Attribution of SST variability in global oceans and the role of ENSO     

Arun Kumar  Bhaskar Jha  Hui Wang 《Climate Dynamics》2014,43(1-2):209-220

Based on a novel design of coupled model simulations where sea surface temperature (SST) variability in the equatorial tropical Pacific was constrained to follow the observed El Niño—Southern Oscillation (ENSO) variability, while rest of the global oceans were free to evolve, the ENSO response in SSTs over the other ocean basins was analyzed. Conceptually the experimental setup was similar to discerning the contribution of ENSO variability to interannual variations in atmospheric anomalies. A unique feature of the analysis was that it was not constrained by a priori assumptions on the nature of the teleconnected response in SSTs. The analysis demonstrated that the time lag between ENSO SST and SSTs in other ocean basins was about 6 months. A signal-to-noise analysis indicated that between 25 and 50 % of monthly mean SST variance over certain ocean basins can be attributed to SST variability over the equatorial tropical Pacific. The experimental setup provides a basis for (a) attribution of SST variability in global oceans to ENSO variability, (b) a method for separating the ENSO influence in SST variations, and (c) understanding the contribution from other external factors responsible for variations in SSTs, for example, changes in atmospheric composition, volcanic aerosols, etc.  相似文献   

Subseasonal forecast skills and biases of global summer monsoons in the NCEP Climate Forecast System version 2   总被引：1，自引：0，他引：1  

Xiangwen Liu  Song Yang  Qiaoping Li  Arun Kumar  Scott Weaver  Shi Liu 《Climate Dynamics》2014,42(5-6):1487-1508

Subseasonal forecast skills and biases of global summer monsoons are diagnosed using daily data from the hindcasts of 45-day integrations by the NCEP Climate Forecast System version 2. Predictions for subseasonal variability of zonal wind and precipitation are generally more skillful over the Asian and Australian monsoon regions than other monsoon regions. Climatologically, forecasts for the variations of dynamical monsoon indices have high skills at leads of about 2 weeks. However, apparent interannual differences exist, with high skills up to 5 weeks in exceptional cases. Comparisons for the relationships of monsoon indices with atmospheric circulation and precipitation patterns between skillful and unskillful forecasts indicate that skills for subseasonal variability of a monsoon index depend partially on the degree to which the observed variability of the index attributes to the variation of large-scale circulation. Thus, predictions are often more skillful when the index is closely linked to atmospheric circulation over a broad region than over a regional and narrow range. It is also revealed that, the subseasonal variations of biases of winds, precipitation, and surface temperature over various monsoon regions are captured by a first mode with seasonally independent biases and a second mode with apparent phase transition of biases during summer. The first mode indicates the dominance of overall weaker-than-observed summer monsoons over major monsoon regions. However, at certain stages of monsoon evolution, these underestimations are regionally offset or intensified by the time evolving biases portrayed by the second mode. This feature may be partially related to factors such as the shifts of subtropical highs and intertropical convergence zones, the reversal of biases of surface temperature over some monsoon regions, and the transition of regional circulation system. The significant geographical differences in bias growth with increasing lead time reflect the distinctions of initial memory capability of the climate system over different monsoon regions.  相似文献   

Comparison of intra-seasonal forecast of Indian summer monsoon between two versions of NCEP coupled models     

D. R. Pattanaik  Arun Kumar 《Theoretical and Applied Climatology》2014,118(1-2):331-345

The real-time forecasting of monsoon activity over India on extended range time scale (about 3 weeks) is analyzed for the monsoon season of 2012 during June to September (JJAS) by using the outputs from latest (CFSv2 [Climate Forecast System version 2]) and previous version (CFSv1 [Climate Forecast System version 1]) of NCEP coupled modeling system. The skill of monsoon rainfall forecast is found to be much better in CFSv2 than CFSv1. For the country as a whole the correlation coefficient (CC) between weekly observed and forecast rainfall departure was found to be statistically significant (99 % level) at least for 2 weeks (up to 18 days) and also having positive CC during week 3 (days 19–25) in CFSv2. The other skill scores like the mean absolute error (MAE) and the root mean square error (RMSE) also had better performance in CFSv2 compared to that of CFSv1. Over the four homogeneous regions of India the forecast skill is found to be better in CFSv2 with almost all four regions with CC significant at 95 % level up to 2 weeks, whereas the CFSv1 forecast had significant CC only over northwest India during week 1 (days 5–11) forecast. The improvement in CFSv2 was very prominent over central India and northwest India compared to other two regions. On the meteorological subdivision level (India is divided into 36 meteorological subdivisions) the percentage of correct category forecast was found to be much higher than the climatology normal forecast in CFSv2 as well as in CFSv1, with CFSv2 being 8–10 % higher in the category of correct to partially correct (one category out) forecast compared to that in CFSv1. Thus, it is concluded that the latest version of CFS coupled model has higher skill in predicting Indian monsoon rainfall on extended range time scale up to about 25 days.  相似文献   

Reduction of structural Fe(III) in nontronite by methanogen Methanosarcina barkeri     

Deng Liu  Michael E. Bishop  Abinash Agrawal  Dennis D. Eberl 《Geochimica et cosmochimica acta》2011,75(4):1057-60

Clay minerals and methanogens are ubiquitous and co-exist in anoxic environments, yet it is unclear whether methanogens are able to reduce structural Fe(III) in clay minerals. In this study, the ability of methanogen Methanosarcina barkeri to reduce structural Fe(III) in iron-rich smectite (nontronite NAu-2) and the relationship between iron reduction and methanogenesis were investigated. Bioreduction experiments were conducted in growth medium using three types of substrate: H₂/CO₂, methanol, and acetate. Time course methane production and hydrogen consumption were measured by gas chromatography. M. barkeri was able to reduce structural Fe(III) in NAu-2 with H₂/CO₂ and methanol as substrate, but not with acetate. The extent of bioreduction, as measured by the 1,10-phenanthroline method, was 7-13% with H₂/CO₂ as substrate, depending on nontronite concentration (5-10 g/L). The extent was higher when methanol was used as a substrate, reaching 25-33%. Methanogenesis was inhibited by Fe(III) reduction in the H₂/CO₂ culture, but enhanced when methanol was used. High charge smectite and biogenic silica formed as a result of bioreduction. Our results suggest that methanogens may play an important role in biogeochemical cycling of iron in clay minerals and may have important implications for the global methane budget.  相似文献   

Winter fog over the Indo-Gangetic Plains: mapping and modelling using remote sensing and GIS   总被引：1，自引：1，他引：0  

Arun K. Saraf  Ajoy K. Bora  Josodhir Das  Vineeta Rawat  Kanika Sharma  Sanjay K. Jain 《Natural Hazards》2011,58(1):199-220

Almost every year in the winter months (December–February), the vast Indo-Gangetic Plain south of the Himalaya is affected by dense fog. This fog is considered as radiational fog, and sometime it becomes smog (when it mixes with smoke). The typical meteorological, topographic and increasing pollution conditions over the Indo-Gangetic Plain are perhaps the common contributing factors for fog formation. In the present study, the North Indian fog has been successfully mapped and analysed using NOAA-AVHRR satellite data. In the winter seasons of 2005–06, 2006–07 and 2007–08, the fog-affected area has been found to cover about 575,800 km², 594,100 km² and 478,000 km², respectively. Less fog in 2007–08 may be the consequence of high fluctuations in the meteorological parameters like temperature, relative humidity and wind speed as related to the prevailing synoptic regime for that season. The dissipation and migration pattern of fog in the study area has also been interpreted on the basis of the analysis of both meteorological and satellite data. Further analysis of the fog-affected area allowed identifying more fog-prone regions. Analysis of past fog-affected days and corresponding meteorological conditions enabled us to identify favourable conditions for fog formation viz. air temperature 3–13°C, relative humidity >87%, wind speed <2 m/s and elevation <300 m. Based on the observations of past fog formation and corresponding governing parameters, fog for few selected days could be predicted in hind-sight and later verified with NOAA images.  相似文献   

CFSv2 ensemble prediction of the wintertime Arctic Oscillation     

Emily E. Riddle  Amy H. Butler  Jason C. Furtado  Judah L. Cohen  Arun Kumar 《Climate Dynamics》2013,41(3-4):1099-1116

Lagged ensembles from the operational Climate Forecast System version 2 (CFSv2) seasonal hindcast dataset are used to assess skill in forecasting interannual variability of the December–February Arctic Oscillation (AO). We find that a small but statistically significant portion of the interannual variance (>20 %) of the wintertime AO can be predicted at leads up to 2 months using lagged ensemble averages. As far as we are aware, this is the first study to demonstrate that an operational model has discernible skill in predicting AO variability on seasonal timescales. We find that the CFS forecast skill is slightly higher when a weighted ensemble is used that rewards forecast runs with the most accurate representations of October Eurasian snow cover extent (SCE), hinting that a stratospheric pathway linking October Eurasian SCE with the AO may be responsible for the model skill. However, further analysis reveals that the CFS is unable to capture many important aspects of this stratospheric mechanism. Model deficiencies identified include: (1) the CFS significantly underestimates the observed variance in October Eurasian SCE, (2) the CFS fails to translate surface pressure anomalies associated with SCE anomalies into vertically propagating waves, and (3) stratospheric AO patterns in the CFS fail to propagate downward through the tropopause to the surface. Thus, alternate boundary forcings are likely contributing to model skill. Improving model deficiencies identified in this study may lead to even more skillful predictions of wintertime AO variability in future versions of the CFS.  相似文献   

General relativistic spectra of accretion discs around rapidly rotating neutron stars: effect of light bending   总被引：1，自引：0，他引：1  

Sudip Bhattacharyya  Dipankar Bhattacharya  Arun V. Thampan 《Monthly notices of the Royal Astronomical Society》2001,325(3):989-994

We present computed spectra, as seen by a distant observer, from the accretion disc around a rapidly rotating neutron star. Our calculations are carried out in a fully general relativistic framework, with an exact treatment of rotation. We take into account the Doppler shift, gravitational redshift and light-bending effects in order to compute the observed spectrum. We find that light bending significantly modifies the high-energy part of the spectrum. Computed spectra for slowly rotating neutron stars are also presented. These results would be important for modelling the observed X-ray spectra of low-mass X-ray binaries containing fast-spinning neutron stars.  相似文献   

X-rays from supernovae remants 3C 400.2 (≡ G53.7-2.2) and MSH 15-56     

P. C. Agrawal  G. R. Riegler  K. P. Singh 《Astrophysics and Space Science》1983,89(2):279-283

Detection of two new soft X-ray sources and their identification with the supernova remnants (RNRs) 3C400.2 and MSH 15-56, is reported. From the observed X-ray flux in the 0.7–2.0 keV energy range, the X-ray luminosity of 3C400.2 is derived to be in the range of (2.7–5.3)×10³⁵ ergs s^?1 and that of MSH 15-56 in the range of (0.9–1.6)×10³⁵ ergs s^?1. If we use the standard adiabatic shockwave model for the SNR's, an age of ～(1.4–3.2)×10⁴ yr is derived for 3C400.2 whereas MSH 15-56 is found to be younger of an age of (5–10)×10³ yr.  相似文献   

Heat and mass transfer in rotating fluid with Hall current,I     

H. L. Agrawal  P. C. Ram  V. Singh 《Astrophysics and Space Science》1983,96(2):245-254

Hall effects on the hydromagnetic free convection resulting from the combined effects of thermal and mass diffusion of an electrically conducting liquid past an infinite vertical porous plate in a rotating system have been analysed. The expressions for the mean velocity, mean temperature in the boundary layer and the mean skin friction, the mean rate of heat transfer on the plate are derived. The effects of magnetic parameterM, Hall parameterm, Schmidt number Sc, and Ekman numberE on the flow field, are discussed with the help of graphs and tables.  相似文献   

Correction to: Stress shadow,stress triggering,and recent earthquake activity in the Kashmir Himalaya,India     

Prasath  R. Arun  Verma  Mithila  Bansal  Brijesh K. 《Journal of Seismology》2022,26(2):415-415

Journal of Seismology -  相似文献