To determine whether resolution of smaller scales is necessary to simulate large-scale ocean climate correctly, I examine results from a global ocean GCM run with different horizontal grid spacings. The horizontal grid spacings span a range from coarse resolutions traditionally used in climate modeling to nearly the highest resolution attained with today's computers. The experiments include four cases employing 4°, 2°, 1° and 1/2° spacing in latitude and longitude, which were run with minimal differences among them, i.e., in a controlled experiment. Two additional cases, 1/2° spacing with a more scale-selective sub-gridscale mixing of heat and momentum, and approximate 1/2° spacing, are also included. The 1/2° run resolves most of the observed mesoscale eddy energy in the ocean. Artificial constraints on the model tend to minimize differences among the different resolution cases. Nevertheless, the simulations show significant changes as resolution increases. These changes generally but not always bring the model into better agreement with observations. Differences are typically more noticeable when comparing the 4° and 2° runs than when comparing the 2° and 1° runs or the 1° and 1/2° runs. A reasonable conclusion to draw for current studies with coupled ocean-atmosphere GCMs is that the ocean grid spacing could be set to about 1° to accrue the benefits of enhanced resolution without paying an excessively steep price in computer-time cost. The model's poleward heat transport at 1/2° grid spacing peaks at about 1 × 1015 W in the Northern Hemisphere and 0.5 × 1015 W in the Southern Hemisphere. These values are significantly below observations, a problem typical of ocean GCMs even when they are less constrained than in the present study. This present problem is alleviated somewhat in the 1/2° run. In this case, however, the eddies resolved by the model generally act to counter rather than to reinforce the heat transport of the mean flow. Improved heat transport may result less from enhanced resolution than from other changes made in this version of the model, such as more accurate wind forcing. 相似文献
The large-scale flux of nitrogen and phosphorus in modern society is coupled to the exponentially growing world population. During the last three decades there has been a several-fold growth in use of nitrogen (N) and phosphorus (P), for example, for agriculture production, in chemical products such as in detergents, and as food additives. Feeding a growing population means increased human nutrient excretion.
The mobility of N differs from that of P. For the easily mobile nitrate ion, river exports are positively correlated to the size of local human populations. P on the other hand is fixed in different systems, at least temporarily. Modern agro-ecosystems accumulate about 60% of the annual input of P. The Stockholm region representing an urban ecosystem served by dephosphatation in sewage, accumulated about 80% of imported P during 1990. With increasing accumulation there is an apparent risk for increasing non-point source pollution.
In some shallow recipient lakes the sediments have been saturated with P. Even after the reduction of external loading these lakes are exporting more P than they are importing. Lakes normally are traps for P.
In order to prevent damage to natural resources and to stop the large-scale flux of nutrients from land to water, the world population growth rate must be decreased and a number of measures taken: agricultural structures must be changed including new concepts for the use of fertilizers; new municipal sewage treatment processes must be developed permitting high-degree recycling of nutrients; unnecessary use of nutrients must be stopped, for example, in detergents and by reducing nutrient additives in food and drinks. Cola drinks, for example, contain 180 mg P l−1, a concentration about 350 times higher than that of the effluent (0.5 mg P l−1) from modern sewage treatment plants operating with dephosphatation. Financial resources and effort are required to stop large-scale eutrophication of marine waters. 相似文献
We review geologic records of both historic and prehistoric tsunami inundations at three widely separated localities that experienced significant damage from the 1964 Alaskan tsunami along the Cascadia margin. The three localities are Port Alberni, Cannon Beach, and Crescent City, representing, respectively, the north, central, and south portions of the study area (1,000 km in length). The geologic records include anomalous sand sheets from marine surges that are hosted in supratidal peaty mud deposits. Paleotsunami sand sheets that exceed the thickness, continuity and/or extent of the 1964 historic tsunami are counted as major paleotsunami inundations. Major paleotsunamis (6–7 in number) at each locality during the last 3,000 years demonstrate mean recurrence intervals of 450–540 years, and within-cluster intervals (three events each) of 270–460 years. It has been 313 years since the last major paleotsunami from a great Cascadia earthquake in AD 1700. We compare the dated sequences of major paleotsunami inundations to the nearest regional records of coastal coseismic subsidence in Willapa Bay in the central margin, Waatch/Neah Bay in the northern margin, and Coquille in the southern margin. Similar numbers of events from both types of records suggest that the major paleotsunamis are locally derived (near-field) from ruptures of the Cascadia margin megathrust fault zone, rather than from transoceanic tsunamis (far-field) originating at other subduction zones around the Pacific Rim. Given the catastrophic hazard of the near-field Cascadia margin tsunamis, we propose a basic rule for reminding the general public of the need for self-initiated evacuation following a great earthquake at the Cascadia margin. 相似文献
Abstract— The first occurrence of stishovite in an iron meteorite, Muonionalusta (group IVA), is reported. The mineral occurs intimately mixed with amorphous silica, forming tabular grains up to ?3 mm wide, with a hexagonal outline. It was identified using X‐ray diffraction and Raman microspectroscopy. The unit‐cell parameters of stishovite are a = 4.165(3) Å and c = 2.661(6) Å, and its chemical composition is nearly pure SiO2. Raman spectra show relatively sharp bands at 231 and 754 cm?1 and a broad band with an asymmetric shape and a maximum around 500 cm?1. The rare grains are found within troilite nodules together with chromite, daubreelite, and schreibersite. From their composition and morphology, and by comparisons with silica inclusions in, e.g., the Gibeon IVA iron, we conclude that these rare grains represent pseudomorphs after tridymite. The presence of stishovite in Muonionalusta is suggested to reflect shock metamorphic conditions in the IVA parent asteroid during a cosmic impact event. 相似文献
Beckerite, a minor component of the amber fossil resins of northern Europe, has previously been classified as being mineralogically distinct from common Baltic amber or succinite on the basis of physical properties such as density and chemical properties such as saponification number. We show that beckerite and succinite are spectroscopically identical, according to infrared and carbon-13 nuclear magnetic resonance methods. Following the suggestion of Schubert, we attribute the deviations of beckerite to the presence of low levels of contaminants, such as decomposed wood and insect excrement. 相似文献