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Bin Wang and Qin Zhang

Abstract

The anomalous Philippine Sea anticyclone (PSAC) conveys impacts of El Niño to east Asian climate during the mature and decay of an El Niño (from the winter to ensuing summer). It is shown that the anomalous PSAC forms in fall about one season prior to the peak El Niño; its strength increases with the El Niño intensity and its sign reverses during a La Niña. The PSAC formation concurs with abnormal deepening of the east Asian trough and with increasing number of northward recurvature of tropical storms in the western Pacific. The PSAC establishment is abrupt, coupling with a swing from a wet to dry phase of an intraseasonal oscillation (ISO) and often concurrent with early retreat of the east Asian summer monsoon. The ISO becomes inactive after PSAC establishment.

The development of the PSAC is attributed to combined effects of the remote El Niño forcing, tropical–extratropical interaction, and monsoon–ocean interaction. The developing El Niño induces off-equatorial ascending Rossby wave responses and land surface cooling in northeast Asia; both deepen the east Asian trough in fall and induces vigorous tropical–extratropical exchange of air mass and heat, which enhances the cold air outbreak and initiation of the PSAC. Through exciting descending Rossby waves, the El Niño–induced Indonesian subsidence generates low-level anticyclonic vorticity over south Asia, which is advected by mean monsoon westerly, instigating the anomalous PSAC. The ISO interacting with the underlying ocean plays a critical role in the abrupt establishment of PSAC. The wind–evaporation/entrainment feedback tends to amplify (suppress) ISO before (after) winter northeasterly monsoon commences, suggesting the roles of atmosphereocean interaction and the seasonal march of background winds in changing the Philippine Sea ISO intensity and maintaining PSAC.

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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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Lijuan Li, Bin Wang, and Guang J. Zhang

Abstract

The weak response of surface shortwave cloud radiative forcing (SWCF) to El Niño over the equatorial Pacific remains a common problem in many contemporary climate models. This study shows that two versions of the Grid-Point Atmospheric Model of the Institute of Atmospheric Physics (IAP)/State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG) (GAMIL) produce distinctly different surface SWCF response to El Niño. The earlier version, GAMIL1, underestimates this response, whereas the latest version, GAMIL2, simulates it well. To understand the causes for the different SWCF responses between the two simulations, the authors analyze the underlying physical mechanisms. Results indicate the enhanced stratiform condensation and evaporation in GAMIL2 play a key role in improving the simulations of multiyear annual mean water vapor (or relative humidity), cloud fraction, and in-cloud liquid water path (ICLWP) and hence in reducing the biases of SWCF and rainfall responses to El Niño due to all of the improved dynamical (vertical velocity at 500 hPa), cloud amount, and liquid water path (LWP) responses. The largest contribution to the SWCF response improvement in GAMIL2 is from LWP in the Niño-4 region and from low-cloud cover and LWP in the Niño-3 region. Furthermore, as a crucial factor in the low-cloud response, the atmospheric stability change in the lower layers is significantly influenced by the nonconvective heating variation during La Niña.

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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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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.

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Yuxuan Yang, Lifeng Zhang, Bin Zhang, Wei You, Mingyang Zhang, and Binpeng Xie

Abstract

The sensitivity of the proper orthogonal decomposition (POD)-based ensemble four-dimensional variational assimilation (4DVar) method (referred to as POD-4DEnVar) to cumulus and microphysics schemes was investigated using the Weather Research and Forecasting (WRF) Model for heavy rainfall in South China. Results show that the choice of the cumulus and microphysics schemes for ensemble samples significantly impacts precipitation prediction and that Doppler radar data assimilation using POD-4DEnVar is sensitive to the parameterization schemes used for the ensemble samples. The cumulus and microphysics schemes primarily affect the vertical velocity and rainwater mixing ratio of the ensemble forecasts. Variations in the ensemble samples caused by different parameterization schemes are introduced into the four-dimensional ensemble variational assimilation by the radar data observation operator. These variations affect the analysis fields and result in variations in precipitation prediction. To obtain the optimal result (smallest forecast error), three methods are designed based on the physical ensemble technique, which can filter out the effects of different parameterization schemes for the ensemble samples through averaging. The results show that the precipitation forecasts from the three assimilation experiments are improved compared with a control experiment, but each physical ensemble method leads to a unique precipitation forecast.

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Tim Li, Bin Wang, C-P. Chang, and Yongsheng Zhang

Abstract

Four fundamental differences of air–sea interactions between the tropical Pacific and Indian Oceans are identified based on observational analyses and physical reasoning. The first difference is represented by the strong contrast of a zonal cloud–SST phase relationship between the warm and cool oceans. The in-phase cloud–SST relationship in the warm oceans leads to a strong negative feedback, while a significant phase difference in the cold tongue leads to a much weaker thermodynamic damping. The second difference arises from the reversal of the basic-state zonal wind and the tilting of the ocean thermocline, which leads to distinctive effects of ocean waves. The third difference lies in the existence of the Asian monsoon and its interaction with the adjacent oceans. The fourth difference is that the southeast Indian Ocean is a region where a positive atmosphere–ocean thermodynamic feedback exists in boreal summer.

A conceptual coupled atmosphere–ocean model was constructed aimed to understand the origin of the Indian Ocean dipole–zonal mode (IODM). In the model, various positive and negative air–sea feedback processes were considered. Among them were the cloud–radiation–SST feedback, the evaporation–SST–wind feedback, the thermocline–SST feedback, and the monsoon–ocean feedback. Numerical results indicate that the IODM is a dynamically coupled atmosphere–ocean mode whose instability depends on the annual cycle of the basic state. It tends to develop rapidly in boreal summer but decay in boreal winter. As a result, the IODM has a distinctive evolution characteristic compared to the El Niño. Sensitivity experiments suggest that the IODM is a weakly damped oscillator in the absence of external forcing, owing to a strong negative cloud–SST feedback and a deep mean thermocline in the equatorial Indian Ocean.

A thermodynamic air–sea (TAS) feedback arises from the interaction between an anomalous atmospheric anticyclone and a cold SST anomaly (SSTA) off Sumatra. Because of its dependence on the basic-state wind, the nature of this TAS feedback is season dependent. A positive feedback occurs only in northern summer when the southeasterly flow is pronounced. It becomes a negative feedback in northern winter when the northwesterly wind is pronounced. The phase locking of the IODM can be, to a large extent, explained by this seasonal-dependent TAS feedback. The biennial tendency of the IODM is attributed to the monsoon–ocean feedback and the remote El Niño forcing that has a quasi-biennial component.

In the presence of realistic Niño-3 SSTA forcing, the model is capable of simulating IODM events during the last 50 yr that are associated with the El Niño, indicating that ENSO is one of triggering mechanisms. The failure of simulation of the 1961 and 1994 events suggests that other types of climate forcings in addition to the ENSO must play a role in triggering an IODM event.

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Wenqing Zhang, Lian Xie, Bin Liu, and Changlong Guan

Abstract

Track, intensity, and, in some cases, size are usually used as separate evaluation parameters to assess numerical model performance on tropical cyclone (TC) forecasts. Such an individual-parameter evaluation approach often encounters contradictory skill assessments for different parameters, for instance, small track error with large intensity error and vice versa. In this study, an intensity-weighted hurricane track density function (IW-HTDF) is designed as a new approach to the integrated evaluation of TC track, intensity, and size forecasts. The sensitivity of the TC track density to TC wind radius was investigated by calculating the IW-HTDF with density functions defined by 1) asymmetric, 2) symmetric, and 3) constant wind radii. Using the best-track data as the benchmark, IW-HTDF provides a specific score value for a TC forecast validated for a specific date and time or duration. This new TC forecast evaluation approach provides a relatively concise, integrated skill score compared with multiple skill scores when track, intensity and size are evaluated separately. It should be noted that actual observations of TC size data are very limited and so are the estimations of TC size forecasts. Therefore, including TC size as a forecast evaluation parameter is exploratory at the present. The proposed integrated evaluation method for TC track, intensity, and size forecasts can be used for evaluating the track forecast alone or in combination with intensity and size parameters. As observations and forecasts of TC size become routine in the future, including TC size as a forecast skill assessment parameter will become more imperative.

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Jianping Duan, Qi-bin Zhang, and Li-Xin Lv

Abstract

The recent increase in the frequency of winter cold extremes has received particular attention in light of the climate's warming. Knowledge about changes in the frequency of winter cold extremes requires long-term climate data over large spatial scale. In this study, a temperature-sensitive tree-ring network consisting of 31 sampling sites collected from seven provinces in subtropical China was used to investigate the characteristics of cold-season temperature extremes during the past two centuries. The results show that the percentage of trees in a year that experienced an abnormal decrease in radial growth relative to the previous year can serve as an indicator of interannual change in January–March temperature in subtropical China. The frequency of extreme interannual decreases in cold-season temperature has increased since the 1930s. The change in cold-season temperature was significantly correlated with the intensity of the Siberian high, yet the correlation was much weaker in the period preceding the 1930s. The findings provide evidence of a frequency change in the occurrence of interannual cold-season temperature extremes in the past two centuries for subtropical China. Particularly, the pattern in the variation of cold-season temperature suggests a change in climate systems around the 1930s.

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Yongsheng Zhang, Tim Li, Bin Wang, and Guoxiong Wu

Abstract

The temporal and spatial structures of the atmospheric circulation associated with the climatology and interannual variations of the summer monsoon onset over the Indochina Peninsula were studied using the observed daily rainfall at 30 stations and the NCEP–NCAR reanalysis from 1951 to 1996. The climatological monsoon onset over Indochina is on 9 May, with a standard deviation of 12 days. The monsoon onset is characterized by the pronounced northeastward progression of the low-level southwesterlies over the Indian Ocean and the intensification and northward extension of the tropical convection from Sumatra. It coincides with the weakening of the midlatitude westerly over south Asia, and the westward propagation of the intraseasonal oscillation (ISO) originated in the South China Sea (SCS) and the western Pacific with a dominant timescale of 12–25 days.

A close relationship between the interannual variations of the monsoon onset and El Niño/La Niña was identified. Years with warm (cold) sea surface temperature (SST) anomalies in the western Pacific and cold (warm) SST anomalies in the central-eastern Pacific in the preceding spring have an early (late) onset. For an early onset year, strong convective activities appear over the southern Indochina Peninsula and the southern SCS in the preceding winter and spring. Associated with the changes of the Walker circulation and the local Hadley circulation related to La Niña, strong convective activities were maintained by the convergence between the anomalous southwesterlies in the Indian Ocean and northeasterlies over the northern SCS. The anomalous southwesterlies in the Indian Ocean were induced by both the anomalous Walker circulation associated with La Niña and anomalous land–sea thermal contrast. The anomalous northeasterlies over the northern SCS were originated in northern winter due to the combined effects of the cold east China land and the warm Philippine Sea, and further maintained by a positive thermodynamic air–sea feedback mechanism related to La Niña. An opposite scenario is found for a late onset year with warm SST anomalies in the central-eastern Pacific (El Niño).

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