Atmospheric and Oceanic Science Letters最新文献

This study investigated the distinct responses of western North Pacific (WNP) tropical cyclone (TC) activity during different decaying El Niño summers. The El Niño events were classified into two types according to the periodicity of the ENSO cycle, with positive SST anomalies in the equatorial central-eastern Pacific maintaining positive values into the following summer as the slow decaying (SD) cases, but transforming to negative values in the following summer as the rapid decaying (RD) cases. Compared with that in SD El Niño summers, the TC occurrence frequency over the WNP is significantly lower in RD El Niño summers, led by a much weaker WNP monsoon trough with more unfavorable environmental factors for TC genesis and development. Further examination showed that the apparent warming over the tropical Indian Ocean basin and cooling over the equatorial central-eastern Pacific contribute together to an enhanced lower-tropospheric anticyclone through modulation of the descending branch of the large-scale Walker circulation over the WNP, which may play a crucial role in suppressing the TC activity during the decaying summer of RD El Niño cases. In contrast, the warming equatorial central-eastern Pacific and remote western Indian Ocean induce a weakening WNP anticyclone and less suppressed deep convection during the decaying summer of SD El Niño cases. Thus, the different evolution of SST anomalies associated with different paces of El Niño decay results in the linkage between the preceding winter El Niño and the decreased WNP TC frequency in summer being more (less) robust for RD (SD) El Niño cases.

摘要

本文分析了El Niño事件衰减速度的差异对衰退年夏季西北太平洋热带气旋 (tropical cyclone, TC) 频数的不同影响. 按照El Niño事件衰减速度不同, 将其划分为迅速衰减 (rapid decaying, RD) 和缓慢衰减 (slow decaying, SD) 的El Niño事件. SD (RD) El Niño事件的衰退年夏季, 赤道中东太平洋海温仍维持正异常 (衰减为负异常) . 与SD El Niño事件相比, RD El Niño事件衰退年夏季西北太平洋TC频数显著减少. 进一步的分析揭示了导致TC频数差异的大尺度环境要素, 指出热带印度洋-太平洋海温异常密切相关的西北太平洋低层反气旋异常在其中起到了关键作用.

In early-to-mid November 2021, a pronounced reversal of surface air temperature (SAT) anomalies (SATAs) occurred over East Asia and Central Siberia, with extreme SATAs that reached up to about 10 °C. Such a synoptic-scale reversal of SATAs was characterized by the alternate emergence of the “colder Central Siberia–warmer East Asia” pattern and the “warmer Central Siberia–colder East Asia” pattern in November 2021. Coinciding with the reversals of the meridional dipole SATAs, large-scale atmospheric circulation anomalies experienced reversed changes. The development of the anomalous cyclonic (anticyclonic) flow over East Aisa (Central Siberia) was crucial for the occurrence of the “warmer Central Siberia–colder East Asia” pattern. Moreover, as the leading mode of daily SAT variability in approximately 56% of the Novembers during 1979–2021, the meridional dipole pattern of warmer (colder) anomalies over Central Siberia and colder (warmer) anomalies over East Asia may be one of the dominant modes of November SAT variability over Eurasia on the synoptic scale.

摘要

2021年11月, 东亚与中西伯利亚经历了相反的冷暖异常转换, 表现为“中西伯利亚偏冷, 东亚偏暖”与“中西伯利亚偏暖, 东亚偏冷”的交替出现. 该偶极型气温异常的天气尺度反转伴随着大尺度大气环流异常的反转. 进一步分析表明, 东亚与中西伯利亚的偶极型气温异常反转是1979–2021年期间11月欧亚气温日变化的主导模态之一(发生概率超过56%).

The Arctic sea-ice cover has decreased in extent, area, and thickness over the last six decades. Most global climate models project that the summer sea-ice extent (SIE) will decline to less than 1 million (mill.) km² in this century, ranging from 2030 to the end of the century, indicating large uncertainty. However, some models, using the same emission scenarios as required by the Paris Agreement to keep the global temperature below 2°C, indicate that the SIE could be about 2 mill. km² in 2100 but with a large uncertainty of ±1.5 mill. km². Here, the authors take another approach by exploring the direct relationship between the SIE and atmospheric CO₂ concentration for the summer–fall months. The authors correlate the SIE and ln(CO₂/CO₂r) during the period 1979–2022, where CO₂r is the reference value in 1979. Using these transient regression equations with an R² between 0.78 and 0.87, the authors calculate the value that the CO₂ concentration needs to reach for zero SIE. The results are that, for July, the CO₂ concentration needs to reach 691 ± 16.5 ppm, for August 604 ± 16.5 ppm, for September 563 ± 17.5 ppm, and for October 620 ± 21 ppm. These values of CO₂ for an ice-free Arctic are much higher than the targets of the Paris Agreement, which are 450 ppm in 2060 and 425 ppm in 2100, under the IPCC SSP1-2.6 scenario. If these targets can be reached or even almost reached, the “no tipping point” hypothesis for the summer SIE may be valid.

The mei-yu season (June–July) rainfall over the mei-yu monitoring domain (MMD) in the Yangtze–Huaihe Basin has shown an increasing trend in recent decades. This study examines the dominant factors responsible for this increasing trend for the period 1979–2020 based on station-observed rainfall and ERA5 reanalysis datasets from the perspective of changes in atmospheric circulation. Although significantly increasing trends exist in the mei-yu season rainfall over the entire MMD, the magnitude of the trends is slightly larger over the eastern MMD (EMMD) than over the western MMD (WMMD). Quantitative diagnoses demonstrate that the relative contributions of anomalous evaporation and moisture advection to the increasing rainfall trend are different between the EMMD and WMMD. The increasing rainfall trend over the WMMD (EMMD) is attributable to increased evaporation (enhanced vertical moisture advection), which is dependent on an anomalous cyclonic circulation in the middle-lower troposphere over the MMD. Such an anomalous cyclone on the northwestern side of the climatological western North Pacific subtropical high facilitates an increase in moisture divergence above 600 hPa over the EMMD, leading to enhanced vertical moisture advection in conjunction with strengthened moisture convergence at 850 hPa. By contrast, the anomalous cyclone favors increasing local evaporation over the WMMD.

摘要

近几十年来, 江淮流域梅雨监测区 (MMD) 的梅雨期 (6–7月) 降水呈增加趋势. 本文基于1979–2020年台站观测降水资料和ERA5再分析数据, 从大气环流变异的角度揭示了这种长期增加趋势的主要影响因素. 发现在MMD范围内, 梅雨期降水趋势的增幅东部大于西部. 水汽收支定量诊断表明, 异常的蒸发和水汽平流对MMD西部和东部降水增加趋势的相对贡献是不同的. MMD西部 (东部) 的降水趋势主要归咎于增强的局地蒸发 (增强的垂直水汽平流) , 后者又取决于MMD对流层中, 低层的异常气旋环流. 这种位于气候平均的西太平洋副热带高压西北侧的异常气旋有助于MMD东部600 hPa以上的水汽辐散增加, 伴随加强的850 hPa水汽辐合, 从而导致垂直水汽平流的增强. 相反, 该异常气旋则有利于增强MMD西部的局地蒸发.

This paper examines truncation and round-off errors in the numerical solution of the 1D advection equation with the Lax–Friedrichs scheme, and accumulation of the errors as they are propagated to high temporal layers. The authors obtain a new theoretical approximation formula for the upper bound of the total error of the numerical solution, as well as theoretical formulae for the optimal grid size and time step. The reliability of the obtained formulae is demonstrated with numerical experimental examples. Next, the ratio of the optimal time steps under two different machine precisions is found to satisfy a universal relation that depends only on the machine precision involved. Finally, theoretical verification suggests that this problem satisfies the computational uncertainty principle when the grid ratio is fixed, demonstrating the inevitable existence of an optimal time step size under a finite machine precision.

摘要

本文对于应用Lax- Friedrichs 格式数值求解一维平流方程, 研究数值求解过程中产生的截断误差与舍入误差, 以及两种误差逐层向高时间层传播的累积, 得到新的数值解总误差上界的理论近似公式, 以及最优格距和最优时间步长的理论公式. 通过数值算例验证了所得公式的可靠性. 然后, 发现了两种不同机器精度下最优时间步长之比满足的一个仅与机器精度有关的普适关系. 最后, 理论验证了在网格比固定的情况下, 此问题满足数值计算的不确定性原理, 以及在机器有限精度下最优时间步长的必然存在.

Fengyun-4 (FY-4), the latest collection of Chinese geostationary meteorological satellites, monitors the Eastern Hemisphere with high spatiotemporal resolutions. This study developed a cloud optical and microphysical property product for the Advanced Geosynchronous Radiation Imager (AGRI) onboard the FY-4 satellites. The product focuses on cloud optical thickness (COT) and cloud effective radius (CER) using a bi-spectral retrieval algorithm, and also includes cloud mask and phase using machine learning (ML) algorithms as prerequisites for COT and CER retrievals. The ML-based algorithm develops four independent models using Random Forest methods for cloud mask, liquid water, ice, and mixed-phase/multi-layer clouds, respectively. A two-habit ice and sphere water cloud model are employed to give their optical properties. Look-up tables of cloud reflectance in the COT and CER sensitive channels are built for efficient forward simulations, and the retrieval is performed by an optimal estimation algorithm. Compared with collocated active observations, the cloud mask and phase results give true positive rates of ∼95% and ∼85% and are more sensitive to mixed-phase clouds. Meanwhile, the AGRI-based COT and CER agree closely with those given by the collocated MODIS and AHI cloud products, and the correlation coefficients between MODIS and the AGRI results are 0.76 and 0.63 for COT and CER, respectively. The COT and CER retrievals will be persistently maintained and improved as the operational product for FY-4/AGRI.

摘要

风云四号作为中国新一代静止气象卫星, 提供了高时空分辨率的监测产品. 本文介绍风云四号搭载的先进地球同步轨道辐射成像仪AGRI的云光学和微物理特性产品. 该产品包含了基于双光谱通道反演的云光学厚度和云粒子有效半径产品, 以及基于机器学习的云识别和云相态产品. 与时空匹配的主动卫星观测结果对比显示, 该产品的云识别和云相态的准确率分别在95%和85%; 该产品提供的云光学厚度和云有效粒径与经典的MODIS产品的相关系数达到0.76和0.63. 团队将持续优化和更新该云光学和微物理特性定量产品, 服务风云四号卫星定量应用.

Previous studies have indicated that the stratospheric quasi-biennial oscillation (QBO) has a global impact on winter weather, but relatively less attention has been paid to its effect in summer. Using ERA5 data, this study reports that the QBO has a significant impact on the tropospheric circulation and surface air temperature (SAT) in the extratropics in Northeast Asia and the North Pacific in early summer. Specifically, a QBO-induced mean meridional circulation prevails from Northeast Asia to the North Pacific in the westerly QBO years, exhibiting westerly anomalies in 20°–35°N and easterly anomalies in 35°–65°N from the lower stratosphere to troposphere. This meridional pattern of zonal wind anomalies can excite positive vorticity and thus lead to anomalous low pressure and cyclonic circulation from Northeast Asia to the North Pacific, which in turn cause northerly wind anomalies and decreased SAT in Northeast Asia in June. Conversely, in the easterly QBO years, the QBO-related circulation and SAT anomalies are generally in an opposite polarity to those in the westerly QBO years. These findings provide new evidence of the impact of the QBO on the extratropical climate, and may benefit the prediction of SAT in Northeast Asia in early summer.

摘要

本文研究了平流层准两年振荡 (QBO) 对东北亚-北太平洋地区初夏对流层环流和地表气温的影响. 在QBO西风位相年, 东北亚至北太平洋地区存在一支由QBO引发的平均经向环流异常, 该经向环流异常可在东北亚至北太平洋地区激发正涡度, 并形成异常气旋式环流. 气旋左侧出现的异常偏北风导致6月东北亚地表气温下降. QBO东风位相年的结果与西风位相年大致相反. 这些结果为QBO对热带外地区天气,气候的影响提供了新的证据, 并为东北亚初夏地表气温的预测提供了新的线索.

Observations have shown a largely enhanced seasonal amplitude of northern atmospheric CO₂ in the past several decades, and this enhancement is attributable to the increased seasonal amplitude of northern net ecosystem productivity (NEP amplitude). In the future, however, the changes in NEP amplitude are not clear, because of the uncertainties in climate change and vegetation dynamics. This study investigated the changes in NEP amplitude north of 45°N under future global warming by using a dynamic global vegetation model (DGVM). The authors conducted two sets of simulations: a present-day simulation (1981–2000) and future simulations (2081–2100) forced by RCP8.5 outputs from CMIP5. The results showed an overall enhanced northern NEP amplitude under the RCP8.5 scenario because of the increased maximum NEP and the decreased minimum NEP. The increases (decreases) in the maximum (minimum) NEP resulted from stronger (weaker) positive changes in gross primary production (GPP) than ecosystem respiration (ER). Changes in GPP and ER are both dominantly driven by surface air temperature and vegetation dynamics. This work highlights the key role of vegetation dynamics in regulating the northern terrestrial carbon cycle and the importance of including a DGVM in Earth system models.

摘要

观测显示过去几十年北半球大气二氧化碳季节幅度大幅增加, 这主要是由北半球陆地净生态系统生产力季节幅度的增加所致. 但是, 因为气候变化和植被动态的不确定性, 未来陆地净生态系统生产力季节幅度的变化还很不清楚. 本工作利用全球植被动力学模式研究了全球变暖背景下北纬45°以北陆地净生态系统生产力季节幅度的变化. 作者做了两大类试验: 当代试验 (1981−2000) 和CMIP5 RCP8.5 变暖情景驱动的未来试验 (2081−2100) . 结果显示, 在RCP8.5变暖情景下北半球中高纬陆地净生态系统生产力季节幅度整体增加, 这是因为陆地净生态系统生产力的月最大值增加且月最小值减小. 最大 (最小) 陆地净生态系统生产力的增加 (减小) 是由于总初级生产力的增加强 (弱) 于生态系统总呼吸. 总初级生产力和生态系统总呼吸的变化都主要受地表气温和植被动态的驱动. 本工作强调了植被动态对北半球中高纬陆地生态系统碳循环的关键调制作用, 也强调了在地球系统模式中包含全球植被动力学模式的重要性.

The modulation of the late-summer Northeast China cold vortex (NCCV) by previous-winter El Niño–Southern Oscillation (ENSO) is investigated using reanalysis data. A significant correlation dominates in the ENSO–NCCV relationship, with El Niño (La Niña) accompanied by a weak (strong) NCCV intensity. The Indian Ocean basin mode (IOBM) plays an essential role in relaying the impact of pre-winter ENSO on the following late-summer atmospheric circulation over Northeast Asia. A positive IOBM responds passively to eastern Pacific–type El Niño and can last till late summer when El Niño has already decayed. It triggers a meridional teleconnection pattern of the cyclone over central China and anticyclone over Northeast Asia, and thereby enables a background circulation adverse to the generation of the NCCV, and vice versa. In addition, the signals of the Indian Ocean during central Pacific–type El Niño and neutral years are extremely weak, which makes their effect on NCCV generation insignificant.

摘要

本文利用再分析资料, 研究了前冬ENSO对夏末东北冷涡 (NCCV) 的调制作用. 结果表明, 前冬ENSO与夏末NCCV强度之间存在显著的相关性, El Niño (La Niña) 对应于弱 (强) 的NCCV. 印度洋海盆模态 (IOBM) 在前冬ENSO对夏末东北亚地区大气环流的影响中起着至关重要的作用. 作为东部型El Niño的被动响应, IOBM可以从前冬一直持续至夏末, 并在夏末激发“中国中部上空气旋—东北亚地区上空反气旋”的经向遥相关模态, 从而不利于NCCV增强. 反之亦然. 此外, 印度洋的信号在中部型El Niño和中性年份相对较弱, 使得它们对于NCCV的影响不显著.

The authors explore the response of the Northern African (NAF) monsoon to orbital forcing in the Last Interglacial (LIG) compared with its response to greenhouses gas (GHG) forcing under the SSP5-8.5 scenario simulated in CMIP6. When the summer surface air temperature increases by 1 °C over the Northern Hemisphere, the NAF monsoon precipitation and its variability during the LIG increase by approximately 51% and 22%, respectively, which is much greater than under SSP5-8.5 (2.8% and 4.3%, respectively). GHG forcing enhances the NAF monsoon mainly by increasing the atmospheric moisture, while the LIG's orbital forcing intensifies the NAF monsoon by changing the monsoon circulation. During the LIG, models and data reconstructions indicate a salient hemispheric thermal contrast between the North and South Atlantic, strengthening the mean-state NAF monsoon precipitation. The interhemispheric temperature contrast enhances atmosphere–ocean interaction and the covariability of the northward sea surface temperature gradient and Saharan low, strengthening the NAF monsoon variability.

摘要

与人为强迫引起的全球变暖相比, 末次间冰期是轨道强迫引起的过去80万年来最暖的一个间冰期, 但鲜有人研究末次间冰期中北非季风的响应. 因此, 本文基于CMIP6多模式模拟结果对比研究了末次间冰期和SSP5–8.5情景下北非季风的响应, 发现末次间冰期下北非季风平均降水及其降水变率均远大于SSP5–8.5情景下的结果. 轨道强迫导致的北大西洋暖于南大西洋增加了北非季风环流和平均降水, 同时, 南北大西洋海温梯度变化通过增强热带北大西洋的海气相互作用增大了海温梯度和撒哈拉低压的变率, 从而增强了北非季风降水变率.