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521.
522.
Possible refugia for reefs in times of environmental stress 总被引:2,自引:0,他引:2
This paper investigates the refuge potential of (1) upwelling areas, (2) coral areas at medium depth, and (3) offshore bank and island reefs in a scenario of increased global warming, and thus increased sea surface temperature (SST) and increased solar UV radiation. (1) Observations on coral health and water temperature in the subtropical Atlantic (Eleuthera and Cat Island, Bahamas) and Indian Ocean (Sodwana Bay, South Africa) suggest a link between cool water delivered by upwelling and coral health. After the 1998 bleaching event, caused by strong SST anomalies, coral health and recovery from the previous year's bleaching was significantly better on the narrow southern Cat Island shelf (70% of corals healthy) where the presence of cold water was observed, which was attributed to small-scale upwelling, than on the wide northern Eleuthera shelf (44% of corals healthy), where downwelling of hot bank waters was believed to have damaged corals. In South Africa, regular, short-term upwelling events in five summers reduced SST to well below bleaching level. (2) In the northern Red Sea (Safaga Bay) and in South Africa (Sodwana Bay), wide areas with either coral frameworks or non-framework communities exist. Calculations show that if the top 10 m (20 m) of the ocean became inhospitable to corals, still 50.4% (17.5%) of the coral area would remain intact in the Red Sea and 99% (40%) in South Africa. (3) Offshore bank and island reefs investigated in the Turks, Caicos, and Mouchoir Banks and Grand and Little Cayman showed high rates of mortality and coral diseases. The most remote sites (Mouchoir Bank) were not the healthiest. Refuge areas appear to exist in (1) and (2), but in (3) only if vigorous water-circulation is encountered. 相似文献
523.
Antonis Polatidis Anastasia Kiratzi Panagiotis Hatzidimitriou Basil Margaris 《Tectonophysics》2003,367(1-2):29-40
Qβ for shear-waves is determined for the inner part of the Hellenic arc, the back-arc area, as a function of frequency in the range 0.6–16 Hz. We used 314 digital records from 32 earthquakes with magnitudes (Mw) ranging from 3.9 to 5.1. Epicentral distances ranged from 65 to 515 km. The data were obtained in 1997 during a 6-month operation of a digital portable network in Greece. The Qβ estimates were made for five frequency bands centred at 0.8, 1.5, 3.0, 6.0 and 12.0 Hz and the Qβ values obtained were 47, 79, 143, 271 and 553, respectively. The results show that Qβ for S-waves increases with frequency taking the form Qβ=55f 0.91 (or Qβ−10.018f−0.91). The high attenuation and the strong frequency dependence found, which is close to the frequency dependence of coda Q for Greece, are characteristic of an area with high seismicity, rapid extension, and in agreement with other similar studies in Greece. 相似文献
524.
The driving force for the basin subsiding against isostatic balance in and around Lake Biwa in the Kinki district, Japan is discussed. The lake region is characterized by strong negative Bouguer anomalies, especially by a steep horizontal gradient zone of gravity anomaly running along the western margin of the lake. The large negative anomaly (>50 mgal) cannot be explained by low-density sediments beneath it. A down-warping structure extending to the Moho depth should be taken into account. This conjecture has been strongly supported by a short-period receiver function imaging, which shows a clear offset of about 8 km for the Moho discontinuity under the steep gravity gradient zone.A question arises as to what is the driving force to create such a large down-warping structure. We consider that the subduction of the shallow-dipping slab under the region (Philippine Sea Slab) may cause crustal deformation by dragging the viscous mantle downward. In order to verify this model, we simulated the induced mantle flow due to the subduction of the Philippine Sea Slab and the pressure distribution on the crust–mantle boundary. This numerical experiment showed that the induced flow makes a strong negative pressure zone under the lake region if the slab has a vertical offset along the direction of subduction. This offset of the slab is consistent with plate models deduced from hypocentral distributions and Sp phases of the deep-focus earthquakes. 相似文献
525.
Yehouda Enzel Revital Bookman David Sharon Haim Gvirtzman Uri Dayan Baruch Ziv Mordechai Stein 《Quaternary Research》2003,60(3):263-273
The Dead Sea is a terminal lake of one of the largest hydrological systems in the Levant and may thus be viewed as a large rain gauge for the region. Variations of its level are indicative of the climate variations in the region. Here, we present the decadal- to centennial-resolution Holocene lake-level curve of the Dead Sea. Then we determine the regional hydroclimatology that affected level variations. To achieve this goal we compare modern natural lake-level variations and instrumental rainfall records and quantify the hydrology relative to lake-level rise, fall, or stability. To quantify that relationship under natural conditions, rainfall data pre-dating the artificial Dead Sea level drop since the 1960s are used. In this respect, Jerusalem station offers the longest uninterrupted pre-1960s rainfall record and Jerusalem rains serve as an adequate proxy for the Dead Sea headwaters rainfall. Principal component analysis indicates that temporal variations of annual precipitation in all stations in Israel north of the current 200 mm yr−1 average isohyet during 1940–1990 are largely synchronous and in phase (70% of the total variance explained by PC1). This station also represents well northern Jordan and the area all the way to Beirut, Lebanon, especially during extreme drought and wet spells. We (a) determine the modern, and propose the past regional hydrology and Eastern Mediterranean (EM) climatology that affected the severity and length of droughts/wet spells associated with multiyear episodes of Dead Sea level falls/rises and (b) determine that EM cyclone tracks were different in average number and latitude in wet and dry years in Jerusalem. The mean composite sea level pressure and 500-mb height anomalies indicate that the potential causes for wet and dry episodes span the entire EM and are rooted in the larger-scale northern hemisphere atmospheric circulation. We also identified remarkably close association (within radiocarbon resolution) between climatic changes in the Levant, reflected by level changes, and culture shifts in this region. 相似文献
526.
根据南海季风试验(SCSMEX)期间南海内区的三个ATLAS(Autonomous Temperature Line Acquisition System)锚碇浮标资料(1998年4月~1999年4月),采用谐波分析方法对南海上层海洋水温年循环、半年循环加以分离,发现无论在年循环还是在半年循环尺度上,18°N附近SCS1站与13°N附近SCS3站的水温变化次表层与表层呈反位相;15°20′N附近SCS2站水温变化基本上次表层与表层同位相.这说明不同区域上层海洋热变化受不同的正压与斜压模态控制.其次,SCS2、SCS3两点水温年循环振幅均在次表层达到极值;而SCS1在表层达到极大值,在100 m深度达到次极大值.3个站位水温半年循环振幅极值均出现在次表层内,这说明该层内的水温半年循环在温度变化趋势中所占的权重比在表层的权重大. 相似文献
527.
从118.75 GHz附近六通道亮温反演大气温度廓线的数值模拟研究 总被引:5,自引:0,他引:5
为了能在静止气象卫星上实现微波被动遥感探测大气温度廓线,并保持一定的地面空间分辨率(如视场小于60 km),就需要使用高频微波及大天线.欧洲和美国下一代静止气象卫星上都已考虑采用118.75 GHz附近通道.为了充分了解118.75 GHz附近通道遥感反演温度廓线的能力,为仪器研制及今后资料的解释反演提供必要的基础数据,作者开展了采用118.75 GHz附近六个通道遥感反演大气温度廓线的数值模拟研究.统计反演的数值试验表明,118.75 GHz附近六通道对温度垂直分布有一定的遥感反演能力;温度反演较好的层次对应于权重函数峰值所在的位置. 相似文献
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