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Hae-Kyung Lee Drbohlav and Bin Wang

Abstract

The structures and mechanism of the northward-propagating boreal summer intraseasonal oscillation (BSISO) in the southern Asian monsoon region are simulated and investigated in a three-dimensional intermediate model (3D model). The horizontal structure of the intraseasonal variability in the 3D model depicts the Kelvin–Rossby wave–type disturbance, which may or may not produce the northward-propagating disturbance in the Indian Ocean, depending on the seasonal-mean background winds.

Two experiments are conducted in order to identify what characteristic of seasonal-mean background can induce the northwestward-tilted band in the Kelvin–Rossby wave, whose overall eastward movement gives the impression of the northward propagation at a given longitude. When the prescribed boreal summer mean winds are excluded in the first experiment, the phase difference between the barotropic divergence tendency and convection disappears. Consequently, the Rossby wave–type convection forms a zonally elongated band. As a result, the northward propagation of convection at a given longitude disappears. When the easterly vertical shear is introduced in the second experiment, the horizontal and the vertical structures of BSISO become similar to that of the northward-propagating one. The reoccurrence of the northwestward-directed convective band and the phase difference between the barotropic divergence tendency and the convection suggest that the summer mean zonal winds in the boreal Indian summer monsoon region are a critical condition that causes the horizontal and vertical structures of northward-propagating BSISO in the southern Asian monsoon region.

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Qingnong Xiao, Xiaolei Zou, and Bin Wang

Abstract

The bogus data assimilation (BDA) scheme designed by Zou and Xiao to specify initial structures of tropical cyclones was tested further on the simulation of a landfalling hurricane—Hurricane Fran (1996). The sensitivity of the simulated hurricane track and intensity to the specified radius of maximum wind of the bogus vortex, the resolution of the BDA assimilation model, and the bogus variables specified in the BDA are studied. In addition, the simulated hurricane structures are compared with the available observations, including the surface wind analysis and the radar reflectivity captured onshore during Fran’s landfall.

The sensitivity study of the BDA scheme showed that the simulations of the hurricane track and intensity were sensitive to the size of the specified bogus vortex. Hurricanes with a larger radius of maximum sea level pressure gradient are prone to a more westward propagation. The larger the radius, the weaker the predicted hurricane. Results of the hurricane initial structures and prediction were also sensitive to the bogus variables specified in the BDA. Fitting the model to the bogused pressure data reproduced the hurricane structure of all model variables more efficiently than when fitting it to bogused wind data. Examining the initial conditions produced by the BDA, it is found that the generation and intensity of hurricane warm-core structure in the model initial state was a key factor for the hurricane intensity prediction.

Initialized with the initial conditions obtained by the BDA scheme, the model successfully simulated Hurricane Fran’s landfall, the intensity change, and some inner-core structures. Verified against the surface wind analysis, the model reproduced the distribution of the maximum wind streaks reasonably well. The model-predicted reflectivity field during the landfall of Hurricane Fran resembled the observed radar reflectivity image onshore.

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Hyung-Jin Kim, Bin Wang, and Qinghua Ding

Abstract

The global monsoon climate variability during the second half of the twentieth century simulated by 21 coupled global climate models (CGCMs) that participated in the World Climate Research Programme’s Coupled Model Intercomparison Project phase 3 (CMIP3) is evaluated. Emphasis was placed on climatology, multidecadal trend, and the response of the global monsoon precipitation to volcanic aerosols. The impact of the atmospheric model’s horizontal resolution on the group ensemble mean (GEM; obtained from the three resolution groups) simulations of global monsoon climate is also examined.

The CMIP3 CGCMs’ multimodel ensemble simulates a reasonably realistic climatology of the global monsoon precipitation and circulation. The GEMs are also able to capture the gross features of the global monsoon precipitation and westerly domains. However, the spreading among the rainfall GEMs is large, particularly at the windward side of narrow mountains (e.g., the western coast of India, the Philippines, Mexico, and the steep slope of the Tibetan Plateau). Main common biases in modeling rainfall climatology include a northeastward shift of the intertropical convergence zone (ITCZ) in the tropical North Pacific and a southward migration of the North Atlantic ITCZ during boreal winter.

The trend in the Northern Hemisphere land monsoon index (NHMI) detected in the CMIP3 models is generally consistent with the observations, albeit with much weaker magnitude. The significant decreasing NHMI trend during 1951–85 and 1951–99 occurs mainly in the models with volcanic aerosols (VOL models). This volcanic signal is detectable by comparison of the forced and free runs. It is estimated that from about one-quarter to one-third of the drying trend in the Northern Hemisphere land monsoon precipitation over the latter half of the twentieth century was likely due to the effects of the external volcanic forcings. On the other hand, the significant increasing trend in the global ocean monsoon index (GOMI) during 1980–99 appears chiefly in those models that are free of volcanic aerosols (No-VOL models). The exclusion of the volcanic aerosols is significant in simulating the positive GOMI trend against the internal variability of each model. These results suggest the climatic importance of the volcanic forcings in the global monsoon precipitation variability.

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Ying Zhao, Bin Wang, and Juanjuan Liu

Abstract

In this study, a new data assimilation system based on a dimension-reduced projection (DRP) technique was developed for the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) modeling system. As an initial step to test the newly developed system, observing system simulation experiments (OSSEs) were conducted using a simulated sea level pressure (SLP) field as “observations” and assimilation experiments using a specified SLP field to evaluate the effects of the new DRP–four-dimensional variational data assimilation (4DVar) method, initialization, and simulation of a tropical storm—Typhoon Bilis (2006) over the western North Pacific. In the OSSEs, the “nature” run, which was assumed to represent the “true” atmosphere, was simulated by the MM5 model, which was initialized with the 1.0° × 1.0° NCEP final global tropospheric analyses and integrated for 120 h. The simulated SLP field was then used as the observations in the data assimilation. It is shown that the MM5 DRP–4DVar system can successfully assimilate the (simulated) model output (used as observations) because the OSSEs resulted in improved storm-track forecasts. In addition, compared with an experiment that assimilated the SLP data fixed at the end of a 6-h assimilation window, the experiment that assimilated the SLP data every 3 min in a 30-min assimilation window further improved the typhoon-track forecasts, especially in terms of the initial vortex location and landfall location. Finally, the assimilation experiments with a specified SLP field have demonstrated the effectiveness of the new method.

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Bin Wang, Renguang Wu, and Xiouhua Fu

Abstract

Observational evidence is presented to show a teleconnection between the central Pacific and East Asia during the extreme phases of ENSO cycles. This Pacific–East Asian teleconnection is confined to the lower troposphere. The key system that bridges the warm (cold) events in the eastern Pacific and the weak (strong) East Asian winter monsoons is an anomalous lower-tropospheric anticyclone (cyclone) located in the western North Pacific. The western North Pacific wind anomalies develop rapidly in late fall of the year when a strong warm or cold event matures. The anomalies persist until the following spring or early summer, causing anomalously wet (dry) conditions along the East Asian polar front stretching from southern China northeastward to the east of Japan (Kuroshio extension).

Using atmospheric general circulation and intermediate models, the authors show that the anomalous Philippine Sea anticyclone results from a Rossby-wave response to suppressed convective heating, which is induced by both the in situ ocean surface cooling and the subsidence forced remotely by the central Pacific warming. The development of the anticyclone is nearly concurrent with the enhancement of the local sea surface cooling. Both the anticyclone and the cooling region propagate slowly eastward. The development and persistence of the teleconnection is primarily attributed to a positive thermodynamic feedback between the anticyclone and the sea surface cooling in the presence of mean northeasterly trades. The rapid establishment of the Philippine Sea wind and SST anomalies implies the occurrence of extratropical–tropical interactions through cold surge–induced exchanges of surface buoyancy flux. The central Pacific warming plays an essential role in the development of the western Pacific cooling and the wind anomalies by setting up a favorable environment for the anticyclone–SST interaction and midlatitude–tropical interaction in the western North Pacific.

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Lijuan Li, Bin Wang, and Guang J. Zhang

Abstract

The weak negative shortwave (SW) radiative feedback α sw during El Niño–Southern Oscillation (ENSO) over the equatorial Pacific is a common problem in the models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5). In this study, the causes for the α sw biases are analyzed using three-dimensional cloud fraction and liquid water path (LWP) provided by the 17 CMIP5 models and the relative roles of convective and stratiform rainfall feedbacks in α sw are explored. Results show that the underestimate of SW feedback is primarily associated with too negative cloud fraction and LWP feedbacks in the boundary layers, together with insufficient middle and/or high cloud and dynamics feedbacks, in both the CMIP and Atmospheric Model Intercomparsion Project (AMIP) runs, the latter being somewhat better. The underestimations of SW feedbacks are due to both weak negative SW responses to El Niño, especially in the CMIP runs, and strong positive SW responses to La Niña, consistent with their biases in cloud fraction, LWP, and dynamics responses to El Niño and La Niña. The convective rainfall feedback, which is largely reduced owing to the excessive cold tongue in the CMIP runs compared with their AMIP counterparts, contributes more to the difference of SW feedback (mainly under El Niño conditions) between the CMIP and AMIP runs, while the stratiform rainfall plays a more important role in SW feedback during La Niña.

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Susan Kemball-Cook, Bin Wang, and Xiouhua Fu

Abstract

Three 15-yr integrations were made with the ECHAM-4 atmospheric GCM (AGCM); in the first integration, the model lower boundary conditions were the observed monthly mean sea surface temperatures, and, in the second, the AGCM was coupled to the University of Hawaii 2.5-layer intermediate ocean model. In the third simulation, the SST climatology generated in the coupled run was used to create monthly mean SSTs, which were then used to drive the AGCM in an uncoupled configuration similar to the first run. The simulation of the intraseasonal oscillation (ISO) in these three runs was compared with data from the NCEP reanalysis and outgoing longwave radiation from NOAA polar-orbiting satellites, with particular emphasis on the boreal summer ISO.

The overall effect of coupling the AGCM to the ocean model is to improve the intraseasonal variability of the model. Upon coupling, the simulated boreal winter ISO becomes more spatially coherent and has a more realistic phase speed. In the May–June Asian monsoon season, the coupled run shows pronounced northward propagation of convection and circulation anomalies over the Indian Ocean, as in the observations, while northward propagation is absent in the uncoupled run. These improvements in the simulated ISO occur despite the fact that the coupled-run SST climatology has a substantial cold bias in both the Indian Ocean and the western Pacific warm pool. The improvement in the model ISO may be attributed to air–sea interaction whose mechanism is increased low-level convergence into the positive SST anomaly ahead of the convection anomaly.

The simulation of the August–October ISO is degraded upon coupling, however. The coupled-run basic state fails to produce the region of easterly vertical shear of the mean zonal wind, which lies on the equator during August–October. This region of easterly shear is critical for the emission of Rossby waves by equatorial convection associated with the ISO. In the absence of easterly shear, the observed northwestward propagation of convection is inhibited in both runs made using the coupled model basic state. The uncoupled AGCM run correctly locates the region of easterly shear and produces an August–October ISO that agrees well with observations.

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Bin Wang, Renguang Wu, and Tim Li

Abstract

Asian–Australian monsoon (A–AM) anomalies depend strongly on phases of El Niño (La Niña). Based on this distinctive feature, a method of extended singular value decomposition analysis was developed to analyze the changing characteristics of A–AM anomalies during El Niño (La Niña) from its development to decay. Two off-equatorial surface anticyclones dominate the A–AM anomalies during an El Niño—one over the south Indian Ocean (SIO) and the other over the western North Pacific (WNP). The SIO anticyclone, which affects climate conditions over the Indian Ocean, eastern Africa, and India, originates during the summer of a growing El Niño, rapidly reaches its peak intensity in fall, and decays when El Niño matures. The WNP anticyclone, on the other hand, forms in fall, attains maximum intensity after El Niño matures, and persists through the subsequent spring and summer, providing a prolonged impact on the WNP and east Asian climate. The monsoon anomalies associated with a La Niña resemble those during an El Niño but with cyclonic anomalies. From the development summer to the decay summer of an El Niño (La Niña), the anomalous sea level pressure, low-level winds, and vertical motion tend to reverse their signs in the equatorial Indian and western Pacific Oceans (10°S–20°N, 40°–160°E). This suggests that the tropospheric biennial oscillation is intimately linked to the turnabouts of El Niño and La Niña.

The remote El Niño forcing alone can explain neither the unusual amplification of the SIO anticyclone during a developing El Niño nor the maintenance of the WNP anticyclone during a decaying El Niño. The atmosphere–ocean conditions in the two anticyclone regions are similar, namely, a zonal sea surface temperature (SST) dipole with cold water to the east and warm water to the west of the anticyclone center. These conditions result from positive feedback between the anomalous anticyclone and the SST dipole, which intensifies the coupled mode in the SIO during El Niño growth and maintains the coupled mode in the WNP during El Niño decay. The interactions in the two anticyclone regions share common wind evaporation/entrainment and cloud–radiation feedback processes but they differ with regard to the oceanic dynamics (vertical and horizontal advection and thermocline adjustment by oceanic waves). The outcome of the interactions in both regions, however, depends crucially on the climatological surface winds. The SIO-coupled mode is triggered by El Niño-induced subsidence and alongshore winds off the coast of Sumatra. However, other independent El Niño local and remote forcing can also trigger this coupled mode.

The traditional view has regarded SST anomalies in the Indian and western Pacific Oceans as causing the A–AM variability. The present analysis suggests that the SST anomalies in these warm ocean regions are, to a large extent, a result of anomalous monsoons. Thus, the atmosphere–warm ocean interaction may significantly modify the impacts of remote El Niño forcing and should be regarded as one of the physical factors that determine the variability of the A–AM.

During the summer of El Niño development, the remote El Niño forcing plays a major role in the A–AM anomalies that exhibit obvious equatorial asymmetry. A tilted anticyclonic ridge originates in the Maritime Continent and extends to southern India, weakening the Indian monsoon while strengthening the WNP monsoon. Numerical modeling experiments suggest that the mean monsoon circulation enhances the equatorial Rossby wave response in the easterly vertical shear region of the Northern Hemisphere and creates the equatorial asymmetry.

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Bin Wang, Tianming Li, and Ping Chang

Abstract

An intermediate tropical Pacific Ocean model is developed to bridge the gap between anomaly models of El Niño and ocean general circulation models. The model contains essential physics for reproducing both the annual and interannual variations of sea surface temperature (SST). A new parameterization scheme for entrained water temperature is shown to work satisfactorily in both the cold tongues and warm pools. This scheme combines the Cane-Zebiak (CZ) model's dynamic framework and mixed layer physics, giving a more realistic description of the active tropical ocean.

Incorporation of the Niiler-Kraus scheme for turbulent entrainment enables the model to better simulate El Niño-Southern Oscillation in the central equatorial Pacific where the CZ model considerably underestimates observed SST variations. It also improves the model's performance on the seasonal cycle, especially in the central-eastern equatorial Pacific and the intertropical convergence zone (ITCZ). The potential energy generation induced by penetrative solar radiation tends to reduce entrainment in the central equatorial Pacific but to enhance mixing in the far eastern equatorial Pacific. Without this process, the model central (eastern) Pacific would be excessively cold (warm).

In response to an idealized sequential westerly burst located in the western equatorial Pacific, the CZ model produces SST oscillations in the eastern equatorial Pacific due to the thermocline oscillation associated with passages of Kelvin waves. In the present model, however, SST variation in the eastern Pacific is insignificant because local entrainment transcends the influence of thermocline oscillation; on the other hand, positive SST anomalies slowly amplify near the date line due to the reduction in wind-induced mixing and surface evaporation.

The annual variations of the oceanic momentum and heat transports associated with the annual march of the ITCZ are shown to have significant impacts on the annual mean state. On the other hand, including an annual mean heat flux correction in the present model does not strongly influence the amplitudes of annual and interannual SST variations. However, it does improve the phase structure of the annual cycle by providing a more accurate annual mean state.

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Yongsheng Zhang, Tim Li, and Bin Wang

Abstract

The decadal change in the spring snow depth over the Tibetan Plateau and impact on the East Asian summer monsoon are investigated using station observations of snow depth data and the NCEP–NCAR reanalysis for 1962–93. During spring (March–April), both the domain-averaged snow depth index (SDI) and the first principal component of the empirical orthogonal function (EOF) analysis exhibit a sharp increase in snow depth after the late 1970s, which is accompanied by excessive precipitation and land surface cooling. The correlation between SDI and precipitation shows a coherent remote teleconnection from the Tibetan Plateau–northern India to western Asia.

It is found that the increased snow depth over the plateau after the mid-1970s is concurrent with a deeper India–Burma trough, an intensified subtropical westerly jet as well as enhanced ascending motion over the Tibetan Plateau. Additional factors for the excessive snowfall include more moisture supply associated with the intensification of the southerly flow over the Bay of Bengal and an increase of humidity over the Indian Ocean. While the extensive changes of the circulation in Eurasia and the Indian Ocean are associated with a climate shift in the Northern Hemisphere after the mid-1970s, some regional factors such as the enhanced coupling between the sea surface temperature (SST) warming in the northern Indian Ocean/Maritime Continent and the tropical convective maximum (TCM), as well as local feedback of the land surface cooling due to excessive snow cover and the atmosphere may contribute to the regional circulation changes. The former enhances the western Pacific subtropical in the South China Sea–Philippine Sea through modulation of the local Hadley circulation and results in stronger pressure gradients and fronts in southeastern and eastern Asia.

A close relationship exists between the interdecadal increase of snow depth over the Tibetan Plateau during March–April and a wetter summer rainfall over the Yangtze River valley and a dryer one in the southeast coast of China and the Indochina peninsula. It is proposed that the excessive snowmelt results in a surface cooling over the plateau and neighboring regions and high pressure anomalies that cause a more northwestward extension of the western Pacific subtropical high in the subsequent summer. Additionally, the increased surface moisture supply provides more energy for the development of the eastward-migrating low-level vortex over the eastern flank of the Tibetan Plateau. Both factors lead to a wetter summer in the vicinity of the Yangtze River valley.

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