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Ho Nam Cheung, Wen Zhou, Hing Yim Mok, and Man Chi Wu

Abstract

This study attempts to assess the possible linkage between Ural–Siberian blocking and the East Asian winter monsoon (EAWM). During the boreal winter, the dominance of blocking thermally enhances cold advection downstream. The frequent occurrence of Ural–Siberian blocking potentially promotes a cold EAWM and vice versa. The seasonal blocking activity can be regarded as the combined effect of the Arctic Oscillation (AO) and the El Niño–Southern Oscillation (ENSO). Weakened (strengthened) meridional flow in the positive (negative) phase of the AO is unfavorable (favorable) for the formation of blocking highs. Because the AO shows a close relationship with the North Atlantic Oscillation (NAO), its teleconnection with Ural–Siberian blocking may exist in the form of an eastward-propagating wave train. Be that as it may, the wave train signal across East Asia may be disturbed by the external effect of a strong ENSO event, which probably enhances (weakens) the westerlies near Siberia in its warm (cold) phase. Consequently, the blocking–EAWM relationship is stronger (weaker) when the AO and ENSO are in phase (out of phase). If both AO and ENSO attain the positive (negative) phase, the Siberian high tends to be weaker (stronger) and the temperature tends to be higher (lower) in East Asia, with less (more) Ural–Siberian blocking. On the other hand, if they are out of phase, they are not strongly linked to the intensity of the Siberian high, and the blocking activity over Ural–Siberia is unclear.

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Hoffman H. N. Cheung, Wen Zhou, Sai-ming Lee, and Hang-wai Tong

Abstract

During the past decade (2004/05–2013/14), the number of cold days in Hong Kong (N CD), as a proxy of the temperature of southern China, appeared to have increased from the historical minimum, in contrast to a remarkable decline in the entire postwar period. This is related to the recent apparent changes in the large-scale circulation upstream and downstream of the East Asian winter monsoon (EAWM) region: the increase in Ural blocking (UB) that enhances cold advection from the polar region and reinforces the Siberian high and the decrease in a western Pacific (WP)-like index that corresponds to increasing meridional gradient of geopotential height over the EAWM region. Overall, UB and WP account for 26.4% of the interannual (≤8 yr) variance and 83.7% of the decadal (>8 yr) variance of N CD for the period 1948/49–2013/14, indicating that further study could lead to improvement in the prediction of N CD.

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Kangmin Wen, Guoyu Ren, Jiao Li, Aiying Zhang, Yuyu Ren, Xiubao Sun, and Yaqing Zhou

Abstract

A dataset from 763 national Reference Climate and Basic Meteorological Stations (RCBMS) was used to analyze surface air temperature (SAT) change in mainland China. The monthly historical observational records had been adjusted for urbanization bias existing in the data series of size-varied urban stations, after they were corrected for data inhomogeneities mainly caused by relocation and instrumentation. The standard procedures for creating area-averaged temperature time series and for calculating linear trend were used. Analyses were made for annual and seasonal mean temperature. Annual mean SAT in mainland China as a whole rose by 1.24°C for the last 55 years, with a warming rate of 0.23°C decade−1. This was close to the warming of 1.09°C observed in global mean land SAT over the period 1951–2010. Compared to the SAT before correction, after-corrected data showed that the urbanization bias had caused an overestimate of the annual warming rate of more than 19.6% during 1961–2015. The winter, autumn, spring, and summer mean warming rates were 0.28°, 0.23°, 0.23°, and 0.15°C decade−1, respectively. The spatial patterns of the annual and seasonal mean SAT trends also exhibited an obvious difference from those of the previous analyses. The largest contrast was a weak warming area appearing in central parts of mainland China, which included a small part of southwestern North China, the northwestern Yangtze River, and the eastern part of Southwest China. The annual mean warming trends in Northeast and North China obviously decreased compared to the previous analyses, which caused a relatively more significant cooling in Northeast China after 1998 under the background of global warming slowdown.

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Richard C. Y. Li, Wen Zhou, Johnny C. L. Chan, and P. Huang

Abstract

The present study investigates the modulation by the Madden–Julian oscillation (MJO) and the impact of the El Niño–Southern Oscillation (ENSO) on tropical cyclone (TC) genesis in the western North Pacific (WNP) during the period 1975–2010. Results reveal a stronger modulation of cyclogenesis by the MJO during El Niño years, while the modulations in neutral and La Niña years are comparable to each other.

The asymmetric background modification by ENSO is found to greatly affect the extent of MJO modulation under different ENSO conditions. First, MJO activity is intensified and extends farther eastward during El Niño years, instead of being confined west of 150°E as in neutral and La Niña periods. Thus, the influence of MJO is stronger and more zonally widespread in El Niño years, causing significant differences in cyclogenesis parameters in most parts of the WNP. In El Niño years, cyclogenesis is further enhanced in the active phase due to synchronization of MJO signals with favorable background ENSO conditions. While in the inactive phase, the dominance of the strong MJO signals leads to further suppression in TC formation. This leads to overall enhancement of the MJO–TC relationship during El Niño years. On the other hand, the MJO signals confined to the western region west of 150°E in neutral and La Niña years lead to changes in TC-related parameters mainly in the western region, which contribute to the comparatively weaker TC modulations. It can thus be concluded that the MJO has an asymmetric modulation on cyclogenesis in the WNP under different ENSO conditions.

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Botao Zhou, Qiuzi Han Wen, Ying Xu, Lianchun Song, and Xuebin Zhang

Abstract

This paper presents projected changes in temperature and precipitation extremes in China by the end of the twenty-first century based on the Coupled Model Intercomparison Project phase 5 (CMIP5) simulations. The temporal changes and their spatial patterns in the Expert Team on Climate Change Detection and Indices (ETCCDI) indices under the RCP4.5 and RCP8.5 emission scenarios are analyzed. Compared to the reference period 1986–2005, substantial changes are projected in temperature and precipitation extremes under both emission scenarios. These changes include a decrease in cold extremes, an increase in warm extremes, and an intensification of precipitation extremes. The intermodel spread in the projection increases with time, with wider spread under RCP8.5 than RCP4.5 for most indices, especially at the subregional scale. The difference in the projected changes under the two RCPs begins to emerge in the 2040s. Analyses based on the mixed-effects analysis of variance (ANOVA) model indicate that by the end of the twenty-first century, at the national scale, the dominant contributor to the projection uncertainty of most temperature-based indices, and some precipitation extremes [including maximum 1-day precipitation (RX1day) and maximum 5-day precipitation (RX5day), and total extremely wet day total amount (R95p)], is the difference in emission scenarios. By the end of the twenty-first century, model uncertainty is the dominant factor at the regional scale and for the other indices. Natural variability can also play very important role.

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Tao Feng, Xiu-Qun Yang, Wen Zhou, Ronghui Huang, Liang Wu, and Dejian Yang

Abstract

Tropical depression (TD)-type waves are the dominant mode of synoptic-scale fluctuations over the western North Pacific. By applying spatiotemporal filters to the observed OLR data and the NCEP–DOE AMIP-II reanalysis data for 1979–2013, this study reveals the characteristics and energetics of convectively coupled TD-type waves under the effects of different circulation patterns in association with vertical wind shear. Results exhibit that different ambient sheared flows significantly affect the vertical structure of westward-propagating TD-type waves, with a lower-tropospheric mode in an easterly sheared background and an upper-tropospheric mode in a westerly sheared background. Energetic diagnoses demonstrate that when the disturbance is trapped in the lower (upper) level by easterly (westerly) shear, the horizontal mean flow in the lower (upper) level favors wave growth by converting energy from the shear of the zonal mean flow (from the convergence of the meridional mean flow). During the penetration of a westward-propagating synoptic-scale disturbance from a westerly sheared flow into an easterly sheared flow, the upper-level disturbance decays, and the lower-level disturbance intensifies. Meanwhile, the upper-level kinetic energy is transferred downward, but the effect induces the wave growth only confined to the midlevels. Consequently, the low-level growth of the westward-propagating upper-level synoptic-scale disturbance is mainly attributed to the barotropic conversion of horizontal mean flow in the lower troposphere.

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Yuntao Jian, Marco Y. T. Leung, Wen Zhou, Maoqiu Jian, Song Yang, and Xiaoxia Lin

Abstract

In this study, the interdecadal variability of the relationship between ENSO and winter synoptic temperature variability (STV) over the Asian-Pacific-American region is investigated based on observational data from 1951 to 2018. An interdecadal shift in the ENSO-STV relationship occurred in the 1980s over Eastern China, changing from significant in Period 1 (P1, 1951-1987) to insignificant in Period 2 (P2, 1988-2018). But the ENSO-STV relationship is significantly stable over North America for the whole period. In addition, a possible reason for this interdecadal shift in the ENSO-STV relationship over Eastern China is also investigated. During P1, the ENSO pattern is significantly correlated to the temperature gradient over Northeast Asia, which is the key region influencing the intensification of extratropical eddies. The intensification of extratropical eddies over Northeast Asia is directly associated with the magnitude of STV over Eastern China. But in P2, the ENSO pattern is not related to the temperature over Northeast Asia. Therefore, the change in the ENSO pattern from P1 to P2 contributes to the interdecadal shift in the ENSO-STV relationship in the 1980s over Eastern China by influencing the temperature gradient over Northeast Asia, while ENSO can influence the temperature gradient over North America for the whole period. Furthermore, the possible role of the ENSO patterns in P1 and P2 is also examined by using an atmospheric general circulation model, highlighting that the pattern of SST variation is a determining factor in regulating STV in different regions.

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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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Richard C. Y. Li, Wen Zhou, C. M. Shun, and Tsz Cheung Lee

Abstract

This study investigates changes in the destructiveness of landfalling tropical cyclones (TCs) over China during 1975–2014. Using four different TC datasets, it is found that TCs making landfall over east China (TCEC) have tended to be more destructive in recent decades, with a significant increase in the power dissipation index (PDI) after landfall. Both time series analysis and diagnostic analysis reveal that such an increase in the PDI of TCEC is associated with concomitant enhancement in landfall frequency as well as landfall intensity over east China. In contrast, changes in the PDI of TCs making landfall over south China (TCSC) are less apparent. Examination of different TC-related parameters shows no obvious changes in terms of landfall frequency, duration, and maximum intensity of TCSC. Diagnostic analysis further suggests that the reduction in TC occurrence over south China offsets considerably the positive effects of the intensity and the nonlinear term.

Further examination of the environmental parameters reveals significant changes in the large-scale steering flow in recent decades, which is characterized by a prominent cyclonic circulation centered over southeast China. The southeasterly flows on the eastern flank of the cyclonic circulation tend to favor subsequent landfall of TCs over east China, resulting in an increase in landfall frequency, which contributes in part to the enhanced PDI of TCs over this region. Meanwhile, the slowing down of the mean translation speed of TCEC and the weakening of vertical wind shear coupled with warmer SSTs in the WNP tend to favor the intensification of TCEC, leading to an increase in intensity and hence the PDI of TCs over east China.

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Marco Y.-T. Leung, Wen Zhou, Dongxiao Wang, P. W. Chan, S. M. Lee, and H. W. Tong

Abstract

In this study, remote influence originating from the tropical western Indian Ocean on June precipitation in South China and the Indochina Peninsula is documented. Based on numerical simulation and statistical analysis, it is noted that the warm anomaly in the tropical western Indian Ocean can induce a weaker-than-normal Walker circulation across the tropical Indian Ocean and western Pacific Ocean. This further leads to a northeast–southwest-oriented western North Pacific subtropical high and a weaker-than-normal monsoon trough in the South China Sea. In addition, the weak monsoon trough is concurrent with an anomalous rising motion in South China and a sinking motion in the Indochina Peninsula. This enhances precipitation in South China and suppresses precipitation in the Indochina Peninsula on an interannual time scale. On the other hand, the warming trend in the tropical western Indian Ocean also supports the long-term trends of precipitation in the two regions.

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