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Guixing Chen, Weiming Sha, Toshiki Iwasaki, and Zhiping Wen

Abstract

Moist convection occurred repeatedly in the midnight-to-morning hours of 11–16 June 1998 and yielded excessive rainfall in a narrow latitudinal corridor over East Asia, causing severe flood. Numerical experiments and composite analyses of a 5-day period are performed to examine the mechanisms governing nocturnal convection. Both simulations and observations show that a train of MCSs concurrently developed along a quasi-stationary mei-yu front and coincided with the impact of a monsoon surge on a frontogenetic zone at night. This process was regulated primarily by a nocturnal low-level jet (NLLJ) in the southwesterly monsoon that formed over southern China and extended to central China. In particular, the NLLJ acted as a mechanism of moisture transport over the plains. At its northern terminus, the NLLJ led to a zonal band of elevated conditionally unstable air where strong low-level ascent overcame small convective inhibition, triggering new convection in three preferred plains. An analysis of convective instability shows that the low-tropospheric intrusion of moist monsoon air generated CAPE of ~1000 J kg−1 prior to convection initiation, whereas free-atmospheric forcing was much weaker. The NLLJ-related horizontal advection accounted for most of the instability precondition at 100–175 J kg−1 h−1. At the convective stage, instability generation by the upward transport of moisture increased to ~100 J kg−1 h−1, suggesting that ascending inflow caused feedback in convection growth. The convection dissipated in late morning with decaying NLLJ and moisture at elevated layers. It is concluded that the diurnally varying summer monsoon acted as an effective discharge of available moist energy from southern to central China, generating the morning-peak heavy rainfall corridor.

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Hainan Gong, Lin Wang, Wen Chen, and Debashis Nath

Abstract

The multidecadal fluctuations in the patterns and teleconnections of the winter mean Arctic Oscillation (AO) are investigated based on observational and reanalysis datasets. Results show that the Atlantic center of the AO pattern remains unchanged throughout the period 1920–2010, whereas the Pacific center of the AO is strong during 1920–59 and 1986–2010 and weak during 1960–85. Consequently, the link between the AO and the surface air temperature over western North America is strong during 1920–59 and 1986–2010 and weak during 1960–85. The time-varying Pacific center of the AO motivates a revisit to the nature of the AO from the perspective of decadal change. It reveals that the North Pacific mode (NPM) and North Atlantic Oscillation (NAO) are the inherent regional atmospheric modes over the North Pacific and North Atlantic, respectively. Their patterns over the North Pacific and North Atlantic remain stable and change little with time during 1920–2010. The Atlantic center of the AO always resembles the NAO over the North Atlantic, but the Pacific center of the AO only resembles the NPM over the North Pacific when the NPM–NAO coupling is strong. These results suggest that the AO seems to be fundamentally rooted in the variability over the North Atlantic and that the annular structure of the AO very likely arises from the coupling of the atmospheric modes between the North Pacific and North Atlantic.

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Wenxin Zeng, Guixing Chen, Yu Du, and Zhiping Wen

Abstract

A succession of MCSs developed during the last week of October 2016 and produced extreme heavy rainfall in central China. The event underwent an evident shift from a mei-yu-like warm scenario to an autumn cold scenario. Diurnal cycles of rainfall and low-level winds may be modulated by the shifting of large-scale atmospheric conditions. We conducted observational analyses and numerical experiments to examine how large-scale circulations influenced rainfall systems through diurnally varying processes. The results show that, in the first half (warm) period of the event, intense rainfall mostly occurred in eastern-central China with an early morning peak. It was closely related to a nocturnal southwesterly low-level jet (NLLJ) on the flank of the western Pacific subtropical high. The NLLJ formed near midnight in southern China where ageostrophic wind rotated clockwise due to Blackadar’s inertial oscillation. The NLLJ extended downstream to central China during the predawn hours due to the horizontal advection of momentum. Both the formation and extension of the NLLJ were supported by an enhanced subtropical high that provided relatively warm conditions with surface heating for boundary layer inertial oscillation and strong background southwesterly winds for momentum transport. The NLLJ induced MCSs at its northern terminus where the low-level ascent, moisture flux convergence, and convective instability were enhanced during the predawn hours. In the second half period with an intrusion of cold air, the diurnal amplitude of low-level winds became small under relatively cold and cloudy conditions. Moderate rainfall tended to occur in western-central China with a peak after midnight, most likely due to frontogenetic processes, upslope lifting, and nighttime cloud-top cooling.

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Hainan Gong, Lin Wang, Wen Chen, Renguang Wu, Wen Zhou, Lin Liu, Debashis Nath, and Xiaoqing Lan

Abstract

The wintertime Arctic Oscillation (AO) pattern in phase 5 of the Coupled Model Intercomparison Project (CMIP5) climate models displays notable differences from the reanalysis. The North Pacific center of the AO pattern is larger in the ensemble mean of 27 models than in the reanalysis, and the magnitude of the North Pacific center of the AO pattern varies largely among the models. This study investigates the plausible sources of the diversity of the AO pattern in the models. Analysis indicates that the amplitude of the North Pacific center is associated with the coupling between the North Pacific and North Atlantic, which in turn is primarily modulated by the strength of the stratospheric polar vortex. A comparative analysis is conducted for the strong polar vortex (SPV) and weak polar vortex (WPV) models. It reveals that a stronger stratospheric polar vortex induces more planetary waves to reflect from the North Pacific to the North Atlantic and more wave activity fluxes to propagate from the North Pacific to the North Atlantic in the SPV models than in the WPV models. Thus, the coupling of atmospheric circulation between the North Pacific and North Atlantic is stronger in the SPV models, which facilitates more North Pacific variability to be involved in the AO variability and induces a stronger North Pacific center in the AO pattern. The increase in vertical resolution may improve the simulation of the stratospheric polar vortex and thereby reduces the model biases in the North Pacific–North Atlantic coupling and thereby the amplitude of the North Pacific center of the AO pattern in models.

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Wen Zhou, Johnny C. L. Chan, Wen Chen, Jian Ling, Joaquim G. Pinto, and Yaping Shao

Abstract

In January 2008, central and southern China experienced persistent low temperatures, freezing rain, and snow. The large-scale conditions associated with the occurrence and development of these snowstorms are examined in order to identify the key synoptic controls leading to this event. Three main factors are identified: 1) the persistent blocking high over Siberia, which remained quasi-stationary around 65°E for 3 weeks, led to advection of dry and cold Siberian air down to central and southern China; 2) a strong persistent southwesterly flow associated with the western Pacific subtropical high led to enhanced moisture advection from the Bay of Bengal into central and southern China; and 3) the deep inversion layer in the lower troposphere associated with the extended snow cover over most of central and southern China. The combination of these three factors is likely responsible for the unusual severity of the event, and hence a long return period.

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Hui-Wen Lai, Hans W. Chen, Julia Kukulies, Tinghai Ou, and Deliang Chen

Abstract

Precipitation over the Tibetan Plateau (TP) has major societal impacts in South and East Asia, but its spatiotemporal variations are not well understood, mainly because of the sparsely distributed in situ observation sites. With the help of the Global Precipitation Measurement satellite product IMERG and the ERA5 dataset, distinct precipitation seasonality features over the TP were objectively classified using a self-organizing map algorithm fed with 10-day averaged precipitation from 2000 to 2019. The classification reveals three main precipitation regimes with distinct seasonality of precipitation: the winter peak, centered at the western plateau; the early summer peak, found on the eastern plateau; and the late summer peak, mainly located on the southwestern plateau. On a year-to-year basis, the winter peak regime is relatively robust, whereas the early summer and late summer peak regimes tend to shift mainly between the central and northern TP but are robust in the eastern and southwestern TP. A composite analysis shows that the winter peak regime experiences larger amounts of precipitation in winter and early spring when the westerly jet is anomalously strong to the north of the TP. Precipitation variations in the late summer peak regime are associated with intensity changes in the South Asian high and Indian summer monsoon. The precipitation in the early summer peak regime is correlated with the Indian summer monsoon together with anticyclonic circulation over the western North Pacific. The results provide a basic understanding of precipitation seasonality variations over the TP and associated large-scale conditions.

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Peiqiang Xu, Lin Wang, Wen Chen, Guosen Chen, and In-Sik Kang

Abstract

The British–Baikal Corridor (BBC) pattern is the dominant waveguide mode trapped along the summertime polar front jet over northern Eurasia. It consists of four geographically fixed centers over the west of the British Isles, the Baltic Sea, western Siberia, and Lake Baikal, respectively. Its intraseasonal variations and dynamics are investigated based on reanalysis datasets. The BBC pattern has a life cycle of about two weeks. Its precursor could be traced back to an upstream wave packet propagating along the Atlantic jet 10 days before its peak, and its life cycle resembles the evolution of a quasi-stationary Rossby wave train. Diagnosis of the streamfunction tendency equation suggests that the growth and decay of the BBC pattern are primarily driven by the nonlinear processes, whereas the quasi-stationary feature of the BBC pattern arises from the cancellation among the linear processes. Energetics analysis indicates that the energy cycle with the transient eddies (TEs) plays an essential role in the growth and decay of the BBC pattern. The BBC pattern first feeds on the barotropic energy provided by the TEs and then returns the energy to TEs in the form of baroclinic energy. It is this nonlinear interaction with the TEs that poses a tough challenge to the current state-of-the-art models to capture the BBC pattern reasonably.

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Shangfeng Chen, Wen Chen, Renguang Wu, Bin Yu, and Hans-F. Graf

Abstract

The present study reveals a close relation between the interannual variation of Aleutian low intensity (ALI) in March and the subsequent winter El Niño–Southern Oscillation (ENSO). When March ALI is weaker (stronger) than normal, an El Niño (a La Niña)–like sea surface temperature (SST) warming (cooling) tends to appear in the equatorial central-eastern Pacific during the subsequent winter. The physical process linking March ALI to the following winter ENSO is as follows. When March ALI is below normal, a notable atmospheric dipole pattern develops over the North Pacific, with an anticyclonic anomaly over the Aleutian region and a cyclonic anomaly over the subtropical west-central Pacific. The formation of the anomalous cyclone is attributed to feedback of the synoptic-scale eddy-to-mean-flow energy flux and associated vorticity transportation. Specifically, easterly wind anomalies over the midlatitudes related to the weakened ALI are accompanied by a decrease in synoptic-scale eddy activity, which forces an anomalous cyclone to its southern flank. The accompanying westerly wind anomalies over the tropical west-central Pacific induce SST warming in the equatorial central-eastern Pacific during the following summer–autumn via triggering eastward-propagating warm Kelvin waves, which may sustain and develop into an El Niño event during the following winter via positive air–sea feedback. The relation of March ALI with the following winter ENSO is independent of the preceding tropical Pacific SST, the preceding-winter North Pacific Oscillation, and the spring Arctic Oscillation. The results of this analysis may provide an additional source for the prediction of ENSO.

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Jiepeng Chen, Xin Wang, Wen Zhou, Chunzai Wang, Qiang Xie, Gang Li, and Sheng Chen

Abstract

Previous research has suggested that the anomalous western North Pacific anticyclone (WNPAC) can generally persist from an El Niño mature winter to the subsequent summer, influencing southern China precipitation significantly, where southern China includes the Yangtze River valley and South China. Since the late 1970s, three extreme El Niño events have been recorded: 1982/83, 1997/98, and 2015/16. There was a sharp contrast in the change in southern China rainfall and corresponding atmospheric circulations in the decaying August between the 2015/16 extreme El Niño event and the earlier two extreme El Niño events. Enhanced rainfall in the middle and upper reaches of the Yangtze River and suppressed rainfall over South China resulted from basinwide warming in the tropical Indian Ocean induced by the extreme El Niño in August 1983 and 1998, which was consistent with previous studies. However, an anomalous western North Pacific cyclone emerged in August 2016 and then caused positive rainfall anomalies over South China and negative rainfall anomalies from the Yangtze River to the middle and lower reaches of the Yellow River. Without considering the effect of the long-term global warming trend, in August 2016 the negative SST anomalies over the western Indian Ocean and cooling in the north tropical Atlantic contributed to the anomalous western North Pacific cyclone and a rainfall anomaly pattern with opposite anomalies in South China and the Yangtze River region. Numerical experiments with the CAM5 model are conducted to confirm that cooler SST in the western Indian Ocean contributed more than cooler SST in the north tropical Atlantic to the anomalous western North Pacific cyclone and anomalous South China rainfall.

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Hainan Gong, Lin Wang, Wen Zhou, Wen Chen, Renguang Wu, Lin Liu, Debashis Nath, and Marco Y.-T. Leung

Abstract

This study revisits the northern mode of East Asian winter monsoon (EAWM) variation and investigates its response to global warming based on the ERA dataset and outputs from phase 5 of the Coupled Model Intercomparison Project (CMIP5) models. Results show that the observed variation in East Asian surface air temperature (EAT) is tightly coupled with sea level pressure variation in the expanded Siberian high (SH) region during boreal winter. The first singular value decomposition (SVD) mode of the EAT and SH explains 95% of the squared covariance in observations from 1961 to 2005, which actually represents the northern mode of EAWM variation. Meanwhile, the first SVD mode of the EAT and SH is verified to be equivalent to the first empirical orthogonal function mode (EOF1) of the EAT and SH, respectively. Since the leading mode of the temperature variation is significantly influenced by radiative forcing in a rapidly warming climate, for reliable projection of long-term changes in the northern mode of the EAWM, we further employ the EOF1 mode of the SH to represent the northern mode of EAWM variation. The models can well reproduce this coupling between the EAT and SH in historical simulations. Meanwhile, a robust weakening of the northern mode of the EAWM is found in the RCP4.5 scenario, and with stronger warming in the RCP8.5 scenario, the weakening of the EAWM is more pronounced. It is found that the weakening of the northern mode of the EAWM can contribute 6.7% and 9.4% of the warming trend in northern East Asian temperature under the RCP4.5 and RCP8.5 scenarios, respectively.

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