ALBERT

Description: Permian hydrothermal activity in the Tarim Basin may have been responsible for the invasion of hot brines into Ordovician carbonate reservoirs. Studies have been undertaken to explain the origin and geochemical characteristics of the diagenetic fluid present during this hydrothermal event although there is no consensus on it. We present a genetic model resulting from the study of δ 13 C, δ 18 O, δ 34 S and 87 Sr/ 86 Sr isotope values and fluid inclusions (FIs) from fracture- and vug-filling calcite, saddle dolomite, fluorite, barite, quartz and anhydrite from Ordovician outcrops in northwest (NW) Tarim Basin and subsurface cores in Central Tarim Basin. The presence of hydrothermal fluid was confirmed by minerals with fluid inclusion homogenization temperatures being 〉10°C higher than the paleo-formation burial temperatures both in the NW Tarim and Central Tarim areas. The mixing of hot (〉 200 °C), high salinity (〉24 wt% NaCl), 87 Sr-rich (up to 0.7104) hydrothermal fluid with cool (60 to 100 °C), low salinity (0 to 3.5 wt% NaCl), also 87 Sr-rich (up to 0.7010) meteoric water in the Ordovician unit was supported by the salinity of fluid inclusions, and δ 13 C, δ 18 O and 87 Sr/ 86 Sr isotopic values of the diagenetic minerals. Up-migrated hydrothermal fluids from the deeper Cambrian strata may have contributed to the hot brine with high sulphate concentrations - which promoted TSR in the Ordovician, resulting in the formation of 12 C-rich (δ 13 C as low as -13.8‰) calcite and 34 S-rich (δ 34 S values from 21.4‰ to 29.7‰) H 2 S, pyrite, and elemental sulfur. Hydrothermal fluid mixing with fresh water in Ordovician strata in Tarim Basin was facilitated by deep-seated faults and up-reaching faults due to the pervasive Permian magmatic activity. Collectively, fluid mixing, hydrothermal dolomitization, TSR and faulting may have locally dissolved the host carbonates and increased the reservoir porosity and permeability, which has significant implications for hydrocarbon exploration. This article is protected by copyright. All rights reserved.

Description: : [1]  The southeastern Tibetan Plateau is one of the predominant summer rainfall regions in the world and is also the crucial water vapor channel of the Asian summer monsoon. The rainfall variability in the region influences not only the local communities but also downstream communities in East Asia. However, previous studies have exhibited large rainfall biases in this region in state-of-the-art climate models. Understanding the observed rainfall variability provides an opportunity to identify the origin of model biases and to lay a foundation for improving model performance. In this study, the interannual variability of the summer precipitation (May-September) over the southeastern Tibetan Plateau was investigated based on NCEP/NCAR reanalysis monthly mean data from 1979 to 2010. The associated atmospheric circulation anomalies of the southeastern Tibetan Plateau summer precipitation (SET_PR) display a North Atlantic Ocean-Europe-Asia teleconnection pattern, indicating a possible role of the Atlantic climate in the SET_PR. Further studies have revealed that the Atlantic sea surface temperature(SST)anomalies have the greatest influence on the SET_PR via the Rossby wave response, whereas the SST anomalies in the Indo-Pacific have less of an influence on the SET_PR because their main impacts are confined to the western North Pacific subtropical high and the monsoonal circulation there. This paper also documents the detailed spatial pattern of the atmospheric circulation anomalies associated with the SET_PR and Atlantic SST year-to-year variability.

Description: The interaction between El Niño–Southern Oscillation (ENSO) and the South China Sea summer monsoon (SCSSM) modulated by the Atlantic multidecadal oscillation (AMO) is investigated in this study. On one hand, the influence of the decaying phase of ENSO on the SCSSM is stronger during negative phases of the AMO than during positive phases. During negative phases of the AMO, El Niño (La Niña) with relatively larger variability leads to a western North Pacific anomalous anticyclone (cyclone) that persists from the ENSO mature winter to the ENSO decaying summer, weakening (strengthening) the SCSSM; on the contrary, during positive phases of the AMO, ENSO with relatively weaker variability cannot influence the SCSSM significantly. On the other hand, the SCSSM has a closer relationship with the subsequent ENSO development during positive phases of the AMO than during negative phases. During positive phases of the AMO, atmospheric teleconnections induced by the warmer North Atlantic result in low pressure and cyclonic anomalies over the South China Sea. Consequently, the stronger than normal SCSSM is accompanied by significant westerly wind anomalies over the western tropical Pacific, which favor the development of El Niño events. However, during negative phases of the AMO, the SCSSM-related westerly wind anomalies are rather weak, having a nonsignificant influence on El Niño development. The results are also demonstrated in model simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5).

Description: Both Arctic sea ice loss and La Niña events can result in cold conditions in midlatitude Eurasia in winter. Since the two forcings sometimes occur simultaneously, determining whether they are independent of each other is undertaken first. The result suggests an overall independence. Considering possible interactions between them, their coordinated impacts on the Northern Hemisphere winter climate are then investigated based on observational data analyses, historical simulation analyses from one coupled model (MPI-ESM-LR) contributing to CMIP5, and atmospheric general circulation model sensitive experiments in ECHAM5. The results show that the impacts of the two forcings are overall linearly accumulated. In comparison with one single forcing, there is intensified cooling response in midlatitude Eurasia along with northern warmer–southern cooler dipolar temperature responses over North America. Despite the additive linearity, additive nonlinearity between the two forcings is identifiable. The nonlinearity causes midlatitude Eurasian cooling weakened by one-tenth to one-fifth as much as their individual impacts in combination. The underlying mechanisms for the weak additive nonlinearity are finally explored by transient adjustment AGCM runs with one single forcing or both the forcings switched on suddenly. The day-to-day evolution of responses suggests that the additive nonlinearity may arise initially from the forced wave dynamics and then be amplified because of the involvement of transient eddy feedbacks.

Description: The Silk Road Pattern (SRP) is an upper-tropospheric teleconnection pattern along the Asian westerly jet in summer on the interannual time scale, and it exerts great influences on the climate of the Eurasian continent. Results in the present study indicate that the SRP exhibits considerable distinctions between early and late summers (i.e., 1 June–9 July and 10 July–31 August, respectively). The SRP is stronger and more geographically fixed in late summer in comparison with its counterpart in early summer. Furthermore, the SRP is closely connected with the summer North Atlantic Oscillation (SNAO) in late summer, but not in early summer. This closer connection in late summer is manifested clearly in the leading mode of upper-tropospheric meridional wind anomalies over the North Atlantic–Eurasian continent domain. The intensified SNAO–SRP relationship in late summer can be explained by the subseasonal change of the SNAO: albeit being a seesaw pattern common in both early and late summers, there is a shift of this pattern toward the northwest–southeast one in late summer from a north–south one in early summer. The southeastern pole of SNAO in late summer extends into the Eurasian continent, and efficiently triggers the SRP to propagate along the Asian jet. By contrast, the south pole of SNAO in early summer is confined over the North Atlantic and is thus less effective to trigger the SRP propagation.

Description: A recent study showed that a tropical Atlantic sea surface temperature (SST) anomaly induces a significant coupled response in late winter [February–April (FMA)] in a coupled model, in which an atmospheric general circulation model is coupled to a slab mixed layer ocean model (AGCM_ML). The coupled response comprises a dipole in the geopotential height, like the North Atlantic Oscillation (NAO), and a North Atlantic tripole in the SST. The simulated NAO response developed 1 or 2 months later in the model than in observations. To determine the possible effects of Ekman heat transport on the development of the coupled response to the tropical forcing, an extended coupled model (AGCM_EML), including Ekman transport in the slab mixed layer ocean, is now used. Large ensembles of AGCM_EML experiments are performed, parallel to the previous AGCM_ML experiments, with the model forced by the same tropical Atlantic SST anomaly over the boreal winter months (September–April). The inclusion of Ekman heat transport is found to result in an earlier development of the coupled NAO–SST tripole response in the AGCM_EML, compared to that in the AGCM_ML. The mutual reinforcement between the anomalous Ekman transport and the surface heat flux causes the tropical forcing to induce an extratropical SST response in November–January (NDJ) in the AGCM_EML that is twice as strong as that in the AGCM_ML. The feedback of this stronger extratropical SST response on the atmosphere in turn drives the development of the NAO response in NDJ. In FMA, the sign of the anomalous surface heat flux is reversed in the Gulf Stream region such that it opposes the anomalous Ekman transport. The resulting equilibrium NAO response in the AGCM_EML is similar to that in the AGCM_ML, but it is reached 1–2 months sooner in the AGCM_EML. Hence, the presence of Ekman transport causes a seasonal shift in the evolution of the coupled response. The faster development of the NAO response in the AGCM_EML suggests that tropical Atlantic SST anomalies should be able to influence the NAO, in nature, on the seasonal time scale, and that efficient interactions with the extratropical ocean play a significant role in determining the coupled response.

Description: The hypothesis of convective quasi-equilibrium (CQE) has dominated thinking about the interaction between deep moist convection and the environment for at least two decades. In this view, deep convection develops or decays almost instantly to remove any changes of convective instability, making the tropospheric temperature always tied to the boundary layer moist static energy. The present study examines the validity of the CQE hypothesis at different vertical levels using long-term sounding data from tropical convection centers. The results show that the tropical atmosphere is far from the CQE with much weaker warming in the middle and upper troposphere associated with the increase of boundary layer moist static energy. This is true for all the time scales resolved by the observational data, ranging from hourly to interannual and decadal variability. It is possibly caused by the ubiquitous existence of shallow convection and stratiform precipitation, both leading to sign reversal of heating from lower to upper troposphere. The simulations by 42 global climate models from phases 3 and 5 of the Coupled Model Intercomparsion Project (CMIP3 and CMIP5) are also analyzed and compared with the observations.