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Jie Song, Chongyin Li, Jing Pan, and Wen Zhou

Abstract

The characteristics of the climatological distribution of the anticyclonic (LC1) and cyclonic (LC2) Rossby wave breaking (RWB) in the Southern Hemisphere (SH) are investigated by calculating the occurrence frequency of the LC1- and LC2-like stratospheric potential vorticity (PV) streamers in the SH during the austral summer [December–February (DJF)] and wintertime [June–August (JJA)] on several isentropic surfaces by using the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) daily dataset. The results show that 1) on the equatorward flank of the climatological midlatitude jet (MLJ), the LC1-like PV streamers are frequently found over the central oceanic regions, whereas the LC2-like PV streamers are almost absent. On the poleward flank of the climatological MLJ, both types of PV streamers are frequently observed and the LC2-like PV streamers predominate; 2) the regions where the occurrences of the PV streamers are frequent overlap the weak zonal wind regions; and 3) in austral winter, a “double-jet” setting is evident in two regions of the SH [the double-jet upstream (DU) and the spilt jet region]. In the double-jet setting regions, the LC1-like PV streamers are frequently found both in the DU and the split-jet regions, while the occurrence of the LC2-like PV streamers is frequent in the split-jet region but is rather infrequent in the DU region.

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Yong Liu, Wen Zhou, Xia Qu, and Renguang Wu

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The Silk Road pattern (SRP) is a well-known teleconnection pattern along the upper-level westerly jet over the Eurasian continent during boreal summer. The SRP has experienced an interdecadal change around the late 1970s. The present study identified a new change of the SRP around the late 1990s, which is characterized by significant weakening and zonal phase shift of the major centers of the SRP during the recent decades. The recent reshaping of the SRP is attributed to an enhanced impact of precipitation anomalies over the northeastern Indian summer monsoon (ISM), which is associated with the leading mode change of the ISM precipitation anomalies around the late 1990s. The interdecadal weakening of the upper-level westerly jet over central and East Asia also favors the southward movement of the SRP during recent periods. The differences of the features, climate impact, and causes related to the recent SRP change from those related to the SRP change around the late 1970s are also contrasted in this study.

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Cheng Qian, Wen Zhou, Soi Kun Fong, and Ka Cheng Leong

Abstract

The Gaussian assumption has been widely used without testing in many previous studies on climate variability and change that have used traditional statistical methods to estimate linear trends, diagnose physical mechanisms, or construct statistical prediction/downscaling models. In this study, the authors carefully test the normality of two hot extreme indices in Macao, China, during the last 100 years based on consecutive daily temperature observational data and find that the occurrences of both hot day and hot night indices are non-Gaussian. Simple least squares fitting is shown to overestimate the linear trend when the Gaussian assumption is violated. Two approaches are further proposed to statistically predict non-Gaussian temperature extremes: one uses a multiple linear regression model after transforming the non-Gaussian predictant to a quasi-Gaussian variable and uses Pearson’s correlation test to identify potential predictors, and the other uses a generalized linear model when the transformation is difficult and uses a nonparametric Spearman’s correlation test to identify potential predictors. The annual occurrences of hot days and hot nights in Macao are used as examples of these two approaches, respectively. The physical mechanisms for these two hot extremes in Macao are also investigated, and the results show that both are related to the interannual and interdecadal variability of a coupled El Niño–Southern Oscillation (ENSO)–East Asian summer monsoon system. Finally, the authors caution other researchers to test the assumed distribution of climate extremes and to apply appropriate statistical approaches.

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Richard C. Y. Li, Wen Zhou, and Tim Li

Abstract

This study investigates the influences of the Pacific–Japan (PJ) teleconnection pattern on synoptic-scale variability (SSV) in the western North Pacific (WNP). The PJ pattern exhibits salient intraseasonal variations, with a dominant peak at 10–50 days. During positive PJ phases, strengthened SSV is found in the WNP, with a much stronger and better organized synoptic wave train structure. Such a synoptic-scale wave train, however, is greatly weakened during negative PJ phases. Examination of the vertical profiles of the observational data suggests that environmental parameters are generally more (less) favorable for the growth of synoptic disturbances under positive (negative) PJ conditions.

Observational results are further verified with an anomaly atmospheric general circulation model, which reveals faster (slower) growth of the synoptic-scale wave train when the environmental anomalies associated with positive (negative) PJ phases are incorporated into the summer mean state of the model. In addition, sensitivity experiments indicate that thermodynamic parameters of the planetary boundary layer (PBL) play a determining role in controlling the development of synoptic disturbances in the WNP. The increase (decrease) in background PBL moisture during positive (negative) PJ phases enhances (suppresses) perturbation moisture convergence and thus the convective heating associated with SSV, leading to strengthened (weakened) synoptic-scale activity in the WNP. Serving as potential seed disturbances for cyclogenesis, the strengthened (weakened) synoptic-scale activity may also contribute to the enhancement (suppression) in intraseasonal TC frequency during positive (negative) PJ phases.

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Xiuzhen Li, Wen Zhou, Deliang Chen, Chongyin Li, and Jie Song

Abstract

The water vapor transport and moisture budget over eastern China remotely forced by the cold-tongue (CT) and warm-pool (WP) El Niño show striking differences throughout their lifetime. The water vapor transport response is weak in the developing summer but strong in the remaining phases of CT El Niño, whereas the opposite occurs during WP El Niño. WP El Niño causes a moisture deficit over the Yangtze River valley (YZ) in the developing summer and over southeastern China (SE) in the developing fall, whereas CT El Niño induces a moisture surplus first over SE during the developing fall with the influential area expanding in the decaying spring and shifting northward in the decaying summer. It is the divergence of meridional water vapor transport that dominates the total water vapor divergence anomaly, with the divergence of zonal transport showing an opposite pattern with smaller magnitude.

Investigation of the vertical profile of moisture budget shows a great baroclinicity, with the strongest abnormal moisture budget occurring in different levels. The moisture transport via the southern boundary plays a crucial role in the regional moisture budget anomalies and is located near the surface over SE, in the lower troposphere over the YZ, and at the lower-middle troposphere over the eastern part of northern China. The enhanced moisture surplus near the surface forced by WP El Niño over SE in the mature winter and decaying spring is offset by a moisture deficit within the lower-middle troposphere due to a diverse response circulation at different vertical levels.

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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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Han Zhou, Wen Zhou, Yuanbo Liu, Yanbin Yuan, Jiejun Huang, and Yongwei Liu

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The spatiotemporal variability of droughts over a region can often be characterized by combining orthogonal climate modes with corresponding temporal coefficients. The temporal characteristics of climate modes have been extensively addressed, while their spatial development remains largely unexplored. Hence, this study investigated the spatiotemporal evolution of meteorological droughts from the perspective of climate modes. Nearly 50 years of monthly precipitation records (1960–2007) from 73 meteorological stations in the Poyang Lake basin, China, were used. The standardized precipitation index at a 1-month scale was applied to quantify meteorological droughts. The dominant modes of the droughts over the basin were identified using principal component analysis, K-means cluster analysis, and analysis of variance. Based on the trajectory migration identification method, the role of the climate modes in spatiotemporal evolution was analyzed. The results showed that four spatial modes of the droughts in the basin were identified. The spatial extents, centroids, and severity of the drought clusters based on the identified modes were significantly consistent with those based on the meteorological stations (R 2 > 0.8, p < 0.05), which indicated that these modes could mostly match the large areas where serious dry/wet conditions occurred. Moreover, their performances in characterizing the spatiotemporal evolutional features (severity, migration distance, and pattern) of drought events were valid, which indicated that they might be considered as the elementary constituents of the historical meteorological drought events across the basin. The findings might offer some implications to understand drought development and causes through possible connections between the dominant modes and climate variability.

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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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Yuntao Jian, Xiaoxia Lin, Wen Zhou, Maoqiu Jian, Marco Y. T. Leung, and Paxson K. Y. Cheung

Abstract

In winter 2018/19, southeastern coastal China experienced extreme warm temperatures that were due to a weak East Asian winter monsoon. On the basis of observations from 10 meteorological stations and reanalysis data, the large-scale circulation patterns associated with this extreme warm winter and the possible driving mechanism of its related sea surface temperature (SST) anomalies are investigated in this study. During this winter, many places in this region reached their highest winter mean temperature record and had more extreme warm days and fewer extreme cold days relative to climatology. According to the circulation patterns during winter 2018/19, several large-scale circulation conditions associated mainly with the weak East Asian winter monsoon are identified: the eastward shift of the Siberian high and a shallower East Asian trough, which is related to the low blocking frequency over the Aleutian region, are both unfavorable for cold-air intrusion southward. Meanwhile, strong low-level southerly wind anomalies over southeastern China are related mainly to the 2018/19 El Niño event. Furthermore, the possible role of SST anomalies over the North Atlantic and tropical western Pacific Oceans is examined by using an atmospheric general circulation model, suggesting that both the “tripole pattern” of North Atlantic SST and tropical western Pacific SST anomalies in winter 2018/19 played a role in influencing the East Asian trough. The combined effect of all of these factors seems to be responsible for this extreme warm winter over southeastern coastal China.

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