Long-term variations in outgoing long-wave radiation (OLR), convective available potential energy (CAPE) and temperature in the tropopause region over India

Relationship of outgoing long-wave radiation (OLR) with convective available potential energy (CAPE) and temperature at the 100-hPa pressure level is examined using daily radiosonde data for a period 1980–2006 over Delhi (28.3°N, 77.1°E) and Kolkata (22.3°N, 88.2°E), and during 1989–2005 over Cochin (10°N, 77°E) and Trivandrum (8.5°N, 77.0°E), India. Correlation coefficient (R _xy) between monthly OLR and CAPE shows a significant (~???0.45) anti-correlation at Delhi and Kolkata suggesting low OLR associated with high convective activity during summer (seasonal variation). Though, no significant correlation was found between OLR and CAPE at Cochin and Trivandrum (low latitude region); analysis of OLR and temperature (at 100-hPa) association suggests that low OLR peaks appear corresponding to low temperature at Delhi (R _xy~ 0.30) and Kolkata (R _xy ~ 0.25) during summer. However, R _xy between OLR and temperature becomes opposite as we move towards low latitudes (~8°–10°N) due to strong solar cycle influence. Large scale components mainly ENSO and quasi-biennial oscillaton (QBO) that contributed to the 100-hPa temperature variability were also analyzed, which showed that ENSO variance is larger by a factor of two in comparison to QBO over Indian region. ENSO warm conditions cause warming at 100-hPa over Delhi and Darwin. However, due to strong QBO and solar signals in the equatorial region, ENSO signal seems less effective. QBO, ENSO, and solar cycle contribution in temperature are found location-dependent (latitudinal variability) responding in consonance with shifting in convective activity regime during El Niño, seasonal variability in the tropical easterly jet, and the solar irradiance.     

External field characterization using CHAMP satellite data for induction studies

Knowledge of external inducing source field morphology is essential for precise estimation of electromagnetic (EM) induction response. A better characterization of the external source field of magnetospheric origin can be achieved by decomposing it into outer and inner magnetospheric contributions, which are best represented in Geocentric Solar Magnetospheric (GSM) and Solar Magnetic (SM) reference frames, respectively. Thus we propose a spherical harmonic (SH) model to estimate the outer magnetospheric contribution, following the iterative reweighted least squares approach, using the vector magnetic data of the CHAMP satellite. The data covers almost a complete solar cycle from July 2001 to September 2010, spanning 54,474 orbits. The SH model, developed using orbit-averaged vector magnetic data, reveals the existence of a stable outer magnetospheric contribution of about 7.39 nT. This stable field was removed from the CHAMP data after transforming to SM frame. The residual field in the SM frame acts as a primary source for induction in the Earth. The analysis of this time-series using wavelet transformation showed a dominant 27-day periodicity of the geomagnetic field. Therefore, we calculated the inductive EM C-response function in a least squares sense considering the 27-day period variation as the inducing signal. From the estimated C-response, we have determined that the global depth to the perfect substitute conductor is about 1132 km and its conductivity is around 1.05 S/m.     

Determining fluid source and possible pathways during burial dolomitization of Maryville Limestone (Cambrian), Southern Appalachians, USA

Detailed petrographic analyses along a depositional transect from a carbonate platform to shale basin reveals that dolomite is the principal burial diagenctic mineral in the Maryville Limestone. This study examines the role of burial dolomitization of subtidal carbonates. Dolomite occurs as a replacement of precursor carbonate and as inter- and intraparticle cements. Four different types of dolomite are identified based on detailed petrographic and gcochemical analyses. Type I dolomite occurs as small, irregular disseminations typically within mud-rich facies.Type II dolomite typically occurs as inclusions of planar euhedral rhombs (ferroan), 5–300 μm in size, in blocky clear ferroan calcite (meteoric) spar. Type II dolomite is non-luminescent. Type I and II dolomite formed during shallow to intermediate burial diagenesis. Type III dolomite consists of subhedral to anhedral crystals 10–150 μm in size occurring as thin seams along stylolites and as thick bands a few millimetres in width. This dolomite consists of dominantly non-luminescent rhombs and, less commonly, orange luminescent and zoned rhombs. Type IV dolomite consists of baroque or saddle-shaped, 100–1500 μm crystals, and is non-luminescent. Type IV dolomite formed during the period of maximum burial. Types III and IV dolomite increase in abundance downslope. Type III dolomite contains 1.2–2.6 wt% Fe and a maximum of 1000 ppm Mn. The distribution of these elements displays no distinct vertical or lateral trends. In contrast, Fe and Mn distributions in Type IV dolomite exhibit distinct spatial trends, decreasing from 3.5–4.5 wl% Fe and 0.1–0.3 wt% Mn in the west (slope/basin) to 1.5–2.5 wt% Fe and less than 600 ppm Mn in the east (shelf margin), a distance of approximately 60 km. Spatial trends in Fe and Mn distributions in Type IV saddle dolomite, suggest a west-east fluid flow during late burial diagenesis. Types III and IV dolomite have a mean δ¹⁸O value of - 7.8%₀₀ and a mean δ¹³C value of + 1.1%₀₀ (relative to the PDB standard). Based on a range of assumed basinal water composition of 2.8%₀₀ SMOW, temperatures calculated from δ¹⁸O values of Types III and IV dolomite range between 75 and 160°C. ⁸⁷Sr/⁸⁶Sr data for Types III and IV dolomite range from 0.7111 to 0.7139. These values are radiogenic when compared to Cambrian marine values and are consistent with the presence of a diagenetic fluid that interacted with siliciclastic sediments. The distribution of Palaeozoic facies in the southern Appalachians indicates a Cambrian shale source for the fluids, whilst burial curves suggest a Middle Ordovician age for burial fluid movement.     

A comparative study on isotopic composition of precipitation in wet tropic and semi-arid stations across southern India

Isotopic composition of monthly composite precipitation samples from Kozhikode (n = 31), a wet tropic station and Hyderabad (n = 25), a semi-arid station across southern India were studied for a period of four years from 2005 to 2008. During the study period, the Kozhikode station recorded an average rainfall of 3500 mm while the Hyderabad station showed an average rainfall of 790 mm. The average stable isotope values in precipitation at the Kozhikode station were δ ¹⁸O = −3.52‰, d-excess = 13.72‰; δ ¹⁸O = −2.94‰, d-excess = 10.57‰; and δ ¹⁸O = −7.53‰, d-excess = 13.79‰, respectively during the pre-monsoon (March–May), monsoon (June–September) and post-monsoon (October–February) seasons. For the Hyderabad station, the average stable isotope values were δ ¹⁸O = −5.88‰, d-excess = 2.34‰; δ ¹⁸O = −4.39‰, d-excess = 9.21‰; and δ ¹⁸O = −8.69‰, d-excess = 14.29‰, respectively for the three seasons. The precipitation at the two stations showed distinctive isotopic signatures. The stable isotopic composition of precipitation at the Hyderabad station showed significant variations from the global trend while the Kozhikode station almost followed the global value. These differences are mainly attributed to the latitudinal differences of the two stations coupled with the differences in climatic conditions.