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The Mid-Piacenzian (MP; approximately 3.3-3.0 Ma) was a relatively warm period in geological time and this past warming climate was in some respects comparable to the near future greenhouse warming climate projections. How the regional summer monsoons responded to the MP warming is an important scientific concern. Using the Pliocene Model Intercomparison Project phase 2 (PlioMIP2) simulations, this study explores the changes in South Asian summer monsoon (SASM) circulation during the MP based on the 3-pattern decomposition of global atmospheric circulation (3P-DGAC) method. The results show that both the zonal and meridional SASM circulations remarkably strengthened during the MP warm period. This is fundamentally different from a weakened SASM circulation response to the near future greenhouse warming forcing that has been reported by previous studies. On the one hand, the enhanced zonal SASM circulation during the MP was closely linked with warmer mid-upper tropospheric temperature over Eastern Eurasia than the tropical Indian Ocean. The increased meridional temperature gradient can induce easterly (westerly) anomalies at the upper-level (low-level) troposphere across the SASM region via the thermal-wind relation, strengthening the zonal SASM circulation. On the other hand, the anomalous convective heating associated with excessively increased North African summer monsoon rainfall during the MP period warms the mid-upper troposphere over North Africa, resulting in an increased west-minus-east temperature gradient across the SASM region. Such a zonal temperature gradient change at the mid-upper troposphere can enhance the meridional SASM circulation as well. Implications of the fundamental difference between the SASM circulation responses to the MP and near future warming climates are discussed.

The present study examines changes in the low-level summer monsoon circulation over the Arabian Sea and their impact on the ocean dynamics using reanalysis data. The study confirms intensification and northward migration of low-level jet during 1979 to 2015. Further during the study period, an increase in the Arabian Sea upper ocean heat content is found in spite of a decreasing trend in the net surface heat flux, indicating the possible role of ocean dynamics in the upper ocean warming. Increase in the anti-cyclonic wind stress curl associated with the change in the monsoon circulation induces downwelling over the central Arabian Sea, favoring upper ocean warming. The decreasing trend of southward Ekman transport, a mechanism transporting heat from the land-locked north Indian Ocean to southern latitudes, also supports increasing trend of the upper ocean heat content. To reinstate and quantify the role of changing monsoon circulation in increasing the heat content over the Arabian Sea, sensitivity experiment is carried out using ocean general circulation model. In this experiment, the model is forced by inter-annual momentum forcing while rest of the forcing is climatological. Experiment reveals that the changing monsoon circulation increases the upper ocean heat content, effectively by enhancing downwelling processes and reducing southward heat transport, which strongly endorses our hypothesis that changing ocean dynamics associated with low-level monsoon circulation is causing the increasing trend in the heat content of the Arabian Sea.

El Niño events vary from case to case with different decaying paces. In this study, we demonstrate that the different El Niño decaying paces have distinct impacts on the East Asian monsoon circulation pattern during post-El Niño summers. For fast decaying (FD) El Niño summers, a large-scale anomalous anticyclone dominates over East Asia and the North Pacific from subtropical to mid-latitude; whereas, the East Asian monsoon circulation display a dipole pattern with anomalous northern cyclone and southern anticyclone for slow decaying (SD) El Niño summers. The difference in anomalous East Asian monsoon circulation patterns was closely associated with the sea surface temperature (SST) anomaly patterns in the tropics. In FD El Niño summers, the cold SST anomalies in the tropical central-eastern Pacific and warm SST anomalies in the Maritime Continent induce the anticyclone anomalies over the Northwest Pacific. In contrast, the warm Kelvin wave anchored over the tropical Indian Ocean during SD El Niño summers plays a crucial role in sustaining the anticyclone anomalies over the Northwest Pacific. In particular, the opposite atmospheric circulation anomaly patterns over Northeast Asia and the mid-latitude North Pacific are mainly modulated by the stationary Rossby wave trains triggered by the opposite SST anomalies in the tropical eastern Pacific during FD and SD El Niño summers. Finally, the effect of distinct summer monsoon circulation patterns associated with the El Niño decay pace on the summer climate over East Asia are also discussed.

This study explores the response of atmospheric temperature to the annular solar eclipse at the summer solstice on 21 June 2020. The radio occultation (RO) technique of the FORMOSAT-7/COSMIC2 (F7/C2) mission observes the temperature in the troposphere and stratosphere. The RO observations show that the temperature decreases significantly (near 4 to 8 °C) between 5 and 8 km altitudes over the Tibetan Plateau area within the 80% obscuration during the eclipse. The tropopause temperature increases by ~ 2 to 5 °C over the same area. By contrast, the tropopause temperature decreases by ~ 4° to 5 °C over the Indian Ocean. The F7/C2 RO technique captured not only the sudden tropospheric cooling and stratospheric warming over Tibet during the eclipse but also the possible response over the Indian Ocean away from the greatest eclipse. Key points RO temperature decreases over the Tibetan Plateau during the eclipse Opposite changes of the tropopause temperatures over Tibet and Indian Ocean Indian summer monsoon circulation was perturbed during the eclipse

The interdecadal enhancement in the interannual variability of summer monsoon meridional circulation (SMMC) over the South China Sea around the early 1990s is investigated. Results show the change in the SMMC variability may arise from the interdecadal shift in the leading modes of low-level geopotential height over East Asia–Australia and Indo–Pacific sea surface temperature anomalies (SSTAs) in boreal summer. Before the early 1990s, the leading mode of Indo–Pacific SSTAs shows a zonal tripole pattern, with abnormally warm eastern Pacific and northern Indian Ocean and cold western Pacific. At the lower level, the western North Pacific cooling and northern Indian Ocean warming generate an anticyclonic anomaly over western North Pacific, while the cooling over the Maritime Continent and east of Australia favors an abnormal anticyclone over Australia. Hence region-wide positive geopotential height anomalies cover East Asia–Australia, which resemble the major mode of geopotential height and generate weak south–north pressure gradient and SMMC variability. After the early 1990s, the leading SSTAs mode shifts to a zonal dipole with abnormally cold western Pacific and warm equatorial central–eastern Pacific. The central Pacific warming induces an anomalous low-level cyclone over Philippines and it is further maintained by the Maritime Continent cooling. Meanwhile, the cooling over the east of Australia and Maritime Continent favors an abnormal Australian anticyclone. The low-level geopotential height thus shows south–north dipole anomalies over East Asia–Australia, resembling its major mode and generating obvious meridional pressure gradient and SMMC variability. The atmospheric responses to different SSTAs modes are confirmed by CAM4 experiments.

Projecting future change of monsoon rainfall is essential for water resource management, food security, disaster mitigation, and infrastructure planning. Here we assess the future change and explore the causes of the changes using 15 models that participated in phase 6 of the Coupled Model Intercomparison Project (CMIP6). The multimodel ensemble projects that, under the shared socioeconomic pathway (SSP) 2–4.5, the total land monsoon rainfall will likely increase in the Northern Hemisphere (NH) by about 2.8% per one degree Celsius of global warming (2.8% °C−1) in contrast to little change in the Southern Hemisphere (SH; −0.3% °C−1). In addition, in the future the Asian–northern African monsoon likely becomes wetter while the North American monsoon becomes drier. Since the humidity increase is nearly uniform in all summer monsoon regions, the dynamic processes must play a fundamental role in shaping the spatial patterns of the global monsoon changes. Greenhouse gas (GHG) radiative forcing induces a “NH-warmer-than-SH” pattern, which favors increasing the NH monsoon rainfall and prolonging the NH monsoon rainy season while reducing the SH monsoon rainfall and shortening the SH monsoon rainy season. The GHG forcing induces a “land-warmer-than-ocean” pattern, which enhances Asian monsoon low pressure and increases Asian and northern African monsoon rainfall, and an El Niño–like warming, which reduces North American monsoon rainfall. The uncertainties in the projected monsoon precipitation changes are significantly related to the models’ projected hemispheric and land–ocean thermal contrasts as well as to the eastern Pacific Ocean warming. The CMIP6 models’ common biases and the processes by which convective heating drives monsoon circulation are also discussed.

The transition of the global atmospheric circulation in the end of 1970's can clearly be detected in the atmospheric temperature, wind velocity, and so on. Wavelet analysis reveals that the temporal scale of this change is larger than 20 years. Studies in this work indicate that the trend of the transition over the mid-latitude Asia is opposite to that of global average for some variables at the middle troposphere. Another finding of this research is that the African-Asian monsoon circulation is weaker and the trade wind over the tropical eastern Pacific is weaker as well after this transition. Such a signal may be found in the summer precipitation over China as well.

The East Asian summer monsoon is unique among summer monsoon systems in its complex seasonality, exhibiting distinct intraseasonal stages. Previous studies have alluded to the downstream influence of the westerlies flowing around the Tibetan Plateau as key to its existence. We explore this hypothesis using an atmospheric general circulation model that simulates the intraseasonal stages with fidelity. Without a Tibetan Plateau, East Asia exhibits only one primary convective stage typical of other monsoons. As the plateau is introduced, the distinct rainfall stages—spring, pre-mei-yu, mei-yu, and midsummer—emerge, and rainfall becomes more intense overall. This emergence coincides with a pronounced modulation of the westerlies around the plateau and extratropical northerlies penetrating northeastern China. The northerlies meridionally constrain the moist southerly flow originating from the tropics, leading to a band of lower-tropospheric convergence and humidity front that produces the rainband. The northward migration of the westerlies away from the northern edge of the plateau leads to a weakening of the extratropical northerlies, which, coupled with stronger monsoonal southerlies, leads to the northward migration of the rainband. When the peak westerlies migrate north of the plateau during the midsummer stage, the extratropical northerlies disappear, leaving only the monsoon low-level circulation that penetrates northeastern China; the rainband disappears, leaving isolated convective rainfall over northeastern China. In short, East Asian rainfall seasonality results from the interaction of two seasonally evolving circulations—the monsoonal southerlies that strengthen and extend northward, and the midlatitude northerlies that weaken and eventually disappear—as summer progresses.

Coupled climate system models consistently show that the low-level southerly wind associated with the East Asian summer monsoon (EASM) is enhanced under anthropogenic greenhouse gas forcing, and the enhanced EASM was attributed to the enhanced land–sea thermal contrast by previous studies. Based on a comparison of the global warming scenarios with the present-day climate in an ensemble of 30 coupled models from phase 5 of the Coupled Model Intercomparison Project (CMIP5), we show evidence that changes in land–sea thermal contrast cannot explain the enhanced EASM circulation in terms of the seasonality. Indeed, the enhanced low-level southerly wind over East Asia is associated with a large-scale anomalous cyclone around the Tibetan Plateau (TP), and numerical simulation by the Linear Baroclinic Model suggests that the enhanced latent heating over the TP associated with enhanced precipitation is responsible for this low-level cyclone anomaly and the enhanced EASM circulation projected by the coupled models. Moisture budget analysis shows that enhanced hydrological recycling and enhanced vertical moisture advection due to increased specific humidity have the largest contribution to the increased precipitation over the TP, and more than half of the intermodel uncertainty in the projected change of EASM circulation is associated with the uncertainty in the changes of precipitation over the TP. Therefore, the TP plays an essential role in enhancing the EASM circulation under global warming through enhanced latent heating over the TP.

Studies conducted at the beginning of this century revealed a warming and humidification trend in northwest China, which lasted for a short period, with some minor variations. The future trend remains uncertain, with uncertainty over the future duration of warming and humidification trends in the region. In this study, a comprehensive index was constructed to quantitatively characterize the degree of humidity. Using observation data, National Center for Atmospheric Research/National Centers for Environmental Prediction (NCAR/NCEP) reanalysis data, and future scenario prediction data we systematically analyzed the temporal and spatial evolution characteristics of the warming and humidification trend in northwest China and its possible causes. The results showed that the temperature in northwest China has been continuously increasing since 1961, and although there was no obvious precipitation trend from 1961 to 1999, it has been increasing nonlinearly since 2000. The wetness index considers the synergy of evaporation and precipitation, and had the same pattern of variation as precipitation. From a spatial perspective, the temperature has consistently increased throughout the whole northwest region, and the degree of warming has gradually increased. Precipitation displayed a trend of increasing in the west and decreasing in the east, but the area with increased rainfall expanded eastward over almost three climatic periods, and the area experiencing humidification extended across the whole of the northwest. There has been an obvious eastward expansion of climate humidification in northwest China since the start of this century. However, the current extent of climate humidification has not changed the basic climate pattern in northwest China. Over the next 30 years, the humidification trend will likely slow down significantly compared with the last 20 years. The eastward expansion of climate humidification since the start of this century may have resulted from the multi-decadal synergistic influence of westerly wind circulation and monsoon circulation.