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Liguang Wu and Bin Wang

Abstract

The recently reported increase in the proportion of intense hurricanes is considerably larger than those projected by the maximum potential intensity (MPI) theory and the results of numerical simulation. To reconcile this discrepancy, the authors examined the best-track datasets for the North Atlantic (NA), western North Pacific (WNP), and eastern North Pacific (ENP) basins. It was found that the changes in the tropical cyclone formation locations and prevailing tracks may have contributed to the changes in the proportion of the intense hurricanes over the past 30 yr. The authors suggest that the changes in the formation locations and prevailing tracks have a profound impact on the basinwide tropical cyclone intensity. Thus, how the atmospheric circulation in the tropical cyclone basins responds to the global warming may be a critical factor in understanding the impacts of global warming on tropical cyclone intensity.

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Bin Wang Xiaofan Li

Abstract

The beta effect on translation of cyclonic and anticyclonic vortices with height-dependent circulation (the beta-drift problem) is investigated via numerical experiments using a dry version of a multilevel primitive equation model (Florida State University model).

The vertical structure of vortex circulation influences steady translation in a manner similar to that of the horizontal structure. Both spatially change the mean relative angular momentum (MRAM) of the vortex. The translation speed and its meridional component are both approximately proportional to the square root of the magnitude of MRAM of the initial (or quasi-steady-state) symmetric circulation. The latitude is another important factor controlling the speed of the beta drift. The meridional component decreases by about 45% when the central latitude of the vortex increases from 10° to 30°N.

The beta-drift speed is intimately related to the axially asymmetric pressure field. During quasi-steady vortex translation the asymmetric pressure field maintains a stationary wavenumber 1 pattern in azimuthal direction with a high in the northeast and a low in the southwest quadrant of a Northern Hemisphere cyclone. The beta-drift velocity is approximately equal to the geostrophic flow implied by the asymmetric pressure gradient at the vortex center. If the Rossby number associated with the asymmetric flow is small, to the lowest order, the asymmetric pressure gradient force at the vortex center is balanced by the Coriolis force associated with the beta drift of the vortex.

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Fei Liu and Bin Wang

Abstract

The eastward-propagating tropical low-frequency disturbances, such as the moist Kelvin waves or the Madden–Julian oscillation (MJO), are often observed to experience convective enhancement when meeting with the westward-propagating 2-day waves. A scale interaction (SI) model is built to understand the nature of the interaction between the 2-day waves and moist Kelvin waves or MJO. In this model, the convective complex of moist Kelvin waves modulates the strength and location of the 2-day waves, which feed back through the upscale eddy transfer. An ageostrophic model describing the 2-day waves is first solved, and the resultant westward-propagating, backward-tilted disturbances are consistent with the observed 2-day waves. An explicit representation of eddy momentum transfer (EMT), eddy heating transfer (EHT), and eddy moisture transfer (EQT) arising from the 2-day waves is then formulated. The SI model shows that the 2-day waves in front of moist Kelvin waves produce an EMT accelerating the low-frequency easterly in the lower troposphere, an EHT cooling down the middle troposphere, and an EQT moistening the middle troposphere. These three transfer terms have comparable magnitude. Although the negative EHT tends to damp the moist Kelvin waves, both the EMT and EQT provide instability sources for the moist Kelvin waves. The 2-day waves also slow down the moist Kelvin waves, mainly through the advective effects of the EMT. So the unstable moist Kelvin waves may exhibit convective enhancement when meeting with the 2-day waves. The theoretical results presented here point to the need to further observe the multiscale structures within the moist Kelvin waves and the MJO.

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Fei Liu and Bin Wang

Abstract

The Madden–Julian oscillation (MJO) is a multiscale system. A skeleton model, developed by Majda and Stechmann, can capture some of planetary-scale aspects of observed features such as slow eastward propagation, nondispersive behavior, and quadrupole-vortex structure. However, the Majda–Stechmann model cannot explain the source of instability and the preferred planetary scale of the MJO. Since the MJO major convection region is leaded by its planetary boundary layer (PBL) moisture convergence, here a frictional skeleton model is built by implementing a slab PBL into the neutral skeleton model. As a skeleton model allowing the scale interaction, this model is only valid for large-scale waves. This study shows that the PBL frictional convergence provides a strong instability source for the long eastward modes, although it also destabilizes very short westward modes. For the long waves (wavenumber less than 5), the PBL Ekman pumping moistens the low troposphere to the east of the MJO convective envelope, and sets up favorable moist conditions to destabilize the MJO and favor only eastward modes. Sensitivity experiments show that a weak PBL friction will enhance the instability slightly. The sea surface temperature (SST) with a maximum at the equator also prefers the long eastward modes. These theoretical analysis results encourage further observations on the PBL regulation of mesosynoptic-scale motions, and exploration of the interaction between PBL and multiscale motions, associated with the MJO to improve the MJO simulation in general circulation models (GCMs).

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Renguang Wu and Bin Wang

Abstract

Climatological summer monsoon onset over the South China Sea (SCS) and the western North Pacific (WNP) (defined as the region of 10°–20°N, 120°–160°E) displays three distinct stages. Around mid-May, monsoon rain commences in the SCS and the Philippines. In early to mid-June, the monsoon rain extends to the southwestern Philippine Sea. After mid-July, the rainy season starts in the northeastern part of the WNP. The onset anomaly, however, displays an in-phase interannual variation across the entire WNP domain. The standard deviation of the onset date increases eastward from 3 pentads in the SCS to 5 pentads in the northeastern part of the domain. The large onset variability in the WNP is mainly attributed to large year-to-year changes of the seasonal cycle. The role of the intraseasonal oscillation is secondary but important especially in the SCS region. The El Niño–Southern Oscillation (ENSO)-related thermal contrast between the WNP and the equatorial central Pacific modulates significantly the seasonal migration of the monsoon trough, the subtropical high, and the convection zone over the WNP during late spring–early summer in the ENSO decay phase. Thus, ENSO plays a dominant role in the interannual variation of the WNP summer monsoon onset.

The general circulation model results suggest that during El Niño events, the warm SST anomalies in the equatorial eastern-central Pacific play a major role in generation of large-scale upper-level convergence and descent anomalies over the WNP. Meanwhile, the cold SST anomalies in the WNP induce lower-level anticyclonic wind anomalies and reduce convective instability. Both the remote and local SST forcing are important for delaying the seasonal movement of the monsoon trough and the western Pacific subtropical high and hence the onset of the monsoon rain. In the La Niña case, the local warm SST anomalies in the WNP are more important than the cold SST anomalies in the equatorial eastern-central Pacific in the generation of lower-level cyclonic wind anomalies and enhancement of convective instability.

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Zhiling Xie and Bin Wang

Abstract

Multiple bias-corrected top-quality reanalysis datasets, gauge-based observations, and selected satellite products are synthetically employed to revisit the climatology and variability of the summer atmospheric heat sources over the Tibetan Plateau (TP). Verification-based selection and ensemble-mean methods are utilized to combine various datasets. Different from previous works, this study pays special attention to estimating the total heat source (TH) and its components over the data-void western plateau (70°–85°E), including the surface sensible heat (SH), latent heat released by precipitation (LH), and net radiation flux (RD). Consistent with previous studies, the climatology of summer SH (LH) typically increases (decreases) from southeast to northwest. Generally, LH dominates TH over most of the TP. A notable new finding is a minimum TH area over the high-altitude region of the northwestern TP, where the Karakoram mountain range is located. We find that during the period of 1984–2006, TH shows insignificant trends over the eastern and central TP, whereas it exhibits an evident increasing trend over the western TP that is attributed to the rising tendency of LH before 1996 and to that of RD after 1996. The year-to-year variation of TH over the central–eastern TP is highly correlated with that of LH, but that is not the case over the western TP. It is also worth noting that the variations of TH in each summer month are not significantly correlated with each other, and hence study of the interannual variation of the TP heat sources should consider the remarkable subseasonal variations.

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Guosen Chen and Bin Wang

ABSTRACT

The eastward propagating Madden–Julian oscillation (MJO) events exhibit various speeds ranging from 1 to 9 m s−1, but what controls the propagation speed remains elusive. This study attempts to address this issue. It reveals that the Kelvin wave response (KWR) induced by the MJO convection is a major circulation factor controlling the observed propagation speed of the MJO, with a stronger KWR corresponding to faster eastward propagation. A stronger KWR can accelerate the MJO eastward propagation by enhancing the low-level premoistening and preconditioning to the east of the MJO deep convection. The strength of the KWR is affected by the background sea surface temperature (SST). When the equatorial central Pacific SST warms, the zonal scale of the Indo-Pacific warm pool expands, which increases the zonal scale of the MJO, favoring enhancing the KWR. This effect of warm-pool zonal scale has been verified by idealized experiments using a theoretical model. The findings here shed light on the propagation mechanism of the MJO and provide a set of potential predictors for forecasting the MJO propagation.

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Xiaofan Li and Bin Wang

Abstract

The movement of a symmetric vortex embedded in a resting environment with a constant planetary vorticity gradient (the beta drift) is investigated with a shallow-water model. The authors demonstrate that, depending on initial vortex structure, the vortex may follow a variety of tracks ranging from a quasi-steady displacement to a wobbling or a cycloidal track due to the evolution of a secondary asymmetric circulation. The principal part of the asymmetric circulation is a pair of counterrotating gyres (referred to as beta gyres), which determine the steering flow at the vortex center. The evolution of the beta gyres is characterized by development/decay, gyration, and radial movement.

The beta gyres develop by extracting kinetic energy from the symmetric circulation of the vortex. This energy conversion is associated with momentum advection and meridional advection of planetary vorticity. The latter (referred to as “beta conversion”) is a principal process for the generation of asymmetric circulation. A necessary condition for the development of the beta gyres is that the anticyclonic gyre must be located to the east of a cyclonic vortex center. The rate of asymmetric kinetic energy generation increases with increasing relative angular momentum of the symmetric circulation.

The counterclockwise rotation of inner beta gyres (the gyres located near the radius of maximum wind) is caused by the advection of asymmetric vorticity by symmetric cyclonic flows. On the other hand, the clockwise rotation of outer beta gyres (the gyres near the periphery of the cyclonic azimuthal wind) is determined by concurrent intensification in mutual advection of the beta gyres and symmetric circulation and weakening in the meridional advection of planetary vorticity by symmetric circulation. The outward shift of the outer beta gyres is initiated by advection of symmetric vorticity by beta gyres relative to the drifting velocity of the vortex.

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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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Juan Li and Bin Wang

Abstract

The present study aims to explore the origins of decadal predictability of East Asian land summer monsoon rainfall (EA-LR) and estimate its potential decadal predictability. As a preliminary study, a domain-averaged EA-LR index (EA-LI) is targeted as it represents the leading mode of variability reasonably well. It is found that the decadal variations of EA-LI are primarily linked to a cooling over the central-eastern tropical Pacific (CEP) and a warming over the extratropical North Pacific and western tropical Pacific (NWP) during May–October. Two numerical experiments suggest that the CEP cooling may be a major driver of EA-LR, while the NWP warming, which is largely a response, cannot be treated as a forcing to EA-LR. However, this does not mean that the NWP sea surface temperature anomalies (SSTAs) play no role. To elaborate on this point, a third experiment is conducted in which the observed cooling is nudged in the CEP but the SST is nudged to climatology in the NWP (i.e., atmosphere–ocean interaction is not allowed). The result shows anomalous northerlies and decreased rainfall over East Asia. Results of the three experiments together suggest that both the forcings from the CEP and the atmosphere–ocean interaction in the NWP are important for EA-LR. Assuming that the tropical and North Pacific SSTAs can be “perfectly” forecasted, the so-called perfect prediction of EA-LI, which is achieved by a physics-based empirical model, yields a significant temporal correlation coefficient skill of 0.70 at a 7–10-yr lead time during a 40-yr independent hindcast (1968–2009), providing an estimation of the lower bound of potential decadal predictability of EA-LI.

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