Onset and end of the summer melt season over sea ice: thermal structure and surface energy perspective from SHEBA   总被引：1，自引：0，他引：1  

P. Ola G. Persson 《Climate Dynamics》2012,39(6):1349-1371

Various measurements from the Surface Heat Flux of the Arctic Ocean (SHEBA) experiment have been combined to study structures and processes producing the onset and end of summer melt over Arctic sea ice. The analysis links the surface energy budget to free-troposphere synoptic variables, clouds, precipitation, and in-ice temperatures. The key results are (1) SHEBA melt-season transitions are associated with atmospheric synoptic events (2) onset of melt clearly occurs on May 28, while the end of melt is produced by a sequence of three atmospheric storm events over a 28-day period producing step-like reductions in the net surface energy flux. The last one occurs on August 22.; (3) melt onset is primarily due to large increases in the downwelling longwave radiation and modest decreases in the surface albedo; (4) decreases in the downwelling longwave radiation occur for all end-of-melt transition steps, while increases in surface albedo occur for the first two; (5) decreases in downwelling shortwave radiation contribute only to the first end-of-melt transition step; (6) springtime free-tropospheric warming preconditions the atmosphere–ice system for the subsequent melt onset; and (7) melt-season transitions also mark transitions in system responses to radiative energy flux changes because of invariant melt-season surface temperatures. The extensive SHEBA observations enable an understanding of the complex processes not available from other field program data. The analysis provides a basis for future testing of the generality of the results, and contributes to better physical understanding of multi-year analyses of melt-season trends from less extensive data sets.  相似文献   

Decreasing Arctic Sea Ice Mirrors Increasing CO2 on Decadal Time Scale          下载免费PDF全文

Ola M Johannessen 《大气和海洋科学快报》2008,1(1):51-56

Arctic sea ice is a keystone indicator of greenhouse-gas induced global climate change, which is expected to be amplified in the Arctic. Here we directly compare observed variations in arctic sea-ice extent and CO2 since the beginning of the 20th century, identifying a strengthening linkage, such that in recent decades the rate of sea-ice decrease mirrors the increase in CO2, with r ~ –0.95 over the last four decades, thereby indicating that 90% (r2 ~ 0.90) of the decreasing sea-ice extent is empirically “accounted for” by the increasing CO2 in the atmosphere. The author presents an empirical relation between annual sea-ice extent and global atmospheric CO2 concentrations, in which sea-ice reductions are linearly, inversely proportional to the magnitude of increase of CO2 over the last few decades. This approximates sea-ice changes during the most recent four decades, with a proportionality constant of 0.030 million km2 per ppmv CO2. When applied to future emission scenarios of the Intergovernmental Panel on Climate Change (IPCC), this relationship results in substantially faster ice decreases up to 2050 than predicted by IPCC models. However, departures from this projection may arise from non-linear feedback effects and/or temporary natural variations on interannual timescales, such as the record minimum of sea-ice extent observed in September 2007.Citation: Johannessen, O. M., 2008: Decreasing arctic sea ice mirrors increasing CO2 on decadal time scale, Atmos. Oceanic Sci. Lett., 1, 51-56  相似文献   

Unprecedented Retreat in a 50-Year Observational Record for Petermann Glacier, North Greenland          下载免费PDF全文

Ola M. JOHANNESSEN  Mohamed BABIKER  Martin W. MILES 《大气和海洋科学快报》2013,6(5):259-265

Petermann Glacier is a marine-terminating outlet glacier that had a 70 km-long floating ice tongue prior to a～270 km2calving event that was observed from satellite sensors in August 2010,shortening the ice tongue by～27 km.Further,in July 2012,another 10 km was lost through calving.In order to understand these events in perspective,here the authors perform a long-term data analysis of Petermann Glacier calving-front variability and ice velocity for each year in the 1990s–2000s,supplemented by available observations from the previous three decades.Five major(on the order of 100 km2)calving events are identified,with～153 km2calved from1959 to 1961,～168 km2in 1991,～71 km2in 2001,～270 km2in 2010,and～130 km2in 2012—as well as～31km2calved in 2008.The increased frequency of major calving events in recent years has left the front terminus position retreated nearly 25 km beyond the range of observed in previous decades.In contrast,stable ice-dynamics are suggested from ice-velocity measurements made each year between 1993–2012,which are on average1063 m yr–1,with limited interannual variability and no significant trend;moreover,there is no apparent relationship between ice-velocity variability and calving events.The degree to which the massive calving events in 2010and 2012 represent natural episodic variability or a response to atmospheric and/or oceanic changes remains speculative;however,melt-induced weakening of the floating ice tongue in recent years is strongly suggested.  相似文献   

An XMM–Newton observation of Ton S180: constraints on the continuum emission in ultrasoft Seyfert galaxies     

S. Vaughan  Th. Boller  A. C. Fabian  D. R. Ballantyne  W. N. Brandt  J. Trümper 《Monthly notices of the Royal Astronomical Society》2002,337(1):247-255

We present an XMM–Newton observation of the bright, narrow-line, ultrasoft type 1 Seyfert galaxy Ton S180. The  0.3–10 keV  X-ray spectrum is steep and curved, showing a steep slope above 2.5 keV  (Γ∼ 2.3)  and a smooth, featureless excess of emission at lower energies. The spectrum can be adequately parametrized using a simple double power-law model. The source is strongly variable over the course of the observation but shows only weak spectral variability, with the fractional variability amplitude remaining approximately constant over more than a decade in energy. The curved continuum shape and weak spectral variability are discussed in terms of various physical models for the soft X-ray excess emission, including reflection off the surface of an ionized accretion disc, inverse Compton scattering of soft disc photons by thermal electrons, and Comptonization by electrons with a hybrid thermal/non-thermal distribution. We emphasize the possibility that the strong soft excess may be produced by dissipation of accretion energy in the hot, upper atmosphere of the putative accretion disc.  相似文献   

Nuclear obscuration in the high-ionization Seyfert 2 galaxy Tol 0109–383     

K. Iwasawa  G. Matt  A.C. Fabian  S. Bianchi  W.N. Brandt  M. Guainazzi  T. Murayama  Y. Taniguchi 《Monthly notices of the Royal Astronomical Society》2001,326(1):119-125

We report on the BeppoSAX detection of a hard X-ray excess in the X-ray spectrum of the classical high-ionization Seyfert 2 galaxy Tol 0109–383. The X-ray emission of this source observed below 7 keV is dominated by reflection from both cold and ionized gas, as seen in the ASCA data. The excess hard X-ray emission is presumably caused by the central source absorbed by an optically thick obscuring torus with N _H∼2×10²⁴ cm⁻² . The strong cold X-ray reflection, if it is produced at the inner surface of the torus, is consistent with the picture where much of the inner nucleus of Tol 0109–383 is exposed to direct view, as indicated by optical and infrared properties. However, the X-ray absorption must occur at small radii in order to hide the central X-ray source but leave the optical high-ionization emission-line region unobscured. This may also be the case for objects such as the Seyfert 1 galaxy Mrk231.  相似文献   

Kgagodi Basin: The first impact structure recognized in Botswana     

D. Brandt  H. Holmes  W. U. Reimold  B. K. Paya  C. Koeberl  P. J. Hancox 《Meteoritics & planetary science》2002,37(12):1765-1779

Abstract— The 3.4 km wide, so‐called Kgagodi Basin structure, which is centered at longitude 27°34.4′ E and latitude 22°28.6′ S in eastern Botswana, has been confirmed as a meteorite impact structure. This crater structure was first recognized through geophysical analysis; now, we confirm its impact origin by the recognition of shock metamorphosed material in samples from a drill core obtained close to the crater rim. The structure formed in Archean granitoid basement overlain and intruded by Karoo dolerite. The crater yielded a gravity model consistent with a simple bowl‐shape crater form. The drill core extends to a depth of 274 m and comprises crater fill sediments to a depth of 158 m. Impact breccia was recovered only between 158 and 165 m depth, below which locally brecciated basement granitoids grade into fractured and eventually undeformed crystalline basement, from ～250 m depth. Shock metamorphic effects were only found in granitoid clasts in the narrow breccia zone. This breccia is classified as suevitic impact breccia due to the presence of melt and glass fragments, at a very small abundance. The shocked grains are exclusively derived from granitoid target material. Shock effects include multiple sets of planar deformation features in quartz and feldspar; diaplectic quartz, and partially and completely isotropized felsic minerals, and rare melt fragments were encountered. Abundances of some siderophile elements and especially, Ir, in suevitic breccia samples are significantly elevated compared to the contents in the target rocks, which provides evidence for the presence of a small meteoritic component. Kgagodi is the first impact structure recognized in the region of the Kalahari Desert in southern Africa. Based on lithological and first palynological evidence, the age of the Kgagodi structure is tentatively assigned to the upper Cretaceous to early Tertiary interval. Thus, the crater fill has the potential to provide a long record of paleoclimatic conditions.  相似文献   

A Holocene sea‐level curve and revised isobase map based on isolation basins from near the southern tip of Norway          下载免费PDF全文

Anders Romundset  Ola Fredin  Fredrik Høgaas 《Boreas: An International Journal of Quaternary Research》2015,44(2):383-400

We report a new Holocene relative sea‐level curve based on the stratigraphy in five closely located isolation basins near Lista in southernmost Norway. The results detail the progress and timing of the mid‐Holocene Tapes transgression, the peak of which in this region represents the highest postglacial sea level, as well as the rate of land emergence since then. One additional cored basin is situated above the marine limit. All the basins have bedrock sills that were levelled using a differential GPS. Isolation and ingression boundaries were identified by macrofossil analysis and radiocarbon dated on terrestrial plant remains. In most cases several dates were obtained from each transition. Relative sea level rose with a mean rate of 7 mm a^?1 during the last part of the Tapes transgression 8600?8200 cal. a BP and then gradually slowed to a mean rate of 1 mm a^?1 from 8200?7000 cal. a BP. Mean sea level reached ～5 m higher than the present level when the transgression culminated. Land emergence took place after this, first slowly at a mean rate of 0.4 mm a^?1 until ～3900 cal. a BP before it increased to 2.6 mm between 3900 and 3400 cal. a BP. Since then it has slowly decreased until today and has been ～0.2 mm a^?1 for the last 2000 years. Based on the new curve we present updated Tapes isobases for the region that are displaced by ～20 km in relation to the existing model. From one basin we also report a 5–10 cm thick layer of sorted, sandy gravel, embedded in a more than 5‐m‐thick deposit of homogeneous shallow‐marine mud. The gravel was deposited ～5500 cal. a BP, which is the same age as a tsunami deposit previously mapped in Shetland. As several typical characteristics of tsunami facies deposits are lacking, the origin of the gravel layer remains inconclusive.  相似文献   

A rate‐dependent cohesive crack model based on anisotropic damage coupled to plasticity     

Per‐Ola Svahn  Kenneth Runesson 《国际地质力学数值与分析法杂志》2006,30(9):887-916

In quasi‐brittle material the complex process of decohesion between particles in microcracks and localization of the displacement field into macrocracks is limited to a narrow fracture zone, and it is often modelled with cohesive crack models. Since the anisotropic nature of the decohesion process in separation and sliding is essential, it is particularly focused in this paper. Moreover, for cyclic and dynamic loading the unloading, load reversal (including crack closure) and rate dependency are essential features that are included in a new model. The modelling of degradation is based on a ‘localized’ version of anisotropic continuum damage coupled to inelasticity. The concept of strain energy equivalence between the states in the effective and nominal settings is adopted in order to define the free energy of the interface. The proposed fracture criterion is of the Mohr type, with a smooth transition of the failure and kinematics (slip and dilatation) characteristics between tension and shear. The chosen potential, of the Lemaitre‐type, for evolution of the dissipative processes is additively decomposed into plastic and damage parts, and non‐associative constitutive equations are obtained. The constitutive equations are integrated by applying the backward Euler rule and by using Newton iteration. The proposed model is assessed analytically and numerically and a typical calibration procedure for concrete is proposed. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

On the existence of small comets and their interactions with planets     

J. C. Brandt  M. F. A'Hearn  C. E. Randall  D. G. Schleicher  E. M. Shoemaker  A. I. F. Stewart 《Earth, Moon, and Planets》1996,72(1-3):243-249

Arguments are presented for a substantial, unexplored population of comets with radii less than 1 km. Known examples confirm this population and extrapolation of any plausible size-distribution function indicates large numbers. However, their accurate numbers, orbital characteristics, and physical properties are unknown. Thus, even though the small comets may be the most frequent cometary bodies impacting the planets, a quantitative evaluation is not currently possible. We advocate an optimized, dedicated search program to characterize this population.  相似文献   

Slow variations in the light of BY Dra     

S. A. Melkonyan  K. Ola  A. V. Oskanyan Jr  V. S. Oskanyan 《Astrophysics》1981,17(2):112-117

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