Changes In Orbital Parameters Research Articles

Interaction of Vegetation and Atmospheric Dynamical Mechanisms in the Mid-Holocene African Monsoon*

Abstract Paleoevidence indicates that generally wetter conditions existed in the Sahara during the mid-Holocene. Climate modeling studies addressing this issue generally agree that mid-Holocene values of the earth’s orbital parameters favored an enhanced North African summer monsoon but also suggest that land surface and vegetation feedbacks must have been important factors. Attempts to reproduce the “green” mid-Holocene Sahara in model studies with interactive vegetation may be interpreted to indicate that the problem is highly sensitive to the atmospheric dynamics of each model employed. In other work, dynamical mechanisms have been hypothesized to affect monsoon poleward extent, particularly ventilation, by import of low-moist static energy air to the continent. Here, interactive vegetation and the ventilation mechanism are studied in an intermediate complexity atmospheric model coupled to simple land and vegetation components. Interactive vegetation is found to be effective at enhancing the precipitation and vegetation amount in regions where the monsoon has advanced because of changes in orbital parameters or ventilation yet not very effective in moving the monsoon boundary if ventilation is strong. The poleward extent of the mid-Holocene monsoon and the steppe boundary are primarily controlled by the strength of ventilation in the atmospheric model. Within this boundary, the largest changes in monsoon precipitation and vegetation occur when interactive vegetation and reduced ventilation act simultaneously, as these greatly reinforce each other.

Understanding the Mid-Holocene Climate

Abstract Paleoclimatic evidence suggests that during the mid-Holocene epoch (about 6000 yr ago) North America and North Africa were significantly drier and wetter, respectively, than at present. Modeling efforts to attribute these differences to changes in orbital parameters and greenhouse gas (GHG) levels have had limited success, especially over North America. In this study, the importance of a possibly cooler tropical Pacific Ocean during the epoch (akin to a permanent La Niña–like perturbation to the present climate) in causing these differences is emphasized. Systematic sets of atmospheric general circulation model experiments, with prescribed sea surface temperatures (SSTs) in the tropical Pacific basin and an interactive mixed layer ocean elsewhere, are performed. Given the inadequacies of current fully coupled climate models in simulating the tropical Pacific climate, this intermediate coupling model configuration is argued to be more suitable for quantifying the contributions of the altered orbital forcing, GHG levels, and tropical Pacific SST conditions to the different mid-Holocene climates. The simulated responses in this configuration are in fact generally more consistent with the available evidence from paleovegetation and sedimentary records. Coupling to the mixed layer ocean enhances the wind–evaporation–SST feedback over the tropical Atlantic Ocean. The net response to the orbital changes is to shift the North Atlantic intertropical convergence zone (ITCZ) northward, and make North Africa wetter. The response to the reduced GHG levels opposes, but does not eliminate, these changes. The northward-shifted ITCZ also blocks the moisture supply from the Gulf of Mexico into North America. This drying tendency is greatly amplified by the local response to La Niña–like conditions in the tropical Pacific. Consistent with the paleoclimatic evidence, the simulated North American drying is also most pronounced in the growing (spring) season.

Response of the intermediate complexity Mars Climate Simulator to different obliquity angles

A climate model of intermediate complexity, named the Mars Climate Simulator, has been developed based on the Portable University Model of the Atmosphere (PUMA). The main goal of this new development is to simulate the climate variations on Mars resulting from the changes in orbital parameters and their impact on the layered polar terrains (also known as permanent polar ice caps). As a first step towards transient simulations over several obliquity cycles, the model is applied to simulate the dynamical and thermodynamical response of the Martian climate system to different but fixed obliquity angles. The model is forced by the annual and daily cycle of solar insolation. Experiments have been performed for obliquities of φ = 15 ∘ (minimum), φ = 25 . 2 ∘ (present), and φ = 35 ∘ (maximum). The resulting changes in solar insolation mainly in the polar regions impact strongly on the cross-equatorial circulation which is driven by the meridional temperature gradient and steered by the Martian topography. At high obliquity, the cross-equatorial near surface flow from the winter to the summer hemisphere is strongly enhanced compared to low obliquity periods. The summer ground temperature ranges from 200 K ( φ = 15 ∘ ) to 250 K ( φ = 35 ∘ ) at 80 ∘ N in northern summer, and from 220 K ( φ = 15 ∘ ) to 270 K ( φ = 35 ∘ ) at 80 ∘ S in southern summer. In the atmosphere at 1 km above ground, the respective range is 195–225 K in northern summer, and 210–250 K in southern summer.

Does interplanetary dust control 100 kyr glacial cycles?

The cause of the 100kyr glacial–interglacial cycles during the past 800kyr is one of the fundamental puzzles in paleoclimatology. The widely accepted Milankovitch theory, relating earth's climate cycles to variations in insolation caused by periodic changes in orbital parameters, has difficulties to explain the predominant 100kyr rhythm. Although earth's eccentricity varies with a period of 100kyr, the resulting change in insolation is too small to produce the corresponding climate cycle by direct forcing (Imbrie et al., Paleoceanography 8 (1993) 699). In order to solve the `100kyr problem', Muller and MacDonald (Nature 377 (1995) 107; Science 277 (1997a) 215; Proc. Nat. Acad. Sci. USA 94 (1997b) 8329) proposed an alternative orbital but non-Milankovitch mechanism attributing the glacial cycles to regular variations in the accretion of interplanetary dust particles (IDP) caused by 100kyr cycles in the orbital inclination of the earth. To test this controversial hypothesis, we study the IDP accumulation in deep-sea sediments from a period in the early Pleistocene. We find apparent 41kyr cycles but no 100kyr periodicity in the IDP accumulation rate. As there is no known mechanism to produce 41kyr cycles in IDP supply from space, we conclude that the 41kyr cycles are caused by the dynamics of sediment accumulation, and that changes in the IDP flux do not drive the Pleistocene glacial cycles.

13 years of timing of PSR B1259−63

This paper summarizes the results of 13 years of timing observations of a unique binary pulsar, PSR B1259$-$63, which has a massive B2e star companion. The data span encompasses four complete orbits and includes the periastron passages in 1990, 1994, 1997 and 2000. Changes in dispersion measure occurring around the 1994, 1997 and 2000 periastrons are measured and accounted for in the timing analysis. There is good evidence for a small glitch in the pulsar period in 1997 August, not long after the 1997 periastron, and a significant frequency second derivative indicating timing noise. We find that spin-orbit coupling with secular changes in periastron longitude and projected semi-major axis ($x$) cannot account for the observed period variations over the whole data set. While fitting the data fairly well, changes in pulsar period parameters at each periastron seem ruled out both by X-ray observations and by the large apparent changes in pulsar frequency derivative. Essentially all of the systematic period variations are accounted for by a model consisting of the 1997 August glitch and step changes in $x$ at each periastron. These changes must be due to changes in the orbit inclination, but we can find no plausible mechanism to account for them. It is possible that timing noise may mask the actual changes in orbital parameters at each periastron, but the good fit to the data of the $x$ step-change model suggests that short-term timing noise is not significant.

Abrupt retreat of summer monsoon at the S1/L1 boundary in China

At a number of localities in the Chinese Loess Plateau, the boundary between the polygenetic soil complex S1 and the overlying loess unit L1 is exceptionally sharp. Both units formed on the near-horizontal surfaces by subaerial deposition of eolian dust and are unaffected by erosion. Since the sedimentation across the boundary is essentially continuous, the feature points to a relatively rapid decline of pedogenic activity, which would otherwise mix the sediment and smooth the transition. In this respect, the boundary is not dissimilar to the “markers” in Central and Western Europe, which separate humous steppe soils of the Early Glacial interstadials from the sterile slope sediments and are interpreted as deposits of a major continental-scale dust storm. The reconstructed rate of environmental change across the boundary, whose age within the precision limits of available dating corresponds to the MIS 5/4 shift, is by an order of magnitude greater than the rate of change of orbital parameters. The deflection of atmospheric circulation in response to the build-up of continental ice sheets would also be likely to take a considerably longer time. We suggest that the most probable cause of the rapid monsoon retreat in the Loess Plateau, documented by the onset of the L1 loess deposition, is the sudden rearrangement of oceanic conveyor belt possibly triggered by the Toba volcanic explosion.

Perturbative solution of the motion of an asteroid in resonance with Jupiter

Using a new technique, we derive general formulae for the rate of change of orbital parameters of an asteroid with negligible mass, perturbed by a massive planet (such as Jupiter) having a commensurable period. The relative strength of the perturbing force defines the perturbation parameter, but our results are also expanded in terms of both eccentricities (of the perturbing and the perturbed body) and the mutual inclination. This approach can serve as a valuable tool in the ongoing study of the so-called stable chaos, and may facilitate its ultimate explanation.

Role of vegetation and soil in the Holocene megathermal climate over China

There was significant difference between vegetation cover in mid‐Holocene and that at present over China based on various evidences as concluded by Shi et al [1992]. These changes were introduced to the climate model to study the role of the changing vegetation and associated soil to the simulated climate of mid‐Holocene at 6000 years before present (BP) with bigger seasonal cycle of insolation than present (caused by the change of orbital parameters). Results show that the changes in vegetation and soil could further strengthen the monsoon rainfall over China. We found that replacing the today's seasonal cycle of insolation by that of mid‐Holocene increases the summer (June‐August) precipitation by 20% in eastern China (100°–120°E, 18°–42°N); while considering both the effect of changing orbital forcing and the vegetation and soil, the total summer precipitation increase is 29% in the same region. Compared with the reconstructed precipitation over China, the result of that including changing vegetation and soil is quantitatively better.

Processes of buildup and retreat of the Greenland Ice Sheet

Sensitivity experiments were conducted to study the influence of several physical processes on the evolution of the Greenland ice sheet over the last 250,000 years. The experiments were carried out by means of a three‐dimensional thermodynamical model. The mass balance is modeled by a surface energy balance model. This means that variations in orbital parameters following from the astronomical (Milankovitch) theory can be incorporated in the ablation calculations. It is shown that with regard to the Greenland ice sheet, variations in shortwave radiation are only of minor importance for the changes in volume over time. Calculations of the various components of the energy balance under glacial and interglacial conditions show that the changes in absorbed shortwave radiation resulting from changes in orbital parameters are considerably smaller than the variations in the sensible heat flux caused by temperature variations. Thermodynamics are only of importance for the volume changes of the ice sheet if the climate, is on average, stable over a long period. In between 110 kyr B.P. and 20 kyr B.P. the ice volume increased slowly due to the cooling of the ice, which resulted in a higher effective viscosity and thus a thicker ice sheet. The long response time of the thermodynamics means that the thermodynamics cannot play a crucial role during transition from a glacial to a deglacial period which occur typically within several thousands of years. The mass balance height feedback has an important function in the buildup of the ice sheet after a warm period like the Eemian. Without isostatic bedrock compensation the ablation will remain very high in some marginal parts, yielding a considerably smaller ice sheet volume for the last glacial period.

Sensitivity Studies of Northern Hemisphere Glaciation Using an Atmospheric General Circulation Model

Abstract The U.K. University Global Atmospheric Modeling Programme GCM is used to investigate whether the growth of Northern Hemisphere ice sheets could have been initiated by changes of orbital parameters and sea surface temperature. Two different orbital configurations, corresponding to the present day and 115 kyr BP are used. The reduced summer solar insulation in the Northern Hemisphere results in a decrease of the surface temperature by 4° to 10°C in the northern continents and to perennial snow in some high-latitude regions. Therefore, the model results support the hypothesis that a deficit of summer insulation can create conditions favorable for initiation of ice sheet growth in the Northern Hemisphere. A decreased sea surface temperature northward of 65°N during the Northern Hemisphere summer may contribute to the maintenance of ice sheets. A simple mixed-layer ocean model coupled to the GCM indicates that the changes of sea surface temperature and extension of sea ice due to insulation changes pl...

A loess-paleosol dating method

Based on the non-diachronic nature of loess- paleosol sequences, a paleosol dating method is suggested. Based on this method, the extant drainage system of the Yellow River was found to be 1.67-1.45 Ma old. During its existence, six fluvial terraces and two alluvial plains were formed. In the deposits of this time interval, studied in 400 sections, 33 cycles were identified in the sequence built of aquatic sediments and paleosol. Correlation of loess, paleosol, and aquatic sediment sequences of China and other regions points to a 0.4 Ma periodicity in the hydrologic regime and in the denudational processes. This periodicity might be related to global climatic changes due to changes of orbital parameters of the Earth.

The Response to Orbital Perturbations in an Atmospheric Model Coupled to a Slab Ocean

The sensitivity of an atmospheric GCM coupled to a mixed-layer ocean to changes in orbital parameters is investigated. Three experiments are compared. One has perihelion at summer solstice and a large obliquity; another has perihelion at winter solstice and a low obliquity. The first of these is favorable for warm summers; the second for cool summers. A third experiment, with perihelion at summer solstice and the lower value of obliquity, is used to examine the relative importance of the changes in perihelion and obliquity. The eccentricity is set at 0.04 in all cases. Surface temperature responses are as large as 15°C, with the largest response over North America in summer. Changes in monsoons and Arctic sea ice are consistent with previous GCM studies. A perpetual summer version of the atmospheric model is used to investigate the positive feedback due to soil moisture. Drying of the soil over North America is found to increase the temperature response by approximately 50% and is also essential to the decrease in summertime precipitation in that region. Soil moisture changes also enhance the precipitation response over central Africa, but have little effect on the model's Asian monsoon. The orbital parameters most favorable for expansion of the Northern Hemisphere glaciers, that is, minimal seasonality, do not produce permanent snow cover. Several model deficiencies that act to accelerate the melting of snow in spring may be responsible.

Cold Periods During the Last Millennium

Studies of temperature change during the last millennium in China and in other regions of the globe were reviewed. Cold periods were identified for East Asia, USSR, Europe, North America, Polar Region and Southern Hemisphere. Most of them were grouped into the same time intervals: first half of12th, second half of 13th, second half of 15th, 17th and 19th century. Temperatures in the last two cold periods were 0.5°C-1.0°C lower than the average of the 20th century. The first three cold periods were less cold and less uniform in geographical distribution and in temporal variations. Therefore, the first three cold periods may be regarded as the transition from the Medieval Warm Period to the Little Ice Age. Long-term variations of solar activity and volcanism were compared with the changes of temperatures. The second to the fourth cold periods seem to relate to the Wolf, Sporer, and Maunder Minima of solar activity. The intensification of explosive volcano eruptions in early15th and 17th century may also have contributed to the occurrence of the third arid fourth cold periods. However, only a slight increase of volcanism and a weak decrease in solar activity in the early 19th century can hardly fully intepret the severe cold period throughout the 19th century. Perhaps neither one nor both of the aforementioned factors can be responsible for all of the five cold periods identified during the last millennium. It is suggested that the changes in cryosphere, biosphere and the oceans may interact with the atmosphere, controlling (together with the external factors) the occurrence of the cold periods. Of course, anthropogenic factors should also be considered, especially for the last one and a half centuries. Finally, factor or factors other than those aforementioned, such as changes in orbital parameters, may also take part in regulation of the climate for the last millennium.

A sensitivity study of IAP AGCM to radiation changes: Climate simulation of 125kyr and 115kyr before present

The IAP AGCM was used to simulate the climate of 125kyr and 115kyr before present. We analysed the results and then studied the sensitivity of the model to the changes of radiation distribution induced by orbital parameter changes. The reasonability of the results was also discussed.

Sensitivity of the Indian monsoon to forcing parameters and implications for its evolution

General-circulation-model simulations used to estimate the sensitivity of the Indian monsoon to changes in orbital parameters, the orography of Tibet—Himalaya, atmospheric C02 concentration and the extent of glacial-age surface boundary conditions show that increased elevations and increased summer solar radiation are most effective in strengthening the monsoon. Strong monsoons (similar to today's) can be induced by strong solar forcing only when the elevation is at least half that of today. These conditions may have been attained in the late Miocene.

Eclipsing millisecond pulsar - Excitation of the companion wind

view Abstract Citations (16) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Eclipsing Millisecond Pulsar: Excitation of the Companion Wind Cheng, Andrew F. Abstract The 1957 + 20 system is a millisecond pulsar in an eclipsing binary. Even if the companion filled its Roche lobe, it could not explain the observed eclipse duration. It has been proposed that the pulsar evaporates a plasma wind from the companion to cause the eclipse and that the companion will eventually be destroyed. This paper presents a model for pulsar evaporation of a wind from a close binary companion. The wind solution is specified by the critical point behavior at the sonic point transition. The lifetime of the 1957 + 20 companion against evaporation is typically less than 3 x 10 to the 8th yr. The incident pulsar radiation that drives the wind is constrained to have penetration depths about 0.008-1 g/sq cm. Observable changes in orbital parameters are expected and may result mainly from mass loss of the companion. Publication: The Astrophysical Journal Pub Date: April 1989 DOI: 10.1086/167296 Bibcode: 1989ApJ...339..291C Keywords: Eclipsing Binary Stars; Pulsars; Stellar Winds; Brown Dwarf Stars; Incident Radiation; Roche Limit; Stellar Orbits; Astrophysics; PULSARS; STARS: ECLIPSING BINARIES; STARS: WINDS full text sources ADS | data products SIMBAD (2)

Quaternary history of the temperate forests of China

Pollen data from 80 sites in North China and Northeast China are reviewed to document the Quaternary history of the deciduous forest and the temperate mixed conifer-hardwood forest of China. During the Late Tertiary the forest in North China consisted of an admixture of temperate deciduous hardwoods, subtropical broadleaved evergreen elements, and ancient conifers of tropical and subtropical affinities. Pollen evidence from long boreholes through Quaternary deposits indicates four or five glacial/interglacial climatic cycles. The first glacial episode, known as the Nangou Cold Period, led to the development of a spruce-fir forest of no modern analogue in North China. Deciduous forest was re-established in North China during the intervening interglacials. The subtropical evergreen elements and Tertiary relicts were successively eliminated during the subsequent glacials. In the last glacial, spruce-fir forests which are now confined to mountain slopes above 1500 m descended to the lowlands of North and Northeast China, implying a temperature depression of at least 8–10°C. Cold steppe occurred at least locally near Beijing in the last glacial maximum, but the data base is insufficient to delimit the spatial and temporal extent of this and other paleovegetation types on a regional scale, or to reconstruct the locations of glacial refugia and the dynamics of plant migration. The Holocene pollen stratigraphies suggest a tripartite division, with a period of maximum warmth, the Hypsithermal, in the mid-Holocene. In both North and Northeast China, the forest became more diverse during the mid-Holocene when thermophilous hardwoods expanded at the expense of pine and, in more southerly locations, birch. Unequivocal evidence for intensified summer monsoon during the early Holocene, as predicted by the Kutzbach model based on orbital parameter changes, remains to be found from the pollen records of the temperate forest regions of China.

The Influence of Changing Orbital Parameters and Surface Boundary Conditions on Climate Simulations for the Past 18 000 Years

Abstract General circulation model experiments at 3000-year intervals for the past 18 000 years were made to estimate the magnitude, timing, and pattern of the climatic response to prescribed changes of orbital parameters (date of perihelion, axial tilt, eccentricity) and glacial-age lower boundary conditions (ice sheets, land albedo, sea ice and sea surface temperature). The experiments used the Community Climate Model (CCM) of the National Center for Atmospheric Research (NCAR). The response of monsoon circulations and tropical precipitation to the orbitally produced solar radiation changes was much larger than the response to changes of glacial-age boundary conditions. The continental interior of Eurasia was 2–4 K warmer in summer, and summer monsoon precipitation of North Africa-South Asia was increased by 10–20% between 12 000 and 6000 yr BP (before present) when perihelion occurred during northern summer (rather than in winter as now) and the earth's axial tilt was larger than now. Southern Hemisphe...

The Martian hydrologic cycle: Effects of CO 2 mass flux on global water distribution

The Martian CO 2 cycle, which includes the seasonal condensation and subsequent sublimation of up to 30% of the planet's atmosphere, produces meridional winds due to the consequent mass flux of CO 2. These winds currently display strong seasonal and hemispheric asymmetries due to the large asymmetries in the distribution of insolation on Mars. It is proposed that asymmetric meridional advection of water vapor on the planet due to these CO 2 condensation winds is capable of explaining the observed dessication of Mars' south polar region at the current time. A simple model for water vapor transport is used to verify this hypothesis and to speculate on the effects of changes in orbital parameters on the seasonal water cycle.

The Martian hydrologic cycle: Effects of CO2 mass flux on global water distribution

The Martian CO 2 cycle, which includes the seasonal condensation and subsequent sublimation of up to 30% of the planet's atmosphere, produces meridional winds due to the consequent mass flux of CO 2 . These winds currently display strong seasonal and hemispheric asymmetries due to the large asymmetries in the distribution of insolation on Mars. It is proposed that asymmetric meridional advection of water vapor on the planet due to these CO 2 condensation winds is capable of explaining the observed dessication of Mars' south polar region at the current time. A simple model for water vapor transport is used to verify this hypothesis and to speculate on the effects of changes in orbital parameters on the seasonal water cycle.