as a basis flow, replaced by the boundary integral equation applying Green theorem over the transient free surface Green function. The resultant boundary integral equation is discretized using quadrilateral elements over which the value of the potential is assumed to be constant and solved using the trapezoidal rule to integrate the memory or convolution part in time. In the structure part of the problem, the finite element method is used to solve the hydroelastic problem. The Mindlin plate as a bending element, which includes transverse shear effect and rotary inertia effect are used. The present numerical results show acceptable agreement with experimental, analytical, and other published numerical results. 相似文献
994.
On the evaluation of time-domain Green function 总被引:1,自引:0,他引:1
An analytical method has been developed to evaluate the wave part of the time-domain Green function and its derivatives. Based on Taylor series expansion, the Green function is obtained by solving a fourth-order ordinary differential equation. The method accelerates the convergence of the summation of an infinite series in the numerical computation. The accuracy of this method was demonstrated by its comparison with other method and its application to solve the radiation problem of a floating hemisphere using a panel-free method. The computed hydrodynamic coefficients agree well with the analytical solutions. 相似文献
995.
西太平洋暖池热含量时频特征的区域性 总被引:2,自引:0,他引:2
根据1955-1999年间的JEDAC月平均海温资料,利用Morlet小波变换方法,对西太平洋暖池东、西代表区热含量的时(间)-频(率)特征进行了分析,并用交叉谱分析方法探讨了两海区热含量间的耦合振荡,结果表明,暖池热含量的时频特征具有明显的区域性,即暖池东、西区热含量的年际和年代际变化都是反相的,且其年际变化的时频特征有较大差异,暖池东、西区热含量间主要存在着年际(43个月)和年代际(172个月)时间尺度的反位相耦合振荡。 相似文献
996.
997.
This paper investigates latching type control on a floating wave energy converter in deep water. An on-board, actively controlled motion-compensated platform is used as a reference (‘active reference’) for power absorption and latching. A variational formulation is used to evaluate an optimal control sequence in the time domain. Time domain simulation results are presented for a heaving buoy in small-amplitude waves. Results are compared with an equivalent system where latching and power absorption are from a sea-bottom-fixed reference. 相似文献
998.
The Pacific Decadal Oscillation 总被引:60,自引:1,他引:60
The Pacific Decadal Oscillation (PDO) has been described by some as a long-lived El Niño-like pattern of Pacific climate variability, and by others as a blend of two sometimes independent modes having distinct spatial and temporal characteristics of North Pacific sea surface temperature (SST) variability. A growing body of evidence highlights a strong tendency for PDO impacts in the Southern Hemisphere, with important surface climate anomalies over the mid-latitude South Pacific Ocean, Australia and South America. Several independent studies find evidence for just two full PDO cycles in the past century: “cool” PDO regimes prevailed from 1890–1924 and again from 1947–1976, while “warm” PDO regimes dominated from 1925–1946 and from 1977 through (at least) the mid-1990's. Interdecadal changes in Pacific climate have widespread impacts on natural systems, including water resources in the Americas and many marine fisheries in the North Pacific. Tree-ring and Pacific coral based climate reconstructions suggest that PDO variations—at a range of varying time scales—can be traced back to at least 1600, although there are important differences between different proxy reconstructions. While 20th Century PDO fluctuations were most energetic in two general periodicities—one from 15-to-25 years, and the other from 50-to-70 years—the mechanisms causing PDO variability remain unclear. To date, there is little in the way of observational evidence to support a mid-latitude coupled air-sea interaction for PDO, though there are several well-understood mechanisms that promote multi-year persistence in North Pacific upper ocean temperature anomalies. 相似文献
999.
We analysed mixed-layer seasonal and interannual variability in phytoplankton biomass and macronutrient (NO3 and Si(OH)4) concentrations from three decades of observations, and nitrogen uptake rates from the 1990s along Line P in the NE subarctic Pacific. Chlorophyll a concentrations near 0.35 mg m−3 were observed year-round along Line P except at the nearshore station (P4) where chlorophyll a concentrations in spring were on average 2.4 times the winter values. In contrast, the temporal variability in carbon-to-chlorophyll ratios at the two main end members of Line P (P4 and OSP) was high. Large seasonal and interannual variability in NO3 and Si(OH)4 concentration were observed along Line P. Highest upper mixed-layer (top 15 m) nutrient concentrations occurred on the continental shelf in late summer and early fall due to seasonal coastal upwelling. Beyond the shelf, maximum nutrient concentrations increased gradually offshore, and were highest in late winter and early spring due to mixing by winter storms. Interannual variations in upper mixed-layer nutrient concentrations beyond the shelf (>128°W) were correlated with E-W winds and the PDO since 1988 but were not correlated with either climate index between 1973 and 1981. Despite differences in nutrient concentration, nutrient utilization (ΔNO3 and ΔSi(OH)4) during the growing season were about 7.5 μM at all offshore stations. Variations in ΔNO3 were correlated with those of ΔSi(OH)4. The annual cycle of absolute NO3 uptake (ρNO3) and NH4 uptake (ρNH4) rates by phytoplankton in the upper mixed-layer showed a weak increasing trend from winter to spring/summer for the period 1992-1997. Rates were more variable at the nearshore station (P4). Rates of ρNO3 were low along the entire line despite abundant NO3 and low iron (Fe), at the offshore portion of Line P and sufficient Fe at the nearshore station (P4). As a result, new production contributed on average to only 32 ± 15% of the total nitrogen (N) uptake along Line P. NO3 utilization in the NE subarctic Pacific is probably controlled by a combination of environmental variables, including Fe, light and ambient NH4 levels. Elevated ambient NH4 concentrations seem to decrease the rates of new production (and f-ratios) in surface waters of the oceanic subarctic NE Pacific. Contrary to expectation, phytoplankton biomass, nutrient utilization (ΔNO3 and ΔSi(OH)4), and nitrogen uptake (ρNO3 + ρNH4) varied relatively little along Line P, despite significant differences in the factors controlling phytoplankton composition assemblages and production. Future studies would benefit from including other variables, especially light limitation, to improve our understanding of the seasonal and interannual variability in phytoplankton biomass and nutrients in this region. 相似文献
1000.