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Ping Huang
and
Ronghui Huang

Abstract

Climatology and interannual variability of convectively coupled equatorial wave (CCEW) activity, including the mixed Rossby–gravity (MRG), tropical-depression-type (TD-type), equatorial Rossby (ER), and Kelvin waves, are investigated using the satellite-observed brightness temperature data from the Cloud Archive User Service. The monthly activity of CCEWs is represented by the root mean square of the daily filtered convections in each month based on the Wheeler–Kiladis filtering method. More precise seasonal cycles of CCEW activity are obtained from the meridional and zonal mean climatology.

Interannual variance of CCEW activity is further investigated. Kelvin wave activity has maximum interannual variance over the eastern Pacific, while the other three waves are most variable in the intertropical convergence zone. The four active CCEWs all have significant correlation with the background convection and local sea surface temperature (SST) over the central and eastern Pacific, but they are not significantly correlated over other regions. The El Niño events may induce more trapped and active CCEWs over the central and eastern Pacific but weaker MRG and TD-type waves over the warm pool. In contrast, the El Niño Modoki has much weaker correlation with CCEW activity. CCEW activity over the southeastern Indian Ocean is negatively correlated with the Indian Ocean dipole, while that over the western and northern Indian Ocean may be determined by atmospheric internal disturbances. The tropical southern Atlantic mode is the strongest Atlantic SST anomaly mode correlated with the Atlantic CCEW activity.

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Qiang Zhang
and
Ronghui Huang

Abstract

Using data observed at Dunhuang, in the Gansu, in the arid region of northwest China in the summer, the characteristics of the soil water content, temperature, and atmospheric humidity were analyzed. It was found that the depth of the active soil temperature layer is about 5 cm, which is much thinner than that of typical soils. In addition, not only is the atmospheric humidity gradient in the surface layer often inverted, but so too is the soil water content gradient in the shallow layer. The diurnal variation of soil water content can be divided into four stages, including wet, water loss, dry, and water gain. It is shown that in soil water content profiles the depth of the active soil layer is about 10 cm, and soil water content inversion is the primary feature in the shallow layer during the “wet” stage. The presence of soil water content inversion indicates that soil in the shallow layer can absorb water from the air through condensation in the nighttime and emit water vapor to the air through evaporation in the daytime. The formation of a soil water content inversion is mainly related to the state of the soil surface temperature.

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Guosen Chen
and
Ronghui Huang

Abstract

Using observational rainfall data and atmospheric reanalysis data, the precipitation variations in Northwest China during July and the corresponding atmospheric teleconnection patterns are studied. The results indicate that the leading modes of July precipitation variations in Northwest China are affected by the Silk Road pattern and the Europe–China (EC) pattern. The analysis suggests that the circumglobal teleconnection (CGT) could be considered as the interannual component of the Silk Road pattern.

To investigate the excitation mechanisms for the CGT pattern and EC pattern on interannual time scales, the singular value decomposition (SVD) analysis is performed between 200-hPa meridional wind velocity over the region of (30°–60°N, 30°–130°E) and tropical rainfall between (15°S and 30°N). The results suggest that the tropical heating anomalies most responsible for the CGT pattern are located over the North Indian Ocean, and the tropical heating anomalies most responsible for EC pattern are located over equatorial central Pacific, Indonesia, and tropical Atlantic. The tropical heating anomalies excite the CGT pattern and EC pattern by inducing divergent flow at the upper troposphere, and the advections of vorticity by the divergent component of the flow act as effective Rossby wave sources. Further analysis indicates that the tropical rainfall anomalies responsible for the CGT pattern and EC pattern are the leading modes of tropical rainfall variations, and these modes of tropical rainfall variations are related to the SST anomalies.

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Guanghua Chen
and
Ronghui Huang

Abstract

The present study investigates the transition from mixed Rossby–gravity (MRG) waves to tropical depression (TD)-type disturbances and its interannual variations over the western North Pacific (WNP), using ECMWF high-resolution data for the years of 1980–2001. As the equatorially trapped MRG waves propagate westward into the WNP, the MRG waves transit to TD-type disturbances because of background flow change. Interannual variations in the transition of MRG waves are related to monsoon circulation change in response to tropical convective heating over the warm pool (WP) region. When the WP is in a warm state, convective heating is enhanced in the western part of the WNP and the monsoon trough retreats westward, which induces a westward shift of the wave transition zone. In contrast, when the WP is in a relatively cold state, the eastward penetration of convection and monsoon trough shifts the wave transition to the eastern part of the WNP.

The zonal wind convergence and shear in the monsoon trough region provide a favorable condition for MRG waves to asymptote to Rossby waves. The asymmetric basic flow contributes to MRG waves moving off the equator toward the northwest. The northeast–southwest-oriented axis of TD-type disturbances in collaboration with the monsoonal environment is favorable for the conversion of eddy kinetic energy from the mean flow. The intensification of the amplitude and shortening of the wavelength during wave transition, to a certain extent, is associated with tropical cyclogenesis over the WNP. Therefore, interannual variations in the longitudinal location of tropical cyclone formation may be interpreted partly by displacement of the wave transition zone. Moreover, this phenomenon of cyclogenesis induced by the wave transition is more common during the cold years in which the monsoon trough penetrates eastward and equatorward.

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Yong Liu
and
Ronghui Huang

Abstract

This study provides a water vapor transport (WVT) perspective on the linkages between the South Asian and East Asian summer monsoons (SASM and EASM) and indicates two robustly coupled modes of the vertical integrated WVT (VIWVT) over the two monsoons that accounts for above 90% of the total squared covariance fraction. The first coupled mode [singular value decomposition mode 1 (SVD1 mode)] depicts a meridional linkage between the meridional dipole VIWVT anomalies over both the SASM and EASM, while the second coupled mode (SVD2 mode) illustrates a zonal connection of an anomalous cyclonic/anticyclonic VIWVT over the SASM and a zonal wavelike VIWVT over the EASM. The SVD1 mode is linked through the anomalous subtropical high over the western North Pacific (WNPSH) and is primarily associated with the transition phase of El Niño/La Niña (ENSO) and simultaneous Indian Ocean basin mode (IOBM) SST warming/cooling. The meridional connection of the VIWVT in the SVD1 mode experienced a clear intensification since the late 1970s that may be attributed to the strengthened impacts of the ENSO/IOBM on the EASM and SASM after the late 1970s. The SVD2 mode is connected by the circumglobal teleconnection (CGT) pattern and related to the developing phase of ENSO and summer North Atlantic tripole (NAT) SST anomalies. The zonal VIWVT connection in SVD2 mode is strongly modulated by the SASM–CGT connections and reveals significant weakening since the late 1970s but reintensifies after the early 1990s. This may be associated with the weakened ENSO–SASM relationship after the late 1970s and interdecadal decreasing of the all Indian summer rainfall since the early 1990s.

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Kaiming Hu
,
Gang Huang
, and
Ronghui Huang

Abstract

Evidence is presented that the boreal summer surface air temperature over south China and northeast China is remotely influenced by the Indian Ocean Basin mode (IOBM) sea surface temperature (SST) anomalies. Above-normal temperature in south China and below-normal temperature in northeast China correspond to a simultaneous Indian Ocean Basin warming. The teleconnection from Indian Ocean SST anomalies to China summer surface air temperature is investigated using observations and an atmospheric general circulation model (AGCM). The results herein indicate that the tropical Indian Ocean Basin warming can trigger a low-level anomalous anticyclone circulation in the subtropical northwest Pacific and an anomalous cyclone circulation in midlatitude East Asia through emanating a baroclinic Kelvin wave. In south China, the reduced rainfall and downward vertical motion associated with the anomalous low-level anticyclone circulation lead to above-normal summer surface air temperature. In northeast China, by contrast, upward vertical motion associated with the anomalous cyclone leads to below-normal summer surface air temperature.

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Ping Huang
,
Chia Chou
, and
Ronghui Huang

Abstract

The seasonal modulation of tropical intraseasonal oscillation (TISO) on tropical cyclone (TC) geneses over the western North Pacific Ocean (WNP) is investigated in three periods of the WNP TC season: May–June (MJ), July–September (JAS), and October–December (OND). The modulation of the TISO–TC geneses over the WNP is strong in MJ, while it appears weaker in JAS and OND. In MJ, TISO propagates northward via two routes, the west route through the South China Sea and the east route through the WNP monsoon trough region, which are two clustering locations of TC geneses. TISO can synchronously influence most TC geneses over these two regions. In JAS, however, the modulation is out of phase between the monsoon trough region and the East Asian summer monsoon region, as well as the WNP subtropical high region, as a result of further northward propagation of TISO and scattered TC geneses. The TISO–TC genesis modulation in each individual region is comparable to that in MJ, although the modulation over the entire WNP in JAS appears weaker. In OND, TISO has a stronger influence on TC geneses west than east of 150°E because TISO decays and its convection center located at the equator is out of the TC genesis region when propagating eastward into east of 150°E. Midlevel relative humidity is the primary contribution to the modulations of TISO on the genesis environment, while vorticity could contribute to the modulation over the subtropics in JAS.

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Guosen Chen
,
Ronghui Huang
, and
Liantong Zhou

Abstract

In this paper, the internal dynamics of the Silk Road pattern has been studied. Since observation indicates that the Silk Road pattern could be considered as stationary external Rossby waves, the quasigeostrophic three-layer model has been used to study the dynamics of external Rossby waves. The three-layer model well captures the essential dynamical features of stationary external Rossby waves in accordance with the observations. Theoretical analysis indicates that the quasi-stationary external modes could be destabilized by the weak thermal damping. For destabilization to occur, the vertical structures of the external modes must have a warm ridge and a cold trough from the lower to middle layers. The effect of thermal damping could be considered as modifying the eddy streamfunction in such way that the eddy streamfunction has a vertical phase tilt, so the eddy could feed on the basic zonal flow by extracting the potential energy. The implications for this baroclinic instability on the self-maintenance of the Silk Road pattern are discussed. The observations imply that this dissipative destabilization mechanism could explain the self-maintenance of the Silk Road pattern.

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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.

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Lin Wang
,
Wen Chen
,
Wen Zhou
, and
Ronghui Huang

Abstract

Interannual variations of the East Asian trough (EAT) axis at 500 hPa are studied with the European Centre for Medium-Range Weather Forecasts 40-yr reanalysis data. The associated circulation pattern and pathway of the East Asian winter monsoon (EAWM) with the EAT axis tilt are specially investigated with a trough axis index, which is closely related to the midlatitude baroclinic process and mainly represents the intensity of the eddy-driven jet over the East Asia–North Pacific sector. When the tilt of EAT is smaller than normal, the EAWM prefers to take the southern pathway and less cold air moves to the central North Pacific. However, the EAWM prefers the eastern pathway and brings more cold air to the North Pacific when the tilt of EAT is larger than normal. These differences induce pronounced changes in both the precipitation and the surface air temperature over East and Southeast Asia. Furthermore, the tilt status of the EAT has a significant modulation effect on the regional climate anomalies related to the intensity of the EAWM. The findings suggest an increase in the temperature anomaly associated with the EAWM intensity and a clear northward–southward shift in its pattern in anomalous tilt phase of the EAT. In addition, the modulation tends to be confined mainly to East Asia and expanded to a larger area during the weak and the strong EAWM winters, respectively. The possible reasons for interannual variations of the EAT tilt are discussed, and it is speculated that the midlatitude air–sea interaction in the North Pacific plays a dominant role. This study on the EAT tilt may enrich knowledge of the East Asian winter monsoon beyond the conventional intensity index and may be helpful to improve regional climate prediction in East Asia.

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