Search Results

You are looking at 71 - 80 of 225 items for

  • Author or Editor: Bin Wang x
  • Refine by Access: All Content x
Clear All Modify Search
Haiyan Teng and Bin Wang

Abstract

A finite-domain wavenumber–frequency analysis was proposed to objectively measure the interannual variability of the boreal summer intraseasonal oscillation (ISO) in the Asian–Pacific region. The strongest interannual variations of the ISO are found in the off-equatorial western North Pacific (WNP). In summers when El Niño is developing, both the westward- and northward-propagating waves with periods of 15–40 and 8–10 days are enhanced in July–October. The northward-propagating ISO in the Indian summer monsoon region, however, has little linkage with El Niño–Southern Oscillation (ENSO).

ENSO affects the northwestward-propagating ISO mode in the WNP through changing the mean circulation. During July–October in the El Niño developing year, the easterly vertical shears over the tropical western Pacific are considerably increased, which in turn promote development and northwestward emanation of Rossby waves away from the equatorial western-central Pacific, reinforcing the WNP ISO. In the Indian summer monsoon region, the ENSO-induced circulation changes are too weak to significantly modify the strong easterly sheared monsoon mean circulation. Therefore, the northward-propagating ISO is insensitive to ENSO.

Unlike the wintertime Madden–Julian oscillation (MJO), which is uncorrelated with ENSO, the May–July MJO is strengthened during El Niño developing years. The questions of why there is a seasonal dependence of the MJO–ENSO relationship and how ENSO directly affects the May–July MJO require further investigations.

Full access
GARY GRUNSEICH and BIN WANG

Abstract

Prediction of the arctic annual sea ice minimum extent and melting patterns draws interest from numerous industries and government agencies but has been an ongoing challenge for forecasters and climate scientists using statistical and dynamical models. Using the dominant independent modes of interannual sea ice concentration (SIC) variability during September–October, a new approach combining statistical analysis with physically derived links to natural climate variability sources is used to predict each mode and the total anomaly pattern. Sea ice patterns associated with each mode are predominantly shaped by the wind-driven advective convergence, forced by circulation anomalies associated with local and remote forms of naturally occurring climate variability. The impacts of the Arctic Oscillation, beginning from the preceding winter, control the leading mode of SIC variability during the annual minimum. In the three final months of the melting period, the broad impacts of the Indian and East Asian summer monsoons produce unique SIC impacts along the arctic periphery, displayed as the second and third modes, respectively. El Niño–Southern Oscillation (ENSO) largely shapes the fourth SIC mode patterns through influencing variability early in the melting period. Using physically meaningful and statistically significant predictors, physical–empirical (P–E) models are developed for each SIC mode. Some predictors directly account for the circulation patterns driving anomalous sea ice, while the monsoon-related predictors convey early season sources of monsoonal variability, which subsequently influences the Arctic. The combined SIC predictions of the P–E models exhibit great skill in matching the observed magnitude and temporal variability along the arctic margins during the annual minimum.

Full access
Fei Liu and Bin Wang

Abstract

The impact of horizontal advection of seasonal-mean moisture (SMM) on Madden–Julian oscillation (MJO) dynamics is investigated here using a theoretical model that includes moisture advection processes. The zonal advection of SMM with an eastward gradient is found to produce planetary-scale instability and promote slow eastward propagation corresponding to an intraseasonal periodicity. This is because the SMM advection by an anomalous easterly of the Kelvin waves generates a moisture source to the east of precipitation, which favors eastward propagation and unstable growth. On the other hand, the advection of SMM with a westward gradient results in a westward-propagating unstable mode. For a realistic SMM distribution, the simulated eastward propagation is enhanced over the Indo-Pacific warm pool, while the westward propagation prevails over the central-eastern Pacific. In contrast to the zonal advection of SMM, the meridional advection of SMM only affects short waves and leaves planetary waves nearly unaffected.

The effect of zonal advection of SMM suggests an important mechanism for explaining the eastward propagation and growth of the MJO over the Indo-Pacific warm pool when the SMM increases eastward. However, this mechanism alone produces unrealistic Kelvin wave–like structure and strong westward propagation in the central-eastern Pacific; both disagree with observations. These caveats, however, can be remitted if the planetary boundary layer (PBL) moisture convergence feedback is included, which couples the Kelvin wave and the Rossby wave via precipitation heating, producing a realistic horizontal structure and also substantially suppressing the unrealistically growing, westward-propagating mode in the central-eastern Pacific.

Full access
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.

Full access
Fei Liu and Bin Wang

Abstract

During late boreal summer (July–October), the intraseasonal oscillation (ISO) exhibits maximum variability over the western North Pacific (WNP) centered in the South China Sea and Philippine Sea, but many numerical models have difficulty in simulating this essential feature of the ISO. To understand why this maximum variability center exists, the authors advance a simple box model to elaborate the potential contribution of the mean-state-dependent atmosphere–ocean interaction. The model results suggest that the WNP seasonal mean monsoon trough plays an essential role in sustaining a strong stationary ISO, contributing to the existence of the maximum intraseasonal variability center. First, the monsoon trough provides abundant moisture supply for the growing ISO disturbances through the frictional boundary layer moisture convergence. Second, the cyclonic winds associated with the monsoon trough provide a favorable basic state to support a negative atmosphere–ocean thermodynamic feedback that sustains a prominent stationary ISO. In an active phase of the ISO, anomalous cyclonic winds enhance the monsoon trough and precipitation, which reduce shortwave radiation flux and increase evaporation; both processes cool the sea surface and lead to an ensuing high pressure anomaly and a break phase of the ISO. In the wintertime, however, the wind–evaporation feedback is positive and sustains the Philippine Sea anticyclone. The result here suggests that accurate simulation of the boreal summer climatological mean state is critical for capturing a realistic ISO over the WNP region.

Full access
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.

Open access
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.

Full access
Fei Liu and Bin Wang

Abstract

This work is an extension and improvement of the minimal Madden–Julian oscillation (MJO) “skeleton” model developed by Majda and Stechmann, which can capture some important features of the MJO—slow eastward propagation, quadrupole-vortex structure, and independence of frequency on wavelength—but is unable to produce unstable growth and selection of eastward-propagating planetary waves. With the addition of planetary boundary layer frictional moisture convergence, these deficiencies can be remedied. The frictional boundary layer “selects” the planetary-scale eastward propagation as the most unstable mode, but the dynamics remains confined to atmospheric processes only. Here the authors study the role of air–sea interaction by implementing an oceanic mixed-layer (ML) model of Wang and Xie into the MJO skeleton model. In this new air–sea coupled skeleton model, the features of the original skeleton model remain; additionally, the air–sea interaction under mean westerly winds is shown to produce a strong instability that selectively destabilizes the eastward-propagating planetary-scale waves. Although the cloud–shortwave radiation–sea surface temperature (CRS) feedback destabilizes both eastward and westward modes, the air–sea feedback associated with the evaporation and oceanic entrainment favors planetary-scale eastward modes. Over the Western Hemisphere where easterly background winds prevail, the evaporation and entrainment feedbacks yield damped modes, indicating that longitudinal variation of the mean surface winds plays an important role in regulation of the MJO intensity in addition to the longitudinal variation of the mean sea surface temperature or mean moist static stability. This theoretical analysis suggests that accurate simulation of the climatological mean state is critical for capturing the realistic air–sea interaction and thus the MJO.

Full access
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).

Full access
Bin Wang and Xihua Xu

Abstract

Using climatological pentad mean outgoing longwave radiation (OLR) and European Centre for Medium-Range Weather Forecasts analysis winds, the authors show that the Northern Hemisphere summer monsoon displays statistically significant climatological intraseasonal oscillations (CISOs). The extreme phases of CISO characterize monsoon singularities—monsoon events that occur on a fixed pentad with usual regularity, whereas the transitional phases of CISO represent the largest year-to-year monsoon variations.

The CISO results from a phase-locking of transient intraseasonal oscillation to annual cycle. It exhibits a dynamically coherent structure between enhanced convection and low-level convergent (upper-level divergent) cyclonic (anticyclonic) circulation. Its phase propagates primarily northward from the equator to the northern Philippines during early summer (May–July), and westward along 15°N from 170°E to the Bay of Bengal during August and September.

The propagation of CISO links monsoon singularities occurring in different regions. Four CISO cycles are identified from May to October. The first cycle has a peak wet phase in mid-May that starts the monsoon over the South China Sea and Philippines. Its dry phase in late May and early June brings the premonsoon dry weather over the regions of western North Pacific summer monsoon (WNPSM), Meiyu/Baiu, and Indian summer monsoon (ISM). The wet phase of Cycle II peaking in mid-June marks the onsets of WNPSM, continental ISM, and Meiyu, whereas the dry phase in early to mid-July corresponds to the first major breaks in WNPSM and ISM, and the end of Meiyu. The wet phase of Cycle III peaking in mid-August benchmarks the height of WNPSM, which was followed by a conspicuous dry phase propagating westward and causing the second breaks of WNPSM (in early September) and ISM (in mid-September). The wet phase of Cycle IV represents the last active WNPSM and withdrawal of ISM in mid-October.

The relationships among ISM, WNPSM, and East Asian Subtropical Monsoon (EASM) are season dependent. During Cycle II, convective activities in the three monsoon regions are nearly in phase. During Cycle III, however, the convective activities are out of phase between ISM and WNPSM; meanwhile, little linkage exists between WNPSM and EASM. The causes of unstable relationships and the phase propagation of CISO are discussed.

Full access