Abrupt changes as indicators of decadal climate variability

 A statistical test has revealed that abrupt regional climate changes are produced in a coupled atmosphere-ocean general circulation model. Abrupt changes are detected over much of the globe although the occurrence frequency is small over the continents. Over the tropical Pacific Ocean and northern Pacific Ocean, surface air temperature (SAT) and sea level pressure (SLP) shift rapidly on decadal time scales. The regional climate changes presented here have been classified into three types. The first type consists of statistically significant shifts in SLP and statistically significant shifts in SAT which are of opposite sign, and which are reinforced through a positive feedback between the atmosphere and the ocean. The second type is for those occurrences where changes are of the same sign. The third type includes those with a significant shift in only one meteorological element. The second and third types are generally generated by changes in air pressure and wind fields induced by changes of the first type. For example, when SLP increases and sea surface temperature (SST) decreases abruptly in the tropical Pacific Ocean, it triggers abrupt regional changes in middle and high latitudes. The abrupt changes in the model climate have characteristics which are very similar to those of observed rapid shifts. Thus, it is concluded that abrupt changes are a predominant part of regional climate change on decadal time scales. Received: 11 February 1999 / Accepted: 18 May 2000     

Transport and deposition of large woody debris in streams: a flume experiment

Large woody debris (LWD) is an integral component of forested streams of the Pacific Northwest and elsewhere, yet little is known about how far wood is transported and where it is deposited in streams. In this paper, we report the results of flume experiments that examine interactions among hydraulics, channel geometry, transport distance and deposition of floating wood. These experiments were carried out in a 1.22-m-wide×9.14-m-long gravel bed flume using wooden dowels of various sizes as surrogate logs. Channel planforms were either self-formed or created by hand, and ranged from meanders to alternate bars. Floating pieces tended to orient with long axes parallel to flow in the center of the channel. Pieces were deposited where channel depth was less than buoyant depth, typically at the head of mid-channel bars, in shallow zones where flow expanded, and on the outside of bends. We hypothesize that the distance logs travel may be a function of the channel's debris roughness, a dimensionless index incorporating ratios of piece length and diameter to channel width, depth and sinuosity. Travel distance decreased as the ratio of piece length to both channel width and radius of curvature increased, but the relative importance of these variables changed with channel planform. Large pieces can move further than our debris roughness models predict if greater than 50% of the active channel area is deeper than the buoyant depth of the piece, or if momentum is high enough to carry pieces across shallows. Our debris roughness model allows first-order prediction of the amount of wood transport under various channel geometries.     

Sensitivity of a GCM simulation to subgrid infiltration and surface runoff

A subgrid parameterization of infiltration and surface runoff was evaluated using a land surface model coupled to an atmospheric general circulation model. Averaged over 5 year simulations, the subgrid parameterization resulted in significantly less infiltration of water into the soil compared to a simulation without subgrid hydrologic processes. As a result, the soils were drier, latent heat flux decreased, and surface air temperature increased. These results are consistent with other studies of subgrid hydrologic parameterizations, which also resulted in drier soils, decreased latent heat flux, and warmer surface temperatures. Several river basins were studied in detail. In the Amazon and Lena basins, the subgrid parameterization resulted in better annual runoff compared to observed annual river flow; surface air temperature was unchanged in the Amazon and better, compared to observations, in the Lena. In the Ob, Yenisey, and Amur basins, the subgrid parameterization resulted in too much annual runoff; July surface air temperature was unchanged or worse (Amur). Annual runoff for the Mississippi basin was better with the subgrid parameterization, but July surface air temperature was worse. These results suggest the utility of subgrid hydrologic parameterizations vary among river basins depending on the relative importance of Horton and Dunne runoff and the geologic factors affecting runoff generation.     

Simulations of precipitation and atmospheric circulation changes associated with warm SSTs: results from an ensemble of long term integrations with idealized anomalies

A general circulation model is used to simulate the atmospheric response to a prescribed, idealized time varying warm sea surface temperature anomaly (SSTA) in the equatorial Pacific characteristic of ENSO events. The model incorporates the full seasonal cycle and an ensemble of five anomaly simulations, each of 14 months duration, is performed and the results compared with those from a ten year control simulation involving climatological SSTs. In a test of the model, simulated perturbations to precipitation patterns are compared with the analysis by Ropelewski and Halpert which identifies regions where precipitation perturbations are associated with the ENSO cycle. Strong and statistically significant perturbations are simulated throughout much of the equatorial Pacific and the Australian region which agree with the analysis. Perturbations over the Americas and the Indian sub-continent show part agreement with observations, while elsewhere, at more remote and high latitude locations, there is less agreement, providing some indication of model limitations in simulating the hydrological cycle. Over the equatorial Pacific most of the anomalous precipitation is accounted for by moisture flux convergence. Only in the vicinity of the maximum anomaly, located in the eastern Pacific, do warmer SSTs contribute to the perturbed circulation. Elsewhere, anomalous wind speeds mainly determine anomalous heat fluxes. As a result, the large scale perturbations to wind, moisture and precipitation appear to be forced indirectly, rather than directly as assumed in simple ocean-atmosphere models.     

Reevaluation of the lake-sediment chronology in the Dead Sea basin, Israel, based on new Th/U dates

The Dead Sea is surrounded by chemical and detrital sediments that were deposited in its larger precursor lakes, Lake Samra and Lake Lisan. The sedimentary history of these lakes was recon-structed by means of ²³⁰Th/²³⁴U ages of 30 samples, mostly of argonite laminae, from 8 columnar sections up to 110 km apart. The general validity of the ages was demonstrated by subjecting them to tests of internal isotopic consistency, agreement with stratigraphic order, and concordance with ¹⁴C ages. In the south, only the part of the Samra Formation older than 170,000 yr is exposed, while the aragonite-detritus rhythmites found in the central and northern region are generally younger than 120,000 yr. The Lisan Formation started accumulating about 63,000 yr B.P., with the clay and aragonite beds in the south-central area reflecting a rise in water level to at least −280 m. The upper part of the Lisan Formation, the aragonite-rich White Cliff Member, started accumulating about 36,000 yr B.P. The lake probably reached its highest level sometime after this, based on the ages of Lisan sediments preserved in the southernmost reaches of the basin.     

Application of particle dynamics to derive a general circulation in low latitudes

Summary In Section 1 are set out the reasons for the use of particle dynamics in this investigation. In Section 2 are solved the equations of motion for particles constrained to move along a horizontal surface under the action of a constant meridional pressure gradient force to obtain (a) velocities in spherical polar co-ordinates with naturally varying Coriolis terms, and (b) velocities and trajectories in plane cartesian co-ordinates in which the Coriolis parameter is assumed constant. Results obtained in each system are compared, showing that the plane co-ordinates as used are not significantly inaccurate, and trajectories of air particles obtained by application significantly inaccurate, and trajectories of air particles obtained by application of this system in steps of 5° of latitude are plotted to show the production of belts of convergence at some distance from the equator under the action of meridional pressure gradient forces.The approximation of these idealised particle trajectories to actual streamlines of the air flow are discussed in Section 3, and the concept of the production of belts of convergence is used to formulate a theory of formation and maintenance of the sub-tropical anticyclones. In Section 4 this theory is applied to observed average temperature data and the calculated results compared with observed average pressure data. A possible synoptic application of the theory is then indicated in Section 5.

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Zusammenfassung Im ersten Abschnitt werden Gründe für die Anwendung der Dynamik von Massenteilchen in der vorliegenden Arbeit dargelegt. Abschnitt 2 enthält die Lösungen der Gleichungen für die Bewegung von Massenteilchen auf horizontalen Flächen unter der Einwirkung eines konstanten meridionalen Druckgradienten. Die Lösungen ergeben a) die Geschwindigkeiten in sphärischen Polarkoordinaten mit variablem Coriolis-Term (entsprechend natürlichen Verhältnissen) und b) Geschwindigkeiten und Trajektorien in ebenen kartesischen Koodinaten in denen der Coriolis-Parameter als konstant angenommen wird. Die Ergebnisse für beide Koordinatensysteme werden verglichen und es zeigt sich, daß bei Verwendung der ebenen Koordinaten keine wesentlichen Ungenauigkeiten auftreten. Zeichnet man schrittweise von 5 zu 5° (Fig. 1) die Trajektorien der Luftteilchen, so ergeben sich als Folge der meridionalen Druckgradienten Konvergenzzonen in einigem Abstand vom Äquator.Die Annäherung dieser idealisierten Trajektorien an tatsächliche Stromlinien der Luft wird in Abschnitt3 diskutiert. An Hand der Vorstellungen von der Entstehung der Konvergenzzonen wird eine Theorie der Entstehung und Erhaltung subtropischer Antizyklonen gegeben. Im vierten Abschnitt wird diese Theorie auf beobachtete Mitteltemperaturen angewendet und die Ergebnisse der Berechnung werden mit beobachteten mittleren Luftdruckwerten verglichen. Auf eine mögliche synoptische Anwendung der vorliegenden Ergebnisse wird in Abschnitt 5 hingewiesen.

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Résumé La première partie contient les motifs de l'emploi de la dynamique de points pesants dans la présente étude. La deuxième partie donne les solutions des équations du mouvement de points pesants sur des surfaces horizontales sous l'effet d'un gradient méridien constant. Ces solutions fournissent les vitesses en coordonnées polaires sphériques avec un terme Coriolis variable, ainsi que les vitesses et les trajectoires en coordonnées cartésiennes planes en admettant un paramètre de Coriolis constant. On compare les résultats pour les deux systèmes de coordonnées et l'on constate que l'emploi des coordonnées planes n'introduit pas d'erreurs appréciables. Si l'on dessine les trajectoires de 5 en 5 degrés on voit apparaître des zones de convergence à quelque distance de l'équateur par suite des gradients méridiens.Dans la troisième partie on discute la similitude plus ou moins grande de ces trajectoires idéalisées avec les lignes de courant réelles. Parant de l'origine possible des zones de convergence, on établit une théorie de l'origine et du maintien des anticyclones subtropicaux. Dans la quatrième partie on applique cette théorie aux températures moyennes observées et l'on compare les résultats avec les valeurs moyennes observées de la pression. Dans la cinquième partie enfin on montre l'application possible des résultats obtenus à la synoptique.

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With 1 Figure.

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This paper is published with the permission of the Director of the meteorological Office.     

Martian craters from earth

Martian craters were evidently observed and described by E. E. Barnard in 1892–1893 and by John E. Mellish in 1915.     

An analytical iterative algorithm for the prediction of special satellite orbit points with the Brouwer orbit theory

Employing a direct recursive algorithm in relation with analytical theories will yield a considerable saving in computer time, as opposed to simulating a point by point integration through repeated evaluations of the orbit theory. As a case in point, we shall compute the set of osculating orbiting elements corresponding to special events within the revolution of an artificial satellite.     

Temperature minimum heating in solar flares by resistive dissipation of Alfvén waves

We examine the possibility that the strong heating produced at temperature-minimum levels during solar flares is due to resistive dissipation of Alfvén waves generated by the primary energy release process in the corona. It is shown how, for suitable parameters, these waves can carry their energy essentially undamped into the temperature-minimum layers and can then produce a degree of heating consistent with observations.Also Department of Applied Physics, Stanford University.