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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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Shu-hua Chen and Wen-yih Sun

Abstract

A fully compressible, three-dimensional, nonhydrostatic model is developed using a semi-implicit scheme to avoid an extremely small time step. As a result of applying the implicit scheme to high-frequency waves, an elliptic partial differential equation (EPDE) has been introduced. A multigrid solver is applied to solve the EPDEs, which include cross-derivative terms due to terrain-following coordinate transformation.

Several experiments have been performed to evaluate the model as well as the performance of the scheme with respect to tolerance number, relaxation choice, sweeps of prerelaxation and postrelaxation, and a flexible hybrid coordinate (FHC).

An FHC with two functions (base and deviation functions) is introduced. The basic function provides constant vertical grid spacing required in the multigrid solver, while the deviation function helps to adjust the vertical resolution.

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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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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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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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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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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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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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