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1.
Transverse secondary circulations involving surface convergence, observed in a well-mixed estuary in North Wales, are made visible by the collection of surface material along an axial line which extends continuously for many kilometres through the estuary. The circulation and axial convergence, however, are seen only during the flood phase of the tide and no similar behaviour has been observed during the ebb phase.Convergent circulations in the estuary are associated with small but steady transverse density gradients in the cross-section, produced by non-uniform advection of the longitudinal gradient through the channel. A diagnostic model, using measured mean distributions of cross-sectional density, indicates surface transverse velocities (~0.1 ms?1) similar to those observed in the estuary. The model further predicts appreciable transverse divergent currents at a fractional depth of 0.75: a prediction which has been tested in the estuary using a vertical array of accurately resolving current direction indicators. 相似文献
2.
This paper describes a wide-field survey made at 34.5 MHz using GEETEE,1 the low frequency telescope at Gauribidanur (latitude
13°36′12′′N). This telescope was used in the transit mode and by per forming 1-D synthesis along the north-south direction
the entire observable sky was mapped in a single day. This minimized the problems that hinder wide-field low-frequency mapping.
This survey covers the declination range of-50° to + 70° (- 33° to +61° without aliasing) and the complete 24 hours of right
ascension. The synthesized beam has a resolution of 26′ x 42′ sec (δ- 14°. 1). The sensitivity of the survey is 5 Jy/beam (1σ). Special care has been taken to ensure that the antenna responds
to all angular scale structures and is suitable for studies of both point sources and extended objects
This telescope is jointly operated by the Indian Institute of Astrophysics, Bangalore and the Roman Research Institute, Bangalore. 相似文献
3.
Sylvie Charbit Masa Kageyama Didier Roche Catherine Ritz Gilles Ramstein 《Global and Planetary Change》2005,48(4):1887
A coupling procedure between a climate model of intermediate complexity (CLIMBER-2.3) and a 3-dimensional thermo-mechanical ice-sheet model (GREMLINS) has been elaborated. The resulting coupled model describes the evolution of atmosphere, ocean, biosphere, cryosphere and their mutual interactions. It is used to perform several simulations of the Last Deglaciation period to identify the physical mechanisms at the origin of the deglaciation process. Our baseline experiment, forced by insolation and atmospheric CO2, produces almost complete deglaciation of past northern hemisphere continental ice sheets, although ice remains over the Cordilleran region at the end of the simulation and also in Alaska and Eastern Siberia. Results clearly demonstrate that, in this study, the melting of the North American ice sheet is critically dependent on the deglaciation of Fennoscandia through processes involving switches of the thermohaline circulation from a glacial mode to a modern one and associated warming of the northern hemisphere. A set of sensitivity experiments has been carried out to test the relative importance of both forcing factors and internal processes in the deglaciation mechanism. It appears that the deglaciation is primarily driven by insolation. However, the atmospheric CO2 modulates the timing of the melting of the Fennoscandian ice sheet, and results relative to Laurentide illustrate the existence of threshold CO2 values, that can be translated in terms of critical temperature, below which the deglaciation is impeded. Finally, we show that the beginning of the deglaciation process of the Laurentide ice sheet may be influenced by the time at which the shift of the thermohaline circulation from one mode to the other occurs. 相似文献
4.
PROPAGATION OF 30—60 DAY LOW FREQUENCY OSCILLATIONS AND THEIR INFLUENCE UPON THE SUBTROPICAL WESTERLIES JET STREAM DURING NORTHERN HEMISPHERE WINTER 下载免费PDF全文
Based on daily ECMWF gridpoint data of two winters during 1981—1983 including an ENSOyear,propagation of low frequency oscillations(LFO)during Northern Hemisphere winters andtheir influences upon 30—60 day oscillations of the subtropical jet stream are studied with the sta-tistical methods as complex empirical orthogonal function(CEOF)and so on.Results show that inthe winter of a normal year(1981—1982),30—60 day oscillations in the subtropical zone aremainly in the northern and southern flanks of exit region of jet stream.In the ENSO year(1982—1983),they are mainly in the vicinity of entrance and exit regions of jet stream.Intraseasonalchanges of subtropical jet stream manifested themselves as latitudinal fluctuation or longitudinalprogression or regression of about 40 day period.There are marked differences between propagat-ing passages of low frequency modes responsible for changes of subtropical jet stream in the normalyear(1981—1982)and in the ENSO year(1982—1983).Changes of oscillation amplitude showobvious phases.In general,the one in late winter is stronger than that in early winter,strongestone occurs in February. 相似文献
5.
D. Prandle 《Estuarine, Coastal and Shelf Science》1985,20(5):615-635
An examination is made of the circulation in narrow estuaries subject to a predominant tidal forcing. Velocity structures are derived separately for residual flow components associated with (a) river flow, (b) wind stress, (c) a well-mixed longitudinal density gradient and (d) a fully stratified saline wedge. Dimensionless parameters are introduced to indicate the magnitude of each component and these parameters are evaluated for 9 major estuaries, thereby revealing their sensitivity to each component.For a channel of constant breadth and depth, formulae are deduced for the length of saline intrusion, L. Comparisons with observed data show that such formulae may be used with confidence to predict changes in L arising from variations in river flow, tidal range or channel depths.The level of stratification is shown to be related to a product of two parameters, one associated with velocity structure and a second involving the square of the ‘flow ratio’ (i.e. residual velocity/amplitude of the tidal velocity). This relationship provides a simple classification system for estuarine stratification which can be used to indicate the sensitivity of any particular estuary to changing conditions. 相似文献
6.
罗德海 《成都信息工程学院学报》1993,(4)
本文在基本气流具有水平切变的情况下,利用摄动法导出了非线性Rossby波所满足的三阶Zakharov方程,然后,考虑了基流具有弱切变的情况,通过使用三阶Zakharov方程研究了Rossby波列的第一类不稳定性问题。结果表明:通过非线性作用,大气中的Rossby波列可产生调制不稳定。同时,本文对这种不稳定的区域,增长率和周期进行了详细的计算,并讨论了波振幅、波数、纬度和基流切变对它们的影响,指出Rossby波列的调制不稳定可以激发30~60天的低频振荡。 相似文献
7.
地球物理现象和太阳活动中的高频振荡 总被引:2,自引:0,他引:2
本文用几种谱分析方法了从1976年7月-1992年9月期间的地球物理资料(日长变化,大气角动量)和太阳活动及1976年7月-1987年12月的日冕指数。结果证实所有序列中呈现出40-60天的振荡,同时也表明:它们的振幅和周期是随时间变化的。本文研究了谱结构的时空分布和讨论地球物理象与太阳活动之间的可能联系。 相似文献
8.
C. Giraudi 《第四纪科学杂志》2004,19(6):537-545
The island of Lampedusa lies on the northern edge of the African continental shelf, but during some Quaternary marine lowstands it was joined to the African continent. The study and dating of the aeolian, alluvial, detrital sediments, calcareous crusts and speleothems have established that the climatic–environmental variations recorded on the island can be related chronologically to those known for northern Libya, Tunisia and the Italian peninsula. During the Last Glacial Maximum, phases of Saharan dust accumulation on Lampedusa occurred, and were coeval with dust accumulation in crater lakes and on high mountains in central‐southern Italy, and with phases of glacial advance in the Apennines and in the Alps. During the late Holocene, accumulation of Saharan dust on Lampedusa occurred but there was little accumulation of dust on the northern side of the Mediterranean Sea. With the new data from Lampedusa, it is possible to envisage two different scenarios of atmospheric circulation relating to the Last Glacial Maximum and to the late Holocene. During the Last Glacial Maximum, southerly atmospheric circulation brought rainfall to the southern slopes of the Alps and to the Apennines. During the late Holocene, a prevalent westerly atmospheric circulation became established in the northern Mediterranean. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
9.
Analyses of sea level and current-meter data using digital filters and a variety of statistical methods show a variety of phenomena related to non-local coastal forcing and local tidal forcing in the northern reach of San Francisco Bay, a partially mixed estuary. Low-frequency variations in sea level are dominated by non-local variations in coastal sea level and also show a smaller influence from tidally induced fortnightly sea level variations. Low-frequency currents demonstrate a gravitational circulation which is modified by changes in tidal-current speed over the spring-neap tidal cycle. Transients in gravitational circulation induce internal oscillations with periods of two to four days. 相似文献
10.
Uranium-series dating of oxygen and carbon isotope records for stalagmite SJ3 collected in Songjia Cave, central China, shows significant variation in past climate and environment during the period 20-10 ka. Stalagmite SJ3 is located more than 1000 km inland of the coastal Hulu Cave in East China and more than 700 km north of the Dongge Cave in Southwest China and, despite minor differences, displays a clear first-order similarity with the Hulu and Dongge records. The coldest climatic phase since the Last Glacial Maximum, which is associated with the Heinrich Event 1 in the North Atlantic region, was clearly recorded in SJ3 between 17.6 and 14.5 ka, in good agreement in timing, duration and extent with the records from Hulu and Dongge caves and the Greenland ice core. The results indicate that there have been synchronous and significant climatic changes across monsoonal China and strong teleconnections between the North Atlantic and East Asia regions during the period 20-10 ka. This is much different from the Holocene Optimum which shows a time shift of more than several thousands years from southeast coastal to inland China. It is likely that temperature change at northern high latitudes during glacial periods exerts stronger influence on the Asian summer monsoon relative to insolation and appears to be capable of perturbing large-scale atmospheric/oceanic circulation patterns in the Northern Hemisphere and thus monsoonal rainfall and paleovegetation in East Asia. Climatic signals in the North Atlantic region propagate rapidly to East Asia during glacial periods by influencing the winter land-sea temperature contrast in the East Asian monsoon region. 相似文献