Linking Arctic sea‐ice and atmospheric circulation anomalies on interannual and decadal timescales

Abstract

The relationship between Arctic sea‐ice concentration anomalies, particularly those associated with the “Great Salinity Anomaly” of 1968–1982, and atmospheric circulation anomalies north of 45°N is investigated. Empirical orthogonal function (EOF) analyses are performed on winter Arctic ice concentration from 1954 to 1990, sea level pressure and 500‐hPa heights from 1947 to 1994, and 850‐hPa temperatures from 1963 to 1994. Variability on both interannual and decadal timescales is apparent in the time series of the leading winter EOFs of all variables. The first EOF of winter sea‐ice concentration was found to characterize the patterns of ice variability associated with the Great Salinity Anomaly in the northern North Atlantic from 1968–82. Spatial maps of temporal correlation coefficients between the time series of the first EOF of winter sea‐ice concentration and the winter atmospheric anomaly fields are calculated at lags of 0 and ±7 year. Maximum correlations were found to exist when the time‐series of this ice EOF 1 leads the atmospheric anomaly fields by one year. A particularly interesting result is the connection between the presence of ice anomalies in the Greenland and Barents Seas and subsequent pressure anomalies of the same sign over the Irminger Basin and the Canadian Arctic. The main emphasis of the paper is to identify connections between Arctic sea‐ice and atmospheric circulation anomalies at interannual time‐scales.     

Phase‐locked and asymmetric correlations of the wintertime atmospheric patterns with the ENSO

Abstract

Teleconnections between sea surface temperature (SST) anomalies over the Pacific and the dominant patterns of wintertime Northern Hemisphere 500‐hPa height are examined by applying statistical techniques such as rotated principal component analysis and composite analysis. It is shown that the Pacific/North American (PNA) patterns in December through March are correlated most significantly with the ENSO‐related SST anomalies in the previous October, while the western Pacific (WP) patterns in December through February are most closely linked to the ENSO‐related SST anomalies in the same season. In addition, the PNA response to the ENSO signal during La Niña events is more significant than that during El Niño events, while the WP response is stronger during El Niño events than during La Niña events. A composite analysis shows that in the El Niño winters the North Pacific centre of the PNA pattern is located about 10 degrees east of its normal position, leading to a less significant correlation between the ENSO signal and the PNA pattern in these winters.

The ENSO‐related SST anomalies include a large centre of action over the tropical Pacific and an oppositely signed anomaly centre over the North Pacific. The North Pacific centre appears to the west of the dateline in September and October. This ENSO‐related seed of SST anomalies slowly moves eastward in the following months, gradually cutting off its connection with SST anomalies over the tropical Pacific and being coupled with the PNA pattern. It is pointed out that, although the wintertime SST anomaly over the North Pacific may appear as a mode linearly independent of the ENSO signal in the same season, it is partially related to the ENSO signal in the preceding autumn.

Possible dynamical explanations of the above results are discussed. It is suggested that the WP pattern can be linked to the tropical Pacific heat source via advection of vorticity by the upper‐tropospheric divergent/convergent flow, and the intensification of vorticity gradients associated with a stronger east Asian jet is likely to be responsible for a more significant WP pattern response to the ENSO signal in the El Niño winters. On the other hand, the ENSO‐related PNA pattern could be considered a manifestation of the eastward extension (El Niño) or westward withdrawal (La Niña) of the east Asian jet stream due to the local Hadley cell over the Pacific. In addition, the ENSO‐related seed of extratropical SST anomaly over the western Pacific in autumn may also play an important role in the development of the PNA pattern in the following winter.     

Magmatic Fluid Inclusions from the Zaldivar Deposit, Northern Chile: The Role of Early Metal-bearing Fluids in a Porphyry Copper System

Abstract. The occurrence of a distinct type of multi-solid, highly-saline fluid inclusions, hosted in igneous quartz phe-nocrysts from the Llamo porphyry, in the Zaldivar porphyry copper deposit of northern Chile is documented. Total homoge-nization of the multi-solid type inclusions occurs at magmatic temperatures (over 750C), well above the typical temperatures of hydro thermal fluids (less than 600C) usually recorded in porphyry copper systems. The analysis of this type of fluid inclusions, using a combination of non-destructive microthermometry, Raman and PIXE techniques and the identification of daughter minerals by SEM method, indicates that the trapped fluid was a dense, complex chloride brine in which Cl, Na, K, Fe, Cu, and Mn are dominant. The high chlorine and metal contents indicate that the metals were separated from the crystallizing magma as homogeneous aqueous chloride-rich solutions that represent the primary magmatic fluids exsolved at high temperatures and depth during the crystallization of the parental intrusive.
The multi-solid type inclusion illustrates the mechanism by which ore components are sequestered from the crystallizing parental magma and concentrated in the exsolved magmatic aqueous fluids. These fluids are significant with respect to the origin of porphyry copper deposits, as they are responsible for the first enrichment of metals and represent the precursors of metal-bearing hydrothermal fluids in a porphyry copper system.     

Rare earth element geochemistry of Thetford Mines ophiolite complex,Northern Appalachians,Canada

The Thetford Mines complex is a complete ophiolite which is part of an ultramafic-mafic belt within Québec Appalachians. These allochtonous bodies were emplaced during the Early Ordovician. The Thetford Mines complex comprises a lower unit of metamorphic harzburgite (in which tabular, dyke-like, dunitic bodies occur) overlain successively by ultramafic cumulates, mafic cumulates, ophitic gabbros, diabase sills and dykes, and basaltic volcanic rocks. Field evidence, petrography and chemical data indicate that the tabular dunitic bodies formed when fractures in the metamorphic harzburgite (which constituted the floor of the magma chamber) filled with early cumulates (i.e., olivine±chromite). Representative rocks from all units were analyzed for major and rare earth elements (REE). Metamorphic harzburgite samples from Thetford Mines complex have U-shaped chondrite-normalized REE patterns. Pyroxenites and wehrlites of the cumulate sequence are all strongly light-REE depleted and have heavy REE ranging from 0.4 to 1.5 times chondrite. REE data from ultramafic and volcanic rocks of Thetford Mines complex and geochemical modelling indicate that the metamorphic harzburgite has the chemical characteristics of depleted upper mantle residues with U-shaped patterns, and that the ultramafic cumulates crystallized from magmas having different La/Yb ratios.     

Wind-driven ocean circulation transition to barotropic instability

This article deals with the ocean circulation driven by steady zonal winds, and damped by bottom and biharmonic friction, when represented by the simple barotropic vorticity equation. A double gyre antisymmetrical wind stress pattern in a square basin is considered. Wind forcing and dissipation parameters are chosen within the ranges of what has been used in previous studies. The flow characteristics for both steady and unsteady situations are tentatively described as functions of model external parameters through the analysis of a large set of numerical experiments. Functional relations are derived for the mid-latitude jet parameters (length, width and transport) on the basis of scaling arguments. With the diagrams established for these quantities in forcing and dissipation parameter relations allow quantitative predictions of model response to a wide range of parameter choices to be made. The transition to barotropic instability is interpreted by analysing and comparing the spin-up phase of different numerical experiments leading either to stable or unstable solutions. Two major types of destabilization are identified, namely through meandering of the mid-latitude eastward jet and Rossby wave radiation from the westward return flow. The characteristics of the flows are shown to be highly sensitive to the external parameter changes. Competition between eddy kinetic energy level and eastward jet extension appears to consttitute the key point of this class of solutions, controlling in particular the intensity of transport in the inner gyres, driven by the eddy field on the two sides of the mid-basin jet, in a very similar manner to that of the more complex multilayered EGCMs.     

Evidence Of Dynamical Coupling Between The Residual Layer And The Developing Convective Boundary Layer

The diurnal cycle of the atmospheric boundary layer (ABL) hasbeen documented on 8 August 1998 in the framework of the Étude et Simulation de la QUalité de l'air en Ile-de-France (ESQUIF) experiment that took place in the Paris area. The ABL structure was documented by means of a ground-based lidar, surface meteorological stations and soundings. The interaction between the residual layer and the convective boundary layer is investigated using the collected data as well as mesoscale modelling. As opposed to the generally accepted concept, we find evidence of entrainment at the top ofthe residual layer. High temporal simulations of the 8 August 1998 casemade with the mesoscale atmospheric model Meso-NH also show evidenceof mixing at the top of the residual layer (RL). This mixing is believed to be related to the presence of convective (gravity) waves in the RL.     

Diffuse bifurcations engraving diverse shear bands in granular materials

Shear bands with characteristic spatial patterns observed in an experiment for a cubic or parallelepiped specimen of dry dense sand were simulated by numerical bifurcation analysis using the Cam‐clay plasticity model. By incorporating the subloading surface concept into the plasticity model, the model became capable of reproducing hardening/softening and contractive/dilative behavior observed in the experiment. The model was reformulated to be compatible with the multiplicative hyperelasto‐plasticity for finite strains. This enhanced constitutive model was implemented into a finite‐element code reinforced by a stress updating algorithm based on the return‐mapping scheme, and by an efficient numerical procedure to compute critical eigenvectors of elastoplastic tangent stiffness matrix at bifurcation points. The emergence of diamond‐ and column‐like diffuse bifurcation modes breaking uniformity of the materials, followed by the evolution of shear bands through strain localization, was observed in the analysis. In the bifurcation analysis of plane strain compression test, unexpected bifurcation modes, which broke out‐of‐plane uniformity and led to 3‐dimensional diamond‐like patterns, were detected. Diffuse bifurcations, which were difficult to observe by experiments, have thus been found as a catalyst creating diverse shear band patterns.     

Long-term evolution of the spin of Mercury: I. Effect of the obliquity and core-mantle friction

The present obliquity of Mercury is very low (less than 0.1°), which led previous studies to always adopt a nearly zero obliquity during the planet’s past evolution. However, the initial orientation of Mercury’s rotation axis is unknown and probably much different than today. As a consequence, we believe that the obliquity could have been significant when the rotation rate of the planet first encountered spin-orbit resonances. In order to compute the capture probabilities in resonance for any evolutionary scenario, we present in full detail the dynamical equations governing the long-term evolution of the spin, including the obliquity contribution.The secular spin evolution of Mercury results from tidal interactions with the Sun, but also from viscous friction at the core-mantle boundary. Here, this effect is also regarded with particular attention. Previous studies show that a liquid core enhances drastically the chances of capture in spin-orbit resonances. We confirm these results for null obliquity, but we find that the capture probability generally decreases as the obliquity increases. We finally show that, when core-mantle friction is combined with obliquity evolution, the spin can evolve into some unexpected configurations as the synchronous or the 1/2 spin-orbit resonance.     

Surface exposure dating and geophysical prospecting of the Holocene Lauvitel rock slide (French Alps)

Large rock falls and rockslides represent a risk for human communities in mountainous areas as they can cause fatalities and destruction of settlements and infrastructures. Assessing the associated hazard requires constraining the time frequency of such events. Since large rockslides are not common, estimating their frequency requires recording them over a long period of time. The Holocene period then appears as pertinent, which implies that rockslide features have to be dated with absolute chronology methods. This paper presents a characterisation and dating of the Lauvitel rockslide, one of the largest Holocene rockslides in the French Alps. Combining field observation with electrical tomography profiles performed on the rockslide deposit that constitutes the Lauvitel Lake dam allows estimating its volume at a minimum of 12?×?10⁶?m³. In addition, cosmic ray exposure dating using in situ-produced ¹⁰Be concentration measurements has been applied to date seven samples collected both on the main sliding surface and on blocks lying on the dam and further downstream. Ages obtained are consistent with a single large rockslide event, which occurred at 4.7?±?0.4 ¹⁰Be-ka and formed two distinct deposits. However, from a mechanical point of view, these clearly separated deposits could hardly result from a single movement. A comparison of their reach angles with those reviewed in the literature highlights that the lower deposit must result from rock avalanches larger than 10⁷?m³, while the upper one (the Lauvitel dam) must result from several events smaller than 10⁶?m³. In the context of hazard assessment for land use planning, these events can, however, be considered as a unique event.     

Surface properties, solubility and dissolution kinetics of bamboo phytoliths

Although phytoliths, constituted mainly by micrometric opal, exhibit an important control on silicon cycle in superficial continental environments, their thermodynamic properties and reactivity in aqueous solution are still poorly known. In this work, we determined the solubility and dissolution rates of bamboo phytoliths collected in the Réunion Island and characterized their surface properties via electrophoretic measurements and potentiometric titrations in a wide range of pH. The solubility product of “soil” phytoliths ( at 25 °C) is equal to that of vitreous silica and is 17 times higher than that of quartz. Similarly, the enthalpy of phytoliths dissolution reaction is close to that of amorphous silica but is significantly lower than the enthalpy of quartz dissolution. Electrophoretic measurements yield isoelectric point pH_IEP = 1.2 ± 0.1 and 2.5 ± 0.2 for “soil” (native) and “heated” (450 °C heating to remove organic matter) phytoliths, respectively. Surface acid-base titrations allowed generation of a 2-pK surface complexation model. Phytoliths dissolution rates, measured in mixed-flow reactors at far from equilibrium conditions at 2 ? pH ? 12, were found to be intermediate between those of quartz and vitreous silica. The dissolution rate dependence on pH was modeled within the concept of surface coordination theory using the equation: