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Guanghua Chen

Abstract

The differences in the transitions of equatorial mixed Rossby–gravity (MRG) waves to off-equatorial tropical depression (TD)-type disturbances during ENSO events are investigated with a global baroclinic anomaly model. The model reproduces reasonably the perturbation evolution within realistic three-dimensional summer mean states corresponding to El Niño (EN) and La Niña (LN) years. Based on wave structure and energetics diagnosis, the results indicate that, following the longitudinal shift of the favorable environmental fields, the wave characteristics are altered accordingly. In the presence of a circulation–convection feedback, the wave train exhibits more rapid growth, a more eastern location of transition, and a more northward-shifting component during EN years than during LN years. The convective heating acts as a leading energy source to supply the wave growth and the increase in eddy kinetic energy is directly attributed to barotropic conversion in the monsoon region.

Sensitivity experiments show that the dynamic effect alone fails to capture the observed wave behaviors although the damped modes also experience a scale contraction and a slight northward migration. The near-surface thermodynamic fields related to sea surface temperature (SST) and low-level specific humidity can play a crucial role in the scale contraction and the propagation characteristics for tropical synoptic waves. The heating feedback scheme combining the actions of SST and low-level moisture can amplify and accelerate the modification of wave characteristics initiated by the dynamic effect, producing a tighter wave structure and steering the wave train toward the warmer and moister ocean.

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Guanghua Chen

Abstract

In a recent paper, Kajikawa and Wang detected the interdecadal shift of the South China Sea summer monsoon (SCSSM) onset with a late SCSSM onset in an earlier epoch (1979–93) and an early SCSSM onset in a later epoch (1994–2008) and attributed this change to enhanced tropical cyclone (TC) activity and intraseasonal variability (ISV) related to 30–80-day and 10–25-day anomalies in the second epoch. This comment assesses the individual impact of TCs and ISV on the interdecadal change of the SCSSM onset by means of the removal of anomalies associated with TCs and ISV. Results herein show that TCs have no significant impact on the SCSSM onset in all years, except 2006 in which a strong and long-lived TC occurred over the South China Sea. After removing the 30–80-day anomaly, the difference in the mean SCSSM onset date in the two epochs decreases to some extent, implying that the 30–80-day anomaly can, in part, play a role in the interdecadal shift of the SCSSM onset. In contrast, the 10–25-day anomaly has an insignificant contribution to the interdecadal shift of the SCSSM onset. The discrepancy of ISV contribution results from the SCSSM background state, the magnitude and spatiotemporal scale of ISV, and the phase relationship between ISV and SCSSM transition from easterly to westerly.

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Guanghua Chen

Abstract

The different modulation of El Niño Modoki and canonical El Niño events on tropical cyclone (TC) frequency over the South China Sea (SCS) during boreal summer and fall for 1960–2009 is investigated. The bootstrap resampling method and two-sample permutation procedure are applied to simulate sampling distributions and conduct statistical tests, respectively. Results from the hypothesis testing indicate that the above-normal TC frequency over the SCS occurs during June–August (JJA) for the El Niño Modoki years, whereas the below-normal TC frequency is significant during September–November (SON) for the canonical El Niño years. The remarkably opposite modulations can be attributed to the different large-scale circulation anomalies, which are consistent with Matsuno–Gill-type responses to the tropical heating source/sink over the western North Pacific (WNP) and Maritime Continent for two kinds of Pacific Ocean warming events. In response to a broad-scale convection anomaly over the WNP during JJA for El Niño Modoki, a zonally elongated cyclonic anomaly dominates the WNP and SCS, leading to enhanced TC activity. In contrast, during SON for the canonical El Niño, a markedly strengthened cooling source centered in the Maritime Continent induces an anticyclonic anomaly over the SCS, resulting in suppressed TC activity.

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Guanghua Chen

Abstract

The role of increased diabatic cooling in secondary eyewall formation (SEF) and eyewall replacement cycle (ERC) is examined using idealized numerical simulation. The experiment with the low-level inner-core diabatic cooling increased by 30% features the low-entropy air and downward motion in the inner-core region whereas the convergence and active convective updrafts are in the outer-core region. In collaboration with the favorable ambient dynamical conditions and boundary layer dynamical processes, the concentric convective ring is initiated with the aid of the outward expansion of strong wind field, and then contracts inward to replace the inner eyewall. Subsequently, the deep-tropospheric radial outflows driven by the large outward-directed agradient force related to the massive strong tangential wind generate a largely outward-tilted eyewall, eventually forming a large-eyed storm. The sensitivity to the strength and radial location of diabatic cooling shows that neither the 20% increase nor 10-km radially inward shift of the low-level cooling produces a pronounced SEF and ERC because of the lack of an evident moat region. In contrast, both the 40% increase and 10-km radially outward shift of cooling lead to the active outer rainbands occurring at a larger radius. In the former case, because of the deep-layer radial outflow above the boundary layer, the largely outward-tilted concentric eyewall shrinks slowly, directly creating a large-eyed structure. In the latter case, the formation of concentric eyewall is delayed because of the low inertial stability at a large radius, but experiences an expeditious ontraction because of the strong radial inflow.

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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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Guanghua Chen and Chia Chou

Abstract

A composite study is performed to examine the differences in equatorial wave behaviors and large-scale background patterns during tropical cyclone (TC) genesis. After removing TC contamination, Madden–Julian oscillation (MJO), equatorial Rossby (ER) wave, mixed Rossby–gravity (MRG) wave, and tropical depression (TD)-type disturbance (jointly referred to as the MT wave) are quantified to evaluate the attribution of TC geneses. Given that TC geneses are attributed to a single wave or multiple waves, the eight categories are specified based on the moderate thresholds. The TC geneses related to multiple waves are roughly twice as many as those related to a single wave. The MT wave alone accounts for a minor proportion of TC geneses without collaboration with other larger-scale waves. The mean TC genesis location related to ER wave shifts to higher latitudes, and the TC geneses attributed to both of MJO and MT waves are more concentrated at the west. The single-wave categories are characterized by a zonally propagating component with a large spatial scale. In contrast, the joint contribution from more than one wave type favors creating a coherent environment with enhanced low-level cyclonic vorticity, horizontal convergence, or vertical easterly shear in a preferred region. Consequently, the waves have a more robust structure and a more northwestward-propagating component. Correspondingly, the TC geneses in the MJO–ER category tend to occur within a monsoon trough dominated by cyclonic circulation. For the MJO–MT category, the background field exhibits a confluence pattern with a monsoon trough to the west and easterly flows to the east. The collaboration of the ER and MT waves facilitates TC geneses within an easterly environment in the southern flank of the subtropical high.

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Xu Wang and Guanghua Chen

Abstract

The propagation dynamics and energetics of the quasi-biweekly oscillation (QBWO) over the South China Sea (SCS) in late summer [August–September (AS)] are investigated in this study. The QBWO originates from east of the Philippines and has a northwestward propagation. After arriving to the east of the SCS, the QBWO shifts to a westward migration and dominates over the SCS. The analyses of the vorticity budget suggest that the meridional wind anomaly could control the spatial migration of the vorticity anomaly through the β-effect term and further influences the movement of the convection anomaly. It implies that the meridional wind is a crucial factor to drive the propagation of the QBWO. The energetics of the QBWO is investigated to understand the maintenance of the QBWO, which indicates that the convection anomaly could affect the circulation anomaly through the energy conversions to maintain the QBWO.

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Donglei Shi and Guanghua Chen

Abstract

The rapid intensification (RI) of supertyphoon Lekima (2019) is investigated from the perspective of balanced potential vorticity (PV) dynamics using a high-resolution numerical simulation. The PV budget shows that the inner-core PV anomalies (PVAs) formed during the RI mainly comprise an eyewall PV tower generated by diabatic heating, a high-PV bridge extending into the eye resulting from the PV mixing, and an upper-tropospheric high-PV core induced by the PV intrusion from stratosphere. The inversion of the total PVA at the end of the RI captures about 90% of changes in pressure and wind fields, indicating that the storm is quasi-balanced. The piecewise PV inversion further demonstrates that the eyewall and mixed PVAs induce the upper-level and midlevel warm cores in the eye region, respectively. The two warm cores cause nearly all the balanced central pressure decrease and thus dominate the RI, with the contribution of the upper warm core being twice that of the midlevel one. In contrast, the upper-tropospheric PV core induces significant warming near the tropopause and deep-layer cooling beneath, reinforcing the upper-level warm core but causing little surface pressure drop.

By comparing the diabatic PV generation due to the convective burst (CB) and non-CB precipitation, we found that the non-CB precipitation accounts for a larger portion for the eyewall PVA and thus the associated upper-level warming, distinct from previous studies that primarily attributed the upper-level warm-core formation to the CB. Nevertheless, CBs act to be more efficient PV generators due to their vigorous latent heat release and are thus favorable for RI.

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Guanghua Chen and Chi-Yung Tam

Abstract

This study investigates the synoptic-scale equatorial response to Rossby wave energy dispersion associated with off-equatorial wave activity sources and proposes a new mechanism for triggering low-level mixed Rossby–gravity (MRG) waves. A case study based on observations in boreal summer 2002 reveals that a vortex related to tropical cyclogenesis generated a coherent wave train through southeastward energy dispersion. The southeastward-propagating energy packet gave rise to the equatorial atmospheric response with a temporal scale similar to the wave train and with a structure consistent with the equatorially trapped MRG wave. A baroclinic multilevel anomaly model is employed to verify the excitation of MRG associated with the energy dispersion originating outside of the equatorial region and to explore the discrepancy in the equatorial responses under the different background flows corresponding to El Niño and La Niña. The results show that the prevalence of the low-level westerly flow, the associated zonal wind convergence, and the easterly vertical wind shear can be more favorable for the enhancement of southeastward-propagating energy dispersion and equatorial MRG response in the low troposphere during El Niño than those during La Niña. In addition, the strength of the mean flow can strongly affect the extent of equatorial wave response and modulate its phase and group velocity due to the Doppler shift effect.

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Guanghua Chen, Yukari N. Takayabu, and Chie Yokoyama

Abstract

Using 10-yr high-resolution satellite and reanalysis data, the synoptic-scale dual structure of precipitable water (PW), in which the southern and northern bands straddled at the ITCZ produce zonally propagating meridional dipoles, is observed over the eastern Pacific (EP) during boreal summer and fall. Composites indicate that the PW dipole, concurrent with the dipole-like filtered divergence, has a shift to the west of the anomalously cyclonic circulation. The vertical structure of filtered meridional wind is characterized by a wavenumber-1 baroclinic mode, and the vertical motion has two peaks situated at 850 and 300 hPa, respectively. To the east of the PW dipole, the shallow convection is embedded within the deep convection, forming a multilevel structure of meridional wind on the ITCZ equatorward side. To the west of the PW dipole, the deep convection tends to be suppressed because of the invasion of midlevel dry air advected by northerly flows. The generation and propagation of the dual PW band can be attributed to the divergence and advection terms related to specific humidity and three-dimensional wind. By comparison, the PW anomalies over the western North Pacific, only exhibiting a single band, coincide with the centers of synoptic disturbances with a barotropic vertical structure. Because of the weakening of lower-level divergence, the vertical motion, and the horizontal gradient of PW, the synoptic-scale PW signal is reduced significantly. The typical cases and statistics confirm that the strong meridional dipoles and westward-propagating disturbances are closely associated with the distortion and breakdown of ITCZ over the EP.

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