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251.
Link between convection and meridional gradient of sea surface temperature in the Bay of Bengal 总被引:2,自引:0,他引:2
We use daily satellite estimates of sea surface temperature (SST) and rainfall during 1998–2005 to show that onset of convection
over the central Bay of Bengal (88–92°E, 14–18°N) during the core summer monsoon (mid-May to September) is linked to the meridional
gradient of SST in the bay. The SST gradient was computed between two boxes in the northern (88–92°E, 18–22°N) and southern
(82–88°E, 4–8°N) bay; the latter is the area of the cold tongue in the bay linked to the Summer Monsoon Current. Convection
over central bay followed the SST difference between the northern and southern bay (ΔT) exceeding 0.75°C in 28 cases. There was no instance of ΔT exceeding this threshold without a burst in convection. There were, however, five instances of convection occurring without
this SST gradient. Long rainfall events (events lasting more than a week) were associated with an SST event (ΔT ≥ 0.75°C); rainfall events tended to be short when not associated with an SST event. The SST gradient was important for the
onset of convection, but not for its persistence: convection often persisted for several days even after the SST gradient
weakened. The lag between ΔT exceeding 0.75°C and the onset of convection was 0–18 days, but the lag histogram peaked at one week. In 75% of the 28 cases,
convection occurred within a week of ΔT exceeding the threshold of 0.75°C. The northern bay SST, T
N
, contributed more to ΔT, but it was a weaker criterion for convection than the SST gradient. A sensitivity analysis showed that the corresponding
threshold for T
N
was 29°C. We hypothesise that the excess heating (∼1°C above the threshold for deep convection) required in the northern
bay to trigger convection is because this excess in SST is what is required to establish the critical SST gradient. 相似文献
252.
Determination of vertical displacements over the coastal area of Korea due to the ocean tide loading using GPS observations 总被引:2,自引:0,他引:2
This paper describes the GPS applicability for detecting the vertical displacements of ground stations caused by ocean tide loading effects. An experiment was carried out using 12 permanent GPS stations located in the coastal area of Korea using data in the period 1 July until 26 August 2003. The relative height differences were calculated from hourly DGPS data processing based on the carrier-phase observation. The power spectra of the M2 and N2 constituents of ocean tide loading were derived using the CLEAN algorithm. The differential vertical displacements generated by the ocean tide loading effect are typically 3–25 mm in coastal area of the Korea. We compared the results from GPS with those of the ocean tide models, NAO.99Jb regional model and GOT00.2, FES99 global models. The M2 (N2) amplitude differences of vertical displacements between GPS and GOT00.2 is 1.22 ± 3.61 mm (1.01 ± 1.48 mm), and that of the M2 (N2) amplitude difference between GPS and FES99 is 0.04 ± 4.64 mm (0.64 ± 1.75 mm), whereas the M2 (N2) amplitude difference between GPS and NAO.99Jb are 0.05 ± 1.03 mm (0.86 ± 1.18 mm). The highest vertical displacements at the PALM station are found for 24.5 ± 0.7 mm from GPS observation, and 22.9 mm from the regional model NAO.99Jb and 13.17 and 10.00 mm from the global models GOT00.2 and FES99, respectively. These values show that the vertical displacements derived from GPS are in good agreement with those of the regional model NAO.99Jb around Korea, more than with the global models. This result indicated that GPS is an effective tool to measure the vertical displacement caused by the ocean tide loading effect in the coastal area, and we need to use the NAO.99Jb ocean tide model rather than the global ocean tide models in and around the Korean peninsula for position determination with permanent GPS installations. This work demonstrates that vertical displacement caused by the M2 and N2 constituents of ocean tide loading can be measured by carrier-phase DGPS. 相似文献
253.
Hideko Takayanagi Yasufumi Iryu Tsutomu Yamada Motoyoshi Oda Kazuyuki Yamamoto Tokiyuki Sato Shun Chiyonobu Akira Nishimura Tsutomu Nakazawa Satoshi Shiokawa 《Island Arc》2007,16(3):394-419
Abstract The lithology of shallow-water carbonates collected from 19 sites on 16 seamounts in six areas of the northwestern Pacific Ocean using the Deep-sea Boring Machine System are described. The areas include the Amami Plateau, Daito Ridge, Oki-Daito Ridge, Urdaneta Plateau, Kyushu-Palau Ridge and Ogasawara Plateau. Chronological constraint is provided by calcareous nannofossil biostratigraphy, planktonic foraminiferal biostratigraphy, larger foraminiferal biostratigraphy and strontium (Sr) isotope stratigraphy. Large amounts of shallow-water carbonates accumulated on the seamounts during the Oligocene, a relatively cool period, whereas limited carbonate deposits formed during the Early Miocene, a relatively warm period. This might indicate that deposition of shallow-water carbonates on seamounts in the northwestern Pacific Ocean was not necessarily controlled by climatic conditions, but was related to volcanism and tectonics that served as foundations for reef/carbonate-platform formation. Remarkable differences in biotic composition exist between Cretaceous and Cenozoic shallow-water carbonates. Late Cretaceous shallow-water carbonates are distinguished by the occurrence of rudists, solenoporacean algae and microencrusters. Middle Eocene to Early Oligocene shallow-water carbonates are dominated by Halimeda or nummulitid and discocyclinid larger foraminifers. Scleractinian corals became common from the Oligocene onward. Nongeniculate coralline algae and larger foraminifers were common to abundant throughout the Eocene to the Pleistocene. The replacement of major carbonate producers in the shallow-water carbonate factory during post-Cretaceous time is in accordance with previous studies and is considered to reflect a shift in seawater chemistry. 相似文献
254.
Several schemes for scalar advection on unstructured triangular grids are assessed for possible use in ocean modelling applications.
Finite element, finite volume and finite volume–element approaches are evaluated. A series of tests, including a numerical
order of convergence analysis, idealized rotating cone and cylinder experiments, and transport of a tracer through the Stommel
Gyre representation of ocean basin-scale circulation, are carried out. Volume element Eulerian–Lagrangian and third-order
Runge-Kutta discontinuous Galerkin schemes are recommended for use in tracer studies. Taylor–Galerkin and second-order Runge–Kutta
discontinuous Galerkin are found to be robust and accurate second-order schemes. When positivity is required, a fluctuation
redistribution scheme was found to be an easily implemented, accurate, and computationally efficient approach.
Responsible editor: Phil Dyke 相似文献
255.
We develop stochastic approaches to determine the potential for tsunami generation from earthquakes by combining two interrelated
time series, one for the earthquake events, and another for the tsunami events. Conditional probabilities for the occurrence
of tsunamis as a function of time are calculated by assuming that the inter-arrival times of the past events are lognormally
distributed and by taking into account the time of occurrence of the last event in the time series. An alternative approach
is based on the total probabilitiy theorem. Then, the probability for the tsunami occurrence equals the product of the ratio,
r (= tsunami generating earthquakes/total number of earthquakes) by the conditional probability for the occurrence of the next
earthquake in the zone. The probabilities obtained by the total probability theorem are bounded upwards by the ratio r and, therefore, they are not comparable with the conditional probabilities. The two methods were successfully tested in three
characteristic seismic zones of the Pacific Ocean: South America, Kuril-Kamchatka and Japan. For time intervals of about 20
years and over the probabilities exceed 0.50 in the three zones. It has been found that the results depend on the approach
applied. In fact, the conditional probabilities of tsunami occurrence in Japan are slightly higher than in the South America
region and in Kuril-Kamchatka they are clearly lower than in South America. Probabilities calculated by the total probability
theorem are systematically higher in South America than in Japan while in Kuril-Kamchatka they are significantly lower than
in Japan. The stochastic techniques tested in this paper are promising for the tsunami potential assessment in other tsunamigenic
regions of the world. 相似文献
256.
A monitoring mission to study the shape and estimate initial dilution of the S. Jacinto outfall plume using an autonomous underwater vehicle (AUV) was performed on July 30, 2002. In order to reduce the uncertainty about plume location and to concentrate the vehicle mission only in the hydrodynamic mixing zone, outputs of a near-field prediction model, based on effective real-time in situ measurements of current speed and direction and density stratification, were opportunistically used to specify in real time the mission transects. The surface characteristics of the outfall plume were found to be influenced strongly by the relatively weak stratification and low current velocities. Dilution was estimated using a temperature–salinity (TS-) diagram with initial mixing lines between wastewater and ambient waters. Effluent dilutions were at least 30:1 in this study. In order to efficiently map the plume dispersion we applied the least-squares collocation method technique. Our results demonstrate that AUVs can provide high-quality measurements of physical properties of effluent plumes in a quite effective manner and valuable considerations about the initial mixing processes under real oceanic conditions can be further investigated. 相似文献
257.
Pb and Th in settling particles in the western Northwest Pacific Ocean: Particle flux and scavenging
A mooring of three conical time-series sediment traps was deployed at two sites in the western Northwest Pacific Ocean for 9 months. Total mass fluxes and activities of 210Pb and 230Th were determined for the settling particles to elucidate their scavenging and transport processes. Sediment samples also were analyzed for 210Pb activities. Total mass fluxes, 210Pb fluxes and 230Th fluxes showed large seasonal variations and their weighted mean fluxes tended to increase with depth, with an especially large increase near-bottom. The ratios of the observed 210Pb fluxes to the 210Pb deficiency fluxes in the upper traps at the two sites were only 0.02 and 0.12, and were attributable to advective export of 210Pb from the surface waters. Those ratios in the near-bottom traps ranged between 1.22 and 2.63. This suggests that these high ratios are due to effective particle scavenging, large lateral 210Pb import and input of resuspended particles that have not become incorporated into the sediments. The mean total 230Th fluxes at the near-bottom traps were 4.2–6.7 times higher than that expected from production in the overlying water column. The 210Pb activities in the surficial sediments were much lower than those in the near-bottom traps. The 210Pb accumulation rates estimated from the excess 210Pb inventory in the sediment column were 40–70% higher than the mean 210Pb fluxes at the near-bottom traps. The ratios of the 210Pb accumulation rates to the 210Pb deficiency fluxes at the near-bottom traps ranged between 2.0 and 3.7. The high fluxes of particulate 210Pb and 230Th at the near-bottom traps reflected a combination of enhanced scavenging of dissolved nuclides and the lateral redistribution of particulate matter by downslope and alongshore transports. However, it was not possible to discriminate among the various processes contributing to high nuclide fluxes. 相似文献
258.
D. R. Pattanaik 《Pure and Applied Geophysics》2007,164(8-9):1527-1545
The present study is an attempt to examine the variability of convective activity over the north Indian Ocean (Bay of Bengal
and Arabian Sea) on interannual and longer time scale and its association with the rainfall activity over the four different
homogeneous regions of India (viz., northeast India, northwest India, central India and south peninsular India) during the
monsoon season from June to September (JJAS) for the 26 year period (1979 to 2004). The monthly mean Outgoing Long-wave Radiation
(OLR) data obtained from National Oceanic and Atmospheric Administration (NOAA) polar orbiting spacecraft are used in this
study and the 26-year period has been divided into two periods of 13 years each with period-i from 1979 to 1991 and period
-ii from 1992 to 2004. It is ascertained that the convective activity increases over the Arabian Sea and the Bay of Bengal
in the recent period (period -ii; 1992 to 2004) compared to that of the former period (period -i; 1979 to 1991) during JJAS
and is associated with a significantly increasing trend (at 95% level) of convective activity over the north Bay of Bengal
(NBAY). On a monthly scale, July and August also show increase in convective activity over the Arabian Sea and the Bay of
Bengal during the recent period and this is associated with slight changes in the monsoon activity cycle over India. The increase
in convective activity particularly over the Arabian Sea during the recent period of June is basically associated with about
three days early onset of the monsoon over Delhi and relatively faster progress of the monsoon northward from the southern
tip of India. Over the homogeneous regions of India the correlation coefficient (CC) of OLR anomalies over the south Arabian
Sea (SARA) is highly significant with the rainfall over central India, south peninsular India and northwest India, and for
the north Arabian Sea (NARA), it is significant with northwest India rainfall and south peninsular rainfall. Similarly, the
OLR anomalies over the south Bay of Bengal (SBAY) have significant CC with northwest India and south peninsular rainfall,
whereas the most active convective region of the NBAY is not significantly correlated with rainfall over India. It is also
found that the region over northeastern parts of India and its surroundings has a negative correlation with the OLR anomalies
over the NARA and is associated with an anomalous sinking (rising) motion over the northeastern parts of India during the
years of increase (decrease) of convective activity over the NARA. 相似文献
259.
南海海平面高度年循环的特征 总被引:10,自引:0,他引:10
根据 TOPEX/ POSEIDON-ERS高度计提供的海平面高度异常资料和并行海洋气候模式(POCM)模拟海平面高度资料,分析了南海海平面高度年循环特征。结果表明:l月,3月和5月海平面高度的异常值分别与7月,9月,11月的异常值相反。l月(7月),深水海区与吕宋海峡的海平面高度为负(正)异常,在大部分陆架区和南海的西和南部,海平面高度为正(负)异常。在3月(9月),除海平面高度异常的量级已减少,且较小的SSH正异常(负异常)出现在南海的中部以外,海平面高度异常的分布型与 1月(7月)类似; SSH的年循环的最大振幅出现在吕宋岛的西北海域;风的季节变化是南海SSH季节变化的主要原因。 相似文献
260.
印度洋偶极子对大气环流和气候的影响 总被引:28,自引:6,他引:28
赤道印度洋SST的分析研究证实了偶极子型振荡的存在,它在9-11月较强而在1-4月较弱。若以海温西高东低为偶极子振荡正位相,以海温东高西低为负位相,则一般是正位相时振荡要强于负位相。印度洋偶极子也存在年际(主要周期为4-5年)和年代际(主要周期为25-30年)变化。分析研究表明,印度洋偶极子对亚洲季风活动有明显影响,因为亚洲地区对流层低层的风场,南亚高压和西太平洋副高强度都与印度洋偶极子有关。另外,印度洋偶极子还对北美和南印度洋(包括澳大利亚和南美)地区的大气环流和气流有影响。 相似文献