The Record-breaking Mei-yu in 2020 and Associated Atmospheric Circulation and Tropical SST Anomalies※

The record-breaking mei-yu in the Yangtze-Huaihe River valley (YHRV) in 2020 was characterized by an early onset, a delayed retreat, a long duration, a wide meridional rainbelt, abundant precipitation, and frequent heavy rainstorm processes. It is noted that the East Asian monsoon circulation system presented a significant quasi-biweekly oscillation (QBWO) during the mei-yu season of 2020 that was associated with the onset and retreat of mei-yu, a northward shift and stagnation of the rainbelt, and the occurrence and persistence of heavy rainstorm processes. Correspondingly, during the mei-yu season, the monsoon circulation subsystems, including the western Pacific subtropical high (WPSH), the upper-level East Asian westerly jet, and the low-level southwesterly jet, experienced periodic oscillations linked with the QBWO. Most notably, the repeated establishment of a large southerly center, with relatively stable latitude, led to moisture convergence and ascent which was observed to develop repeatedly. This was accompanied by a long-term duration of the mei-yu rainfall in the YHRV and frequent occurrences of rainstorm processes. Moreover, two blocking highs were present in the middle to high latitudes over Eurasia, and a trough along the East Asian coast was also active, which allowed cold air intrusions to move southward through the northwestern and/or northeastern paths. The cold air frequently merged with the warm and moist air from the low latitudes resulting in low-level convergence over the YHRV. The persistent warming in the tropical Indian Ocean is found to be an important external contributor to an EAP/PJ-like teleconnection pattern over East Asia along with an intensified and southerly displaced WPSH, which was observed to be favorable for excessive rainfall over YHRV.     

A study of track deflection associated with the landfall of Tropical Cyclone Sidr (2007) over the Bay of Bengal and Bangladesh

In this study, the dynamics of track deflection associated with Tropical Cyclone (TC) Sidr (2007) are explored using a numerical weather prediction model. It is found that (a) the simulated track of Sidr is sensitive to flow, orographic, and model vertical structure that change the environmental steering flow leading to the track deflection. In particular, the track of TC Sidr is deflected northwestward for cases with lower domain height, horizontal domain covering only part of Himalaya mountains, and varying mountain heights; (b) the simulated track of TC Sidr, when compared with GFS reanalysis data, is mainly controlled by its deep-layer environmental steering flow as a point vortex; (c) the northwestward deflection with lower domain height is caused by an artificially larger high pressure at lower levels in the vicinity of the Himalayas, due to the upward propagation of wave energy being reflected by the upper domain boundary; (d) the significant northwestward deflection associated with the varying mountain height cases is due to the cyclone vortex being advected by the northeasterly monsoonal flow, which is blocked by the mountains in the corresponding cases with mountains; (e) the northeastward track deflection after the landfall of Sidr is explained by the addition of the frictional force.In summary, the model vertical domain height and the Himalaya mountain representation play key roles in influencing the accuracy of TC Sidr track simulation, compared with other factors, such as the vertical resolution, at least for TC Sidr.     

Effects of vertical wind shear and storm motion on tropical cyclone rainfall asymmetries over the North Indian Ocean

Tropical cyclone (TC) rainfall asymmetry is often influenced by vertical wind shear and storm motion. This study examined the effects of environmental vertical wind shear (200-850 hPa) and storm motion on TC rainfall asymmetry over the North Indian Ocean (NIO): the Bay of Bengal (BoB) and the Arabian Sea (AS). Four TC groups were used in this study: Cyclonic Storm (CS), Severe Cyclonic Storm (SCS), Very Severe Cyclonic Storm (VSCS) and Extreme Severe Cyclonic Storm (ESCS). The Fourier coefficients for wave number-1 was used to analyze the structure of TC rainfall asymmetry. Results show that the maximum TC rainfall asymmetry was predominantly in the downshear left quadrant in the BoB, while it placed to downshear right quadrant in the AS, likely due to the different primary circulation strength of the TC vortex. For the most intense cyclone (ESCS), the maximum TC rainfall asymmetry was in the upshear left quadrant in the BoB, whereas it was downshear right quadrant in the AS. It is evident for both basins that the magnitude of TC rainfall asymmetry declined (increased) with TC intensity (shear strength). This study also examined the collective effects of vertical wind shear and storm motion on TC rainfall asymmetry. Here, the analysis in case of the strong shear environment (>7 m s^-1) omitted for the AS because the maximum value for this basin was about 7 m s^-1. The result showed that the downshear left quadrant was dominant in the BoB for the maximum TC rainfall asymmetry. In a weak shear environment (<5 m s^-1), on the other hand, downshear right quadrant is evident for the maximum TC rainfall asymmetry in the BoB, while it placed dominantly downshear left quadrant in the AS. In the case of motion-relative wavenumber-1, the maximum TC rainfall asymmetry was dominantly downshear for both basins.     

Role of external forcing on the seasonal and interannual variability of mixed layer depth over the Bay of Bengal using reanalysis datasets during 1980–2015

The monsoon reversal winds in different seasons and high influx of freshwater from various rivers make the Bay of Bengal (BoB) a unique region. Thus, the knowledge of the dynamics of the mixed layer over this region is very important to assess the climatic variation of the Indian subcontinent. A comprehensive study of the role of external forcing on the seasonal and interannual mixed layer depth (MLD) variability over the BoB is carried out for 36 years (1980–2015) using reanalysis products. A weak and strong seasonality of MLD is observed over the northern and the southern BoB (NBoB and SBoB) respectively. The partial correlation suggests that the net heat flux (Qnet) is the major contributor to the deepening of MLD over the NBoB, whereas the wind stress controls the deepening over the SBoB. The seasonal variability reveals the deepening of MLD during summer and winter monsoon and the shallowing during pre- and post-monsoon over the BoB. The relation of the interannual MLD variability and the different phases of the Indian Ocean Dipole (IOD) reveals that the negative phase of IOD is associated with deeper MLD over BoB while the positive phase of IOD depicts shallower MLD. In addition, the opposing characteristic of MLD is highly prominent during October-December. This is majorly contributed by variations related to the second downwelling Kelvin and associated Rossby waves over BoB for the opposing phases of the IOD years.     

海洋酸化对颗石藻的影响

工业革命以来人类活动产生了大量二氧化碳气体（CO2）并释放到大气中。CO2溶于海水,造成海水pH值降低,改变海洋碳酸系统的平衡。海洋酸化对海洋生态系统特别是钙化生物构成威胁。颗石藻作为主要的钙化浮游生物,在海洋碳循环过程中起着重要的作用。大多数培养实验表明CO2浓度上升会促进颗石藻光合作用。而海洋酸化对不同种或不同品系颗石藻钙化作用产生不同的影响。     

渤海海底地震仪探测试验及初步成果

利用大容量气枪枪阵震源和海底地震仪在渤海首次开展人工地震深部地球物理探测试验。本次试验布设一条NWW-SEE向垂直构造走向的勘测线,共投放海底地震仪51台,回收成功50台。试验结果和数据分析表明,所使用的枪阵有足够的能量输出,海底地震仪记录震相丰富,可识别到Ps,Pg,PmP,Pn等多种震相。初至波层析成像结果表明,9 km以上地层速度结构存在明显的横向不均匀性,渤中地区新生代沉积基底埋深5～6 km,结晶基底埋深约9 km,郯庐断裂带内存在"U"型下凹的相对低速体并有向下切割的趋势。此次试验是我国在渤海深部探测中的成功示范,有效填补了渤海海域深地震测深数据的空白,为渤海深部地壳结构研究及含油气盆地形成演化研究提供了重要的基础资料。     

HDD铺管引起地表工后变形的数值模拟分析

水平定向钻进（HDD）是一种重要的非开挖铺管技术,在非开挖领域应用非常广泛。由于其会扰动周围地层以及造成地层损失,进而造成一定程度的地表变形,如地表沉降、地表水平变形等。本文采用有限差分数值模拟方法,运用FLAC3D软件分析了HDD铺管的引起的地表工后变形问题;综合考虑了不同地层内聚力、地表荷载及扩孔级数情况下的地表工后变形。     

四川汶川地震断裂带科学钻探2号孔（WFSD-2）岩性特征和断裂带的结构

以WFSD-2钻孔岩心为研究对象,通过详细的岩心编录和岩石学、构造地质学等研究,识别出该钻孔岩心具有6段岩性,从上向下依次为彭灌杂岩（0-599.31m）、三叠系须家河组二段（599.31-1211.49m）、彭灌杂岩（1211.49-1679.51m）、三叠系须家河组三段（1679.51-1715.48m）、彭灌杂岩（1715.48-2081.47m）、三叠系须家河组四段（2081.47-2283.56m）。彭灌杂岩主要以花岗岩和火山岩为主,三叠系须家河组沉积岩以砂岩、粉砂岩、泥岩、页岩、煤层（线）和砾岩为主。3套彭灌杂岩与三叠系须家河组沉积岩重复出现,时代较老的岩性段逆冲覆盖在新的地层之上,表明龙门山构造带由一系列逆冲岩片叠置而成。岩心中断裂岩较为发育,主要为断层角砾岩、碎裂岩和断层泥,反映出脆性变形作用的特点。通过对断裂岩的统计分析,厘定了20余条产状不同、规模不等的次级断裂带,断裂带宽度和断裂密度峰值显示FZ600、FZ720、FZ782、FZ817、FZ922、FZ951、FZ1449、FZ1681、FZ2082为主要断裂带,其中FZ1681系规模最大的一条断裂。依据断裂岩的组合特征可以将岩心中断裂带的结构以断层泥为核部划分为两大类：对称型断裂带和不对称型断裂带。根据地表破裂带、WFSD-1钻孔岩心中主滑移带位置的几何关系、岩性分层等因素,可推断汶川地震主滑移带应位于FZ1134、FZ1449或FZ1681之中,同时也暗示该地区经常发生类似汶川地震的大地震活动。研究表明,龙门山地区经历了强烈的构造缩短和快速隆升作用,暗示龙门山地区构造活动非常强烈。     

Provenance of the Langjiexue Group to the south of the Yarlung-Tsangpo Suture Zone in southeastern Tibet: Insights on the evolution of the Neo-Tethys Ocean in the Late Triassic

ABSTRACT

The Upper Triassic Langjiexue Group, which lies immediately south of the Yarlung-Tsangpo Suture Zone in the Shannan area of southeastern Tibet, represents an important part of the Tethyan Himalayan Sequence (THS). Its provenance and palaeogeography have been the subject of debate. We present new data on petrographic composition, whole-rock geochemistry, and detrital zircon U–Pb geochronology to constrain the provenance of the Langjiexue Group. The dominance of quartz grains and felsic volcanic lithic fragments suggests that the sandstones are litho-quartzose. The trace element geochemical signatures (V–Ni–Th*10, Co/Th–La/Sc, Eu/Eu*–Th/Sc) suggest derivation from felsic igneous sources. The detrital zircon age spectra display three major peaks: a Meso-to-Neoproterozoic peak (1200–900 Ma, 7–18%), a Neoproterozoic-to-Late Cambrian peak (750–500 Ma, 32–65%), and a Late Carboniferous-to-Late Triassic peak (300–200 Ma, 11–33%). The maximum depositional age of early Carnian (236–235 Ma) is obtained by calculating weighted average ages of the youngest zircons (≤250 Ma). The youngest age cluster (300–200 Ma) is incompatible with sources from neighbouring terranes, including the South Qiangtang terrane, Lhasa terrane, THS, and Higher Himalayan Crystalline. Correlations of the Permian–Triassic zircons with those of time-equivalent strata in northwest Australia, west Burma, and the Banda Arc unveil a potential connection to the Tasmanides along the convergent margin of eastern Australia. The New England Orogen (300–230 Ma) could have supplied the Langjiexue Group with magmatic materials via continent-scale drainage systems or a submarine fan complex. This scenario provides a new perspective into the transport of detritus from distal orogens to sedimentary basins thousands of kilometres away.     

The tectono-magmatic evolution of the West Junggar terrane (NW China) unravelled by U–Pb ages of detrital zircons in modern river sands

ABSTRACT

The West Junggar terrane (WJT) is an outstanding laboratory for studying the tectonic evolution of the Junggar–Balkhash Ocean, because it contains widespread Paleozoic magmatism in different tectonic settings. We attempt to reconstruct the tectono-magmatic evolution of WJT through U–pb analysis of detrital zircons from three modern river sand samples from the Harabura, Baibuxie, and Aletengyemule rivers in the Barleik Mountains of the central WJT. A total of 232 concordant spots show Th/U ratios of 0.14–1.69, typical of igneous origin, and they contain abundant Paleozoic (96%) and few Precambrian (4%) ages, with major age populations at 450–530, 400–430, 320–380, and 265–320 Ma. The first two groups may be derived from the early subduction- and accretion-related magmatic rocks of the WJT, whereas the third group is congruent with magmatic activities related to the final subduction and basin-filling processes within a framework of the remnant Junggar–Balkhash Ocean. By combining with the regional data, the last group of magmatic events is referred to as post-subduction magmatism. The missing Mesozoic–Cenozoic magmatism clearly indicates a pre-Permian closure for the Junggar–Balkhash Ocean, nearly coeval with the closure of other oceans in the southwestern Palaeo-Asian Ocean.