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We investigate the formation process and pathways of deep water masses in a coupled ice–ocean model of the Arctic and North
Atlantic Oceans. The intent is to determine the relative roles of these water masses from the different source regions (Arctic
Ocean, Nordic Seas, and Subpolar Atlantic) in the meridional overturning circulation. The model exhibits significant decadal
variability in the deep western boundary current and the overturning circulation. We use detailed diagnostics to understand
the process of water mass formation in the model and the resulting effects on the North Atlantic overturning circulation.
Particular emphasis is given to the multiple sources of North Atlantic Deep Water, the dominant deep water masses of the world
ocean. The correct balance of Labrador Sea, Greenland Sea and Norwegian Sea sources is difficult to achieve in climate models,
owing to small-scale sinking and convection processes. The global overturning circulation is described as a function of potential
temperature and salinity, which more clearly signifies dynamical processes and clarifies resolution problems inherent to the
high latitude oceans. We find that fluxes of deep water masses through various passages in the model are higher than observed
estimates. Despite the excessive volume flux, the Nordic Seas overflow waters are diluted by strong mixing and enter the Labrador
Sea at a lighter density. Through strong subpolar convection, these waters along with other North Atlantic water masses are
converted into the densest waters [similar density to Antarctic Bottom Water (AABW)] in the North Atlantic. We describe the
diminished role of salinity in the Labrador Sea, where a shortage of buoyant surface water (or excess of high salinity water)
leads to overly strong convection. The result is that the Atlantic overturning circulation in the model is very sensitive
to the surface heat flux in the Labrador Sea and hence is correlated with the North Atlantic Oscillation. As strong subpolar
convection is found in other models, we discuss broader implications. 相似文献
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Sirpa Tani Kalle Juuti Seija Kairavuori 《Norsk geografisk tidsskrift. Norwegian journal of geography》2013,67(3):172-178
The article investigates the potential and challenges of integrating geography with other school subjects. The analysis is based on the outcomes of a course that introduced the principles of discipline-based integration. The course was included in multicultural-class teacher education programme at the University of Helsinki. For a project within the course, the students worked in small groups and made short films that integrated three subjects – geography, physics, and visual arts – in order to evoke questions of space in children's minds. The students were asked to write essays in which they analysed their understanding of space from the three subjects’ viewpoints, describe the aims and contents of their short films, and lastly reflect on their experiences of the potential and possible problems in discipline integration. In this article, the authors analyse the students’ experience of the integration between the geography, physics, and visual arts. In general, the students experienced discipline-based integration as an interesting way of working. Moreover, the project seemed to deepen their understanding of different grounds for integrative pedagogy. Although the students reported some difficulties regarding the subject knowledge required for fruitful integration, the process enabled them to construct a better understanding of the identity of each discipline. 相似文献
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Sirpa Rasmus David Gustafsson Harri Koivusalo Ari Laurén Achim Grelle Olli‐Kalle Kauppinen Ola Lagnvall Anders Lindroth Kai Rasmus Magnus Svensson Per Weslien 《水文研究》2013,27(20):2876-2891
Leaf area index (LAI) and canopy coverage are important parameters when modelling snow process in coniferous forests, controlling interception and transmitting radiation. Estimates of LAI and sky view factor show large variability depending on the estimation method used, and it is not clear how this is reflected in the calculated snow processes beneath the canopy. In this study, the winter LAI and sky view fraction were estimated using different optical and biomass‐based approximations in several boreal coniferous forest stands in Fennoscandia with different stand density, age and site latitude. The biomass‐based estimate of LAI derived from forest inventory data was close to the values derived from the optical measurements at most sites, suggesting that forest inventory data can be used as input to snow hydrological modelling. Heterogeneity of tree species and site fertility, as well as edge effects between different forest compartments, caused differences in the LAI estimates at some sites. A snow energy and mass balance model (SNOWPACK) was applied to detect how the differences in the estimated values of the winter LAI and sky view fraction were reflected in simulated snow processes. In the simulations, an increase in LAI and a decrease in sky view fraction changed the snow surface energy balance by decreasing shortwave radiation input and increasing longwave radiation input. Changes in LAI and sky view fraction affected directly snow accumulation through altered throughfall fraction and indirectly snowmelt through the changed surface energy balance. Changes in LAI and sky view fraction had a greater impact on mean incoming radiation beneath the canopy than on other energy fluxes. Snowmelt was affected more than snow accumulation. The effect of canopy parameters on evaporation loss from intercepted snow was comparable with the effect of variation in governing meteorological variables such as precipitation intensity and air temperature. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
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Previous numerical simulations have shown that under certain conditions rotating gas clouds break up in two equal parts. We explore the consequences, if this process continues and a planar, equal mass four-body system forms. The results of the four-body calculations are used in Monte Carlo simulation of fragmentation of a rotating gas cloud in a galactic nucleus. Then it appears possible that clusters of large numbers of objects form; the dynamical evolution of these clusters have been calculated. The results were applied to ejection of supermassive objects from galactic nuclei. We find that several statistical properties of double radio sources may be understood as resulting from the fragmentation process. 相似文献
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As part of a major international study, 219 first‐year geography students at three Australian universities drew sketch maps of the world. These maps were overwhelmingly Sinocentric in orientation. In terms of content, Australian students tended to include slightly fewer countries than students elsewhere. The level of inclusion of individual countries in the sketch maps was influenced greatly by the area and population of the countries in question but secondary factors such as proximity to Australia, Commonwealth nation status, historical/cultural factors and level of economic development were also important. In comparison to students throughout the world, Australian students demonstrated a fair knowledge of East Asia and ASEAN nations, Western Europe and Commonwealth nations, and relatively poor knowledge of Latin America, non‐Commonwealth Africa and South‐west Asia. 相似文献
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Tapio Tuukkanen Leena Stenberg Hannu Marttila Leena Finér Sirpa Piirainen Harri Koivusalo Bjørn Kløve 《地球表面变化过程与地形》2016,41(13):1841-1853
Ditch cleaning in drained peatland forests increases sediment loads and degrades water quality in headwater streams and lakes. A better understanding of the processes controlling ditch erosion and sediment transport in such systems is a prerequisite for proper peatland management. In order to relate hydrological observations to key erosion processes in headwater peatlands drained for forestry, a two‐year study was conducted in a nested sub‐catchment system (treated with ditch cleaning) and at two reference sites. The treated catchment was instrumented for continuous discharge and turbidity monitoring, erosion pin measurements of changes in ditch bed and banks and time‐integrated sampling of suspended sediment (SS) composition. The results showed that ditch cleaning clearly increased transient suspended sediment concentrations (SSCs) and suspended sediment yields (SSYs), and resulted in temporary storage of loosely deposited organic sediment in the ditch network. After exhaustion of this sediment storage, subaerial processes and erosion from ditch banks became dominant in producing sediment for transport. Recorded SSCs were higher on the rising limbs of event hydrographs throughout the study period, indicating that SS transport was limited by availability of erosion‐prone sediment. A strong positive correlation (R2 = 0.84, p < 0.001) between rainfall intensity (above a threshold of 1 mm h?1) and average SSC obtained on the rising limb of hydrographs for the sub‐catchment showed that soil detachment from ditch banks by raindrop impact can directly increase SSC in runoff. At the main catchment outlet, variation in SSC was best explained (R2 = 0.67, p < 0.05) by the linear combination of initial discharge (?), peak discharge (+) and the lag time from initial to peak discharge (?). Based on these factors, ditch cleaning slightly increased peak discharges and decreased transit times in the study catchment. The implications of the results for water pollution management in peatland forests are discussed. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
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Jarmo J. Meriläinen Virve Kustula Allan Witick Eeva Haltia-Hovi Timo Saarinen 《Journal of Paleolimnology》2010,44(2):531-545
In order to assess the importance of different pollution sources for the increase in element concentration and accumulation, historical changes in selected elements were studied in the annually laminated sediment of Lake Korttajärvi in Central Finland (62°20′N; 25°41′E). The sediment chronology based on varve counting (256 BC to AD 2005) provided a unique opportunity to explore and date signals of metal emissions, including the ancient metallurgical activities of the Roman Empire at the beginning of the Current Era. Records of this kind are mostly lacking in Finland and northernmost Europe. The stratigraphic sequence of element concentrations did not reflect any major changes in the lake, but changes in element accumulation rates provided distinct pollution signals caused by airborne fallout, catchment erosion, and to some extent municipal wastewater loading. The maximum bulk sedimentation recorded in the twentieth century was 11-fold and organic sedimentation 4-fold higher than the mean background sedimentation rate (256 BC to AD 1019). The increase in the accumulation rates of the majority of the elements, such as Cd, Sn, Pb, Si, Ni, B, Cu, Zn, Sr, Na, K, Sb, Ca, Cr, U and Mg, in descending order, was at least equal to that of bulk sedimentation or much greater, especially for Cd, Sn, and Pb. Changes in the accumulation of Co, Fe, Mn, Mo and As were small and mainly followed those of organic sedimentation. The earliest pollution signals were those of Pb recorded in AD 1055–1141. A weak signal of Pb pollution from the Roman Era was detected in metal concentrations, but this could not be confirmed by the accumulation rate data for Pb. 相似文献
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