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Ruidan Chen, Zhiping Wen, and Riyu Lu

Abstract

Southern China, located in the tropical–subtropical East Asian monsoonal region, presents a unique anticyclonic–cyclonic circulation pattern during extreme heat (EH), obviously different from the typical anticyclone responsible for EH in many other regions. Associated with the evolution of EH in southern China, the anticyclonic–cyclonic anomalies propagate northwestward over the Philippines and southern China. Before the EH onsets, the anticyclonic anomaly dominates southern China, resulting in stronger subsidence over southern China and stronger southerly (southwesterly) flow over the western (northern) margins of southern China. The southerly (southwesterly) flow transports more water vapor to the north of southern China, thus, together with the local stronger subsidence, resulting in drier air condition and accordingly favoring the occurrence of EH. Conversely, after the EH onsets, the cyclonic component approaches southern China and offsets the high temperature.

The oscillations of temperature and circulation anomalies over southern China exhibit a periodicity of about 10 days and indicate the influence of a quasi-biweekly oscillation, which originates from the tropical western Pacific and propagates northwestward. Therefore, the 5–25-day-filtered data are extracted to further analyze the quasi-biweekly oscillation. It turns out that the evolution of the filtered circulation remarkably resembles the original anomalies with comparable amplitudes, indicating that the quasi-biweekly oscillation is critical for the occurrence of EH in southern China. The quasi-biweekly oscillation could explain more than 50% of the intraseasonal variance of daily maximum temperature T max and vorticity over southern China and 80% of the warming amplitude of EH onsets. The close relationship between the circulation of the quasi-biweekly oscillation and the EH occurrence indicates the possibility of medium-range forecasting for high temperature in southern China.

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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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Xiuzhen Li, Wen Zhou, and Yongqin David Chen

Abstract

A combination of Ward’s and k-means clustering was applied to a 3-month standardized precipitation index (SPI-03), and eight divisions of homogeneous drought variation throughout China were identified from the perspective of meteorological and agricultural droughts. A greater meridional gradient appeared over eastern China (six divisions) than over western China (two divisions).

The climate division facilitated the evaluating of not only regional but also widespread droughts. Trend evaluation showed that western north China (WNC) has become increasingly wet in recent decades, while northern northeast China (NNE) has become increasingly dry. The Yangtze River valley (YZ) tended to experience less and weaker drought after the late 1970s. Southern northeast China (SNE) and the southwestern China–Tibetan Plateau (SW-TP) showed a decreasing trend in long-term but not short-term SPIs, implying that long-term drought conditions might develop continuously, thus allowing the following droughts to develop more rapidly and with a stronger intensity. Examination of the drought risk under El Niño revealed that northern regions were likely to suffer from drought rather than flood in the developing phase and the reverse in the decaying phase. Southeastern China (SE) and the YZ were vulnerable to flood rather than drought in the mature and decaying spring, with SE subjected to drought in the decaying summer. Such a distinctive regional pattern of drought risks was closely connected with the abnormal moisture supply patterns modulated by ENSO in different phases.

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Peiqiang Xu, Lin Wang, and Wen Chen

Abstract

The British–Baikal Corridor (BBC) pattern, a new teleconnection along the summertime upper-tropospheric polar front jet (PFJ), is investigated based on observational and reanalysis datasets. The BBC pattern consists of four geographically fixed centers over the west of the British Isles, the Baltic Sea, western Siberia, and Lake Baikal, respectively. It features a zonally oriented and meridionally confined wavelike structure with a zonal wavenumber 5, and it influences the climate along its route significantly. The BBC pattern forms from the trapped effect of the PFJ waveguide that is characterized by a strong meridional gradient of stratification. As a preferred dynamical mode inherent in the PFJ, it is maintained through the baroclinic energy conversion from the basic flow and the feedback forcing of high-frequency transient eddies. Meanwhile, its geographical location is determined by the barotropic energy conversion, which is sensitive to the configuration of the basic flow. The interannual variability of the BBC pattern is dominated by atmospheric internal dynamics considering its loose relation with immediate atmospheric external forcing. Further analyses suggest that the BBC pattern is excited by the active multiscale interactions among the climatological mean flow, the low-frequency flow, and the synoptic-scale transient eddies in the exit region of the North Atlantic jet, which may also determine the preferential upstream forcing region and anchor the BBC pattern geographically. Budget analyses on vorticity, temperature, and water vapor are performed to interpret the physical nature of the BBC pattern. The possible linkage to the North Atlantic Oscillation is also discussed.

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Juan Feng, Lin Wang, and Wen Chen

Abstract

Modulation of the Pacific decadal oscillation (PDO) on the behavior of the East Asian summer monsoon (EASM) in El Niño decaying years has been studied. When El Niño is in phase with the PDO (El Niño/high PDO), the low-level atmospheric anomalies are characterized by an anticyclone around the Philippines and a cyclone around Japan, inducing an anomalous tripolar rainfall pattern in China. In this case, the western Pacific subtropical high (WPSH) experiences a one-time slightly northward shift in July and then stays stationary from July to August. The corresponding anomalous tripolar rainfall pattern has weak subseasonal variations. When El Niño is out of phase with the PDO (El Niño/low PDO), however, the anomalous Philippines anticyclone has a much larger spatial domain, thereby causing an anomalous dipole rainfall pattern. Accordingly, WPSH experiences clearly two northward shifts. Therefore, the related dipole rainfall pattern has large subseasonal variations. One pronounced feature is that the positive rainfall anomalies shift northward from southern China in June to central China in July and finally to northern China in August.

The different El Niño–EASM relationships are caused by the influences of PDO on the decaying speed of El Niño. During the high PDO phase, El Niño decays slowly and has a strong anchor in the north Indian Ocean warming, which is responsible for the anomalous EASM. Comparatively, during the low PDO phase, El Niño decays rapidly and La Niña develops in summer, which induces different EASM anomalies from that during the high PDO phase. Additionally, PDO changes El Niño behaviors mainly via modifying the background tropical winds.

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Xiuzhen Li, Yongqin David Chen, and Wen Zhou

Abstract

The response of moisture circulation to the daily evolution of the India–Burma Trough (IBT) and the modulation of disturbances along the South Asian waveguide are analyzed to seek a potential precursor of winter precipitation over south China. Daily observational precipitation and reanalysis data from ERA-Interim during 1979–2012 are employed. It is found that moisture circulation in response to the IBT is part of the zonally oriented wave trains along the South Asian waveguide, but it persists longer and migrates farther eastward than other lobes. Cyclonic moisture transport enhances the moisture supply to south China as a strong IBT develops, and shifts eastward abruptly after the peak of IBT with enhanced precipitation shifting from southwest to southeast China. This response is a joint effect of synoptic, intraseasonal, and interannual components that show similar wave train structures, whereas slight differences still occur. The synoptic component shows a shorter wavelength, more southerly path, faster phase speed, and group velocity, with the signal from the North Atlantic to the Bay of Bengal (BoB) in 6 days, implying that a disturbance over the North Atlantic is a potential precursor of winter precipitation over south China. The synoptic moisture convergence is more intensive than that at other scales upstream except over Southeast Asia, where all components are comparable. This might result from the constrained moisture source from BoB at the synoptic scale because of a short wavelength, while widespread sources from BoB–western North Pacific (WNP) at other scales as wavelengths are longer.

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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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Xiuzhen Li, Zhiping Wen, Deliang Chen, and Zesheng Chen

Abstract

The El Niño–Southern Oscillation (ENSO) cycle has a great impact on the summer moisture circulation over East Asia (EA) and the western North Pacific [WNP (EA-WNP)] on an interannual time scale, and its modulation is mainly embedded in the leading mode. In contrast to the stable influence of the mature phase of ENSO, the impact of synchronous eastern Pacific sea surface temperature anomalies (SSTAs) on summer moisture circulation is negligible during the 1970s–80s, while it intensifies after 1991. In response, the interannual variation of moisture circulation exhibits a much more widespread anticyclonic/cyclonic pattern over the subtropical WNP and a weaker counterpart to the north after 1991. Abnormal moisture moves farther northward with the enhanced moisture convergence, and thus precipitation shifts from the Yangtze River to the Huai River valley. The decadal shift in the modulation of ENSO on moisture circulation arises from a more rapid evolution of the bonding ENSO cycle and its stronger coupling with circulation over the Indian Ocean after 1991. The rapid development of cooling SSTAs over the central-eastern Pacific, and warming SSTAs to the west over the eastern Indian Ocean–Maritime Continent (EIO-MC) in summer, stimulates abnormal descending motion over the western-central Pacific and ascending motion over the EIO-MC. The former excites an anticyclone over the WNP as a Rossby wave response, sustaining and intensifying the WNP anticyclone; the latter helps anchor the anticyclone over the tropical–subtropical WNP via an abnormal southwest–northeast vertical circulation between EIO-MC and WNP.

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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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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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