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Prediction of summer precipitation in north China (NCP) has long been a challenge partly because its low correlation with previous sea surface temperature (SST) anomalies (SSTA) limits the application of SST in NCP prediction. This study aims to extract optimal predictors of NCP from the SST field using an objective method—empirically optimal screening (EOS). It finds that the optimal precursory signal of NCP lies in the change of SSTA from winter to spring rather than the SSTA itself. This study identifies two optimal precursory signs predicting a positive (negative) NCP anomaly: the anomalous SST cooling (warming) from winter to spring in the coastal area of Somalia and Peru. Interestingly, these two presummer conditions have considerable independence, but they lead to a similar summer development of La Niña (El Niño). In summer, the tropical precipitation anomaly pattern associated with La Niña (El Niño) development excites a meridional wave train over the western Pacific and the circumglobal teleconnection in the Northern Hemisphere. Both of the anomalous wave trains show abnormal high (low) pressure over northeast Asia, which induces the south (north) wind anomalies over north China and produces abundant (deficient) precipitation there. These results highlight the importance of the SST evolution from winter to spring, break through the limitation of SST application in NCP prediction, and thus bring a prospect of improving NCP forecast skills. Significance Statement: Sea surface temperature (SST) anomalies are most used as predictors in climate prediction. However, the forecast of summer precipitation in north China is limited by its low correlation with prior SST anomalies. In this paper, we find that the optimal precursory signal of north China precipitation (NCP) is not the SST anomaly itself, but the changes of SST anomalies from winter to spring in the coastal area of Somalia and Peru. These two precursory signals are almost independent yet indicate similar summer situations leading to NCP anomaly. These results highlight the importance of the dynamic evolution of sea surface temperature in improving the forecast skill of NCP. [ABSTRACT FROM AUTHOR]

The spatial distribution of summer rainfall anomalies over eastern China often shows a tripole pattern with rainfall anomalies over the Yangtze River basin varies in opposite phase with North China and South China. It is not clear whether this tripole pattern is an intrinsic atmospheric mode or it is remotely forced. Using two sets of model outputs from 20 models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5), this paper investigates the driving mechanisms of this leading rainfall mode and its major influencing factors. One set (piControl) is fully coupled atmosphere–ocean simulations under constant preindustrial forcing and the other (sstClim) is atmosphere-alone models forced by prescribed climatological sea surface temperatures (SSTs). By comparing results from these two different sets of simulations, it is found that the tripole pattern is the leading mode of summer precipitation variability over eastern China with or without oceanic forcing. It can be regarded as an intrinsic atmospheric mode although air–sea interaction can modify its temporal variability. The cyclonic–anticyclonic atmospheric circulation anomaly over the northern North Pacific is identified as a key factor in both experiments. As atmospheric internal variability, it is related to a circumglobal zonal wave train propagating along the westerly jet stream. When air–sea interactions involved, modulation from SST anomalies is exerted through the meridional Pacific–Japan/East Asia–Pacific wave train propagating along the East Asian coast. Our results suggest that the North Pacific could be another key region providing potential predictability to the East Asian monsoon in addition to the Indo-Pacific. [ABSTRACT FROM AUTHOR]

Any skillful prediction is of great benefit to North China, a region that is densely populated and greatly impacted by droughts. This paper reports potential predictability of North China summer drought 1 month ahead based on hindcasts for 1961–2005 from the "ENSEMBLES" project. Correlation scores of the standardized precipitation–evapotranspiration index and standardized precipitation index reach 0.49 and 0.39, respectively. The lower-level northwestern Pacific cyclonic circulation anomaly (NWPCCA) and East Asian upper-tropospheric temperature (UTT) cooling are the crucial circulations with regard to summer drought. Two sources of predictability are identified: 1) Pacific–Japan and Silk Road teleconnections forced by well-established eastern Pacific Ocean El Niño sea surface temperature anomalies (SSTA) in summer, when the two key circulations are both well predicted because of a good prediction of enhanced equatorial central Pacific (CP) rainfall and Indian rainfall deficit, and 2) the subtropical atmosphere–ocean coupling associated with CP El Niño developing, when the skill mainly arises from the reasonable prediction of NWPCCA. In observations, the NWPCCA persists from the preceding spring to summer through a wind–evaporation–SST feedback related to the Pacific meridional mode (PMM). In predictions, the persistence of the NWPCCA is mainly forced by the enhanced convection over the subtropical central North Pacific due to the persistence of the PMM-related meridional SSTA gradient over the CP. This predicted SSTA suppresses the equatorial Pacific rainfall, contributing to low prediction skill for the East Asian UTT cooling. This study demonstrates the importance of extratropical signals from the preceding season in North China summer drought prediction. [ABSTRACT FROM AUTHOR]

The intensity of interannual variation of spring precipitation over southern China during 1979–2014 and possible reasons for it are investigated in this paper. There is a significant interdecadal change in the intensity of interannual variation of spring precipitation over southern China around 1995/96. The intensity of interannual variation of spring rainfall over South China is stronger during 1979–95 than that during 1996–2014. The possible reason may be the larger amplitude of the sea surface temperature anomaly (SSTA) in the western Pacific Ocean (WP) before 1995/96. The cooler (warmer) SSTA in WP may trigger an abnormal local anticyclone (cyclone) at lower levels. The anomalous southwesterly (northeasterly) flow at the northwestern flank of the WP anticyclone (cyclone) covers South China, transporting more (less) moisture to South China. Meanwhile, the anomalous winds converge (diverge) in South China at lower levels and diverge (converge) at upper levels, which causes the anomalous ascent (descent) to enhance (reduce) the precipitation over there. However, during 1996–2014, the intensity of interannual variation of spring rainfall over the middle and lower reaches of the Yangtze River valley becomes much stronger than that during 1979–95, which is related to the intensified interannual variation of the atmospheric circulation in the middle and high latitudes over Eurasia. The weak (strong) Siberian high and East Asian trough may reduce (enhance) the northerly wind from the middle and high latitudes. As a result, the middle and lower reaches of the Yangtze River valley are subjected to the anomalous southerly wind, favoring more (less) precipitation over there. [ABSTRACT FROM AUTHOR]

The Indian Ocean basin (IOB) mode is the dominant mode of the interannual sea surface temperature (SST) variability in the Indian Ocean, with the Indian Ocean dipole (IOD) as the second mode. An IOB event normally occurs after an El Niño or a concurrent IOD–El Niño event, the dynamics of which are traditionally believed as forced by ENSO through the Walker circulation anomalies over the tropical Indian Ocean. A strong IOB in 2020 took place after the strongest 2019 IOD on record but independent of El Niño, which challenges the traditional atmospheric bridge dynamics of the IOB event. In this study, the dynamics of the 2020 IOB event are investigated using the numerical seasonal climate prediction system of the National Marine Environmental Forecasting Center of China. It is found that the initialization of the Indian Ocean subsurface temperature during the 2019 IOD event has led to the outburst of the 2020 IOB event successfully, the dynamics of which are the propagation and the western boundary reflection of the equatorial and off-equatorial Rossby waves, inducing heat content recharge over the tropical Indian Ocean upper thermocline. In comparison, experiments of SST initialization over the tropical Indian Ocean, with the subsurface temperature in a climatological state, were unable to reproduce the onset of the 2020 IOB event, suggesting that the local air–sea interaction within the Indian Ocean basin is of secondary importance. The numerical experiments suggest that the thermocline ocean wave dynamics play an important role in forcing the IOB event. The revealed thermocline dynamics are potentially useful in climate prediction associated with IOB events. [ABSTRACT FROM AUTHOR]

An atmospheric river (AR) is a highly accumulated line of moisture, which often preconditions heavy rainfall events. Considering the short-lived nature of ARs, we aim to understand their characteristics over the western North Pacific (WNP) in boreal summer, focusing on the difference between the post–El Niño (pEN) and non-post–El Niño (npEN) cases in weekly segments rather than the seasonal mean. We found that during AR-active weeks in pEN summers, a meridional dipole pattern of anomalous geopotential height with a more westward-extended WNP subtropical high dominates, favoring AR activity over southern China to the central Pacific. In npEN summers, in contrast, a zonal dipole pattern with an anomalous low over southeast China and an anomalous high over subtropical WNP is pronounced during the AR-active weeks, which is accompanied with warm sea surface temperature anomalies and greater downward shortwave radiation to the southeast of Japan. Such anomalous circulation leads to enhanced AR occurrence over the northern South China Sea (SCS) to Japan. The above differences in the AR features between the two cases appear as the distinct regionality in the AR-related rainfall over eastern China and Japan. Temporal decompositions reveal that both seasonal mean and submonthly variations play dominant roles in the anomalous circulation during AR-active weeks in pEN summers, while the submonthly variations are more pronounced in npEN summers. Further analysis also suggests that tropical cyclones could impact AR occurrence in both cases with different patterns of atmospheric responses, inducing remarkable AR enhancement over the northern SCS in npEN summers. [ABSTRACT FROM AUTHOR]

The abundant precipitation over Southern China during the pre–flood season (PFS) [i.e., April–June (AMJ)] has important socioeconomic impacts on this densely populated region. Using observational and reanalyzed datasets, this study explores how El Niño affected the subsequent PFS precipitation over Southern China during 1961–2020. The results show that the El Niño–related anomalies in sea surface temperature forced a northwestern Pacific anomalous anticyclone (NPAAC) in the decaying AMJ. This NPAAC featured southwesterly wind anomalies in its northwestern flank, which could transport moisture from the South China Sea, and accompanying the NPAAC there was abnormal descending motion over the tropical western Pacific, resulting in weakened regional Hadley circulation with abnormal ascending motion over subtropical East Asia. Before the 1990s, this abnormal ascending motion was located mainly to the east of Southern China with insignificant impacts on the PFS precipitation there. In contrast, after the early 1990s, El Niño–related warm sea surface temperature anomalies were stronger and longer-lasting with westward extension. This enhanced the NPAAC with a decadal westward extension, and consequently, the anomalous regional Hadley circulation was more evident over Southern China after the early 1990s during the El Niño decaying AMJ, causing strong abnormal upward motion and excessive precipitation there. The present results emphasize an enhancing influence of El Niño on the subsequent PFS precipitation over Southern China since the early 1990s, offering better understanding of the interannual precipitation variability over Southern China and with important implications for regional seasonal climate prediction. Significance Statement: The precipitation over Southern China during the pre–flood season (PFS) contributes between 40% and 50% of the local annual precipitation, with severe floods occurring frequently and tremendous socioeconomic impacts on the region, including on its agriculture, water resources, food security, ecosystems, disaster mitigation, infrastructure construction, and human health. This study reveals a decadal enhancement in the effect of El Niño on the subsequent PFS precipitation over Southern China since the early 1990s. This is ascribed to a decadal enhancement of El Niño–related warm sea surface temperature anomalies over the equatorial central Pacific, as a result of El Niño warming occurring more frequently over the central Pacific against the backdrop of global warming. To the extent that the central Pacific El Niño events would occur even more frequently for the projected future warming scenarios, a stronger effect of El Niño on the PFS precipitation over Southern China would be expected, implying potentially enhanced seasonal predictability of the regional climate in the future. [ABSTRACT FROM AUTHOR]

This study investigates the impact of boreal winter–peaked La Niña on the following-summer precipitation in East Asia through intermediate sea surface temperature (SST) anomalies playing the role of relay in observation and numerical models. There are widespread dry conditions in both North and South China and wet conditions in coastal areas of central and eastern China. Such a pattern is mainly attributed to an anomalous low pressure over the western tropical Pacific and an anomalous anticyclone over northeast Asia. It is found that the delayed impact of La Niña on the East Asian climate is operated through intermediate SST anomalies—the Z-shape cold SST anomalies in the tropical North Pacific. There might be three ways for the SST anomalies to operate. First, they produce tropical atmospheric perturbations that can penetrate into the subtropical jet through the westerly trough over the northeast subtropical Pacific, the wave train being then excited along the jet. Second, perturbations created through the monsoon trough over the western Pacific can directly stimulate northward-propagating Rossby waves along the East Asian coast, mainly at low level. And third, perturbations over the tropical Atlantic–northwest Africa can also trigger downstream propagating waves along the subtropical jet. The observation effects of the intermediate SST anomalies and their possible impact mechanisms on atmospheric circulation are largely reproduced within numerical simulations performed with the Community Earth System Model. [ABSTRACT FROM AUTHOR]

The multidecadal decrease of precipitation in North China since the 1960s has caused a significant impact on the ecological environment and social development. It has become a key concern for meteorologists and national decision-makers to understand what is driving this. To better understand the oceanic impact on the interdecadal variation of summer precipitation in North China, we analyze two 1200-yr global climate simulations from state-of-the-art coupled models with an explainable convolutional neural network (CNN). The accuracy of the CNN model in correctly diagnosing the increased and decreased summer precipitation anomaly in North China reaches 98% and 96.6% in two CMIP6 models, respectively. We find that a sea surface temperature anomaly (SSTA) in the northern Pacific, associated with the Pacific decadal oscillation (PDO), and an SSTA in the northern Atlantic, associated with the Atlantic multidecadal oscillation (AMO), play essential roles in the interdecadal variation of summer precipitation in North China. The modulations of the PDO and AMO on summer precipitation in North China are consistent with the results derived from limited observations and simulations, which means these mechanisms are robust. In particular, when the PDO is in a negative phase and the AMO is in a positive phase, the positive anomaly of precipitation and the associated circulation anomalies are intensified. This study also confirms that with the explainable deep learning method, a convolutional neural network can be used to identify multidecadal variability in North China summer precipitation, although limited datasets such as observational records make it more challenging to apply deep learning methods. [ABSTRACT FROM AUTHOR]

In this study, the fast response of East Asian summer precipitation to COVID-19–induced aerosol emission reductions is examined using the Community Earth System Model, version 2.2 (CESM2.2). The emission reductions decreased aerosol optical depth and cloud cover over northern China in June 2020. The troposphere became warmer, strengthening the land–sea thermal contrast and anomalous southerly winds. The subtropical westerly jet accelerated and shifted southward, favoring low-level convergence, upward air motions, and subsequent condensational heating over the Yangtze River basin (YRB). The feedback of condensational heating in return strengthened the convergence and ascent. The western North Pacific subtropical high was intensified, which further enhanced the moisture advection and convergence over the YRB. Both the enhanced moisture convergence and ascent increased precipitation over the YRB during June 2020. Furthermore, local and remote emission reductions show different impacts on convection and moisture transport over the YRB. The emission reductions over China caused stronger convective precipitation (1.15 vs 0.63 mm day-1 ) but weaker larger-scale precipitation (1.17 vs 2.24 mm day-1 ) than the emission reductions outside China. In addition to the emission reductions, the sea surface temperature (SST) anomalies in 2020 also play an important role in increasing precipitation over the YRB, contributing about 42.8%. The relative contribution of SST anomalies also increases under the COVID-19–induced emission scenario. [ABSTRACT FROM AUTHOR]

In this paper, the authors have identified three coherent modes of summertime rainfall variability over China and global sea surface temperature (SST) for the period of 1955-98 by Singular Value Decomposition. Based on these modes, the impacts of the 1997-98 El Niño on major drought and flood occurrences over China have been assessed. The first mode can be identified with the growing phase of El Niño superimposed on a warming trend since the mid-1950s. This mode strongly influences rainfall over northern China. The second mode comprises a quasi-biennial (QB) variability manifested in alternate wet and dry years over the Yangtze River Valley (YRV) of central China. The third mode is dominated by a quasi-decadal oscillation in eastern China between the Yangtze River and the Yellow River, with an opposite tendency in southern China. Using a mode-by-mode reconstruction, the contributions of these leading modes to the 1997 and 1998 observed rainfall anomalies are evaluated. It is found that the severe drought in northern China, and to a lesser degree the flood in southern China, in 1997 is likely a result of the influence of anomalous SST forcing during the growing phase of the 1997-98 El Niño. The severe flood over YRV in 1998 is associated with the biennial tendency of basin-scale SST anomaly during the transition from El Niño to La Niña in 1997-98. In addition, the prolonged dry tendency over northern China and wet tendency over YRV since the 1970s may be associated with a long-term warming trend in the tropical Indian Ocean and western Pacific. The long-term dry background exacerbated the drought situation over northern China in 1997, and the wet background exacerbated the flood situation over YRV in 1998, under the impacts of the 1997-98 El Niño. In contrast, the rainfall variability in southern China is most chaotic, with no clear dominance of either El Niño or QB signals. The significance, reliability, and... [ABSTRACT FROM AUTHOR]

The features and causes of the leading intermonth modes of winter surface air temperature anomalies (SATA) over China are investigated, and associated prediction models are developed. The first three intermonth modes of winter SATA over China are obtained by extended empirical orthogonal function (i.e., EEOF1–3) analysis. The results show that EEOF1 represents consistent variations in the whole winter, with a variance contribution of 32.3%, whereas EEOF2 and EEOF3 show spatiotemporally inconsistent changes, and their variance contributions are 16.9% and 12.5%, respectively. EEOF2 has out-of-phase variations between December and January–February, and EEOF3 exhibits a temporal warm–cold alternating pattern, with spatially reversing changes over northwestern and southern China. However, the Climate Forecast System, version 2 (CFSv2) presents a limited prediction skill for winter SATA over China and their intermonth modes. Further investigations indicate that the September sea ice over the Barents–Laptev Seas, the November snow cover over western Europe and East Asia, and the November northern Atlantic sea surface temperature can be, respectively, adopted to develop prediction schemes for the consistent mode (EEOF1 scheme) and two inconsistent modes (EEOF2 and EEOF3 schemes) based on specific mechanisms. These schemes show effective performances in predicting both individual modes and the reconstruction field of SATA over China. The temporal correlation coefficients (TCCs) between cross-validation results and observations are 0.48, 0.51, and 0.31 for the EEOF1–3 modes, respectively (the 90% confidence level is 0.27). For the reconstruction field, the TCCs are 0.40, 0.27, and 0.45 in December, January, and February, respectively, which are much higher than those of the CFSv2 outputs (0.23, −0.16, and −0.09). [ABSTRACT FROM AUTHOR]

This study aimed to explore the changing characteristics of summer, compound, extreme hot–humid events over eastern China in recent decades and the associated underlying mechanisms. Results showed that the summer extreme hot–humid events over eastern China have tended to increase but with strong interannual variability in recent decades. Further analyses indicated that the summer sea surface temperature (SST) anomalies over the northwest Pacific (NW Pacific) and western tropical Pacific, and the May Arctic sea ice concentration (SIC) anomalies are the key elements that are closely connected to the variations in extreme hot–humid events over eastern China. The positive SST anomalies over the NW Pacific in summer, on the one hand, could strengthen the local circulation patterns that lead to the anomalous anticyclonic circulation over eastern China. On the other hand, the positive SST anomalies over the western tropical Pacific excite an anomaly pattern similar to the Pacific–Japan pattern that weakens the westerly jet and enhances the anticyclone circulation anomalies over the upper level of eastern China. Both of them provide favorable conditions for extreme hot–humid events over eastern China with increased moisture and temperatures. The sensitivity experiments by the Community Atmosphere Model, version 5 (CAM5), with SST forcing yield similar influencing processes to the observations. Additionally, the effect of SIC may be potentially responsible for the hot conditions in extreme hot–humid events over eastern China. The signal of declining May SIC could persist to summer and then influence the SST anomalies to its south, which could also produce a beneficial background for extreme hot–humid events to increase over eastern China by exciting the Rossby wave train propagating southeastward across the Mongolia–Lake Baikal region. [ABSTRACT FROM AUTHOR]

As subsystems of the Asian summer monsoon, summer precipitation variations in India and the northern part of eastern China (NEC) are physically connected. This study noted that the connection has been significantly enhanced after 1999 compared to 1979-98, which is due to the strengthened water vapor transportation connection between the two regions. That is associated with interdecadal variations of the combined effects of El Niño-Southern Oscillation (ENSO) and sea surface temperature anomalies (SSTAs) over the tropical Indian Ocean (TIO) on the northwest Pacific subtropical high (NWPSH) and the Indo-Pacific Walker cell. Against the background of La Niña, the strengthened NWPSH and Indo-Pacific Walker cell favor water vapor transport to India and the NEC since 1999. Accordingly, summer precipitation in the two regions increases simultaneously, leading to the enhancement of the summer precipitation teleconnection between them. In addition, the influence of TIO SSTAs and the Indian Ocean dipole (IOD) on Indo-Pacific circulations decreases, which further enhances the relative importance of ENSO on the summer precipitation in the two regions. However, during 1979-98, La Niña SSTAs has weak effects on the NWPSH and Indo-Pacific Walker cell, the negative TIO SSTAs significantly weaken NWPSH, and the negative IOD also obstructs the westward extension of the Indo-Pacific Walker cell. Circulations and water vapor transportation related to the Indian Ocean and NEC summer precipitation are inconsistent, resulting in a weak precipitation teleconnection between them. The above conclusions are also validated by extreme case analysis and CMIP6 models. [ABSTRACT FROM AUTHOR]

Eastern China (EC) suffered an extreme drought with long-lasting duration and record-breaking intensity in late summer–autumn 2019. Our diagnosed results show that the central Pacific (CP) El Niño, in tandem with warm sea surface temperature anomalies (SSTAs) over the Kuroshio Extension (KE) region, induces the meridionally elongated cyclonic circulation anomalies stretching from the western North Pacific (WNP) to the Yellow Sea. Its western flank corresponds to overwhelming low-level northerly wind anomalies over EC, which result in deficient moisture and anomalous descent over EC and hence cause the extreme drought in 2019. To investigate the relative contributions of SSTAs over different regions, we performed sensitivity experiments, and analyzed the relationship between extreme drought like what occurred in 2019 (a 2019Drought-like event) and the SSTAs in CMIP6 historical simulations. Modeling evidences reveal that both warm SSTAs over the central equatorial Pacific and the KE region are indispensable for shaping the meridionally elongated cyclone anomaly. Specifically, the cyclone anomaly over the WNP induced by CP El Niño aligns with the cyclone anomaly over the Yellow Sea induced by the warm SSTAs over the KE region, merging into a meridionally stretched cyclone anomaly to the east of EC. Consequently, the northerly anomalies stretch across EC, leading to unfavorable atmospheric conditions and the rainfall deficit there. Projection results show that the occurrence probability of a 2019Drought-like event will increase by 20% (decrease by 40%–50%) under a high (medium-low) emission scenario compared to present-day climate, indicating the nonlinear response of extreme drought to different emission scenarios and the urgency of carbon emission reduction. Significance Statement: An extreme drought hit the Eastern China (EC) region in 2019 and caused tremendous losses. This study proposed that both the 2019 CP El Niño and the warm SST anomalies over the Kuroshio Extension (KE) region induce the meridionally elongated circulation anomalies and the resultant extreme drought. We showed that the typical circulation anomalies induced by central Pacific (CP) El Niño cannot totally explain the meridionally elongated circulation anomalies in 2019. Our modeling evidences confirmed the indispensable role of warm SST anomalies over KE region in the 2019 extreme drought's formation. The projection results show that extreme drought like that in 2019 will occur more (less) frequently under a high (medium-low) emission scenario compared to modern-day level, indicating the urgency of carbon emission reduction. [ABSTRACT FROM AUTHOR]

This study reveals that the relationship between autumn north tropical Atlantic (NTA) sea surface temperature (SST) anomalies and Northeast China's winter snowfall (NECWS) undergoes a remarkable interdecadal enhancement after 2001. Previous research confirmed that the relationship between the NTA SST anomaly and atmospheric circulation experienced interdecadal changes after the 2000s and suggested various reasons for this phenomenon. During 1961–2000, the NTA SST anomaly has a significantly positive correlation with other oceans, especially the tropical Indian Ocean (TIO), and the latter modulates the former's impact on atmospheric circulations over the Eurasian continent with a cancelling effect, which results in a weaker relationship of the NTA SST anomaly and NECWS. In contrast, the warm NTA SST anomaly is relatively independent from other oceans during 2001–20, and it proves to be the forcing factor for NECWS since its solo influence on the winter atmospheric circulations initiated from the North Atlantic to East Asia is more robust, featuring the negative phase of the North Atlantic Oscillation and a downstream quasi-barotropic Rossby wave train over the mid- to high latitudes of Eurasian continent. Accordingly, together with the deepened East Asian trough and the strongly northward transported humid and warm air from the western Pacific, the local significantly enhanced ascending motions with cooling temperature favor much more NECWS. The linear baroclinic model simulates the effects of NTA and TIO SST anomalies on winter atmospheric circulations, corroborating the aforementioned results. Such results can be used for the prediction of NECWS with respect to the precursor of the autumn NTA SST anomaly. [ABSTRACT FROM AUTHOR]

Precipitation variations over southern China in February and March have profound influences on local agricultural activities. Therefore, the leading intermonthly variation mode of February-to-March precipitation over southern China is investigated in this study using the extended empirical orthogonal function (EEOF) method. The first leading EEOF (EEOF1) pattern shows a persistent precipitation anomaly from February to March over southern China. Mechanistic analysis indicates that EEOF1's different phases are related to different factors. A dipole sea surface temperature (SST) pattern with warm anomalies in the South China Sea (SCS) and cold anomalies in the western tropical Pacific (WTP) can lead to negative EEOF1 by exciting an anomalous Philippine Sea cyclone. In contrast, positive EEOF1 is influenced jointly by dipole patterns of SST in the southeast Indian Ocean (SEIO)–WTP area and atmospheric circulation over Europe and West Asia. The individual influence of the SST and atmospheric dipole patterns cannot induce persistent precipitation anomalies over southern China from February to March. Furthermore, we check the prediction skill of the National Center for Atmospheric Research (NCEP) Climate Forecast System (CFSv2) for positive and negative EEOF1. The evaluation results show that CFSv2 can predict the relationship between negative EEOF1 and the SCS–WTP dipole SST pattern, consequently showing skillful prediction for negative EEOF1 at the 1-month lead, with a correct rate of approximately 60%. However, CFSv2 cannot reproduce the combined effect of the aforementioned atmospheric dipole pattern and the SEIO–WTP dipole SST pattern and is therefore unskillful for predicting positive EEOF1, with a correct rate of only approximately 36%. [ABSTRACT FROM AUTHOR]

Tropical cyclone (TC) translation distance, proportional to its duration and translation speed, basically determines the spatial extent of TC-impacted marine and terrestrial areas. Although a long-term slowdown of TCs has recently been reported, changes in translation distance of TCs over the western North Pacific (WNP) and their driving mechanisms remain poorly understood. Using multiple TC datasets, here we find that the trends are opposite in overland translation distance of landfall WNP TCs over China and excluding China, with the number of landfalls in the two groups being almost the same. However, the increase in overland translation distance of landfall TCs over China is offset by a greater decrease in that excluding China, with the result that the overland translation distance of TCs over the WNP has declined (−4.0% decade−1). The oversea translation distance of TCs over the WNP also shows a decreasing trend (−2.4% decade−1), and thus the genesis-to-demise translation distance of TCs over the WNP has significantly decreased (by 17.7%) during 1961–2019. The shorter duration and reduced translation speed of TCs directly and jointly contribute (by 76.9% and 23.1%, respectively) to their shortened translation distance. The gradual increase in westerly and northerly winds and weaker moisture transport during the days following TC generation tend to inhibit long-distance travel after TC genesis. Declining TC translation distance is likely associated with the long-term increases in genesis potential index and an anomalous cyclonic steering flow over the WNP. Our findings suggest that these changes in TC translation distance may alter regional patterns of TC-related drying and wetting of the WNP. Significance Statement: The spatial extent affected by a tropical cyclone (TC) is largely determined by TC translation distance. For example, TCs passing through marine areas could cool sea surface temperature and modulate local air–sea heat fluxes exchange; TCs with longer translation distances over land are more likely to trigger widespread flooding or terminate droughts in inland areas. Over the western North Pacific (WNP), we find that observational evidence supports opposite trends in overland translation distance of landfall TCs over China and excluding China. Nevertheless, the genesis-to-demise translation distance of TCs shows a significant decline over the WNP during the past six decades. Understanding the impacts of changes in TC translation distance has particular importance given the extensive footprint of TC-related drying and wetting of the marine and terrestrial areas. [ABSTRACT FROM AUTHOR]

Air quality over the North China Plain (NCP) and rainfall over southern China exhibit robust and correlated interannual variations during winter from 1979 to 2018. A correlation coefficient of 0.7 between the PM2.5 concentration over the NCP and rainfall in southern China indicates that poor air quality and rainfall are linked (referred to as the south rainfall–north pollution pattern), which is likely due to the strong eastern Pacific (EP) El Niño. In the mature phase of the strong EP El Niño, the Northeast Asian anomalous anticyclone (NAAA) is strengthened by two Rossby wave trains modulated by warmer sea surface temperatures. One wave train is directly related to the strong EP El Niño and originates in the tropical eastern Pacific, propagating eastward to East Asia along the middle and high latitudes, which weakens the Ural ridge and strengthens the NAAA. The other wave train is derived from the northern Indian Ocean and propagates into East Asia along the great circle route, which strengthens an anomalous cyclone in southern China and the NAAA. This results in an anomalous horizontal trough of 850 hPa being captured in eastern China at ∼30°N. The southwesterly flow along the southern margin of the trough transports abundant moisture to southern China, which leads to heavy rainfall in southern China combined with an anomalous ascending motion related to the anomalous cyclone. The southeasterly flow to the north of the trough weakens the East Asia winter monsoon due to a weakened Ural ridge, which then leads to a high PM2.5 concentrations over the NCP with the help of an anomalous descending motion related to the NAAA. [ABSTRACT FROM AUTHOR]

The spring tripole sea surface temperature (SST) anomalies in North Atlantic are an outstanding regional mode of interannual variability. Based on the observed and reanalyzed datasets during 1979–2019, this study reveals the relationship and linking mechanism between the spring tripole North Atlantic SST anomalies and the central China July precipitation (CCJP). Results show that the tripole SST anomalies, especially the warm SST anomalies in the tropical North Atlantic (TNA) and the subpolar North Atlantic (SNA), often cause surplus CCJP through the tropical and extratropical pathways. On the one hand, the spring TNA SST warming induces a pan-tropical climate response with the cooling in the central equatorial Pacific and the warming in the Indo-western Pacific until the following July through a series of air–sea interactions, helping maintain an anomalous anticyclone over the northwest Pacific and transport more warm humid flows to central China. On the other hand, the spring TNA and SNA SST warming persist into the following July and then emanate a wave train extending from the SNA throughout the Eurasian continent to East Asia, which induces an anomalous anticyclone over North China with its southeast flank transporting more cold air to central China. The warm humid flows from the south against the cold air from the north are conductive to the local ascending motion, favoring the increased CCJP. Our results highlight both the tropical and extratropical teleconnection pathways of the North Atlantic SST anomalies affecting the CCJP. This suggests an important seasonal predictor of the regional climate. Significance Statement: July is the peak rainy month of central China, with heavy precipitation occurring frequently and often causing serious impacts on the local production and livelihood of millions of people. This study finds that the spring tripole sea surface temperature anomalies in North Atlantic induced by the North Atlantic Oscillation can exert significant impacts on the following July precipitation over central China through both the tropical and extratropical pathways. This improves our understanding of the causes of the surplus July precipitation over central China and has important implications for the seasonal predictability of the regional climate. [ABSTRACT FROM AUTHOR]

Spring rainfall is important for agriculture and economics in North China (NC). Thus, there is an imperative need for accurate seasonal prediction of the spring precipitation. This study implements a novel rainfall framework to improve understanding of NC spring rainfall. The framework is built based on a three-cluster normal mixture model. Distribution parameters are sampled using Bayesian inference and a Markov chain Monte Carlo algorithm. The probability behaviors of light, moderate, and heavy rainfall events can be reflected by the three rainfall clusters, respectively. Analysis of 61-yr data indicates that moderate rainfall makes the largest contribution (67%) to the total rainfall amount. The moderate rainfall intensity is mainly influenced by the sea surface temperature anomaly (SSTA) in the previous season over the equatorial eastern Pacific, and rainfall frequency is influenced by geopotential height anomaly in the mid- to high latitudes in spring. It is also found that more extreme precipitation events can be observed in the spring following an eastern Pacific El Niño event in the previous autumn and winter. Based on these relationships, we develop a multiple linear regression model. Hindcasts for spring precipitation using the model indicates that its anomaly correlation is 0.48, significant at the 99% confidence level. The result suggests that the newly developed model can well predict spring rainfall amount in NC. [ABSTRACT FROM AUTHOR]

The interannual variability in summer precipitation intraseasonal oscillation intensity over southwest China (SWC) is investigated in this study. The results indicate that the 7–20-day period dominates the intraseasonal variability in summer SWC precipitation. The leading mode of summer SWC precipitation 7–20-day oscillation intensity (SPOI) is a north–south dipole pattern with prominent interannual variability. The atmospheric circulation anomalies from both tropics and mid- to high latitudes are responsible for the interannual variability in the dipole pattern. In the tropics, an enhanced local Hadley cell and an anomalous anticyclone over southern China and the northwest Pacific contribute to the north-positive–south-negative pattern of SPOI by inducing moisture convergence (divergence) over northern (southern) SWC in the background state. In the mid- to high latitudes, the 7–20-day Rossby wave trains along the subtropical jet are crucial for the 7–20-day precipitation over northern SWC. Further analyses suggest that the sea surface temperature (SST) anomalies over the Maritime Continent (MC) and the North Atlantic (NA) are associated with the SPOI dipole pattern. The MC SST warming causes convection anomalies over the tropical Indo-Pacific, consequently triggering a Matsuno–Gill-type atmospheric response conducive to the north-positive–south-negative pattern of SPOI. The NA SST tripole triggers a Rossby wave train across Eurasia that strengthens the East Asian westerly jet and enhances 7–20-day atmospheric variability, consequently favoring the variability of 7–20-day precipitation over northern SWC. Diagnoses of moisture and vorticity budgets further indicate the importance of the interaction between intraseasonal fluctuations and atmospheric background in the formation of the north–south difference in 7–20-day precipitation variability over SWC. [ABSTRACT FROM AUTHOR]

Impacts of El Niño on the following summer East Asian precipitation are dependent on its peak-time characteristics. Three types are identified and denoted as eastern Pacific (EP), mixed-type Pacific (MP), and central Pacific (CP) El Niño. For EP, excessive rainfall occurs in the Yangtze River valley, in relation to an anomalous anticyclone over the western tropical Pacific. For MP, a dipolar precipitation with anomalous wet conditions in northern China and dry conditions in east-central China is mainly due to an anomalous cyclone over Northeast Asia and a weak anticyclone over the Sea of Japan. For the case of CP El Niño, wetness along the Yangtze–Huaihe River valley and dryness in Southeast China are ascribed to an anomalous cyclone in Northeast China and a strong anticyclone over the western tropical Pacific. It is also revealed that the precipitation anomalies are fundamentally caused by different intermediate sea surface temperature (SST) anomalies from other basins while El Niño decays in the following summer. In the case of EP El Niño, the joint action from warm SST in the tropical Indian Ocean, the Niño-1.2 region, and the North Atlantic is responsible for the delayed effect of El Niño. For MP, the main delayed effect comes from warm SST in the northeast subtropical Pacific, developing La Niña, and cold SST in the southern tropical Atlantic. Finally, CP El Niño exerts its delayed influence through warm SST in the northern tropical Atlantic. [ABSTRACT FROM AUTHOR]

The first rainy season (April–June) of South China includes the phases before and after the onset of the South China Sea summer monsoon (SCSSM). Abundant moisture supply is the key dynamic process for precipitation formation. Thus, we employ the FLEXPART model to explore the corresponding moisture sources for the two phases. Before the onset of SCSSM, land regions contribute more moisture to the precipitation over South China than the ocean sources. The main source regions are Southeast Asia (27.01%), the South China Sea (25.96%), South China (11.12%), and the southern part of the northwestern Pacific (10.23%). Land sources (66.87%) play a more important role than ocean sources (33.13%) in the interannual variations, with the contributions mainly from Southeast Asia (47.56%) and the South China Sea (28.79%). After the onset of SCSSM, the climatological contribution of ocean sources is larger than that of land regions, and the main source regions are the South China Sea (20.78%), Southeast Asia (17.51%), the Bay of Bengal (13.76%), and South China (11.21%). For the interannual variations, the contributions of land sources and ocean regions are comparable, and mainly from Southeast Asia (33.53%) and the Bay of Bengal (32.26%). The moisture transports for the interannual variations in first rainy season precipitation over South China before and after the onset of SCSSM are significantly correlated with the east–west contrast of sea surface temperature anomalies over northern part of North Pacific and the uniform warming over Indian Ocean, respectively. This study provides important guidance in improving the regional precipitation predictions and understanding the water resources changes. [ABSTRACT FROM AUTHOR]

The reported decreasing trend of the annual tropical cyclone (TC) landfalls in southern China and increasing trend in southeastern China in recent decades are confirmed to be an abrupt shift occurring at the end of the twentieth century, based on a statistical analysis. The opposite trends in the two adjacent regions are often considered to be a result of tropical cyclone landfalls in southern China being deflected northward. However, it is demonstrated in this study that they are phenomenally independent. In fact, the abrupt decrease of TC landfalls in southern China occurs as a result of an abrupt decrease of the westward events in the postpeak season (October–December), which in turn is a consequence of a significant decrease of the TC genesis frequency in the southeastern part of the western North Pacific (WNP) Ocean basin. On the other hand, the abrupt increase of TC landfalls in southeastern China occurs because of an abrupt increase of the northwest events in the peak season (July–September), as the consequence of a statistically westward shift of TC genesis. The relevant variations of TC genesis are shown to be mainly caused by decreased relative vorticity and increased vertical wind shear, which, however, are intrinsically related to the accelerated zonal atmospheric circulation driven by a La Niña–like sea surface warming pattern over the WNP that developed after the end of twentieth century. [ABSTRACT FROM AUTHOR]

Because spring precipitation in East Asia is critical for recharging water resources after dry winters, its spatiotemporal variations and related mechanisms need in-depth research. This study analyzed a leading spatiotemporal variability of precipitation over East Asia for boreal spring (March-May) during 1979 to 2017. We found that a dipole mode dominates the anomalous spring precipitation between southern China and Southeast Asia with significant interannual and decadal variations. The interannual dipole mode is attributable to the eastern Pacific (EP)-type El Niño-Southern Oscillation (ENSO) while the decadal dipole mode is related to the decadal variation of the central Pacific (CP)-type ENSO. In the El Niño phases of both time scales, the anticyclonic anomaly over the South China Sea and Philippines causes moisture convergence (divergence) over southern China (Southeast Asia), resulting in positive (negative) precipitation anomalies therein; the opposite occurs in the La Niña phases. The ensemble experiments using the Community Atmosphere Model version 5.1 confirmed that the tropical sea surface temperature (SST) in the EP- and CP-type ENSO can be the major drivers of the interannual and decadal dipole modes, respectively. About half of 15 climate models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5) showed that the El Niño phase of dipole mode will become dominant in the future. The individual models' future projections however considerably vary, implying that there is still large uncertainty. [ABSTRACT FROM AUTHOR]

The dynamic origin of the interannual variability of West China autumn rainfall (WCAR), a special weather/climate phenomenon over western-central China in September and October, was investigated via observational diagnosis and numerical simulations. Here we found that the interannual variability ofWCARis closely related to the local horizontal trough, which is passively induced by two lower-level anticyclonic (high pressure) anomalies over East Asia. The anticyclonic anomaly over the south is a Gill-type response to the central and eastern Pacific diabatic cooling, while that over the north is part of the mid- to high-latitude barotropic Rossby wave train, which could be induced by either the thermal forcing of the central and eastern Pacific Ocean sea surface temperature (SST) cooling or that of the subtropical northern Atlantic Ocean SST warming. The quasi-barotropic high pressure anomaly over East Asia acts as an ''invisible mountain'' that steers the low-level anomalous southwesterly into a southeasterly and hinders the water vapor going farther to the north, leading to enhanced WCAR. However, the real mountain ranges in the region (the Qinglin and Ba Mountains) have no essential impact on the formation and interannual variability of WCAR. [ABSTRACT FROM AUTHOR]

The influence and mechanism of volcanic eruptions on decadal megadroughts over eastern China during the last millennium were investigated using a control (CTRL) and five volcanic eruption sensitivity experiments (VOLC) from the Community Earth System Model (CESM) Last Millennium Ensemble (LME) archive. The decadal megadroughts associated with the failures of the East Asian summer monsoon (EASM) are associated with a meridional tripole of sea surface temperature anomalies (SSTAs) in the western Pacific from the equator to high latitudes, suggestive of a decadal-scale internal mode of variability that emerges from empirical orthogonal function (EOF) analysis. Composite analyses further showed that, on interannual time scales, within a decade after an eruption the megadrought was first enhanced but then weakened, due to the change from an El Niño state to a La Niña state. The impacts of volcanic eruptions on the magnitudes of megadroughts are superposed on internal variability. Therefore, the evolution of decadal megadroughts coinciding with strong volcanic eruptions demonstrate that the impacts of internal variability and external forcing can combine to influence hydroclimate. [ABSTRACT FROM AUTHOR]

We simulate the response of Asian summer climate to Atlantic multidecadal oscillation (AMO)-like sea surface temperature (SST) anomalies using an intermediate-complexity general circulation model (IGCM4). Experiments are performed with seven individual AMO SST anomalies obtained from CMIP5/PMIP3 global climate models as well as their multimodel mean, globally and over the North Atlantic Ocean only, for both the positive and negative phases of the AMO. During the positive (warm) AMO phase, a Rossby wave train propagates eastward, causing a high pressure and warm and dry surface anomalies over eastern China and Japan. During the negative (cool) phase of the AMO, the midlatitude Rossby wave train is less robust, but the model does simulate a warm and dry South Asian monsoon, associated with the movement of the intertropical convergence zone in the tropical Atlantic. The circulation response and associated temperature and precipitation anomalies are sensitive to the choice of AMO SST anomaly pattern. A comparison between global SST and North Atlantic SST perturbation experiments indicates that East Asian climate anomalies are forced from the North Atlantic region, whereas South Asian climate anomalies are more strongly affected by the AMO-related SST anomalies outside the North Atlantic region. Experiments conducted with different amplitudes of negative and positive AMO anomalies show that the temperature response is linear with respect to SST anomaly but the precipitation response is nonlinear. [ABSTRACT FROM AUTHOR]

The relationship between the tropical west Pacific (TWP) and East Asian summer monsoon/precipitation has been documented in previous studies. However, the stability for the signals of midsummer precipitation in the TWP sea surface temperature (SST_TWP), which is important for climate variation, has drawn little attention. This study identifies a strengthened relationship between the leading empirical orthogonal function mode (EOF1) of midsummer precipitation over Northeast China (NEC) and the SST_TWP after the mid-1990s. The EOF1 mode shows a significant positive correlation with the SST_TWP for 1996–2016, whereas the relationship is statistically insignificant for 1961–90. Further results indicate that the North Pacific multidecadal oscillation (NPMO) shifts to a positive phase after the 1990s. In the positive NPMO phase, the anomalous circulation over the northeast Pacific expands westward over the central North Pacific–Aleutian Islands region. Concurrently, the SST_TWP-associated wavelike pattern propagates northeastward from the west Pacific to the northwest Pacific and farther to the North Pacific, facilitating the poleward expansion and intensification of the SST_TWP-related circulation anomalies over the North Pacific. Therefore, the SST_TWP has an enhanced influence on NEC precipitation through the modulation of the circulation anomalies over the central North Pacific–Aleutian Islands region after the mid-1990s. Additionally, the tropical anticyclone/cyclone associated with the SST_TWP expands westward to South China, exerting an intensified impact on meridional wind anomalies along eastern China and on moisture transport over NEC. These conditions jointly contribute to the strengthened relationship between the SST_TWP and the EOF1 mode of NEC midsummer precipitation after the mid-1990s. [ABSTRACT FROM AUTHOR]

This study investigates the association between summer high temperature extremes (HTEs) over China and the Pacific meridional mode (PMM) that is characterized by an anomalous north–south sea surface temperature gradient and an anomalous surface circulation over the northeastern subtropical Pacific. It is found that the HTE activities over most parts of southern China (particularly eastern China) are prominently intensified during the positive PMM phase and weakened during the negative phase. Further examinations suggest that thePMMis linked with HTEs in China through processes that entail both eastward and westward development of signals emanating from the PMM site. The westward development is associated with the formation of an anomalous low-level cyclone over the western North Pacific (WNP), which may be viewed as a Matsuno–Gill-type response to the off-equatorial heating in the eastern Pacific. This circulation change is accompanied by anomalous ascent overWNPand northern China, and subsidence over eastern China. On the other hand, the eastward development process is linked to the PMM-induced displacement of the East Asian jet stream and the generation of a midlatitude Rossby wave train. In the positive PMM phase, the above circulation changes are accompanied by anomalous air subsidence and enhanced adiabatic heating, reduced precipitation, anomalous lower-level anticyclone, and rising surface pressure over the eastern part of China. Moreover, the land surface of that region receives more solar radiation. Opposite changes are discernible over northern China. These changes are favorable for the occurrence and persistence of HTEs over eastern China and tend to suppress HTEs over northern China. [ABSTRACT FROM AUTHOR]

In recent years, haze pollution has become the most concerning environmental issue in China due to its tremendous negative effects. In this study, we focus on the enhanced responses of December–January haze days in North China to September–October sea ice in the Beaufort Sea during 1998–2015. Via both observation and numerical approaches, compared with an earlier period (1980–97), the sea ice concentration in the Beaufort Sea presented large variability during 1998–2015. During 1980–97, the Beaufort Sea was mainly covered by perennial ice, and the ablation and freezing of sea ice mainly occurred at the south edge of the Beaufort Sea. Thus, heavy sea ice in autumn induced negative sea surface temperature anomalies across the Gulf of Alaska in November. However, the colder sea surface in the Gulf of Alaska only induced a weak influence on the haze-associated atmospheric circulations. In contrast, during 1998–2015, a drastic change in sea ice existed near the center of the Arctic Ocean, due to the massive melting of multiyear sea ice in the western Beaufort Sea. The perennial ice cover in the western Beaufort Sea was replaced by seasonal ice. The broader sea ice cover resulted in positive sea surface temperature anomalies in the following November. Then, suitable atmospheric backgrounds were induced for haze pollution in December and January. Simultaneously, the response of the number of haze days over North China to sea ice cover increased. These findings were verified by the CESM-LE simulations and aided in deepening the understanding of the cause of haze pollution. [ABSTRACT FROM AUTHOR]

Recent studies demonstrated the existence of a conspicuous atmospheric combination mode (C-mode) originating from nonlinear interactions between El Niño–Southern Oscillation (ENSO) and the Pacific warm pool annual cycle (AC). Here we find that the C-mode exhibits prominent decadal amplitude variations during the ENSO decaying boreal spring season. It is revealed that the Atlantic multidecadal oscillation (AMO) can largely explain this waxing and waning in amplitude. A robust positive correlation between ENSO and the C-mode is detected during a negative AMO phase but not during a positive phase. Similar results can also be found in the relationship of ENSO with 1) the western North Pacific (WNP) anticyclone and 2) spring precipitation over southern China, both of which are closely associated with the C-mode. We suggest that ENSO property changes due to an AMO modulation play a crucial role in determining these decadal shifts. During a positive AMO phase, ENSO events are distinctly weaker than those in an AMO negative phase. In addition, El Niño events concurrent with a positive AMO phase tend to exhibit a westward-shifted sea surface temperature (SST) anomaly pattern. These SST characteristics during the positive AMO phase are both not conducive to the development of the meridionally asymmetric C-mode atmospheric circulation pattern and thus reduce the ENSO/C-mode correlation on decadal time scales. These observations can be realistically reproduced by a coupled general circulation model (CGCM) experiment in which North Atlantic SSTs are nudged to reproduce a 50-yr sinusoidally varying AMO evolution. Our conclusion carries important implications for understanding seasonally modulated ENSO dynamics and multiscale climate impacts over East Asia. [ABSTRACT FROM AUTHOR]

The present study investigates the interdecadal changes in the relationship between El Niño–Southern Oscillation (ENSO) and midlatitude North Atlantic (MNA) sea surface temperature (SST) with northwest China (NWC) winter precipitation (WP) variability and the plausible causes. Results show that ENSO and MNA SST have weak correlations with NWC WP before the mid-1990s, whereas the connections are enhanced sharply afterward, with above (below) normal precipitation occuring when there are positive (negative) ENSO SST and negative (positive) MNA SST anomalies (SSTA). Remarkable differences are found in the atmospheric circulations. After the mid-1990s, there is a pronounced Pacific–North American–Eurasian (PNA-EU)-like pattern in the Northern Hemisphere, whereas an Arctic Oscillation–like pattern is found before the mid-1990s. The change in the relationships between NWC WP and SSTs is likely attributable to the enhanced connection between ENSO and MNA SST after the mid-1990s. It is found that ENSO and MNA SSTA can cause NWC WP variation independently through atmospheric teleconnections. In addition, significant precipitation anomalies also occur when concurrent but oppositely signed SSTs anomalies in the two regions are observed. The reinforced negative correlations between ENSO and MNA SST after the mid-1990s act in concert on NWC WP by exciting a PNA-EU-like pattern. This information would help us to better understand the physical processes of the teleconnections between NWC WP variability and the ENSO/MNA SST, in which the strength of the correlation between ENSO and MNA SST should be taken into account. [ABSTRACT FROM AUTHOR]

Based on the long-term reanalysis datasets and the multivariate copula method, this study reveals that the frequency of summer hot drought events (SHDEs) over northeastern China (NEC) shows interdecadal variations during 1925–2010. It is revealed that the summer sea surface temperature (SST) over the North Atlantic has a significant positive correlation with the frequency of SHDEs over NEC on the decadal time scale, indicating a potential influence of the Atlantic multidecadal oscillation (AMO). Further analyses indicate that during the positive phases of the AMO, the warming SST over the North Atlantic can trigger a stationary Rossby wave originating from the North Atlantic, which splits into two wave trains propagating along two different routes. One is a zonally orientated wave train that resembles the Silk Road pattern, whereas the other is an arching wave train that resembles the polar–Eurasian pattern. A negative (positive) phase of the Silk Road pattern (polar–Eurasian pattern) may result in the weakened westerly wind along the jet stream, the downward vertical motion, and the anomalous positive geopotential center over NEC, providing favorable conditions for precipitation deficiency and high temperature and resulting in increased SHDEs. Thus, the Silk Road pattern and the polar–Eurasian pattern serve as linkages between the AMO and SHDEs over northeastern China in summer on the interdecadal time scale. Model simulations from CAM4 perturbed with warmer SST in the North Atlantic show precipitation deficiency and high temperature conditions over northeastern China in summer, supporting the potential impacts of the North Atlantic SST on SHDEs over northeastern China. The results suggest that the phase of the AMO should be taken into account in the decadal prediction of SHDEs over northeastern China in summer. [ABSTRACT FROM AUTHOR]

This study investigates the trend in destructive potential of landfalling tropical cyclones (TCs) in terms of power dissipation index (PDI) over mainland China in the period of 1980–2018. Results show that both the accumulated PDI and averaged PDI after landfall show significant increasing trends. The increasing trends are found to be contributed primarily by the increasing mean duration of TCs over land and the increasing TC intensity at landfall. Further analyses indicate that the increase in landfalling TC intensity prior to and at landfall, the decrease in intensity weakening rate after landfall, and the northward shift of landfalling TC track density all contribute to the longer duration of TCs after landfall. Moreover, the conducive large-scale conditions, such as the increases in coastal sea surface temperature and land surface temperature and soil moisture, the decrease in low-level vertical wind shear, and the increase in upper-level divergence, are all favorable for intense landfalling TCs and their survival after landfall, thus contributing to the increasing destructive potential of landfalling TCs over China. [ABSTRACT FROM AUTHOR]

An empirical orthogonal function (EOF) analysis was conducted for spring precipitation gauge data over northeast China (NEC). The first EOF mode is characterized by a homogenous rainfall pattern throughout NEC. The corresponding principal component has both significant interannual and interdecadal variations. This leading mode explains a large portion of the total NEC spring rainfall (NECSR) variances and is statistically independent from other higher modes. The physical processes responsible for the interannual and interdecadal variabilities were investigated via observational diagnoses and numerical experiments. On the interannual time scale, NECSR is mainly affected by the SST anomalies (SSTAs) in the northern tropical Atlantic Ocean. When the SSTAs are positive, the subsequently induced positive precipitation and convection can stimulate two quasi-barotropic Rossby wave trains over the mid- to high latitudes. A cyclonic anomaly center of the Rossby wave train appears over northeastern Asia, leading to a positive rainfall anomaly in the region. On the interdecadal time scale, NECSR is mainly influenced by the SSTAs over the warm-pool region. Positive SSTAs in the warm-pool region result in enhanced convection (ascending motion) around the Maritime Continent and suppressed convection (descending motion) over the central equatorial Pacific Ocean. This zonal dipole convection pattern stimulates a quasi-barotropic circulation pattern with an anticyclonic anomaly over the Tibetan Plateau and a cyclonic anomaly over northeastern Asia. The cyclonic anomaly over northeastern Asia enhances the NECSR. Numerical experiments further suggested that the convective heating anomaly over the Maritime Continent, rather than cooling over the central equatorial Pacific, plays a more essential role in driving the interdecadal rainfall variability of NECSR. [ABSTRACT FROM AUTHOR]

Seasonal covariability of the dryness/wetness in China and global sea surface temperature (SST) is investigated by using the monthly self-calibrated Palmer drought severity index (PDSI) data and other data from 1950 to 2014. The singular value decomposition (SVD) analysis shows two recurring PDSI-SST coupled modes. The first SVD mode of PDSI is associated with the warm phases of the eastern Pacific-type El Niño-Southern Oscillation (ENSO), the interdecadal Pacific oscillation (IPO) or Pacific decadal oscillation (PDO), the Indian Ocean basin mode (IOBM) in the autumn and winter, and the cold phase of the IOBM in the spring. Meanwhile, the Atlantic multidecadal oscillation (AMO) pattern appears in every season except the autumn. The second SVD mode of PDSI is accompanied by a central Pacific-type El Niño developing from the winter to autumn over the tropical Pacific and a positive phase of IPO or PDO from the winter to summer. Moreover, an AMO pattern is observed in all seasons except the summer, whereas the SST over the tropical Indian Ocean shows negligible variations. The further analyses suggest that AMO remote forcing may be a primary factor influencing interdecadal variability of PDSI in China, and interannual to interdecadal variability of PDSI seems to be closely associated with the ENSO-related events. Meanwhile, the IOBM may be a crucial factor in interannual variability of PDSI during its mature phase in the spring. In general, the SST-related dryness/wetness anomalies can be explained by the associated atmospheric circulation changes. [ABSTRACT FROM AUTHOR]

In the early 1990s, the mei-yu rainfall over South China in early boreal summer exhibited an abrupt change and northward extension. This change altered the pattern of East Asian summer rainfall from a dipole-like to a monopole-like pattern; that is, the out-of-phase relationship between the rainfall in the south and that in the north of the Yangtze and Huaihe River valley changed to an in-phase relationship. The physical processes potentially responsible for triggering this abrupt change were analyzed in this study. Our observations revealed that the western North Pacific subtropical high (WNPSH), sea surface temperature (SST) in the subtropical eastern North Pacific (SENP), and the mei-yu rainfall in South China exhibited an abrupt increase in the early 1990s, suggesting that these factors are correlated. From the observations and results of numerical experiments, we proposed that the abrupt SST warming in the SENP in the early 1990s generated an east-west overturning circulation anomaly in the Pacific Ocean and that the anomalous downward motion in the western North Pacific consequently triggered the abrupt increase and westward extension of the WNPSH in the early 1990s. The enhanced and westward extension of WNPSH created a low-level southeasterly anomaly that transported considerable humid and warm air into East Asia and sequentially triggered the abrupt increase of mei-yu rainfall in the South China in the early 1990s. [ABSTRACT FROM AUTHOR]

A linear regression model is constructed to predict the April–May precipitation in central China (PCC) with a lead time of 1–2 months. This model not only reproduces the historical April–May PCC from 1985 to 2006 but also demonstrates strong robustness and reliability during the independent test period of 2007–16. Two preceding factors are selected to build the model, the February–March Arctic stratospheric ozone (ASO) and Indian Ocean sea surface temperature (IOSST), indicating a synergistic impact of Arctic and tropical signals on the midlatitude climate. A possible mechanism of ASO changes affecting Chinese precipitation is that the stratospheric circulation anomalies related to ASO changes may downward influence circulation over North Pacific, and then extend westward to influence East Asia, leading to changes in Chinese precipitation. Anomalies of the other predictor, IOSST, are associated with a baroclinic structure over central China. For example, warm IOSST causes anomalous convection over central China and affects the warm and humid airstream flowing from the Pacific to China. These processes related to the two predictors result in the April–May PCC anomalies. Sensitivity experiments and a transient experiment covering a longer period than the observations/reanalysis support the results from our statistical analysis based on observations. It implies that this statistical model could be applied to the output of seasonal forecasts from observations and improve the forecasting ability of April–May PCC in the future. [ABSTRACT FROM AUTHOR]

Here we investigate the response of boreal spring precipitation over southern China (SPSC) to central Pacific (CP) El Niño based on observational datasets. While there is enhanced precipitation over southern China during the decaying boreal spring of eastern Pacific (EP) El Niño events, so far no clear precipitation response has been detected during the same decaying stage for CP El Niño composites. Here we show that around half of the CP El Niño events coincide with enhanced SPSC (wet CP El Niño), while the other half are accompanied by reduced SPSC (dry CP El Niño). These two types of CP El Niño events bear dramatically different evolution features in their respective tropical sea surface temperature anomaly (SSTA) patterns. Wet CP El Niño events are characterized by an SSTA longitudinal position confined to the tropical central-eastern Pacific. In contrast, dry CP El Niño events exhibit a clear westward propagation of SSTAs during their evolution, with maximum SSTAs located to the west of the date line after their mature phase. These different longitudinal positions of positive SSTAs during their decaying phase result in distinct meridional structures of the tropical Pacific convection anomalies as well as the ENSO combination mode (C-mode) response. An anomalous low-level anticyclone is evident over the western North Pacific during wet CP El Niño events during their decaying phase, while an anomalous cyclonic circulation is found for dry CP El Niño events. We emphasize that the impacts of CP El Niño on the SPSC depend crucially on the simultaneous zonal location of warm SSTAs in the tropical Pacific. [ABSTRACT FROM AUTHOR]

It is known that an increase of water vapor over the Arctic is one of most plausible causes driving Arctic amplification. However, debate continues with regard to the explanation of the underlying mechanisms driving the increase of moisture over the Arctic region in the observations. Here, we used the Community Atmosphere Model with prescribed sea surface temperature along with reanalysis datasets to examine the role of fast atmospheric responses to the increase of anthropogenic sulfate aerosol concentrations in China. We found that it plays an additive role in moisture transport from the midlatitudes, resulting in warming of the Arctic region, especially around the Barents–Kara Seas. Specifically, sulfate aerosol forcing in China reduces the meridional temperature gradient and leads to the increase of moisture transport into the Arctic by altering atmospheric circulation. The resulting increase of moisture then leads to surface warming through the enhancement of the downwelling longwave radiation. This implies that Arctic warming around the Barents–Kara Seas has been accelerated, at least in part, by a fast atmospheric response to anthropogenic sulfate aerosol emissions in China in the recent past. [ABSTRACT FROM AUTHOR]

Haze pollution is among the most serious disasters in the North China Plain, dramatically damaging human health and the social economy. The frequency of haze events in February typically varies from the number of haze days in the winter. To improve the understanding of haze pollution in February, this study not only showed the large-scale atmospheric circulations associated with the variation in the haze, but also analyzed its connection with Arctic sea ice. The observational and large ensemble model results both illustrated that the preceding increase in the early-winter Chukchi Sea ice might intensify the February haze pollution. The accumulated sea ice over the Chukchi Sea resulted in a steeper meridional sea surface temperature gradient and a significant and persistent westerly thermal wind. In February, the responsive pattern in the atmosphere developed into a Rossby wave–like pattern, linking the Chukchi Sea ice and the February haze pollution. Modulating by the induced large-scale atmospheric circulations, the horizontal and vertical atmospheric ventilation conditions and the hygroscopic growth conditions enhanced the frequency of haze pollution events. [ABSTRACT FROM AUTHOR]

Observational analysis indicates significant decadal changes in daytime, nighttime, and compound (both daytime and nighttime) heat waves (HWs) over China across the mid-1990s, featuring a rapid increase in frequency, intensity, and spatial extent. The variations of these observed decadal changes are assessed by the comparison between the present day (PD) of 1994–2011 and the early period (EP) of 1964–81. The compound HWs change most remarkably in all three aspects, with frequency averaged over China in the PD tripling that in the EP and intensity and spatial extent nearly doubling. The daytime and nighttime HWs also change significantly in all three aspects. A set of numerical experiments is used to investigate the drivers and physical processes responsible for the decadal changes of the HWs. Results indicate the predominant role of the anthropogenic forcing, including changes in greenhouse gas (GHG) concentrations and anthropogenic aerosol (AA) emissions in the HW decadal changes. The GHG changes have dominant impacts on the three types of HWs, while the AA changes make significant influences on daytime HWs. The GHG changes increase the frequency, intensity, and spatial extent of the three types of HWs over China both directly via the strengthened greenhouse effect and indirectly via land–atmosphere and circulation feedbacks in which GHG-change-induced warming in sea surface temperature plays an important role. The AA changes decrease the frequency and intensity of daytime HWs over Southeastern China through mainly aerosol–radiation interaction, but increase the frequency and intensity of daytime HWs over Northeastern China through AA-change-induced surface–atmosphere feedbacks and dynamical changes related to weakened East Asian summer monsoon. [ABSTRACT FROM AUTHOR]

The mei-yu onset over the middle to lower reaches of the Yangtze River Valley (MLYRV) varies considerably from early June to mid-July, which leads to large interannual changes in rainy-season length, total summer rainfall, and flooding potential. Previous studies have investigated the impact of El Niño–Southern Oscillation (ENSO) on the mei-yu onset. This study shows that a strong (weak) East Asian and western North Pacific (EAWNP) intraseasonal oscillation (ISO) in spring leads to an early (late) onset of the mei-yu over the MLYRV, and this ISO–mei-yu relationship is attributed to different types of ENSO in the preceding winter. A strong EAWNP ISO in spring is related to an eastern Pacific El Niño (EP El Niño) in the previous winter, and negative sea surface temperature (SST) anomalies in the eastern Indian Ocean and the South China Sea (SCS) in May, which can cause an early onset of the South China Sea summer monsoon that also favors an early mei-yu onset. In contrast, a weak EAWNP ISO in spring is associated with a central Pacific El Niño (CP El Niño) before April, but with an EP El Niño after April, and positive SST anomalies in both the eastern Indian Ocean and the SCS in May. A statistical forecast model combining the intensity of spring EAWNP ISO, CP ENSO, and EP ENSO indices shows a high prediction skill of the observed mei-yu onset date. [ABSTRACT FROM AUTHOR]

Robust surface warming with distinct interdecadal variations has been observed in the offshore area of China and adjacent seas (hereafter, offshore China) during winter and summer of the period 1958-2014. Acceleration of this warming during 1980-99 at rates greater than the global mean warming rate was accompanied by a weakening of the East Asian monsoon (EAM) and a strengthening of the west Pacific subtropical high (WPSH). It was determined that the sea surface temperature (SST) variation in offshore China correlates very well with changes in the EAM wind on interdecadal time scales. It was also established that the enhanced oceanic lateral heat transfer, mainly attributed to the leading empirical orthogonal function (EOF1), weakening EAM wind mode, has a central role in robust interdecadal winter surface warming in offshore China. However, except for the effect of oceanic lateral heat transfer, the increased surface heat flux through radiative heating related to the third EOF (EOF3) strengthening EAM anticyclone wind mode (WPSH) in summer appears to have a greater contribution to interdecadal summer surface warming in offshore China. These results help clarify the relationship between interdecadal SST variations, EAM, oceanic currents, and sea surface flux in offshore China. [ABSTRACT FROM AUTHOR]

The teleconnection between the summer (June-August) Palmer drought severity index (PDSI) in China and seasonal global sea surface temperatures (SSTs) is investigated at both spatial and temporal scales during 1901-2012. Three pairs of coupled spatial patterns for China's PDSI and global SST anomalies are identified using the singular value decomposition (SVD) method. With a combination of ensemble empirical mode decomposition (EEMD) and multiple linear regression (MLR) analysis, it is found that the first mode, the sea ice loss-global warming pattern, causes wetness over north and northeastern China and drying over Inner Mongolia. The North Pacific Current (NPC) mode shows that a warmer NPC corresponds to a wetter summer over eastern China and a drier one over the Tibetan Plateau. Both NPC and Pacific decadal oscillation (PDO) affect moisture variability in northern China and over the Tibetan Plateau, with the NPC mode more important in the centennial scale, while the PDO mode is more important in the multidecadal scale. [ABSTRACT FROM AUTHOR]

The influence of the tropical Pacific sea surface temperature (SST) on the wintertime surface air temperature (SAT) in China is investigated using both the observational data and the output of coupled ocean-atmosphere numerical models during the period from 1960 to 2006. A singular value decomposition analysis (SVD) is applied between 500-hPa geopotential height (Z500) in the Northern Hemisphere and SST in the tropical Pacific Ocean to get the tropical Pacific SST-forced atmospheric patterns. The association of the SAT over China and the tropical Pacific SST is measured by calculating the temporal correlation coefficient (TCC) between the SAT and the expansion coefficient of the atmospheric component of the leading two SVD modes. Results show that the SAT over China is significantly correlated to the second SVD mode (SVD2). The SST component of SVD2 is characterized by negative tropical Pacific SST anomalies centered over the midequatorial Pacific Ocean. The atmospheric component of SVD2 (ASVD2) shares many similarities in spatial structures to the Arctic Oscillation (AO). The time variation of ASVD2, however, is found more closely correlated to the variation of SAT over China than the AO. When SVD2 is in its positive phase, the SAT over China tends to be warmer than normal. Further analysis indicates that the TCC between the SAT in China and ASVD2 is largely decreased after the long-term climate trend is removed. The time variability of the tropical Pacific SST-forced large-scale atmospheric patterns and its relationship to SAT are reasonably captured by the multimodel ensemble (MME) seasonal forecasts. An examination of the MME forecast skill indicates that ASVD2 contributes significantly to the TCC skill of MME forecasts. [ABSTRACT FROM AUTHOR]

This study investigates the influence of different sea surface temperature (SST) modes on the winter temperature in China using the generalized equilibrium feedback assessment (GEFA). It is found that the second EOF mode of winter temperature in China during 1958-2010 shows a typical northeast-southwest (NE-SW) pattern, which is a major spatial mode of Chinese winter temperature at interannual scales. The winter temperature of the NE-SW pattern is forced mainly by SST modes in the tropical Pacific and Atlantic. For 2009/10, the tropical Pacific El Niño mode and tropical Atlantic tripole mode have the largest contribution to the response. The physical mechanism of the cold northeast-warm southwest (CNE-WSW) pattern is also explained in terms of GEFA of the responses of the atmospheric circulation. The northerly flow at the low level transports cold air to northern and northeastern China, resulting in a lower temperature there. Meanwhile, the anomaly meridional wind advects warm air from the southern oceans to southwestern China, leading to warming there. [ABSTRACT FROM AUTHOR]

A regional atmosphere-ocean coupled model is developed, based on the Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model in conjunction with the Princeton Ocean Model, to investigate atmosphere-ocean coupled processes that might occur over the Yellow and East China Sea shelves in winter. To examine how the coupled processes actually work in the ocean, sea surface temperatures (SSTs) computed in both coupled and uncoupled models are compared with SSTs synthesized from multiple satellite observations. The results indicate that the coupled model significantly improves the negative SST bias in shallow waters around the Chinese coast produced by the uncoupled model. Cool and dry northerly winds from the Asian landmass reduce SST in these shallow waters through intensive upward heat loss. Thereafter, the horizontal gradient of sea level pressure (SLP) around the Chinese coast moderates because the land-ocean heat contrast weakens owing to the reduced SST in the coastal waters. As a result, the wind speed weakens, in line with the moderated horizontal SLP gradient. Moreover, northerly winds can reduce the transport of cool and dry air from the Asian landmass. Hence, upward heat flux around the coastal waters is reduced because of the weakening of the northerly winds and the decreased cool and dry air. This negative feedback thereby suppresses excessive SST cooling along the Chinese coast during winter. [ABSTRACT FROM AUTHOR]