Variability patterns of Rossby wave source

Rossby waves (RW) propagation due to a local forcing is one of the mechanisms responsible for wave trains configurations known as teleconnections. The term teleconnection refers to anomalies patterns that are correlated in several regions of the world, causing large-scale changes in atmospheric waves patterns and temperature and precipitation regimes. The aim of teleconnections studies is to provide a better understanding of atmospheric variability and their mechanisms of action in order to identify patterns that can be tracked and predicted. The purpose of this study was to characterize seasonal and spatial variability of atmospheric RW sources. The RW source at 200?hPa was calculated for the four seasons with reanalysis data of zonal and meridional wind. In the Northern Hemisphere (NH), the RW sources were located on East Asia, North America, North Atlantic and Pacific. The main RW sources in the Southern Hemisphere (SH) were located over Intertropical, South Pacific, South Atlantic and South Indian Convergence Zones. Extratropical regions were also identified, mainly to the south of Australia. The vortex stretching term (S1) and the advection of absolute vorticity by the divergent wind (S2) were analyzed to discuss the physical mechanisms for RW generation. In the NH, the source at East Asia in DJF changed to a sink in JJA, related to the dominance of S1 term in DJF and S2 term in JJA. In the SH, the vortex stretching term had the dominant contribution for RW source located to the south of Australia. The main forcing for RW sources at east of Australia was the advection of absolute vorticity by divergent flow. Over South America, both terms contributed to the source in DJF. The main modes of RW source variability were discussed by using empirical orthogonal functions analysis. RW variability was characterized by wave trains configurations in both hemispheres over regions of jet streams and storm tracks, associated with favorable and unfavorable areas for RW generation.     

Experimental determination of F and Cl partitioning between lherzolite and basaltic melt

We experimentally determined F and Cl partition coefficients together with that of 19 trace elements (including REE, U-Th, HFSE and LILE) between basaltic melt and lherzolite minerals: olivine, orthopyroxene, clinopyroxene, plagioclase and garnet. Under conditions from 8 to 25 kbars and from 1,265 to 1,430°C, compatibilities of F and Cl are globally ordered as D ^Cpx/melt > D ^Opx/melt > D ^Grt/melt > D ^Ol/melt > D ^Plag/melt, and D _F ^mineral/melt is larger than D _Cl^mineral/melt. Four other major results were brought to light. (1) Chlorine partition coefficients positively correlate with the jadeite component in orthopyroxene, and increase of the CaTs component promotes Cl incorporation in clinopyroxene. (2) Variations of fluorine partition coefficients correlate strongly with melt viscosity. (3) F and Cl partition coefficients correlate with the Young’s modulus (E ₀) of pyroxene octahedral sites (M sites) and with Raman vibrational modes of pyroxenes. This demonstrates a fundamental interaction between the M site of pyroxenes and the incorporation of F and Cl. (4) We also determined the parameters of the lattice-strain model applied to 3+ cation trace elements for the two M sites in orthopyroxene and clinopyroxene: D ₀^M1, D ₀^M2, r ₀^M1, r ₀^M2, E ₀^M1 and E ₀^M2.     

Granulite formation in a Gondwana fragment: petrology and mineral equilibrium modeling of incipient charnockite from Mavadi, southern India

We report a new occurrence of incipient charnockite from Mavadi in the Trivandrum Granulite Block (TGB), southern India, and discuss the petrogenesis of granulite formation in an arrested stage on the basis of petrography, geothermobarometry, and mineral equilibrium modeling. In Mavadi, patches and lenses of charnockite (Kfs?+?Qtz?+?Pl?+?Bt?+?Grt?+?Opx?+?Ilm?+?Mag) of about 30 to 220 cm in length occur within Opx-free Grt-Bt gneiss (Kfs?+?Qtz?+?Pl?+?Bt?+?Grt?+?Ilm). The application of mineral equilibrium modeling on the charnockite assemblage in the NCKFMASHTO system to constrain the conditions of charnockitization defines a P–T range of 800 °C at 4.5 kbar to 850 °C at 8.5 kbar, which is broadly consistent with the results from the conventional geothermobarometry (810–880 °C at 7.7–8.0 kbar) on these rocks. The P–T conditions are lower than the peak metamorphic conditions reported for the ultrahigh-temperature granulites from this area (T?>?900 °C). The heterogeneity in peak P–T conditions within the same crustal block might be related to local buffering of metamorphic temperatures by the Opx-Bt-Kfs-Qtz assemblage. The result of T versus mole H₂O (M(H₂O)) modeling demonstrated that the Opx-free assemblage in the Grt-Bt gneiss is stable at M(H₂O)?=?0.3 to 1.5 mol%, and orthopyroxene occurs as a stable mineral at M(H₂O) <0.3 mol%, which is consistent with the petrogenetic model of incipient charnockite related to the lowering of the water activity and stabilization of orthopyroxene through the breakdown of biotite by dehydration caused by the infiltration of CO₂-rich fluid from external sources. We also propose a possible alternative mechanism to form charnockite from Grt-Bt gneiss through slight variations in bulk-rock chemistry (particularly for the K- and Fe-rich portion of Grt-Bt gneiss) that can enhance the stability of orthopyroxene rather than that of biotite, with K-metasomatism playing a possible role.     

Chromitite and peridotite from Rayat, northeastern Iraq, as fragments of a Tethyan ophiolite

Ophiolitic rocks (chromitites and serpentinized peridotites) were petrologically examined in detail for the first time from Rayat, in the Iraqi part of the Zagros thrust zone, an ophiolitic belt. Almost all the primary silicates have been altered out, but chromian spinel has survived from alteration and gives information about the primary petrological characteristics. The protolith of the serpentinite was clinopyroxene-free harzburgite with chromian spinel of intermediate Cr# (= Cr/[Cr + Al] atomic ratio) of 0.5 to 0.6. The harzburgite with that signature is the most common in the mantle section of the Tethyan ophiolites such as the Oman ophiolite, and is the most suitable host for chromitite genesis. Except for one sample, which has Cr# = 0.6 for spinel, the Cr# of spinel is high, around 0.7, in chromitite. The variation in Cr# of spinel in chromitite observed here has been also reported in the Oman ophiolite. The peridotite with chromitite pods exposed at Rayat was derived from an ophiolite similar in petrological character to the Oman ophiolite, one of the typical Tethyan ophiolites (fragments of Tethyan oceanic lithosphere). This result is consistent with the previous interpretation based on geological analysis.     

Quantification of the effect of forest harvesting versus climate on streamflow cycles and trends in an evergreen broadleaf catchment

ABSTRACT

A new method known as Unobserved Components–Dynamic Harmonic Regression (UC-DHR) was applied to a 39-year record of rainfall and streamflow for three sub-catchments of the Sarukawa Experimental Watershed in southwestern Japan. Some 25% of the timber was harvested from one of the sub-catchments in May–July 1982 and the objective was to quantify the magnitude of this effect relative to the effects of climate cycles (e.g. Southern Oscillation Index). The observed effects of inter-annual climate cycles (i.e. 0.89–1.36 mm/d) were seen to be comparable (i.e. 0.70–1.17 mm/d) to the effects of harvesting 25% of the standing timber. This result underlines the importance of always quantifying the effect of climate on streamflow response when harvesting impacts are studied.

Editor D. Koutsoyiannis; Associate editor T. Okruszko     

Short-term variation in copepod community and physical environment in the waters adjacent to the Kuroshio Current

A continuous survey examined short-term variations in the zooplankton community and physical ocean environment from the northeastern Izu Islands to Boso Peninsula in Japan. High copepod abundance and small upwellings in the surface layer and salinity minimum layer in the subsurface were observed on the north side of coastal fronts in the westernmost transect, moving southward as the Kuroshio Current left the Boso Peninsula. Thus, the salinity minimum layer might be a key factor forming upwelling and the fronts, leading to large abundance of coastal copepods off the northeastern Izu Islands. A community structure analysis of calanoid copepods revealed an intermediate belt assemblage between coastal and offshore (Kuroshio) assemblages. Copepod abundance was remarkably low and Ctenocalanus vanus dominated (nearly 37%) in the intermediate belt zone, indicating that C. vanus has a relatively high tolerance to adverse environments for calanoid copepods. As the Kuroshio Current left the Boso Peninsula, the coastal assemblage expanded in the same direction, and the intermediate belt assemblage off the northeastern Izu Islands disappeared. The largest population of Calanus sinicus was found along the two western transects off the northeastern Izu Islands (>1000 m depth), which was assumed to be transported from Sagami Bay and advanced southwestward while growing from copepodite stages CIII to CV. Larvae of C. sinicus would be an important food for fish larvae in addition to Paracalanus parvus s.l., the numerically dominant species in the coastal assemblage, and C. vanus under the adverse conditions for coastal copepods.     

Bed morphological changes around a finite patch of vegetation

For the appropriate management and restoration of rivers, isolated vegetation is often a practical means for improving stream habitat and ecology. The effect of a finite vegetation patch on flow and bed morphology in an open channel was investigated using laboratory experiments. The patch containing emergent and submerged vegetation was modeled using circular cylinders and located mid‐channel along a side wall. Several configurations of the patch and submergence ratio (i.e. water depth to the height of vegetation), and two flow conditions (i.e. below and above the sediment motion threshold) were considered. For flows below the sediment motion threshold, erosion occurred primarily on the opposite side of the patch and near the leading edge of the patch. The degree of scouring depth observed in both these regions was affected by the submergence ratio and it increased with the non‐dimensional flow blockage (i.e. the product of the patch density and width). In contrast, for flows above the sediment motion threshold, sediment accumulated within and around the patch due to a reduction in bed shear stress, which was strongly influenced by the flow blockage and the obstruction ratio (i.e. the ratio of patch width to channel width). The eroded area observed within the patch was consistent with the interior adjustment region where the deceleration and diversion of flow occurred through the patch. As the flow blockage increased or as the obstruction ratio decreased, the deposition rate within and behind the patch decreased. Furthermore, the deposition rate increased with an increase in the ratio of flow rate through the patch to total flow rate regardless of the submergence ratio. Copyright © 2014 John Wiley & Sons, Ltd.     

Seismic velocity structure around the Hyuganada region, Southwest Japan, derived from seismic tomography using land and OBS data and its implications for interplate coupling and vertical crustal uplift

The Hyuganada region, a forearc region of Southwest Japan, is characterized by several interesting geological and geophysical features, i.e., significant aseismic crustal uplift of 120 m during the past 120 thousand years at the Miyazaki Plain, negative free-air gravity anomalies with the maximum magnitude of −130 mgal, and relatively less cohesive interplate coupling compared with that for off the Shikoku and Kii Peninsula. In order to examine the causes of these observations, we determined a detailed three-dimensional seismic velocity structure based on the seismic data observed by ocean bottom seismometers (OBS) and land stations. P- and S-wave tomographic velocity structures clearly indicate the subducting slab and also the zones of high Poisson's ratio at 25–35 km depth along the coastline of the northeastern part of the Hyuganada. The region with high Poisson's ratio may correspond to the serpentinized mantle wedge as suggested for other mantle wedges, and appears to be coincident with the zone for observed aseismic slips such as the slow-slip and after-slip events. Also, the detection may be related to a relatively weak interplate coupling in the Hyuganada region. The tomographic structures also indicate low velocity zones with a horizontal scale comparable to the Kyushu-Palau Ridge in and around the subducting slab. If we assume that the low velocity zones correspond to the subducted Kyushu-Palau Ridge, then the predicted gravity anomaly due to the density contrast between the low velocity zones and the surrounding region can explain about 60% of the gravity anomaly in the Hyuganada region. The buoyancy is probably an important factor for the crustal uplift observed in the Miyazaki Plain, the steep bending of the subducting slab and the normal fault-type earthquakes around the Hyuganada region.