The evaluation of ocean simulations against observed datasets is essential to assess their realism and to guide model development, but often remains qualitative, and ignores certain datasets. This paper presents a three-dimensional, quantitative comparison of a 1/6° Atlantic numerical simulation (CLIPPER) with the WOCE current meter dataset in terms of mean velocity and eddy kinetic energy. Our metrics reveal the good behaviour of CLIPPER open boundary conditions and forcing with respect to full-depth current records. Due to its still moderate resolution, however, the model globally underestimates the observed mean speeds and eddy activity. This discrepancy is barely noticeable at low latitudes but increases toward the poles, probably since the poleward decrease of the Rossby radius exceeds that of the horizontal grid step. At least in this eddy-admitting regime, it is suggested that the numerics of geopotential-coordinate models like ours dissipate mean and eddy momentum at depth and adversely affect current–topography interactions. 相似文献
A series of experiments on the fluid-absent melting of a quartz-rich aluminous metagreywacke has been carried out. In this
paper, we report the chemical composition of the phases present in the experimental charges as determined by electron microprobe.
This analytical work includes biotite, plagioclase, orthopyroxene, garnet, cordierite, hercynite, staurolite, gedrite, oxide,
and glass, over the range 100–1000 MPa, 780–1025 °C. Biotites are Na- and Mg-rich, with Ti contents increasing with temperature.
The compositions of plagioclase range from An17 to An35, with a significant orthoclase component, and are always different from the starting minerals. At high temperature, plagioclase
crystals correspond to ternary feldspars with Or contents in the range 11–20 mol%. Garnets are almandine pyrope grossular
spessartine solid solutions, with a regular and significant increase of the grossular content with pressure. All glasses are
silicic (SiO2 = 67.6–74.4 wt%), peraluminous, and leucocratic (FeO + MgO = 0.9–2.9 wt%), with a bulk composition close to that of peraluminous
leucogranites, even for degrees of melting as high as 60 vol.%. With increasing pressure, SiO2 contents decrease while K2O increases. At any pressure, the melt compositions are more potassic than the water-saturated granitic minima. The H2O contents estimated by mass balance are in the range 2.5–5.6 wt%. These values are higher than those predicted by thermodynamic
models. Modal compositions were estimated by mass balance calculations and by image processing of the SEM photographs. The
positions of the 20 to 70% isotects (curves of equal proportion of melt) have been located in the pressure-temperature space
between 100 MPa and 1000 MPa. With increasing pressure, the isotects shift toward lower temperature between 100 and 200 MPa,
then bend back toward higher temperature. The melting interval increases with pressure; the difference in temperature between
the 20% and the 70% isotects is 40 °C at 100 MPa, and 150 °C at 800 MPa. The position of the isotects is interpreted in terms
of both the solubility of water in the melt and the nature of the reactions involved in the melting process. A comparison
with other partial melting experiments suggests that pelites are the most fertile source rocks above 800 MPa. The difference
in fertility between pelites and greywackes decreases with decreasing pressure. A review of the glass compositions obtained
in experimental studies demonstrates that partial melting of fertile rock types in the crust (greywackes, pelites, or orthogneisses)
produces only peraluminous leucogranites. More mafic granitic compositions such as the various types of calk-alkaline rocks,
or mafic S-type rocks, have never been obtained during partial melting experiments. Thus, only peraluminous leucogranites
may correspond to liquids directly formed by partial melting of metasediments. Other types of granites involve other components
or processes, such as restite unmixing from the source region, and/or interaction with mafic mantle-derived materials.
Received: 11 July 1995 / Accepted: 27 February 1997 相似文献
The behaviour of two silicates (albite and olivine) toward aqueous dissolution has been investigated with emphasis to two aspects: (1) similarities and differences with silicate glasses; and (2) influence of ion bombardment which transforms the initially crystalline structure to a glass-like one. The occurrence on leached materials of hydrated layers enriched with metallic elements is checked by using high-energy ion beam analytical techniques.
It is shown that for both amorphous and crystalline silicates, dissolution involves competing processes responsible for either hydrated-layer generation (hydration by ion exchange and/or water permeation), or its destruction (hydrolysis of siloxane bonds and detachment of silica units). At moderate temperature ( 100°C), the latter processes dominate for crystalline silicates and the resulting dissolution is congruent. However, these silicates tend to behave like glasses upon ion bombardment above a critical dose. At higher temperature (> 200°C), hydration is markedly enhanced and thick altered layers are generated even on undamaged minerals such as albite and orthoclase, where alkali cations are easily exchangeable. 相似文献
The two-step character of the last deglaciation is well recognized in Western Europe, in Greenland and in the North Atlantic. For example, in Greenland, a gradual temperature decrease started at the Bölling (B) around 14.5 ky BP, spanned through the Alleröd (A) and was followed by the cold Younger Dryas (YD) event which terminated abruptly around 11.5 ky BP. Recent results suggest that this BA/YD sequence may have extended throughout all the Northern Hemisphere but the evidence of a late transition cooling is still poor for the Southern Hemisphere. Here we present a detailed isotopic record analyzed in a new ice core drilled at Dome B in East Antarctica that fully demonstrates the existence of an Antarctic cold reversal (ACR). These results suggest that the two-step shape of the last deglaciation has a worldwide character but they also point to noticeable interhemispheric differences. Thus, the coldest part of the ACR, which shows a temperature drop about three times weaker than that recorded during the YD in Greenland, may have preceded the YD. Antarctica did not experienced abrupt changes and the two warming periods started there before they started in Greenland. The links between Southern and Northern Hemisphere climates throughout this period are discussed in the light of additional information derived from the Antarctic dust record. 相似文献
Simultaneous measurements of soluble and insoluble impurities were made on the 950 m deep Vostok (78°30′S, 106°54′E, 3420 m a.s.l.) ice core, spanning roughly 50000 yr, using various analytical techniques. We observed higher continental (×37) and marine (×5.1) inputs during the last glacial age than during the Holocene stage. A study of microparticle compositions and of volcanic indicators (Zn, H2SO4), shows that the high observed crustal input is not due to enhanced volcanism, but is rather of continental eolian origin. For the first time, the ionic balance along a deep ice core is established, mainly used in discussing the evolution of the Cl to Na ratio over central East Antarctica with changing climatic conditions: the presence of relatively high amounts of Na2SO4 in the marine aerosol at the Vostok site during the Holocene is demonstrated. Comparison with the Dome C (74°39′S, 124°10′E, 3040 m a.s.l.) results confirms the chronology of the major events: (i) maximum terrestrial input around the last glacial maximum (~18 ka BP); (ii) end of the high continental flux over Antarctica near 13 ka BP; (iii) marine input varying in an opposing manner to isotopic fluctuations with rather high concentrations beginning to decrease when isotopic values increase and reaching Holocene values at the end of the transition between cold and warmer climate conditions. Detailed comparison with results provided by deep ice cores from other sites which are probably more influenced by oceanic air masses seems to indicate that most of the aerosol reaching central East Antarctica travel over large distance probably at rather high altitude through the troposphere. We can consider that central East Antarctica is well representative of the upper part of the troposphere (higher than i.e., 3000 m) and should, therefore, provide valuable data for global and Antarctic paleoclimatological models. 相似文献
Two Holocene ice core records from East Antarctica (Vostok and EPICA-Dome C) were analysed for dust concentration and size
distribution at a temporal resolution of 1 sample per ~50 years. A series of volcanic markers randomly distributed over the
common part of the ice cores (from 9.8 to 3.5 kyear BP) ensures accurate relative dating (±33 years). Dust-size records from
the two sites display oscillations structured in cycles with sub-millennial and secular scale frequencies that are apparently
asynchronous. The power spectra of the composite sum (Σ) of the two dust-size records display spectral energy mostly for 150-
to 500-year periodicities. On the other hand, the 200-year band is common to both records and the 200 year components of the
two sites are out-of-phase (100-year lead or lag) over ~5.5 kyear, a phenomenon also reflected by a significant (>99% conf.
lev.) band in the power spectra of the composite difference (Δ) of the two size records. During long-range transport, mineral
dust originating from the Southern Hemisphere continents is graded to a variable extent depending on the altitude and duration
of atmospheric transport. Relatively coarse dust is associated with air mass penetration from the middle–lower troposphere
and conversely relatively fine dust with upper troposphere air masses or the influence of subsidence over the Antarctic plateau,
a hypothesis already proposed for the changes that occurred during the Last Glacial Maximum to Holocene transition (Delmonte
et al. 2004b). Moreover, we assume that the overall fluctuation of air mass advection over Antarctica depends on the meridional pressure
gradient with respect to low latitudes, i.e. the Antarctic Oscillation (AAO). We therefore suggest a regional variability
in atmospheric circulation over East Antarctica. The 150–500 year power spectrum of the composite (Σ) parameter represents
the long term variability of the AAO, imprinted by secular internal oscillations probably related to the southern ocean-climatic
system. On the other hand, the Δ dust composite parameter suggests a persistent atmospheric dipole over East Antarctica delivering
coarser (finer) dust particles alternatively to Vostok and Dome C regions with a bi-centennial periodicity. Indeed, a seesaw
phenomenon in dust size distribution was already observed at three East Antarctic sites during the last deglaciation (Delmonte
et al. 2004b) and was interpreted as a progressive reduction of the eccentricity of the polar vortex with respect to the geographic south
pole. Interestingly, the Δ parameter shows a pronounced 200-year oscillation mode, throwing new light on the unresolved question
of a possible relationship between climate and solar activity. 相似文献
The progress of the joint Pilot Project for time transfer, formed by the International GPS Service (IGS) and the Bureal International
des Poids et Mesures (BIPM), was recently reviewed. Three notable milestones were set. (1) The IGS will implement, at least
in a test mode, an internally realized time scale based on an integration of combined frequency standards within the IGS network.
This will eventually become the reference time scale for all IGS clock products (instead of the current GPS broadcast time).
(2) A new procedure for combined receiver and satellite clock products will be implemented officially in November 2000. Receiver
clocks are an entirely new product of the IGS. (3) The BIPM will coordinate an effort to calibrate all Ashtech Z12-T (and
possibly other) receivers suitable for time transfer applications, either differentially or absolutely. Progress reports will
be presented publicly in the spring 2001. ? 2001 John Wiley & Sons, Inc. 相似文献