Search Results

You are looking at 131 - 140 of 225 items for

  • Author or Editor: Bin Wang x
  • Refine by Access: All Content x
Clear All Modify Search
Alejandro Ludert, Bin Wang, and Mark A. Merrifield

Abstract

The U.S.-Affiliated Pacific Islands (USAPIs), located in the tropical western Pacific, are very susceptible to severe drought. Dry season (December–May) rainfall anomalies have different relationships to ENSO for USAPIs north and south of 7°N. South of 7°N, rainfall exhibits a canonical negative correlation with the Oceanic Niño Index (ONI) (i.e., dry conditions during warm periods). To the north, the dry season falls into either “canonical” or “noncanonical” (positively correlated with ONI) regimes. Noncanonical years pose an important forecasting challenge as severe droughts have occurred during cool ONI conditions (referred to here as “cool dry” cases). Composite analysis of the two regimes shows that for noncanonical cool dry years, anticyclonic circulation anomalies over the tropical western North Pacific (TWNP), with a band of anomalous dry conditions extending from the central Pacific toward Micronesia, result in unexpected droughts. In contrast, canonical “cool wet” events show cyclonic TWNP circulation and increased rainfall over the northern USAPIs. Maximum SST anomalies are located near the date line during noncanonical years, and farther east during canonical years. While both regimes show negative rainfall and TWNP anticyclonic circulation anomalies before the onset of the December–May dry season, during the dry season these anomalies persist during noncanonical events but rapidly reverse sign during canonical events. SST anomalies in the noncanonical regime extend eastward from the central Pacific rather than intensify in place over the eastern Pacific in the canonical regime. Differences in the evolution of circulation, precipitation, and SST anomalies suggest distinct physical mechanisms governing the two ENSO regimes, with possible ramifications for seasonal forecasts.

Full access
Chengsi Liu, Qingnong Xiao, and Bin Wang

Abstract

An ensemble-based four-dimensional variational data assimilation (En4DVAR) algorithm and its performance in a low-dimension space with a one-dimensional shallow-water model have been presented in Part I. This algorithm adopts the standard incremental approach and preconditioning in the variational algorithm but avoids the need for a tangent linear model and its adjoint so that it can be easily incorporated into variational assimilation systems. The current study explores techniques for En4DVAR application in real-dimension data assimilation. The EOF decomposed correlation function operator and analysis time tuning are formulated to reduce the impact of sampling errors in En4DVAR upon its analysis. With the Advanced Research Weather Research and Forecasting (ARW-WRF) model, Observing System Simulation Experiments (OSSEs) are designed and their performance in real-dimension data assimilation is examined. It is found that the designed En4DVAR localization techniques can effectively alleviate the impacts of sampling errors upon analysis. Most forecast errors and biases in ARW are reduced by En4DVAR compared to those in a control experiment. En3DVAR cycling experiments are used to compare the ensemble-based sequential algorithm with the ensemble-based retrospective algorithm. These experiments indicate that the ensemble-based retrospective assimilation, En4DVAR, produces an overall better analysis than the ensemble-based sequential algorithm, En3DVAR, cycling approach.

Full access
Peng Zhang, Bin Wang, and Zhiwei Wu

Abstract

According to the sea surface temperature anomaly (SSTA) intensity in the Niño-3.4 region and the east–west gradient across the Pacific, three types of El Niño are identified in this work. An event with larger than average intensity is defined as a strong El Niño, all others are considered to be weak events. Almost all strong El Niños are concurrent with a large gradient, which is featured by negative SSTAs in the western Pacific and positive SSTAs in the equatorial eastern Pacific (EP) and Indian Ocean (IO). According to the east–west gradient, the weak events can be subdivided into gradient-weak (GW) El Niño and equatorial-weak (EW) El Niño. The GW El Niño characterizes a great east–west gradient without a significant IO SSTA. In contrast, the EW event features a positive SSTA over the tropical IO and EP. The impact of GW El Niño on the North Atlantic–Eurasia continent (NA–Eurasia) displays a negative North Atlantic Oscillation (NAO)-like atmospheric anomaly, resulting in a drier and cooler-than-normal winter over Eurasia. Observational and numerical evidence indicate that the prolonged subtropical jet from the North Pacific to NA acts as a waveguide that captures the planetary Rossby waves generated by the GW El Niño. This waveguide favors the propagation of the perturbations into the downstream regions, which would affect the NA–Eurasian climate. However, the EW El Niño is accompanied by a relatively weak subtropical jet that cannot impact the NA–Eurasian climate significantly. For the strong El Niño, the absence of the NAO signal can be attributed to the counteracting of the teleconnections triggered by the Pacific and the tropical IO.

Full access
Liguang Wu, Bin Wang, and Scott A. Braun

Abstract

While the previous studies of the impacts of air–sea interaction on tropical cyclones (TCs) generally agree on significant reduction in intensity and little change in track, they did not further explore the relative roles of the weak symmetric and strong asymmetric sea surface temperature (SST) anomalies relative to the TC center. These issues are investigated numerically with a coupled hurricane–ocean model in this study.

Despite the relatively small magnitude compared to the asymmetric component of the resulting cooling, the symmetric cooling plays a decisive role in weakening TC intensity. A likely reason is that the symmetric cooling directly reduces the TC intensity, while the asymmetric cooling affects the intensity through the resulting TC asymmetries, which are mainly confined to the lower boundary and much weaker than those resulting from large-scale environmental influences.

The differences in TC tracks between the coupled and fixed SST experiments are generally small because of the competing processes associated with the changes in TC asymmetries and the beta drift induced by air–sea interaction. The symmetric component of the SST drop weakens the TC intensity and outer strength, leading to a more northward beta drift. On the other hand, since the asymmetric component of the SST cooling is negative in the rear and positive in the front of a TC in the coupled experiments, the enhanced diabatic heating is on the southern side of a westward-moving TC, tending to shift the TC southward. In the coupled model the westward TCs with relatively weak (strong) outer strength tend to turn to the north (south) of the corresponding TCs without air–sea interaction.

Full access
Bin Wang, Renguang Wu, and K-M. Lau

Abstract

Analyses of 50-yr NCEP–NCAR reanalysis data reveal remarkably different interannual variability between the Indian summer monsoon (ISM) and western North Pacific summer monsoon (WNPSM) in their temporal–spatial structures, relationships to El Niño, and teleconnections with midlatitude circulations. Thus, two circulation indices are necessary, which measure the variability of the ISM and WNPSM, respectively. A weak WNPSM features suppressed convection along 10°–20°N and enhanced rainfall along the mei-yu/baiu front. So the WNPSM index also provides a measure for the east Asian summer monsoon. An anomalous WNPSM exhibits a prominent meridional coupling among the Australian high, cross-equatorial flows, WNP monsoon trough, WNP subtropical high, east Asian subtropical front, and Okhotsk high. The WNP monsoon has leading spectral peaks at 50 and 16 months, whereas the Indian monsoon displays a primary peak around 30 months. The WNPSM is weak during the decay of an El Niño, whereas the ISM tends to abate when an El Niño develops. Since the late 1970s, the WNPSM has become more variable, but its relationship with El Niño remained steady; in contrast, the ISM has become less variable and its linkage with El Niño has dramatically declined. These contrasting features are in part attributed to the differing processes of monsoon–ocean interaction.

Also found is a teleconnection between a suppressed WNPSM and deficient summer rainfall over the Great Plains of the United States. This boreal summer teleconnection is forced by the heat source fluctuation associated with the WNPSM and appears to be established through excitation of Rossby wave trains and perturbation of the jet stream that further excites downstream optimum unstable modes.

Full access
Sihua Huang, Bin Wang, and Zhiping Wen

Abstract

The upper-level tropical easterly jet (TEJ) is a crucial component of the summer monsoon system and tropical general circulation. The simulation and projection of the TEJ, however, have not been assessed. Here we evaluate models’ fidelity and assess the future change of the TEJ by utilizing 16 models that participated in phase 6 of the Coupled Model Intercomparison Project (CMIP6). Most of the models can reproduce the TEJ reasonably well in terms of climatology, seasonal evolution, and interannual variability. Nevertheless, underestimation of the TEJ’s intensity and extent is identified, with the maximum bias occurring in the jet centers over the tropical Indian Ocean (IO) and the tropical eastern Pacific (EP). Under the shared socioeconomic pathway 5–8.5, the multimodel ensemble projects a remarkable reduction in the central TEJ intensity by about 18% over the IO and 77% over the EP toward the end of the twenty-first century. The mean intensity of TEJ will weaken by about 11%, and the extent will reduce by 6%, suggesting a significantly weakened upper-level monsoon circulation in the future climate. The projected El Niño–like warming pattern over the tropical Pacific may play a critical role in the future weakening of the TEJ via inducing suppressed rainfall over the tropical eastern IO and Central America. The model uncertainties in the projected TEJ changes may arise from the uncertainties in the models’ projected tropical EP warming. The sensitivity of future projections to model selection is also examined. Results show that the selection of models based on different physical considerations does not yield a significantly different projection.

Restricted access
Yuqing Wang, Shang-Ping Xie, Haiming Xu, and Bin Wang

Abstract

A regional climate model is used to simulate boundary layer stratocumulus (Sc) clouds over the southeast Pacific off South America during August–October 1999 and to study their dynamical, radiative, and microphysical properties and their interaction with large-scale dynamic fields. Part I evaluates the model performance against satellite observations and examines physical processes important for maintaining the temperature inversion and Sc clouds in the simulation.

The model captures major features of the marine boundary layer in the region, including a well-mixed marine boundary layer, a capping temperature inversion, Sc clouds, and the diurnal cycle. The Sc clouds develop in the lower half of and below the temperature inversion layer that increases its height westward off the Pacific coast of South America. The strength of the capping inversion is determined not only by large-scale subsidence and local sea surface temperature (SST), but also by cloud–radiation feedback. A heat budget analysis indicates that upward longwave radiation strongly cools the upper part of the cloud layer and strengthens the temperature inversion. This cloud-top cooling further induces a local enhancement of subsidence in and below the inversion layer, resulting in a dynamical warming that strengthens the temperature stratification above the clouds.

While of secondary importance on the mean, solar radiation drives a pronounced diurnal cycle in the model boundary layer. Consistent with observations, boundary layer clouds thicken after sunset and cloud liquid water content reaches a maximum at 0600 local time just before the sunrise.

Full access
Zhuo Wang, C-P. Chang, Bin Wang, and Fei-Fei Jin

Abstract

Rossby wave propagation theory predicts that Rossby waves in a tropical easterly flow cannot escape from the Tropics to the extratropics. Here the authors show that a southerly flow component in the basic state (a southerly conveyor) may transfer a Rossby wave source northward; thus, a forcing embedded in the deep tropical easterlies may excite a Rossby wave response in the extratropical westerlies. It is shown that the southerly conveyor determines the location of the effective Rossby wave source and that the extratropical response is relatively insensitive to the location of the tropical forcing, provided that the tropical response can reach the southerly conveyor. A stronger southerly flow favors a stronger extratropical response, and the spatial structure of the extratropical response is determined by the extratropical westerly basic flows.

Full access
Dongliang Wang, Xudong Liang, Ying Zhao, and Bin Wang

Abstract

The impact of two bogussing schemes on tropical cyclone (TC) forecasts is compared. One scheme for bogussing TCs into the initial conditions of the nonhydrostatic version of the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) is proposed by NCAR and the Air Force Weather Agency (AFWA), and four-dimensional variational data assimilation technology is employed for the other bogus data assimilation (BDA) scheme. The initial vortex structure adjusted by the NCAR–AFWA (N–A) scheme is more physically realistic, while the BDA scheme produces an initial vortex structure that is more consistent with the model. The results from 41 forecasts of TCs occurring over the western North Pacific (WNP) in 2002 suggest that the adjustment of the initial structure in the BDA scheme produces a greater benefit to the subsequent track and intensity forecasts, and the improvements in the track and intensity forecasts are significant using the BDA scheme. It seems that when using a model with 45-km grid length, the N–A scheme has a negative impact on the track forecasts for the recurving TCs and on the intensity predictions after 24 h.

Full access
Lu Wang, Tim Li, Eric Maloney, and Bin Wang

Abstract

This study investigates the fundamental causes of differences in the Madden–Julian oscillation (MJO) eastward propagation among models that participated in a recent model intercomparison project. These models are categorized into good and poor groups characterized by prominent eastward propagation and nonpropagation, respectively. Column-integrated moist static energy (MSE) budgets are diagnosed for the good and the poor models. It is found that a zonal asymmetry in the MSE tendency, characteristic of eastward MJO propagation, occurs in the good group, whereas such an asymmetry does not exist in the poor group. The difference arises mainly from anomalous vertical and horizontal MSE advection. The former is attributed to the zonal asymmetry of upper-midtropospheric vertical velocity anomalies acting on background MSE vertical gradient; the latter is mainly attributed to the asymmetric zonal distribution of low-tropospheric meridional wind anomalies advecting background MSE and moisture fields. Based on the diagnosis above, a new mechanism for MJO eastward propagation that emphasizes the second-baroclinic-mode vertical velocity is proposed. A set of atmospheric general circulation model experiments with prescribed diabatic heating profiles was conducted to investigate the causes of different anomalous circulations between the good and the poor models. The numerical experiments reveal that the presence of a stratiform heating at the rear of MJO convection is responsible for the zonal asymmetry of vertical velocity anomaly and is important to strengthening lower-tropospheric poleward flows to the east of MJO convection. Thus, a key to improving the poor models is to correctly reproduce the stratiform heating. The roles of Rossby and Kelvin wave components in MJO propagation are particularly discussed.

Full access