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Guixing Chen, Yu Du, and Zhiping Wen

Abstract

This study revisits the long-term variabilities of the East Asian summer monsoon (EASM) in 1958–2017 through examination of diurnal cycles. We group monsoon days into four dynamic quadrants (Q1 to Q4), with emphasis on the strong daily southerlies coupled with a large (Q1) or small (Q4) diurnal amplitude over Southeast China. The occurrence day of Q1 increases in June–July with the seasonal progress of the EASM. It is most pronounced in the 1960s to the 1970s and declines to the lowest in the 1980s to the 1990s, whereas the Q4 occurrence increases notably from the 1970s to the 1990s; both groups return to normal in recent years. The interdecadal decrease (increase) of Q1 (Q4) occurrence corresponds well to the known weakening of EASM in the twentieth century, and it also coincides with the rainfall anomalies over China shifting from a “north flooding and south drought” to a “north drought and south flooding” mode. The rainfall under Q1 (Q4) can account for ~60% of the interannual variance of summer rainfall in northern (southern) China. The contrasting effects of Q1 and Q4 on rainfall are due to their remarkably different regulation on water vapor transport and convergence. The interannual/interdecadal variations of Q1 (Q4) occurrence determine the anomalous water vapor transports to northern (southern) China, in association with the various expansion of the western Pacific subtropical high. In particular, Q1 conditions can greatly intensify nighttime moisture convergence, which is responsible for the long-term variations of rainfall in northern China. The results highlight that the diurnal cycles in monsoon flow act as a key regional process working with large-scale circulation to regulate the spatial distributions and long-term variabilities of EASM rainfall.

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Wen Chen, Juan Feng, and Renguang Wu

Abstract

The present study investigates the roles of El Niño–Southern Oscillation (ENSO) and the Pacific decadal oscillation (PDO) in the relationship between the East Asian winter monsoon (EAWM) and the following East Asian summer monsoon (EASM). The variability of the EAWM is divided into an ENSO-related part named EAWMEN and an ENSO-unrelated part named EAWMres. Corresponding to a weak EAWMEN, an anomalous low-level anticyclone forms over the western North Pacific (WNP) and persists from winter to the following summer. This anticyclone enhances southerlies over the coast of East Asia in summer. Hence, a weak EAWMEN tends to be followed by a strong EASM and vice versa. As such, a link is established between the EAWMEN and the EASM. The persistence of this WNP anticyclone may be mainly attributed to the sea surface temperature anomalies associated with the ENSO-related EAWM part in the tropical Indian Ocean and the extratropical North Pacific. In contrast, corresponding to a weak EAWMres, the anomalous WNP anticyclone is only seen in winter, and there is no obvious relationship between the EAWMres and the following EASM. Therefore, the observed EAWM–EASM relationship is dominated by the winter monsoon variability associated with ENSO. It is found that the EAWMEN–EASM relationship is modulated by the PDO. There tends to be a much stronger EASM after a weak EAWMEN during the positive PDO phases than during the negative PDO phases.

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Juan Feng, Wen Chen, and Xiaocong Wang

Abstract

The El Niño Modoki–induced anomalous western North Pacific anticyclone (WNPAC) undergoes an interesting reintensification process in the El Niño Modoki decaying summer, the period when El Niño Modoki decays but warm sea surface temperature (SST) anomalies over the tropical North Atlantic (TNA) and cold SST anomalies over the central-eastern Pacific (CEP) dominate. In this study, the region (TNA or CEP) in which the SST anomalies exert a relatively important influence on reintensification of the WNPAC is investigated. Observational analysis demonstrates that when only anomalous CEP SST cooling occurs, the WNPAC experiences a weak reintensification. In contrast, when only anomalous TNA SST warming emerges, the WNPAC experiences a remarkable reintensification. Numerical simulation analysis demonstrates that even though the same magnitude of CEP SST cooling and TNA warming is respectively set to force the atmospheric general circulation model, the response of the WNPAC is still much stronger in the TNA warming experiment than in the CEP cooling experiment. Further analysis demonstrates that this difference is caused by the distinct location of the effective tropical forcing between the CEP SST cooling and TNA SST warming for producing a WNPAC. The CEP cooling-induced effective anomalous diabatic cooling is located in the central Pacific, by which the forced anticyclone becomes gradually weak from the central Pacific to the western North Pacific. Thus, a weak WNPAC is produced. In contrast, as the TNA SST warming–induced effective anomalous diabatic cooling is just located in the western North Pacific via a Kelvin wave–induced Ekman divergence process, the forced anticyclone is significant and powerful in the western North Pacific.

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Weiwen Wang, Wen Zhou, and Deliang Chen

Abstract

This study investigates summer high temperature extremes (HTEs) in southeast China and their linkage with the El Niño–Southern Oscillation (ENSO) and atmospheric circulations in the East Asian summer monsoon (EASM). An interdecadal change in HTEs associated with the abrupt shift of the ENSO–monsoon climate in the late 1980s is demonstrated. Before this interdecadal shift, the interannual variability of HTEs was linked mainly to temperature adjustments associated with the meridional displacement of the East Asian jet stream (EAJS), whereas after the shift HTEs were found to follow an ENSO cycle, which may be due to intensified and persistent ENSO activities, tropical Indian Ocean (TIO) warming, and changes in atmospheric teleconnections. Impacts of the EAJS, the South Asian high (SAH), and the western North Pacific subtropical high (WNPSH) on HTEs are further investigated based on empirical orthogonal function (EOF) analysis. It is found that mainly the first leading EOF mode with a homogeneous spatial pattern shows dominance before the interdecadal shift, whereas both of the first two leading EOF modes show dominance after the interdecadal shift. A possible mechanism of how HTEs in southeast China are linked to the EAJS, the SAH, and the WNPSH in the ENSO–monsoon coupled system is proposed.

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Xi Cao, Tim Li, Melinda Peng, Wen Chen, and Guanghua Chen

Abstract

The effects of intraseasonal oscillation (ISO) of the western North Pacific (WNP) monsoon trough on tropical cyclone (TC) formation were investigated using the Advanced Research Weather Research and Forecasting (ARW) Model. A weak vortex was specified initially and inserted into the background fields containing climatological-mean anomalies associated with active and inactive phases of monsoon trough ISOs.

The diagnosis of simulations showed that monsoon trough ISO can modulate TC development through both dynamic and thermodynamic processes. The dynamic impact is attributed to the lower–midtropospheric large-scale vorticity associated with monsoon trough ISO. Interactions between cyclonic vorticity in the lower middle troposphere during the active ISO phase and a vortex lead to the generation of vortex-scale outflow at the midlevel, which promotes the upward penetration of friction-induced ascending motion and thus upward moisture transport. In addition, the low-level convergence associated with active ISO also helps the upward moisture transport. Both processes contribute to stronger diabatic heating and thus promote a positive convection–circulation–moisture feedback. On the other hand, the large-scale flow associated with inactive ISO suppresses upward motion near the core by inducing the midlevel inflow and the divergence forcing within the boundary layer, both inhibiting TC development. The thermodynamic impact comes from greater background specific humidity associated with active ISO that allows a stronger diabatic heating. Experiments that separated the dynamic and thermodynamic impacts of the ISO showed that the thermodynamic anomaly from active ISO contributes more to TC development, while the dynamic anomalies from inactive ISO can inhibit vortex development completely.

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Shangfeng Chen, Renguang Wu, Wen Chen, and Kai Li

Abstract

This study reveals a pronounced out-of-phase relationship between surface air temperature (SAT) anomalies over northeast Eurasia in boreal winter and the following summer during 1980–2017. A colder (warmer) winter over northeast Eurasia tends to be followed by a warmer (cooler) summer of next year. The processes for the out-of-phase relation of winter and summer SAT involve the Arctic Oscillation (AO), the air–sea interaction in the North Atlantic Ocean, and a Eurasian anomalous atmospheric circulation pattern induced by the North Atlantic sea surface temperature (SST) anomalies. Winter negative AO/North Atlantic Oscillation (NAO)-like atmospheric circulation anomalies lead to continental cooling over Eurasia via anomalous advection and a tripolar SST anomaly pattern in the North Atlantic. The North Atlantic SST anomaly pattern switches to a dipolar pattern in the following summer via air–sea interaction processes and associated surface heat flux changes. The summer North Atlantic dipolar SST anomaly pattern induces a downstream atmospheric wave train, including large-scale positive geopotential height anomalies over northeast Eurasia, which contributes to positive SAT anomalies there via enhancement of downward surface shortwave radiation and anomalous advection. Barotropic model experiments verify the role of the summer North Atlantic SST anomalies in triggering the atmospheric wave train over Eurasia. Through the above processes, a colder winter is followed by a warmer summer over northeast Eurasia. The above processes apply to the years when warmer winters are followed by cooler summers except for opposite signs of SAT, atmospheric circulation, and SST anomalies.

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Ching-Sen Chen, Wen-Sheen Chen, and Zensing Deng

Abstract

The field program TAMEX (Taiwan Area Mesoscale Experiment) was held during May and June 1987. One of its objectives was to study the cited of terrain on precipitation systems. On 7 June 1987 a band of radar echo, orientated from north to south, developed during the afternoon along the western slope and mountainous area of Taiwan island. Before this system moved eastward toward the Pacific Ocean in the late afternoon, it dumped more than 100 mm of precipitation at a few stations in only a few hours. The analysis of radar data from CAA radar revealed that the precipitation occurred over western-sloped terrain and a mountain plateau in the early afternoon. The system was wider than 60 km in the east-west direction, and the echo top was higher than 10 km. The maximum reflectivity was over 50 dBZ along the steep slope and near the mountain peak. The precipitation system over the mountain area extended eastward with the passage of time; meanwhile, new echoes continually formed along the western-sloped area and moved eastward. They intensified as they moved toward the mountain peak merging with the precipitation system. Through this mechanism the precipitation system could maintain itself for several hours and produce a large amount of rainfall.

A two-dimensional numerical cloud model with a terrain-following coordinate system, similar to the one developed by Durran and Klemp, was used to investigate the topographic effect on the precipitation system. A smoother terrain feature was used for the lower boundary, with a 30-km-wide mountain plateau (of less than 1 km in height) and sloped terrain on the western and eastern sides. Surface heating and boundary-layer moisture supply were parameterized in the model. Simulation results indicated that during the early simulation a cell formed near the foothills of the west slope and moved eastward. As it climbed up the sloping terrain it intensified. Its speed decreased and its high intensity was maintained over the slope and the mountain plateau. At the same time, a new cell formed west of the older cell and moved eastward. Finally this new cell merged into the western side of the older one near the mountain peak to form one precipitation system and moved eastward slowly. Thus, the intensity of the merged system was enhanced over the mountain plateau. While this system maintained its high intensity and moved eastward, new cells continually formed along the western slope and moved eastward to merge into the western side of the precipitation system over the mountainous area. The intensity of the precipitation system was enhanced for a few hours over the mountain itself and became a long-lasting system. Toward the end of the simulation, this long-lasting system had moved near the eastern slope and had still maintained its intensity. At the same time, the low-level temperature decreased over the mountainous area as a result of precipitation evaporation. When new cells, forming over the western slope, moved toward the mountain plateau, they entered their decaying stage 45 min after their occurrence. They did not merge into the existing system on the eastern part of the mountain; therefore, the precipitation over the mountain plateau became weaker.

Several sensitivity tests have been made to study the effect of varying the magnitude of surface heating, the boundary-layer moisture supply, the height of the terrain, and the temperature, moisture, and wind profiles on the simulation result. The result indicated that low-level and midlevel moisture were important for the formation of new cells over the western slope and a long-lasting system over the mountain area, respectively. The initial wind speed of 7 m s−1 below 4 km and calm wind above 4 km was used in the model; then a long-lasting precipitation system over the mountainous area appeared. If the wind speed was reduced to 3.5 m s−1, only new cells formed over the western slope. If the maximum height of the terrain was decreased from 1 to 0.5 km, then only new cells formed over the slope area. Hence, sensitivity tests indicated that the combination of the adequate thermodynamic structure, the westerly wind pattern, and the correct size of the mountain could help form both the new cells over the sloped terrain and a long-lasting system over mountain areas as in northern Taiwan on 7 June 1987 during TAMEX. The surface heating effect played the role of creating the upslope wind and augmentation of this precipitation system.

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Shangfeng Chen, Renguang Wu, Wen Chen, and Shuailei Yao

Abstract

The present study reveals a marked enhancement in the relationship between Eurasian winter and spring atmospheric interannual variability since the early 1990s. Specifically, the dominant mode of winter Eurasian 500-hPa geopotential height anomalies, with same-sign anomalies over southern Europe and East Asia and opposite-sign anomalies over north-central Eurasia, is largely maintained to the following spring after the early 1990s, but not before the early 1990s. The maintenance of the dominant atmospheric circulation anomaly pattern after the early 1990s is associated with a triple sea surface temperature (SST) anomaly pattern in the North Atlantic that is sustained from winter to the subsequent spring. This triple SST anomaly pattern triggers an atmospheric wave train over the North Atlantic through Eurasia during winter through spring. Atmospheric model experiments verify the role of the triple SST anomaly in maintaining the Eurasian atmospheric circulation anomalies. By contrast, before the early 1990s, marked SST anomalies related to the winter dominant mode only occur in the tropical North Atlantic during winter and they disappear during the following spring. The triple SST anomaly pattern after the early 1990s forms in response to a meridional atmospheric dipole over the North Atlantic induced by a La Niña–like cooling over tropical Pacific, and its maintenance into the following spring may be via a positive air–sea interaction process over the North Atlantic. Results of this analysis suggest a potential source for the seasonal prediction of the Eurasian spring climate.

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Shangfeng Chen, Wen Chen, Renguang Wu, and Linye Song

Abstract

Previous studies indicated that spring Arctic Oscillation (AO) can influence the following East Asian summer monsoon (EASM). This study reveals that the Atlantic multidecadal oscillation (AMO) has a pronounced modulation of the spring AO–EASM connection. Spring AO has a close relation with the EASM during the negative AMO (−AMO) phase. However, during the positive AMO (+AMO) phase, the spring AO–EASM connection is weak. During the −AMO phase, a marked dipole atmospheric anomaly pattern (with an anticyclonic anomaly over the midlatitudes and a cyclonic anomaly over the subtropics) and a pronounced tripole sea surface temperature (SST) anomaly pattern is formed in the North Pacific during positive spring AO years. The cyclonic anomaly, SST, and precipitation anomalies over the subtropical western North Pacific (WNP) maintain and propagate southwestward in the following summer via a positive air–sea feedback, which further impacts the EASM variation. During the +AMO phase, the Pacific center of the spring AO (i.e., the anticyclonic anomaly over the midlatitudes) is weak. As such, the cyclonic anomaly cannot be induced over the subtropical WNP by the spring AO via wave–mean flow interaction. Hence, the spring AO–EASM connection disappears during the +AMO phase. The AMO impacts the Pacific center of the spring AO via modulating the Aleutian low intensity and North Pacific storm track intensity. The observed AMO modulation of the spring AO–EASM connection and Pacific center of the spring AO can be captured by the long historical simulation in a coupled global climate model.

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Jinling Piao, Wen Chen, Shangfeng Chen, Hainan Gong, and Qiong Zhang

Abstract

Previous studies found a seesaw pattern of summer precipitation between northeast Asia and east Siberia on an interannual time scale, which is associated with an eastward-propagating atmospheric wave train over Eurasia and corresponding water vapor transport circulations. Using a general circulation model with an embedded water-tagging module, the main water vapor sources of the two regions, as well as the relative contributions of each source region to the total precipitation for both the climatological mean and interdecadal variation, are further compared in this study. The model simulation results show that local evaporation, the Pacific Ocean, and East Asia are the dominant moisture sources for northeast Asian precipitation. In contrast, for east Siberia, moisture mainly originates from the Pacific Ocean, northeast Asia, west Siberia, and local evaporation. This suggests that the local evaporation and Pacific Ocean are both crucial to the moisture supply of the two regions, implying the important roles of the land processes and adjacent oceanic sources. In addition, northeast Asia appears to be the major moisture source for east Siberia, whereas east Siberia has weak impacts on the moisture input for northeast Asia. Further analysis finds that the model simulation can capture interdecadal changes in summer precipitation over the two regions around the late 1990s. This interdecadal change is mainly manifested in the moisture supplies from the Pacific Ocean, North Atlantic Ocean, and east Siberia, which suggests a link with the circulation anomalies under the combined impacts of the Pacific decadal oscillation and the Atlantic multidecadal oscillation.

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