雅鲁藏布江流域夏季降水的年际变化及其原因

本文通过多套观测与再分析降水资料的比较,分析了雅鲁藏布江流域夏季降水的特征,从水汽含量与水汽输送的角度检验了雅鲁藏布江水汽通道的特点,研究了流域夏季降水的年际变化及其原因。分析表明:（1）该流域夏季降水大值位于雅鲁藏布江出海口至大峡谷一带,观测中流域平均降水可达5.8 mm d-1。不同资料表现的降水空间分布一致,但再分析降水普遍强于观测,平均为观测的2倍左右。（2）该流域夏季的水汽主要来自印度洋和孟加拉湾的偏南暖湿水汽输送,自孟加拉湾出海口沿布拉马普特拉河上溯至大峡谷,即雅鲁藏布江水汽通道。水汽收支诊断表明,夏季流域南部（即水汽通道所在处）是水汽辐合中心,流域平均的辐合约9.5 mm d-1,主要来自风场辐合与地形坡度的贡献。（3）不同再分析资料表现的流域降水和水汽分布特征总体一致,但量值差异较大。NCEP（美国国家环境预报中心）气候预报系统再分析资料CFSR、日本气象厅再分析资料JRA-25较欧洲中期天气预报中心再分析ERA-Interim资料更适于研究该流域（青藏高原东南部）的水汽特征,因为后者给出的流域降水和水汽偏强。（4）近30年该流域夏季降水无显著趋势,以年际变率为主。年际异常的水汽辐合（约为气候态的35.4%）源自异常西南风导致的局地水汽辐合（纬向、经向辐合分别贡献了16.5%、83.5%）,地形作用很小。流域夏季降水的年际变化是由印度夏季风活动导致的异常水汽输送造成的,其关键系统是印度季风区北部的异常气旋（反气旋）式水汽输送。     

Intraseasonal and interannual zonal circulations over the Equatorial Indian Ocean

El Ni?o Southern Oscillation (ENSO) and given phases of the Madden?CJulian Oscillation (MJO) show similar regional signatures over the Equatorial Indian Ocean, consisting in an enhancement or reversing of the convective and dynamic zonal gradients between East Africa and the Maritime Continent of Indonesia. This study analyses how these two modes of variability add or cancel their effects at their respective timescales, through an investigation of the equatorial cellular circulations over the central Indian Ocean. Results show that (1) the wind shear between the lower and upper troposphere is related to marked regional rainfall anomalies and is embedded in larger-scale atmospheric configurations, involving the Southern Oscillation; (2) the intraseasonal (30?C60?days) and interannual (4?C5?years) timescales are the most energetic frequencies that modulate these circulations, confirming the implication of the MJO and ENSO; (3) extreme values of the Indian Ocean wind shear result from the combination of El Ni?o and the MJO phase enhancing atmospheric convection over Africa, or La Ni?a and the MJO phase associated with convective activity over the Maritime Continent. Consequences for regional rainfall anomalies over East Africa and Indonesia are then discussed.     

Long-term trend in potential vorticity intrusion events over the Pacific Ocean: Role of global mean temperature rise

In this study, we examine a long-term increasing trend in subtropical potential vorticity (PV) intrusion events over the Pacific Ocean in relation to the global mean temperature rise, based on multiple reanalysis datasets. The frequency of the PV intrusions is closely related to the upper-tropospheric equatorial westerly duct and the subtropical jet (STJ). An overall strengthening of the westerly duct and weakening of the STJ are found to be driven by the warming-induced strengthening of Walker circulation and regional changes in Hadley circulation on multi-decadal timescale, leading to an increase in the PV intrusion frequency over the tropics. The results are robust in all datasets. The multi-decadal strengthening in the Pacific Walker circulation is consistent with the global mean temperature rise. In this way, the PV intrusions are correlated with the warming related global mean temperuate rise. When the interannual variability of ENSO is removed from the intrusion time series, the long-term trend in PV intrusions due to external forcing associated with anthropogenic warming (global mean temperature rise) becomes clearer. The link between the global mean temperature rise and intrusion frequency is further verified by performing a correlation analysis between the two. The significant (> 95%) correlation coefficient is 0.85, 0.94, 0.84, 0.83, and 0.84 for ERA-40, ERA-Interim, NCEP-NCAR, JRA-55, and JRA-25, respectively. This unequivocally indicates that the global mean temperature rise can explain around 69%–88% of the variance related to the long-term increase in PV intrusion frequency over the Pacific Ocean.     

多套大气再分析资料的南亚高压强度变化特征及其与海表温度异常关系的比较分析

以往的研究中多采用NCE/NCAR再分析资料来讨论南亚高压的变化特征及其与海表温度的关系,鉴于其分析结果具有一定的片面性,本文采用ERA40、ERA—Interim、NCEWNCAR、NCEP—DOE和JRA．25五套再分析资料,以及应用全球、热带印度洋和热带大西洋1978--2008年逐月观测海表温度分别驱动NCARCAM5．1全球大气环流模式的数值模拟结果,比较了它们的夏季南亚高压强度变化特征及其与海表温度的关系。再分析资料问的比较结果表明,NCEWNCAR、NCEP—DOE两套再分析资料与ERA40、ERA—Interim、JRA-25三套再分析资料的南亚高压强度变化在20世纪70年代末至90年代初存在非常明显的差异,前两套再分析资料揭示的该时段南亚高压强度显著偏高,可能是不真实的,进而导致南亚高压强度与海表温度异常的关系与后三套再分析资料的结果差异明显。ERA40、ERA—Interim和JRA-25三套再分析资料和数值试验结果均表明,20世纪70年代末以后,夏季南亚高压强度异常与前期冬季、春季及同期夏季的热带印度洋海表温度异常关系持续密切,表明热带印度洋是影响夏季南亚高压强度变化的关键海区。当热带印度洋偏暖时,热带地区对流层温度增暖,南亚高压强度增强、面积增大、南扩、东伸西展,反之亦然。     

中国探空观测与第3代再分析大气湿度资料的对比研究

为评估中国探空观测与第3代再分析大气湿度资料的差异,基于ERA-Interim、JRA-55、MERRA和CFSR再分析和中国118探空站1979-2015年逐月850-300 hPa大气比湿和相对湿度的原始值及均一化序列,通过分析探空与再分析资料的相对偏差、相关系数、标准差比和变化趋势,研究了两者在多年平均值、年际变率、离散度及长期变化趋势等方面的差异。结果表明,中国探空原始湿度序列存在显著的非均一性问题,均一化提高了序列的连续性,但存在显著的低偏差,总体较原始湿度偏低5%-43%。再分析中国平均对流层大气比湿和相对湿度较探空观测偏高,相对湿度的偏差幅度（7%-48%）较比湿大（4%-13%）,对流层高层较低层大,春秋季偏差较夏季显著。各再分析资料间的差别较小,JRA-55在对流层高层较其他再分析资料偏低,与探空观测较接近。再分析与均一化后中国探空比湿和相对湿度年际变率和离散度在对流层低层较为一致,对流层中高层再分析资料的离散度明显高于探空。再分析与均一化探空中国平均比湿在对流层低层一致呈上升趋势;对流层中层探空为上升趋势,再分析资料为下降趋势。再分析与均一化探空相对湿度变化趋势差异较大,探空为上升趋势且对流层中高层上升显著,对流层再分析为下降趋势。      

冬季和春季印度洋海温异常年际变率模态对中国东部夏季降水的可能影响过程

印度洋热力状况是影响全球气候变化和亚洲季风变异的一个重要的因素,但以往研究更多关注热带印度洋海温的变化,对南印度洋中高纬地区海温变化关注不够,由此限制了我们对印度洋的全面认识.本文研究了年际尺度上整个印度洋海温异常主导模态的特征及其对我国东部地区夏季降水的可能影响过程,以期望为气候变异研究及预测提供理论依据.研究结果表明:全印度洋海温异常年际变率的主导模态特征是在南印度洋副热带地区海温异常呈现西南—东北反向变化的偶极子模态,西极子位于马达加斯加以东南洋面,东极子位于澳大利亚以西洋面;同时,热带印度洋海温异常与东极子一致.当西极子为正的海温异常,东极子、热带印度洋为负异常时定义为正的印度洋海温异常年际变率模态;反之,则为负的印度洋海温异常年际变率模态.从冬至春,印度洋海温异常年际变率模态具有较好的季节持续性;与我国长江中游地区夏季降水显著负相关,而与我国华南地区夏季降水显著正相关.其可能的影响过程为:对于正的冬、春季印度洋海温异常年际变率模态事件,印度洋地区异常纬向风的经向大气遥相关使得热带印度洋盛行西风异常,导致春、夏季海洋性大陆对流减弱,使夏季西太平洋副热带高压强度偏弱、位置偏东偏北,造成华南地区夏季降水增多,长江中游地区降水减少;反之亦然.同时,印度洋海温异常年际变率模态可通过改变印度洋和孟加拉湾向长江中游地区的水汽输送而影响其夏季降水.     

青藏高原西侧地区冬季降水的年际变率及其影响因子

基于全球降水气候中心（GPCC）和全球降水气候计划（GPCP）的降水数据及ERA-interim再分析资料,分析了1979~2012年冬季青藏高原（简称高原）西侧地区降水的基本特征及影响其年际变率的潜在因子。结果表明高原冬季降水主要发生在其西侧地区且为全区变化一致型,降水所需的水汽主要来自上游地区,从该区域的西边界输入。然而,高原西侧地区冬季降水的年际变率主要由水汽输送的动力过程所决定,表现为高原西侧的西南风异常。此外,高原西侧冬季降水的年际变率与其上游典型的大气内部变率北大西洋涛动和北极涛动相关性不强,而与赤道西印度洋和热带中东太平洋的海温显著相关。热带中东太平洋海温异常通过影响大气环流变化,在印度洋北部激发一个反气旋式的环流异常,使得高原西侧地区出现异常西南风,从而加强了水汽通量输送的动力作用。同时在赤道异常东风的作用下,暖水也向印度洋西部输送堆积。赤道中东太平洋海温的异常可进一步导致西风急流发生南北移动,从而也在一定程度上影响了高原西侧冬季水汽输送以及降水的年际变率。     

Evaluation of the trend uncertainty in summer ozone valley over the Tibetan Plateau in three reanalysis datasets

Trend uncertainty in the ozone valley over the Tibetan Plateau (OVTP) and the South Asian high (SAH) during 1979–2009 in ERA-Interim (interim reanalysis data from the ECMWF), JRA-55 (55-yr reanalysis data from the Japan Meteorological Agency), and NCEP-CFSR (Climate Forecast System Reanalysis) datasets was evaluated. The results showed that the NCEP-CFSR OVTP became strong in the summers of 1979–2009, whereas it became weak according to ERA-Interim and JRA-55. Satellite data merged with TOMS (Total Ozone Mapping Spectrometer) and OMI (Ozone Monitoring Instrument) agreed with the OVTP trend of NCEP-CFSR. The OVTP strengthening in NCEP-CFSR may have been caused by SAH intensification, a rising tropopause, and increasing ozone over non-TP (non-Tibetan Plateau) areas (27°–37°N, < 75°E and > 105°E). Analogously, the OVTP weakening in ERA-Interim and JRA-55 may have been affected by weakening SAH, descending tropopause, and decreasing non-TP ozone.     

夏季北印度洋海温异常对西北太平洋低层反气旋异常的影响

采用1957—2002年850 hPa风场的ERA-40再分析资料,分析得知西北太平洋低层环流存在着明显的年际变化。这种年际变化表征了西北太平洋夏季风的年际变化,并且会影响东亚夏季风的变化。用Hadley海表面气压以及海表温度资料诊断得到,这种夏季西北太平洋反气旋异常（WPAC,northwest Pacific anomalous anticyclone）的年际变化与北印度洋同期海表温度变化存在很好的相关。用偏相关方法消除N ino3.4信号的同期线性影响,这种同期相关更加显著,而西南热带印度洋的同期海温与WPAC的相关并不显著。数值试验结果表明,北印度洋存在正海温异常时,北印度洋降水偏多,同时伴随着西北太平洋反气旋异常。当只有西南热带印度洋有正海温异常时,北印度洋会出现东风异常且降水减少,而西北太平洋有弱的气旋异常。数值模式结果与观测数据的诊断结果相吻合,说明当夏季北印度洋海表温度为正异常时,可能会产生西北太平洋反气旋异常。     

Understanding Interannual Variations of the Local Rainy Season over the Southwest Indian Ocean

Located at the southern boundary of the tropical rainfall belt within the South Africa monsoon regime, Rodrigues Island, ～2500 km east of East Africa, is ideally located to investigate climatic changes over the southwest Indian Ocean(SWIO). In this study, we investigate the climatic controls of its modern interannual rainfall variability in terms of teleconnection and local effects. We find that increased rainfall over the SWIO tends to occur in association with anomalously warm(cold) SSTs over the equatorial central Pacific(Maritime Continent), resembling the central Pacific El Ni?o, closely linked with the Victoria mode in the North Pacific. Our analyses show that the low-level convergence induced by warm SST over the equatorial central Pacific leads to anomalous low-level divergence over the Maritime Continent and convergence over a large area surrounding the Rodrigues Island, which leads to increased rainfall over the SWIO during the rainy season. Meanwhile, the excited Rossby wave along the tropical Indian Ocean transports more water vapor from the tropical convergence zone into the SWIO via intensified northwest wind. Furthermore, positive feedback induced by the Rossby wave response to the increased rainfall in the region contributes to the large interannual variations over the SWIO.     

Precipitation climatology over India: validation with observations and reanalysis datasets and spatial trends

Changing rainfall patterns have significant effect on water resources, agriculture output in many countries, especially the country like India where the economy depends on rain-fed agriculture. Rainfall over India has large spatial as well as temporal variability. To understand the variability in rainfall, spatial–temporal analyses of rainfall have been studied by using 107 (1901–2007) years of daily gridded India Meteorological Department (IMD) rainfall datasets. Further, the validation of IMD precipitation data is carried out with different observational and different reanalysis datasets during the period from 1989 to 2007. The Global Precipitation Climatology Project data shows similar features as that of IMD with high degree of comparison, whereas Asian Precipitation-Highly-Resolved Observational Data Integration Towards Evaluation data show similar features but with large differences, especially over northwest, west coast and western Himalayas. Spatially, large deviation is observed in the interior peninsula during the monsoon season with National Aeronautics Space Administration-Modern Era Retrospective-analysis for Research and Applications (NASA-MERRA), pre-monsoon with Japanese 25 years Re Analysis (JRA-25), and post-monsoon with climate forecast system reanalysis (CFSR) reanalysis datasets. Among the reanalysis datasets, European Centre for Medium-Range Weather Forecasts Interim Re-Analysis (ERA-Interim) shows good comparison followed by CFSR, NASA-MERRA, and JRA-25. Further, for the first time, with high resolution and long-term IMD data, the spatial distribution of trends is estimated using robust regression analysis technique on the annual and seasonal rainfall data with respect to different regions of India. Significant positive and negative trends are noticed in the whole time series of data during the monsoon season. The northeast and west coast of the Indian region shows significant positive trends and negative trends over western Himalayas and north central Indian region.     

Projected changes in vertical temperature profiles for Australasia

The vertical temperature profile in the atmosphere reflects a balance between radiative and convective processes and interactions with the oceanic and land surfaces. Changes in vertical temperature profiles can affect atmospheric stability, which in turn can impact various aspects of weather systems. In this study, we analyzed recent-past trends of temperature over the Australian region using a homogenized monthly upper-air temperature dataset and four reanalysis datasets (NCEP, ERA-Interim, JRA-55 and MERRA). We also used outputs of 12 historical and future regional climate model (RCM) simulations from the NSW/ACT (New South Wales/Australian Capital Territory) Regional Climate Modelling (NARCliM) project and 6 RCM simulations from the CORDEX (Coordinated Regional Downscaling Experiment) Australasian project to investigate projected changes in vertical temperature profiles. The results show that the currently observed positive trend in the troposphere and negative trend in the lower stratosphere will continue in the future with significant warming over the whole troposphere and largest over the middle to upper troposphere. The increasing temperatures are found to be latitude-dependent with clear seasonal variations, and a strong diurnal variation for the near surface layers and upper levels in tropical regions. Changes in the diurnal variability indicate that near surface layers will be less stable in the afternoon leading to conditions favoring convective systems and more stable in the early morning which is favorable for temperature inversions. The largest differences of future changes in temperature between the simulations are associated with the driving GCMs, suggesting that large-scale circulation plays a dominant role in regional atmospheric temperature change.

     

The Antarctic oscillation-East Asian summer monsoon connections in NCEP-1 and ERA-40

Connections between the spring Antarctic Oscillation(AAO)and the East Asian summer monsoon (EASM)in two reanalysis datasets—NCEP-1(NCEP/NCAR Reanalysis 1)and ERA-40(ECMWF 40- year Reanalysis)—are investigated in this study.Both show significant correlation between AAO and EASM rainfall over the Yangtze River valley,especially after about 1985.Though ERA-40 shows weaker anomalous signals connecting AAO and EASM over southern high latitudes than NCEP-1,both datasets reveal similar connecting patterns between ...     

Intraseasonal Oscillation of Tropospheric Ozone over the Indian Summer Monsoon Region

Boreal summer intraseasonal oscillation(BSISO) of lower tropospheric ozone is observed in the Indian summer monsoon(ISM) region on the basis of ERA-Interim reanalysis data and ozonesonde data from the World Ozone and Ultraviolet Radiation Data Centre. The 30–60-day intraseasonal variation of lower-tropospheric ozone shows a northwest–southeast pattern with northeastward propagation in the ISM region. The most significant ozone variations are observed in the Maritime Continent and western North Pacific. In the tropics, ozone anomalies extend from the surface to 300 hPa; however, in extratropical areas, it is mainly observed under 500 hPa. Precipitation caused by BSISO plays a dominant role in modulating the BSISO of lower-tropospheric ozone in the tropics, causing negative/positive ozone anomalies in phases 1–3/5–6. As the BSISO propagates northeastward to the western North Pacific, horizontal transport becomes relatively more important, increasing/reducing tropospheric ozone via anticyclonic/cyclonic anomalies over the western North Pacific in phases 3–4/7–8.As two extreme conditions of the ISM, most of its active/break events occur in BSISO phases 4–7/1–8 when suppressed/enhanced convection appears over the equatorial eastern Indian Ocean and enhanced/suppressed convection appears over India, the Bay of Bengal, and the South China Sea. As a result, the BSISO of tropospheric ozone shows significant positive/negative anomalies over the Maritime Continent, as well as negative/positive anomalies over India, the Bay of Bengal,and the South China Sea in active/break spells of the ISM. This BSISO of tropospheric ozone is more remarkable in break spells than in active spells of the ISM, due to the stronger amplitude of BSISO in the former.     

基于再分析资料与观测资料的中国低温阈值变化特征研究

采用1979—2013年中国192站逐日最低温度观测资料和NCEP/NCAR、NCEP/DOE、JRA-55、ERA-Interim再分析资料及1979—2004年均一化资料,分别计算低温阈值并对比分析其气候态、年际和年代际变化、长期趋势等特征。结果表明:与观测结果相比,均一化资料阈值在东北、内蒙古西部和两广等地偏低,在青藏高原东侧、新疆北部和黄河中下游偏高,线性趋势则相反;再分析资料阈值在南方偏低、东北偏高,在东部的可信度高于西部;再分析资料能显示内蒙古中西部的降温趋势和青藏高原的增温趋势,但在数值和范围上有差异,且均低估了观测资料反映的华北地区的显著升温现象;再分析资料能体现观测资料阈值的全区一致性、东北与其他地区反相的空间分布及其年际变化特征,仅JRA-55和ERA-Interim可再现低温阈值的年代际变化特征。     

Toward Understanding the Extreme Floods over Yangtze River Valley in June?July 2020: Role of Tropical Oceans※

The extreme floods in the Middle/Lower Yangtze River Valley (MLYRV) during June?July 2020 caused more than 170 billion Chinese Yuan direct economic losses. Here, we examine the key features related to this extreme event and explore relative contributions of SST anomalies in different tropical oceans. Our results reveal that the extreme floods over the MLYRV were tightly related to a strong anomalous anticyclone persisting over the western North Pacific, which brought tropical warm moisture northward that converged over the MLYRV. In addition, despite the absence of a strong El Ni?o in 2019/2020 winter, the mean SST anomaly in the tropical Indian Ocean during June?July 2020 reached its highest value over the last 40 years, and 43％ (57％) of it is attributed to the multi-decadal warming trend (interannual variability). Based on the NUIST CFS1.0 model that successfully predicted the wet conditions over the MLYRV in summer 2020 initiated from 1 March 2020 (albeit the magnitude of the predicted precipitation was only about one-seventh of the observed), sensitivity experiment results suggest that the warm SST condition in the Indian Ocean played a dominant role in generating the extreme floods, compared to the contributions of SST anomalies in the Maritime Continent, central and eastern equatorial Pacific, and North Atlantic. Furthermore, both the multi-decadal warming trend and the interannual variability of the Indian Ocean SSTs had positive impacts on the extreme floods. Our results imply that the strong multi-decadal warming trend in the Indian Ocean needs to be taken into consideration for the prediction/projection of summer extreme floods over the MLYRV in the future.     

The Indo-Australian monsoon and its relationship to ENSO and IOD in reanalysis data and the CMIP3/CMIP5 simulations

A large spread exists in both Indian and Australian average monsoon rainfall and in their interannual variations diagnosed from various observational and reanalysis products. While the multi model mean monsoon rainfall from 59 models taking part in the Coupled Model Intercomparison Project (CMIP3 and CMIP5) fall within the observational uncertainty, considerable model spread exists. Rainfall seasonality is consistent across observations and reanalyses, but most CMIP models produce either a too peaked or a too flat seasonal cycle, with CMIP5 models generally performing better than CMIP3. Considering all North-Australia rainfall, most models reproduce the observed Australian monsoon-El Niño Southern Oscillation (ENSO) teleconnection, with the strength of the relationship dependent on the strength of the simulated ENSO. However, over the Maritime Continent, the simulated monsoon-ENSO connection is generally weaker than observed, depending on the ability of each model to realistically reproduce the ENSO signature in the Warm Pool region. A large part of this bias comes from the contribution of Papua, where moisture convergence seems to be particularly affected by this SST bias. The Indian summer monsoon-ENSO relationship is affected by overly persistent ENSO events in many CMIP models. Despite significant wind anomalies in the Indian Ocean related to Indian Ocean Dipole (IOD) events, the monsoon-IOD relationship remains relatively weak both in the observations and in the CMIP models. Based on model fidelity in reproducing realistic monsoon characteristics and ENSO teleconnections, we objectively select 12 “best” models to analyze projections in the rcp8.5 scenario. Eleven of these models are from the CMIP5 ensemble. In India and Australia, most of these models produce 5–20 % more monsoon rainfall over the second half of the twentieth century than during the late nineteenth century. By contrast, there is no clear model consensus over the Maritime Continent.     

Vertical cloud structures of the boreal summer intraseasonal variability based on CloudSat observations and ERA-interim reanalysis

The boreal summer intraseasonal variability (BSISV), which is characterized by pronounced meridional propagation from the equatorial zone to the Indian Continent, exerts significant modulation of the active/break phases of the south Asian monsoon. This form of variability provides a primary source of subseasonal predictive skill of the Asian summer monsoon. Unfortunately, current general circulation models display large deficiencies in representing this variability. The new cloud observations made available by the CloudSat mission provide an unprecedented opportunity to advance our characterization of the BSISV. In this study, the vertical structures of cloud water content and cloud types associated with the BSISV over the Indian Ocean and subcontinent are analyzed based on CloudSat observations from 2006 to 2008. These cloud structures are also compared to their counterparts as derived from ERA-interim reanalysis. A marked vertical tilting structure in cloud water is illustrated during the northward propagation of the BSISV based on both datasets. Increased cloud liquid water content (LWC) tends to appear to the north of the rainfall maximum, while ice water content (IWC) in the upper troposphere slightly lags the convection. This northward shift of increased LWC, which is in accord with local enhanced moisture as previously documented, may play an important role in the northward propagation of the BSISV. The transition in cloud structures associated with BSISV convection is further demonstrated based on CloudSat, with shallow cumuli at the leading edge, followed by the deep convective clouds, and then upper anvil clouds. Some differences in cloud water structures between CloudSat and ERA-interim are also noted, particularly in the amplitudes of IWC and LWC fields.     

Toward Understanding the Extreme Floods over Yangtze River Valley in June?July 2020: Role of Tropical Oceans※

The extreme floods in the Middle/Lower Yangtze River Valley (MLYRV) during June?July 2020 caused more than 170 billion Chinese Yuan direct economic losses. Here, we examine the key features related to this extreme event and explore relative contributions of SST anomalies in different tropical oceans. Our results reveal that the extreme floods over the MLYRV were tightly related to a strong anomalous anticyclone persisting over the western North Pacific, which brought tropical warm moisture northward that converged over the MLYRV. In addition, despite the absence of a strong El Ni?o in 2019/2020 winter, the mean SST anomaly in the tropical Indian Ocean during June?July 2020 reached its highest value over the last 40 years, and 43％ (57％) of it is attributed to the multi-decadal warming trend (interannual variability). Based on the NUIST CFS1.0 model that successfully predicted the wet conditions over the MLYRV in summer 2020 initiated from 1 March 2020 (albeit the magnitude of the predicted precipitation was only about one-seventh of the observed), sensitivity experiment results suggest that the warm SST condition in the Indian Ocean played a dominant role in generating the extreme floods, compared to the contributions of SST anomalies in the Maritime Continent, central and eastern equatorial Pacific, and North Atlantic. Furthermore, both the multi-decadal warming trend and the interannual variability of the Indian Ocean SSTs had positive impacts on the extreme floods. Our results imply that the strong multi-decadal warming trend in the Indian Ocean needs to be taken into consideration for the prediction/projection of summer extreme floods over the MLYRV in the future.     

Interannual variability of summertime outgoing longwave radiation over the Maritime Continent in relation to East Asian summer monsoon anomalies

The Maritime Continent (MC) is under influences of both the tropical Pacific and the Indian Ocean. Anomalous convective activities over the MC have significant impacts on the East Asian summer monsoon (EASM) and climate in China. In the present study, the variation in convective activity over the MC in boreal summer and its relationship to EASM anomalies are investigated based on regression analysis of NCEP–NCAR reanalysis and CMAP [Climate Prediction Center (CPC) Merged Analysis of Precipitation] data, with a focus on the impacts of ENSO and the Indian Ocean Dipole (IOD). The most significant interannual variability of convective activity is found over 10°S–10°N, 95°–145°E, which can be roughly defined as the key area of the MC (hereafter, KMC). Outgoing longwave radiation anomaly (OLRA) exhibits 3- to 7-yr periodicities over the KMC, and around 70% of the OLRA variance can be explained by the ENSO signal. However, distinct convection and precipitation anomalies still exist over this region after the ENSO and IOD signals are removed. Abnormally low precipitation always corresponds to positive OLRA over the KMC when negative diabatic heating anomalies and anomalous cooling of the atmospheric column lead to abnormal descending motion over this region. Correspondingly, abnormal divergence occurs in the lower troposphere while convergence occurs in the upper troposphere, triggering an East Asia–Pacific/Pacific–Japan (EAP/PJ)-like anomalous wave train that propagates northeastward and leads to a significant positive precipitation anomaly from the Yangtze River valley in China to the islands of Japan. This EAP/PJ-like wave pattern becomes even clearer after the removal of the ENSO signal and the combined effects of ENSO and IOD, suggesting that convective anomalies over the KMC have an important impact on EASM anomalies. The above results provide important clues for the prediction of EASM anomalies and associated summer precipitation anomalies in China.