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Min Wen and Renhe Zhang

Abstract

The quasi-biweekly oscillation (QBWO) of the tropical convection around Sumatra and its relation to the low-level circulation over the tropical Indian Ocean in boreal spring is investigated. From March to May, the convection over northern Sumatra increases continuously and oscillates with a pronounced period of 10–20 days. Time-lag cross correlations among the QBWOs of the convection, the apparent heat source, and winds in the lower troposphere reveal a possible mechanism of QBWO maintenance. In the strongest phase of the QBWO of the convection around Sumatra, there is an anomalous convective heating symmetric about the equator. The atmospheric Rossby wave response to the heating produces twin cyclones straddling the equator in the west of the convection area. The development of the twin cyclones induces an anomalous southerly north of the equator and a northerly south of the equator at 850 hPa, giving rise to the divergence of the low-level wind field, which weakens the convection around Sumatra. The weakening of the convection leads to the negative phase of convection. In the weakest phase, the Rossby wave response to the anomalous convective cooling produces twin anticyclones symmetric about the equator, resulting in the convergence of the low-level winds and, in turn, enhancing the convection around Sumatra. Consequently, the feedbacks among convection, the Rossby wave response, and the associated wind field at the lower troposphere may be important maintenance mechanisms of the tropical QBWO. The appearance of a tropical westerly is a crucial index of the Asian summer monsoon onset. In the northern equatorial region, the westerly first occurs just to the west of Sumatra, and then extends westward in boreal spring. The westerly around the equator associated with the Rossby wave response to the convective heating of the QBWO of the convection around Sumatra displays a notable intraseasonal feature, which may play an important role in modulating the process of the Asian summer monsoon onset.

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Qingye Min and Renhe Zhang

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Despite the fact that great efforts have been made to improve the prediction of El Niño events, it remains challenging because of limited understanding of El Niño and its precursors. This research focuses on the influence of South Pacific atmospheric variability on the development of the sea surface temperature anomaly (SSTA) in the tropical Pacific. It is found that as early as in the boreal spring of El Niño years, the sea level pressure anomaly (SLPA) shows a configuration characterized by two significant negative anomaly centers in the north and a positive anomaly center in the south between the subtropics and high latitudes in South Pacific. Such an anomalous SLPA pattern becomes stronger in the following late boreal spring and summer associated with the strengthening of westerly anomalies in the tropical Pacific, weakening the southeasterly trade winds and promoting the warming of tropical eastern Pacific, which is conducive to the development of El Niño events. It is demonstrated that the SLPA pattern in boreal spring revealed in this study is closely associated with boreal summer South Pacific Oscillation (SPO) and South Pacific meridional mode (SPMM). As a precursor in boreal spring, the prediction skill of the South Pacific SLPA in boreal spring for the SSTA in the eastern equatorial Pacific is better than that of the SPMM. This study is helpful to deepen our understanding of the contribution of South Pacific extratropical atmospheric variability to El Niño occurrence.

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Renhe Zhang and Zhiyan Zuo

Abstract

Numerous studies have been conducted on the impact of soil moisture on the climate, but few studies have attempted to diagnose the linkage between soil moisture and climate variability using observational data. Here, using both observed and reanalysis data, the spring (April–May) soil moisture is found to have a significant impact on the summer (June–August) monsoon circulation over East Asia and precipitation in east China by changing surface thermal conditions. In particular, the spring soil moisture over a vast region from the lower and middle reaches of the Yangtze River valley to north China (the YRNC region) is significantly correlated to the summer precipitation in east China. When the YRNC region has a wetter soil in spring, northeast China and the lower and middle reaches of the Yangtze River valley would have abnormally higher precipitation in summer, while the region south of the Yangtze River valley would have abnormally lower precipitation. An analysis of the physical processes linking the spring soil moisture to the summer precipitation indicates that the soil moisture anomaly across the YRNC region has a major impact on the surface energy balance. Abnormally wet soil would increase surface evaporation and hence decrease surface air temperature (Ta). The reduced Ta in late spring would narrow the land–sea temperature difference, resulting in the weakened East Asian monsoon in an abnormally strengthened western Pacific subtropical high that is also located farther south than its normal position. This would then enhance precipitation in the Yangtze River valley. Conversely, the abnormally weakened East Asian summer monsoon allows the western Pacific subtropical high to wander to south of the Yangtze River Valley, resulting in an abnormally reduced precipitation in the southern part of the country in east China.

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Xiaoyan Wang and Renhe Zhang
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Ruonan Zhang, Renhe Zhang, and Zhiyan Zuo

Abstract

In this study, the relationship between Eurasian spring snow decrement (SSD) and East Asian summer precipitation and related mechanisms were investigated using observational data and the Community Atmospheric Model, version 3.1 (CAM3.1). The results show that a west–east dipole pattern in Eurasian SSD anomalies, with a negative center located in the region between eastern Europe and the West Siberia Plain (EEWSP) and a positive center located around Baikal Lake (BL), is significantly associated with East Asian summer precipitation via triggering an anomalous midlatitude Eurasian wave train. Reduced SSD over EEWSP corresponds to anomalously dry local soil conditions from spring to the following summer, thereby increasing surface heat flux and near-surface temperatures. Similarly, the increase in SSD over BL is accompanied by anomalously low near-surface temperatures. The near-surface thermal anomalies cause an anomalous meridional temperature gradient, which intensifies the lower-level baroclinicity and causes an acceleration of the subtropical westerly jet stream, leading to an enhanced and maintained Eurasian wave train. Additionally, the atmospheric response to changed surface thermal conditions tends to simultaneously increase the local 1000–500-hPa thickness, which further enhances the Eurasian wave train. Consequently, significant wave activity flux anomalies spread from eastern Europe eastward to East Asia and significantly influence the summer precipitation over China, with more rainfall over northeastern China and the Yellow River valley and less rainfall over Inner Mongolia and southern China.

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Li Liu, Renhe Zhang, and Zhiyan Zuo

Abstract

As important parameters in the land–atmosphere system, both soil moisture (SM) and vegetation play a significant role in land–atmosphere interactions. Using observational data from clay and sand stations over central eastern China, the relationship between leaf area index (LAI) and SM (LAI–SM) in different types of soil was investigated. The results show that the LAI–SM correlation is significantly positive in clay but not significant in sand. The physical causes for the discrepant LAI–SM correlations in different types of soil were explored from the perspectives of evapotranspiration (ET) and soil water retention. In clay stations, increasing LAI is associated with greater soil-water-retention capacity. Although the increasing LAI corresponds to increasing ET, the impact of ET on SM is weak because of the small particle size of soil. Consequently, the LAI–SM relationship in clay is significantly positive. In sand stations, ET is negatively correlated with SM owing to the large soil particle size, resulting in a negative LAI–SM correlation in sand. However, soil water retention is weakened by the increased LAI, which may be an important factor causing the insignificant LAI–SM correlation in sand.

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Bingyi Wu, Renhe Zhang, and Rosanne D’Arrigo

Abstract

Two distinct modes of the East Asian winter monsoon (EAWM) have been identified, and they correspond to real and imaginary parts of the leading mode of the EAWM, respectively. Analyses of these modes used the National Centers for Environment Prediction (NCEP) and National Center for Atmospheric Research (NCAR) monthly mean reanalysis datasets for the period 1968–2003, as well as the Southern Oscillation index (SOI), North Atlantic Oscillation index, and eastern equatorial Pacific sea surface temperature (SST) data. Results were obtained by resolving a complex Hermite matrix derived from 850-hPa anomalous wind fields, and determining the resulting modes’ associations with several climate variables. The first distinct mode (M1) is characterized by an anomalous meridional wind pattern over East Asia and the western North Pacific. Mode M1 is closely related to several features of the atmospheric circulation, including the Siberian high, East Asian trough, East Asian upper-tropospheric jet, and local Hadley circulation over East Asia. Thus, M1 reflects the traditional EAWM pattern revealed in previous studies. The second distinct EAWM mode (M2), which was not identified previously, displays dominant zonal wind anomalies over the same area. Mode M2 exhibits a closer relation than M1 to sea level pressure anomalies over the northwestern Pacific southeast of Japan and with the SOI and equatorial eastern Pacific SST. Unlike M1, M2 does not show coherent relationships with the Siberian high, East Asian trough, and East Asian upper-tropospheric jet. Since atmospheric circulation anomalies relevant to M2 exhibit a quasi-barotropic structure, its existence cannot simply be attributed to differential land–sea heating. El Niño events tend to occur in the negative phase of M1 and the positive phase of M2, both corresponding to a weakened EAWM. The Arctic Oscillation does not appear to impact the EAWM on interannual time scales. Although the spatial patterns for the two modes are very different, the two distinct modes are complementary, with the leading EAWM mode being a linear combination of the two. The results herein therefore demonstrate that a single EAWM index may be inappropriate for investigating and predicting the EAWM.

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Yangxing Zheng, Renhe Zhang, and Mark A. Bourassa

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Composite analysis from NCEP–NCAR reanalysis datasets over the period 1948–2007 indicates that stronger East Asian winter monsoons (EAWM) and stronger Australian summer monsoons (ASM) generally coexist in boreal winters preceding the onset of El Niño, although the EAWM tend to be weak after 1990, probably because of the decadal shift of EAWM and the change in El Niño events from cold-tongue type to warm-pool type. The anomalous EAWM and ASM enhance surface westerlies over the western tropical Pacific Ocean (WTP). It is proposed that the enhanced surface westerlies over the WTP prior to El Niño onset are generally associated with the concurrent anomalous EAWM and ASM. A simple analytical atmospheric model is constructed to test the hypothesis that the emergence of enhanced surface westerlies over the WTP can be linked to concurrent EAWM and ASM anomalies. Model results indicate that, when anomalous northerlies from the EAWM converge with anomalous southerlies from the ASM, westerly anomalies over the WTP are enhanced. This result provides a possible explanation of the co-impact of the EAWM and the ASM on the onset of El Niño through enhancing the surface westerly over the WTP.

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Qingye Min, Jingzhi Su, and Renhe Zhang

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An interannual variability mode in the southeast Pacific with a physical interpretation similar to that of the Pacific meridional mode (PMM) in the North Pacific was recently identified. Both modes have been shown to influence the subsequent development of El Niño–Southern Oscillation (ENSO) events. This study investigates the relationship between ENSO and the two PMMs using observational and reanalysis data. The results show that the South Pacific meridional mode (SPMM) mainly favors the development of sea surface temperature anomalies (SSTAs) in the eastern equatorial Pacific, whereas the North Pacific meridional mode (NPMM) mainly favors the development of SSTAs in the central equatorial Pacific. Both of the meridional modes are considered to be analogous in terms of their physical interpretation and can be important predictors of ENSO when considering different flavors of ENSO. Neither the NPMM nor the SPMM can be precluded as accurate indicators when forecasting particular flavors of ENSO.

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Yali Luo, Renhe Zhang, and Hui Wang

Abstract

Seasonal variations in the occurrence frequency, vertical location, and radar reflectivity factor (dBZ) of hydrometeors covering eastern China and the Indian monsoon region are described using two CloudSat standard products [Geometrical Profiling Product (GEOPROF) and GEOPROF-lidar] during the period July 2006–August 2007. The 14-month averaged hydrometeor occurrence frequency is 80% (for eastern China) and 70% (for Indian region), respectively, to which multilayer (mostly double or triple layers) hydrometeors contribute 37% and 47%. A significant increase in the multilayer hydrometeor amount from winter to summer in the Indian region causes a pronounced seasonal variation in its total hydrometeor amount. The nearly opposite phases in the seasonal variations of single- and multilayer hydrometeor amounts result in little change with season in total hydrometeor amount in eastern China. Although the passive sensor-based satellite cloud product is able to provide the major seasonal features in the hydrometeor occurrence frequency (HOF) as revealed by the CloudSat/Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) product, it generally underestimates the HOF.

The maxima in the amounts of both high-level and thick hydrometeor layers occur during summer in both regions, reflecting the impact of the Asian summer monsoon. The abundance of low-level cloud layers and scarcity of hydrometeors at higher levels in eastern China during autumn to winter reflect the general subsidence motion in the middle and upper troposphere. The hydrometeors are geometrically thin in both regions. Cirrus containing small ice crystals is the most common cloud type in the Indian region over the year, while the eastern China hydrometeors are located lower and distributed more evenly in the dBZ–altitude phase space. Although the Indian region has deeper convection and more anvils than eastern China during summer, the averaged dBZ–altitude distributions of deep convection and anvils are nearly identical between the two regions.

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