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Bin Wang and LinHo

Abstract

To date, the monsoon-research community has not yet reached a consensus on a unified definition of monsoon rainy season or on the linkage between the onsets over the Asian continent and the adjacent oceans. A single rainfall parameter is proposed, and a suite of universal criteria for defining the domain, onset, peak, and withdrawal of the rainy season are developed. These results reveal a cohesive spatial–temporal structure of the Asian–Pacific monsoon rainy season characteristics, which will facilitate validation of monsoon hydrological cycles simulated by climate system models and improve our understanding of monsoon dynamics.

The large-scale onset of the Asian monsoon rainy season consists of two phases. The first phase begins with the rainfall surges over the South China Sea (SCS) in mid-May, which establishes a planetary-scale monsoon rainband extending from the south Asian marginal seas (the Arabian Sea, the Bay of Bengal, and the SCS) to the subtropical western North Pacific (WNP). The rainband then advances northwestward, initiating the continental Indian rainy season, the Chinese mei-yu, and the Japanese baiu in early to mid-June (the second phase). The heights of the rainy seasons occur primarily in three stepwise phases: in late June over the mei-yu/baiu regions, the northern Bay of Bengal, and the vicinity of the Philippines, in late July over India and northern China; and in mid-August over the tropical WNP. The rainy season retreats northward over east Asia, yet it moves southward over India and the WNP.

Clear distinctions in the characteristics of the rainy season exist among the Indian, east Asian, and WNP summer monsoon regions. Nevertheless, the rainy seasons of the three subsystems also show close linkage. The causes of the regional distinctions and linkages are discussed. Also discussed are the atypical monsoon rainy seasons, such as the skewed and bimodal seasonal distributions found in various places of Asian monsoon domain.

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Fei Liu and Bin Wang

Abstract

This study investigates the moisture and wave feedbacks in the Madden–Julian oscillation (MJO) dynamics by applying the general three-way interaction theoretical model. The three-way interaction model can reproduce observed large-scale characteristics of the MJO in terms of horizontal quadrupole-vortex structure, vertically tilted structure led by planetary boundary layer (PBL) convergence, slow eastward propagation with a period of 30–90 days, and planetary-scale circulation. The moisture feedback effects can be identified in this model by using diagnostic thermodynamic and momentum equations, and the wave feedback effects are investigated by using a diagnostic moisture equation. The moisture feedback is found to be responsible for producing the MJO dispersive modes when the convective adjustment process is slow. The moisture feedback mainly acts to reduce the frequency and growth rate of the short waves, while leaving the planetary waves less affected, so neglecting the moisture feedback is a good approximation for the wavenumber-1 MJO. The wave feedback is shown to slow down the eastward propagation and increase the growth rate of the planetary waves. The wave feedback becomes weak when the convective adjustment time increases, so neglecting the wave feedback is a good approximation for the MJO dynamics during a slow adjustment process. Sensitivities of these two feedbacks to other parameters are also discussed. These theoretical findings suggest that the two feedback processes, and thus the behaviors of the simulated MJO mode, should be sensitive to the parameters used in cumulus parameterizations.

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Liguang Wu and Bin Wang

Abstract

In order to understand the roles of various physical processes in baroclinic tropical cyclone (TC) motion and the vertical coupling between the upper- and lower-level circulations, a new dynamical framework is advanced. A TC is treated as a positive potential vorticity (PV) anomaly from environmental flows, and its motion is linked to the positive PV tendency. It is shown that a baroclinic TC moves to the region where the azimuthal wavenumber one component of the PV tendency reaches a maximum, but does not necessarily follow the ventilation flow (the asymmetric flow over the TC center). The contributions of individual physical processes to TC motion are equivalent to their contributions to the wavenumber one PV component of the PV tendency. A PV tendency diagnostic approach is described based on this framework. This approach is evaluated with idealized numerical experiments using a realistic hurricane model. The approach is capable of estimating TC propagation with a suitable accuracy and determining fractional contributions of individual physical processes (horizontal and vertical advection, diabatic heating, and friction) to motion. Since the impact of the ventilation flow is also included as a part of the influence of horizontal PV advection, this dynamical framework is more general and particularly useful in understanding the physical mechanisms of baroclinic and diabatic TC motion.

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Guosen Chen and Bin Wang

Abstract

Current theoretical studies have a debate on whether the Madden–Julian oscillation (MJO) has a zero or westward group velocity. A recent analysis of the observed Hovmöller diagram of MJO signals suggested that the MJO has a significant westward group velocity. Here it is shown that the observed MJO has a negligibly small group velocity, which is manifested in two aspects. First, on the wavenumber–frequency spectra diagram the precipitation spectra indicate quasi independence of the MJO frequency on wavenumber, suggesting a nearly vanishing group velocity. Second, on the Hovmöller diagram of the regressed intraseasonal daily precipitation, the MJO group velocity is defined by the propagation of the wave envelopes of the precipitation and is shown to be negligibly small for the eastward propagating signals. The causes of the discrepancy between this study and the recent study mentioned above are the calculating method and the data filtering process. The group velocity in the recent study is calculated by the propagation of local convection extrema, which does not necessarily indicate the propagation of the wave envelopes. More importantly, the westward propagation of the local convection extrema is an artifact of the data filtering. The Hovmöller diagram in the recent study was constructed by using only the eastward propagating wavenumber-1–5 signals. This truncation of data onto the planetary scales of the eastward wavenumber domain fails to resolve the Maritime Continent “barrier effect,” causing significant artificial westward propagation of local convection extrema.

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Guosen Chen and Bin Wang

Abstract

Well-organized eastward propagation of the Madden–Julian oscillation (MJO) is found to be accompanied by the leading suppressed convection (LSC) over the Maritime Continent (MC) and the western Pacific (WP) when the MJO convection is in the Indian Ocean (IO). However, it remains unclear how the LSC influences the MJO and what causes the LSC. The present study shows that the LSC is a prevailing precursor for eastward propagation of the MJO across the MC. The LSC enhances the coupling of IO convection and the Walker cell to its east [front Walker cell (FWC)] by increasing the zonal heating gradient. The enhanced FWC strengthens the low-level easterly, which increases boundary layer (BL) convergence and promotes congestus convection to the east of the deep convection; the enhanced congestus convection preconditions the lower to middle atmosphere, which further promotes the transition from congestus to deep convection and leads to eastward propagation of the MJO. The MJO ceases eastward propagation once the FWC decouples from it. Further analysis reveals that LSC has two major origins: one comes from the eastward propagation of the preceding IO dry phase associated with the MJO, and the other develops concurrently with the IO convection. In the latter case, the development of the LSC is brought about by a two-way interaction between the MJO’s tropical heating and the associated tropical–extratropical teleconnection: the preceding IO suppressed convection induces a tropical–extratropical teleconnection, which evolves and forms an anomalous western North Pacific cyclone that generates upper-level convergence and induces significant LSC.

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Bin Wang and Hualan Rui

Abstract

A simple theoretical analysis on the stability of a resting tropical atmosphere to semigeostrophic perturbations is given using a free atmosphere–boundary layer coupled model on an equatorial β-plane.

An unstable mode emerges when sea surface temperature is higher than a critical value. The growing mode is a moist Kelvin wave modified through coupling with a Rossby wave of the lowest meridional index. The modified Rossby modes, however, remain damped even for high SST. The unstable mode selection can be explained in terms of wave energy generation due to the latent heating induced by frictional moisture convergence.

The horizontal mode-coupling has profound impacts on wave instability. It favors the amplification of long planetary-scale waves, slows down eastward propagation, and suppresses unrealistically fast growth of the uncoupled moist Kelvin mode by creating substantial meridional flows. These effects make the coupled unstable mode more resemble observed equatorial intraseasonal disturbances.

The results also demonstrate that when maximum SST moves from the equator to 7.5°N, the growth rate of the unstable wave is significantly reduced, suggesting that the annual march of the “thermal equator” and associated convective heating is likely responsible for annual variations of the equatorial 40–50 day wave activity.

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Bin Wang and Tianming Li

Abstract

Tropical boundary-layer flows interact with the free tropospheric circulation and underlying sea surface temperature, playing a critical role in coupling collective effects of cumulus heating with equatorial dynamics. In this paper a unified theoretical framework is developed in which convective interaction with large-scale circulation includes three mechanisms: convection–wave convergence (CWC) feedback, evaporation–wind (EW) feedback, and convection–frictional convergence (CFC) feedback. We examine the dynamic instability resulting from the convective interaction with circulation, in particular the role of CFC feedback mechanism.

CFC feedback results in an unstable mode that has distinctive characteristics from those occurring from CWC feedback or EW feedback in the absence of mean flow. The instability generated by CFC feedback is of low frequency with a typical growth rate on an order of 10−6 s−1. The unstable mode is a multiscale wave packet; a global-scale circulation couples with a large-scale (several thousand kilometers) convective complex. The complex is organized by boundary-layer convergence and may consist of a few synoptic-scale precipitation cells. The heating released in the complex in turn couples the moist Kelvin wave and the Rossby wave with the gravest meridional structure, forming a dispersive system. The energy propagates slower than the individual cells within the wave packet. A transient boundary layer is shown to favor planetary-scale instability due to the fractionally created phase shift between the maximum vertical motion and the heating associated with boundary-layer convergence.

The implications of the theory to the basic dynamics of tropical intraseasonal oscillation are discussed.

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Baozhen Zhu and Bin Wang

Abstract

The tropical Indian and western Pacific oceans are two prominent action centers for tropical 30–60-day convective variability. When convection is enhanced over the equatorial Indian Ocean, the tropical western Pacific often experiences an abnormal dry condition (phase I), whereas the development of the convection over the tropical western Pacific tends to be accompanied by suppressed convection in the equatorial Indian Ocean (phase II). This convection seesaw is a fundamental characteristic of the tropical 30–60-day oscillation.

The seesaw is intimately associated with the activity of propagating low-frequency convective systems (LFCSs). Its formation process is season dependent. Typical boreal summer seesaw results from a time-lagged development of two systems: a western system that originates in the equatorial Indian Ocean and moves eastward and/or northward and an eastern system that develops in the western Pacific monsoon region and moves westward and/or northward. The boreal winter seesaw, on the other hand, is caused by the longitudinal dependence of the evolution of eastward-moving LFCSs that strongly amplify in the equatorial Indian Ocean, weaken and/or split when rapidly passing over the maritime continent, and reintensify in the South Pacific convergence zone (SPCZ).

There are two phases of the seesaw. During the first phase, the LFCSs interact with the Indian monsoon in boreal summer and Indonesian–Australian monsoon in boreal winter. Likewise, during the second phase, the LFCSs interplay with monsoon circulations over the western Pacific monsoon trough in boreal summer and over the SPCZ in boreal winter. The convection seesaw activity is closely tied to the corresponding active-break monsoon cycles over the two polar regions of the seesaw.

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Xiaosu Xie and Bin Wang

Abstract

The stability of equatorial Rossby waves in the presence of mean flow vertical shear and moisture convergence-induced heating is investigated with a primitive equation model on an equatorial β plane.

A vertical shear alone can destabilize equatorial Rossby waves by feeding mean flow available potential energy to the waves. This energy transfer necessitates unstable waves’ constant phase lines tilt both horizontally (eastward with latitude) and vertically (against the shear). The preferred most unstable wavelength increases with increasing vertical shear and with decreasing heating intensity, ranging typically from 3000 to 5000 km. The instability strongly depends on meridional variation of the vertical shear. A broader meridional extent of the shear allows a faster growth and a less-trapped meridional structure. When the shear is asymmetric relative to the equator, the unstable Rossby wave is constrained to the hemisphere where the shear is prominent. Without boundary layer friction the Rossby wave instability does not depend on the sign of the vertical shear, whereas in the presence of the boundary layer, the moist Rossby wave instability is remarkably enhanced (suppressed) by easterly (westerly) vertical shears. This results from the fact that an easterly shear confines the wave to the lower level, generating a stronger Ekman-pumping-induced heating and an enhanced meridional heat flux, both of which reinforce the instability.

The moist baroclinic instability is a mechanism by which westward propagating rotational waves (Rossby and Yanai waves) can be destabilized, whereas Kelvin waves cannot. This is because the transfer of mean potential energy to eddy requires significant magnitude of barotropic motion. The latter is a modified Rossby wave and can be resonantly excited only by the westward propagating rotational waves. The common features and differences of the equatorial Rossby wave instability and midlatitude baroclinic instability, as well as the implications of the results are discussed.

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Li Dong and Bin Wang

Abstract

A Lagrangian linear advection scheme, which is called the trajectory-tracking scheme, is proposed in this paper. The continuous tracer field has been discretized as finite tracer parcels that are points moving with the velocity field. By using the inverse distance weighted interpolation, the density carried by parcels is mapped onto the fixed Eulerian mesh (e.g., regular latitude–longitude mesh on the sphere) where the result is rendered. A renormalization technique has been adopted to accomplish mass conservation on the grids. The major advantage of this scheme is the ability to preserve discontinuity very well. Several standard tests have been carried out, including 1D and 2D Cartesian cases, and 2D spherical cases. The results show that the spurious numerical diffusion has been eliminated, which is a potential merit for the atmospheric modeling.

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