Search Results

You are looking at 11 - 20 of 32 items for

  • Author or Editor: Zhiping Wen x
  • Refine by Access: All Content x
Clear All Modify Search
Liang Wu
,
Zhiping Wen
,
Ronghui Huang
, and
Renguang Wu

Abstract

The present study investigates the influence of the monsoon trough (MT) on the interannual variability of tropical cyclone (TC) activity over the western North Pacific during July–November for the period 1979–2007. It is shown that the TC activity is closely related to the MT location. During the years when the MT extends eastward (retreats westward), more (less) TCs form within the southeastern quadrant of the western North Pacific. Such a relationship can be explained by the changes in large-scale environmental factors associated with the movement of the MT. An eastward extension of the MT coincides with warmed ocean surface, enhanced convection, increased relative humidity in the lower and midtroposphere, reduced vertical shear of zonal wind, intensified upper-level divergence, and low-level anomalous cyclonic vorticity over the southeast quadrant of the western North Pacific. These conditions associated with the eastern extension of the MT are favorable for TC genesis, while those associated with the westward retreat of the MT are not. Diagnosis of the barotropic energy conversion indicates that synoptic-scale disturbances moving westward from tropical eastern Pacific will gain the energy from the mean flow when they meet with the eastward-extending MT. This is an important reason for the linkage between MT variability and TC genesis over the western North Pacific.

Full access
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.

Full access
Wei Wu
,
Zhiping Wen
,
Renguang Wu
, and
Tongmei Wang

Abstract

In the present study, monthly mean objectively analyzed air–sea fluxes (OAFlux) and NCEP–Department of Energy (DOE) reanalysis datasets are employed to investigate air–sea interaction over the subtropical North Pacific during the El Niño–Southern Oscillation (ENSO) transition phase. A coupled low-frequency mode is identified, for which surface net heat flux and atmospheric circulation changes are strongly coupled during the ENSO transition phase. This mode features anomalous cooling (warming) and low-level anomalous cyclonic (anticyclonic) circulation over the subtropical North Pacific. When this mode is prominent, the atmospheric circulation anomalies lead to SST cooling (warming) through surface heat flux anomalies associated with increases (decreases) in the sea–air temperature and humidity differences induced by anomalous cold (warm) advection. In turn, positive heat flux anomalies induce more surface heating, and the SST cooling (warming) causes less (more) deep convective heating. The anomalous surface heating and deep convective heating contribute significantly to anomalous circulation through the thermal adaptation mechanism (adaptation of atmospheric circulation to vertical differential heating). This positive feedback favors the maintenance of these anomalous winds over the subtropical North Pacific.

Full access
Xiao Feng
,
Renguang Wu
,
Jiepeng Chen
, and
Zhiping Wen

Abstract

The present study investigates the year-to-year variations of September–October rainfall in Hainan, China, for the period 1965–2010. The dominant circulation anomalies feature a cyclone (an anticyclone) over the Indochina Peninsula and northern South China Sea, an anticyclone (a cyclone) over subtropical western North Pacific and lower-level convergence (divergence) over the Maritime Continent in the wet (dry) years. These circulation anomalies are responses to an east–west sea surface temperature (SST) anomaly pattern with negative (positive) SST anomalies in the equatorial central Pacific and positive (negative) SST anomalies around the Maritime Continent in the wet (dry) years. Although the SST anomaly pattern is similar (but with opposite anomaly), the SST signal in the equatorial central Pacific is more significant in the dry years than in the wet years. This difference indicates a larger case-to-case variability in the wet years than in the dry years. The large variability in the wet years is attributed to contributions of tropical cyclones (TCs) and intraseasonal oscillations (ISOs). There are more TCs impinging on Hainan and the TC tracks are closer to the island in the wet years than in the dry years. The rainfall shows large intraseasonal variations with periods of 10–20 and 30–60 days during September–October in the wet years. The 10–20-day ISO originates from the Maritime Continent, whereas the 30–60-day ISO develops over tropical Indian Ocean and propagates northeastward to northern South China Sea. In contrast, the ISO signal is much weaker in the dry years.

Full access
Renguang Wu
,
Zhiping Wen
,
Song Yang
, and
Yueqing Li

Abstract

The present study documents a pronounced interdecadal change in summer rainfall over southern China around 1992/93 and explores the plausible reasons for this change. The summer rainfall is persistently below normal during 1980–92 and above normal during 1993–2002. Coherent changes in atmospheric circulation are identified over East Asia and the South China Sea (SCS)–western North Pacific (WNP). The increase in rainfall is accompanied by an increase in lower-level convergence, midtropospheric ascent, and upper-level divergence over southern China. The changes in lower-level winds feature two anomalous anticyclones: one over the SCS–subtropical WNP, and the other over north China–Mongolia. The outflows from the two anomalous anticyclones converge over southern China, leading to anomalous moisture convergence, enhanced ascent, and increased rainfall. The development of the northern anticyclone is related to an increase in the Tibetan Plateau snow cover in the preceding winter–spring that leads to a contrast in temperature change between the plateau and the surrounding regions. The relatively small temperature change over the plateau, coupled with increases in temperature to the west and the east, leads to an increase in surface pressure extending northward from the plateau. The development of the southern anticyclone is related to an increase in sea surface temperature in the equatorial Indian Ocean that enhances lower-level convergence and ascent. The accompanying upper-level divergent flows from the tropical Indian Ocean to the SCS–WNP lead to the development of anomalous descent and lower-level anomalous anticyclone over the SCS–WNP.

Full access
Ruidan Chen
,
Zhiping Wen
,
Riyu Lu
, and
Wenjun Liu

Abstract

This study reveals the interdecadal changes in the interannual variability of the summer temperature over Northeast Asia (NEA), which presents an enhancement around the early 1990s and a reduction after the mid-2000s. The stronger NEA temperature variability after the early 1990s is favored by the enhanced influence of the Pacific–Japan (PJ) teleconnection, which is remotely modulated by the southeastern tropical Indian Ocean (SETIO). After the early 1990s, the mean state over the SETIO presents relatively warmer SST and ascending motion, favoring a good relationship between the local SST and convection. Therefore, the SETIO SST could prominently influence the local convection and subsequently modulate the convection over the western North Pacific (WNP) via a cross-equatorial overturning circulation. The abnormal convection over the WNP further triggers the PJ teleconnection to influence NEA. However, these ocean–atmosphere processes disappear before the early 1990s. In this period, the mean state over the SETIO features relatively colder SST and subsiding motion, accompanied by a poor relationship between the local SST and convection. Therefore, the variability of convection over the SETIO is weak, and thus the atmospheric variability over the WNP is also weakened and the PJ teleconnection presents a different distribution that could not influence NEA. The reduced variability of NEA temperature after the mid-2000s is related to the feeble influence of the PJ teleconnection and the reduced variability of the SETIO SST, which is modulated by the SST over the tropical central–eastern Pacific during the preceding winter to spring.

Full access
Sihua Huang
,
Bin Wang
,
Zhiping Wen
, and
Zesheng Chen

Abstract

Previous studies found a tight connection between the tropical easterly jet (TEJ) and Indian summer monsoon rainfall (ISMR). Here we show that the TEJ–ISMR relationship is nonstationary and breaks down from 1994 to 2003 (epoch P2), in contrast to the significant positive correlation during epochs P1 (1979–93) and P3 (2004–16). The breakdown of the TEJ–ISMR relationship concurs with the increased rainfall variability over the tropical eastern Indian Ocean (TEIO). The enhanced TEIO rainfall anomalies excite a significant lower-level cyclonic circulation that reduces the ISMR and meanwhile strengthen the upper-level divergence and excite a pair of upper-level anticyclones to the west of the TEIO as Rossby wave responses, both accelerating the TEJ. Thus, the TEIO rainfall plays a more important role than the ISMR in TEJ variability during P2, causing the breakdown of the TEJ–ISMR relationship. In contrast, a relatively weak amplitude of the TEIO rainfall during P1 and P3 was unable to change the positive TEJ–ISMR relationship. The changes in the TEIO rainfall variability are mainly attributed to the increased SST variability over the tropical southeastern Indian Ocean, but their cause remains elusive.

Full access
Yuanyuan Guo
,
Zhiping Wen
,
Yu Zhu
, and
Xiaodan Chen

Abstract

Tropical sea surface temperature (SST) and associated precipitation, acting as diabatic heat forcing, has far-reaching climatic impacts across the globe through exciting poleward-propagating Rossby waves. It is found that the leading mode of tropical Pacific forcing in austral autumn experiences a significant interdecadal shift from an eastern Pacific (EP) to a central Pacific (CP) type around the late 1990s. More specifically, the EP-type precipitation anomaly mode before 1998 drives a quadrupole-like teleconnection pathway emanating from the tropical Pacific to the Ross Sea and Amundsen–Bellingshausen Seas (ABS) region, whereas the CP-type mode after 1999 excites a Pacific–South American (PSA)-like teleconnection orienting along a great circle. Divergent flows induced by different precipitation anomaly modes primarily determine the generation of Rossby waves by means of the vortex stretching and vorticity advection processes. Furthermore, the synoptic high-frequency transient eddy activity along with its dynamic forcing effect differs greatly before and after 1998/99, contributing to different locations of the teleconnection lobes at mid- to high latitudes. In contrast, the subseasonal low-frequency transient eddy activity exerts a limited influence. Our findings also indicate that the EP-type (CP-type) tropical forcing mode could significantly modulate the zonal displacement (strength) of the Amundsen Sea low, which could lead to distinct climate responses of West Antarctica and the Antarctic Peninsula in austral autumn.

Open access
Xiuzhen Li
,
Zhiping Wen
, and
Wan-Ru Huang

Abstract

Two extremely wet winters in 2015/16 and 2018/19 over Southeast China are compared in this study. South-to-north discrepancies appear in the spatial distribution of precipitation, with anomalous precipitation centered over the southeast coast in 2015/16 and the lower reaches of Yangtze River valley in 2018/19, respectively. Both instances of enhanced precipitation are ascribed mainly to warm and moist advection from the south, with transport in 2015/16 partly by a deepened India–Burma trough to the west, whereas with transport in 2018/19 mainly by a subtropical western North Pacific anticyclone (WNPAC). Both the India–Burma trough and WNPAC are maintained by the wave trains propagating along the South Asian jet, which are zonally offset by a quarter-wavelength. Further study of the wave train sources in 2015/16 and 2018/19 shows that they both tend to originate from extremely strong storm-track activity over the North Atlantic but have different displacement. The former is located more northeastward than the mean storm track and is modulated by a strong positive NAO, whereas the latter lies over the midlatitude central North Atlantic along with a circumglobal teleconnection. These differences further result in a quarter-wavelength offset in the Rossby wave source near the entrance of the South Asian jet by the convergence of upper-level divergent wind.

Free access
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.

Full access