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This paper utilizes reanalysis data and statistical methods to elucidate the physical mechanisms of intraseasonal variations of precipitation over southern China during the boreal winter, as well as the two primary factors influencing these variations. Anomalous southerly (northerly) wind over southern China will result in the strong moisture convergence (divergence) in enhanced (suppressed) scenarios, and the stronger positive (negative) horizontal advection of absolute vorticity at upper troposphere induces the anomalous ascending (descending) motion in enhanced (suppressed) scenarios. The Madden–Julian oscillation (MJO) in the tropics and a wave train over the North Atlantic and Eurasia with four activity centres in the mid–high latitudes (referred as NAE‐wave train) are the two primary factors that modulate the intraseasoanl precipitation over southern China. The intraseasonal circulation induced by MJO and NAE‐wave train exhibits distinct features, resulting in different physical mechanisms of their influences. The moisture flux divergence anomalies over southern China associated with MJO mainly stem from the meridional moisture convergence, whereas the meridional moisture advection is the first contributor in the influence of NAE‐wave train. The anomalous vorticity advection at upper troposphere associated with MJO is primarily influenced by the meridional advection, but that associated with NAE‐wave train is dominated by the zonal advection. Anomalous precipitation induced by MJO is concentrated mainly over South China, while that caused by NAE‐wave train is primarily located in the mid–lower Yangtze River basin. Therefore, different phases locking of MJO and NAE‐wave train could lead to various precipitation anomalies over southern China. [ABSTRACT FROM AUTHOR]

This paper uses monthly surface air temperature (SAT) data from the Climatic Research Unit dataset and the Coupled Model Intercomparison Project phase 6 multimodel ensemble simulations to investigate the variations in the annual‐mean and four seasons SAT across the Eurasian and North Africa landmasses and the possible causes. The SAT over multiple timescales has significant seasonal and regional differences. The trend, variance of decadal variation (DV), and variance of interannual variability (IV) show that the differences between Siberia and mid‐low latitudes are much larger in winter and spring than in summer and autumn. The SAT exhibits a robust warming trend over Siberia in winter (0.21–0.42°C·decade−1) than over mid–low latitudes in summer (<0.07°C·decade−1). The cause is the combined diversity effects of greenhouse gases (GHG) and anthropogenic aerosols (AA) in four seasons over different regions. The warming trend of Siberia in winter is affected by the combination of strong warming effect by GHG and the weak cooling effect by AA, and the trend of mid–low latitudes in summer is opposite. The leading modes of the DV show a consistent spatial pattern over the whole Eurasia in four seasons while the IV show a meridional dipole pattern across 40°N except in summer. The internal climate variability is the main driver of the SAT IV and DV, but the effect of anthropogenic activity and natural forcing may reinforce the IV and DV in some regions. The combined role of GHG and AA forcings can significantly enlarge the DV in many parts of Eurasia, and the AA and natural forcing have significant effect on the IV in the high latitudes. [ABSTRACT FROM AUTHOR]

The regional amplitude and phase characteristics of surface (2 m) air temperature (SAT) annual cycle (AC) over the Northern Hemisphere land area are analysed by the clustering method for the period 1953–2018. The five clusters are found in the nonlinear changes of AC amplitude and the six clusters are found in the AC phase. The spatial distributions of clusters are not the same for amplitude and phase, but the similarity is that both the amplitude and phase have remarkably different trends in the mainland of Eurasia and North America, even though these two regions locate over the similar latitudinal zone. Descending trends of AC amplitude are found for the whole period 1953–2018 in most areas, except in and around Greenland. However, when checking the nonlinear variation and making a piece‐wise linear fit, the abnormal increasing trend (stronger seasonality) appears in the major part of Eurasia after the year 1990. The comparisons of surface radiation fluxes in Eurasia and North America indicate that the long‐term reduction of the reflection and absorption of clouds and aerosols with the declined surface albedo after 1980–1985 may lead to the amplitude amplification after 1990 in Eurasia. In contrast, the variation of surface radiation in North America remains unchanged, where the decreasing trend of amplitude may result from other causes. The overall phase trend is increasing (toward earlier season), but in North America after 1990, the phase shift becomes delayed. The phase variations are correlated to North Atlantic Oscillation (NAO) cold season index over almost all regions, especially in Europe. [ABSTRACT FROM AUTHOR]

The two dominant patterns of the winter (December–February) intraseasonal surface air temperature (SAT) over central Asia is derived by empirical orthogonal function analysis, with the first (mode 1) featuring a regional monopole and the second (mode 2) featuring a northwest–southeast‐orientated dipole. Mode 1 is characterized with warmer (colder) Arctic and colder (warmer) Eurasia along with an anomalous high (low) over the Ural Mountains, while mode 2 is characterized with northwest–southeast dipole over Eurasia along with wave train‐like atmospheric circulation anomalies from the western Atlantic to Asia. Whether the two intraseasonal modes are modulated by lower‐boundary forcing is then investigated, and the results suggest that mode 1 is significantly modulated by sea ice concentration over the Greenland Sea, but by central‐Pacific ENSO for mode 2. Finally, the underlying mechanisms are diagnosed, and the results suggest that the diabatic heating related with decreased sea ice in Greenland Sea induce a positive height anomaly over Urals which favours the formation of Ural blocking and therefore tends to intensify the occurrence of mode 1. Also, central‐Pacific El Niño excites the Pacific–North American teleconnection, which alters North Atlantic subtropical jet extension, favours intensified energy conversion to intraseasonal wave train and subsequently intensifies the occurrence of mode 2. This study highlights the joint role of Arctic sea ice and the SST in the tropical central Pacific. [ABSTRACT FROM AUTHOR]

Previous studies indicated that El Niño–Southern Oscillation (ENSO) significantly impacts the zonal shift of the South Asian High (SAH) in boreal summer. This analysis reveals that the influence of ENSO on the zonal shift of the SAH has experienced a significant interdecadal weakening around the early 1980s. Specifically, ENSO has a significant relation with the SAH's zonal shift before the early 1980s. However, after the early 1980s, the impact of the ENSO on the SAH is weak. We further examined the physical processes for the interdecadal change of the ENSO–SAH relation. It is indicated that interdecadal change of relationship between ENSO and SAH zonal shift is attributable to change in the connection between ENSO and Indian summer monsoon (ISM). Before the early 1980s, ENSO could exert significant impacts on the ISM and lead to significant condensational latent heating anomalies around the northern Indian Peninsula. The atmospheric heating anomalies around the northern Indian Peninsula trigger an atmospheric wave train extending eastward from North Africa toward the Iranian Plateau (IP), which further leads to changes in the zonal location of the SAH. Hence, the ENSO exerts significant impacts on the summer SAH zonal shift. By contrast, after the early 1980s, ENSO had a weak impact on the ISM and could not lead to apparent atmospheric heating anomalies over IP. As such, ENSO has a weak effect on the SAH zonal shift. Further analysis indicates that interannual variation of the SAH zonal shift is related to an atmospheric wave train over mid–high latitudes of Eurasia after the early 1980s. [ABSTRACT FROM AUTHOR]

There is a lack of studies elucidating the characteristics of interannual variability in different models, especially for northern Eurasia. Therefore, spatial patterns of interannual variability in summer precipitation over northern Eurasia across 15 models of Coupled Model Intercomparison Project Phase 5 (CMIP5) were investigated using empirical orthogonal function (EOF) analysis and cluster analysis. The spatial patterns of EOF1‐3 obtained from the models exhibited similar features across the models. The spatial pattern characteristics of the interannual variability of models were similar to those of the actual observations. However, their contribution rates and the order of magnitude of the EOFs in each model differ among models and from observations. Despite the similarity between the patterns of the EOF in the models and the observations, the atmospheric circulations causing the pattern differed in the models and actual observations, resulting in uncertainty in the models. This indicates that in the analysis of extreme events and their impact, the reproducibility and uncertainty of climate models with regard to interannual variability need to be adequately evaluated. In addition, similar to many climate models, the spatial pattern of interannual variability, which is different from any of EOF1‐3 for the observations, has been more strongly represented than the actual observations. Clarification of the factors governing the common but unsuitable variability across models may be expected to decrease the uncertainty of the models in terms of interannual variability. [ABSTRACT FROM AUTHOR]

The relationship between the spring precipitation in the middle and lower reaches of the Yangtze River valley and the North Atlantic Oscillation in the preceding winter is investigated based on data diagnosis. Through affecting the dynamics and thermodynamics in near‐surface atmosphere over Eurasia, the North Atlantic Oscillation in January can exert significant influence on the Eurasian snow cover to form a dipole pattern of snow anomalies. The dipole pattern of snow anomalies stimulates the Rossby wave train in the middle and high latitudes of the Northern Hemisphere in the following spring. Two activity centres associated with the wave train, one in the middle latitudes of Northwest Pacific with an anomalous anticyclone circulation and another in Mongolia with an anomalous cyclone circulation, bring the water vapour from the south and converge over the middle and lower reaches of the Yangtze River, resulting in the excessive precipitation over there. The results indicate that the North Atlantic Oscillation in January can act as a precursor signal for predicting the spring rainfall anomalies in the middle and lower reaches of the Yangtze River. [ABSTRACT FROM AUTHOR]

Temporal evolution of relationships between surface temperature and modes of low‐frequency circulation variability is compared between five reanalyses (20CRv3, 20CRv2c, ERA‐20C, JRA‐55, and NCEP‐1) in winter from 1958 to 2010 over the northern Extratropics. The relationships are evaluated using 15‐year running correlations between temperature anomalies (from the CRU TS v. 4.03 data set) and the intensity of circulation modes (detected in 500 hPa heights by rotated principal component analysis). The analysis, utilizing mean absolute differences between time series of running correlations, points to the large agreement between ERA‐20C, JRA‐55, and NCEP‐1. Circulation modes in those reanalyses are highly similar, which in turn lead to the agreement in temporal development of correlations. In contrast, relationships of some circulation modes with temperature in 20CRv3 and 20CRv2c differ due to differences in the position, strength, and shape of the action centres. This concerns circulation modes located over Eurasia and the Atlantic, mainly North Atlantic Oscillation and Eurasian mode type 1 (EU1). Composite maps, calculated for all running periods, indicate dissimilar temporal evolution of action centres in both 20CR reanalyses. Increased differences in correlations occur mainly during periods when the position and strength of action centres diverge the most. Relationships of circulation modes located over North America and the Pacific with temperature share large resemblance between all reanalyses, including those from the 20CR family. Differences appear to be smaller in 20CRv3 compared to the preceding version, 20CRv2(c), suggesting that the development of the 20CR reanalysis has succeeded in correcting and diminishing biases. [ABSTRACT FROM AUTHOR]

The observed spatial–temporal variations in daily snowfall over the Eurasian continent from 1980 to 2019 are explored in this study. The results show that the snowfall amount and its number of days are mainly centred in the high latitudes and high terrain regions of the Eurasian continent (such as North Asia), while they are relatively small in Southern Europe, Central Asia and East Asia. During the past decades, a decreasing trend can be observed for snowfall amounts and the numbers of days over most regions of Eurasia, while increases are clear in terms of the mean intensity. In response to climate warming in recent decades, the first date of snowfall occurrence postponed and the ending date advanced across the Eurasian continent. Furthermore, the change characteristics of the different grades of snowfall are also investigated across this region, including light snowfall, moderate snowfall, heavy snowfall and snowstorm. The light snowfall and number of days show a decreasing tendency, which mainly results in a decrease in the total snowfall amount and number of days. However, the amounts of heavy snowfall and snowstorms and their numbers of days are significantly increased, implying that the Eurasian continent has experienced increased severe snowfall events during recent decades. [ABSTRACT FROM AUTHOR]

The middle and lower reaches of Yangtze River (MLYR) experienced a historically high number of persistent cloudy and rainy days throughout winter 2018/2019. The persistent cold and rainy weather caused severe social impacts and economic losses. Climatic monitoring shows that a weak central Pacific (CP) El Niño event initiated in November 2018 with an increase in the sea surface temperature (SST) in the central and eastern tropical Pacific. During the mature phase of the CP El Niño, the western North Pacific Subtropical High (WNPSH) was stronger than normal and the western boundary moved westward. However, the weak CP El Niño event alone is not enough to explain the abnormally strong and northward displacement of WNPSH and associated heavy rainfall in winter of 2018/2019. This study highlights the eastward propagating Rossby wave train in the middle‐upper troposphere along with the Asian westerly jet‐induced anomalous high pressure and anticyclonic circulation over East Asia, which contributed to the intensification and northward displacement of WNPSH. The synergic influence of both the CP El Niño and Rossby wave train over Eurasia resulted in an intensified and northward shifted WNPSH, which, together with the intensification of the India Burma trough, strengthened the transport of water vapour into the MLYR basin. Accompanied by frequent outbreaks of cold air and confluence of cold and warm air over the MLYR basin, cold and rainy weather persisted in winter 2018/2019 in this region. [ABSTRACT FROM AUTHOR]

The sea surface temperature anomalies (SSTAs) in the tropical Pacific (TP) and North Atlantic (NA) are both important factors regulating the winter rainfall over southern China (SC). Our results show the combined impact of SSTAs exhibits an offset effect before 1985, after a decadal transition period, the two SSTAs patterns form an additive effect on the interannual timescale after 1995. These distinct impacts of the TP and NA result in the southward shift of the winter rainfall variability, which is associated with the decadal change of the TP and NA. Both the background modes in TP and NA show a phase transition during the period 1985–1995, accompanied by the interannual variations of the SSTAs in two ocean basins before 1985 and after 1995. Before 1985, the canonical El Niño‐related southerly winds of the anticyclone over the western North Pacific transport sufficient water vapour and enhance rainfall in SC. While the concurrent dipole NA SSTA‐related wave train propagates to Eurasia, causing a cyclone and divergence of water vapour fluxes suppressing the local rainfall. After 1995, the El Niño‐related warm SSTAs move westward and strengthen the rainfall anomalies in the Yangtze River basin. Meanwhile, a tripole mode of SSTAs replaces the dipole counterpart in NA, resulting in the anomalous cyclonic circulation over South Asia with more rainfall in the south edge of SC. The decadal changes of the TP and NA mode combination could affect the seasonal distribution and prediction of China winter rainfall. [ABSTRACT FROM AUTHOR]

In this study, we investigated the interdecadal variability (IDV) of winter precipitation in the Greater Mekong Subregion (GMS) between 1892 and 2016. The results showed that the precipitation IDV was characterized mainly by opposite variations between the southern and northern parts of GMS. This IDV pattern was closely linked to the Atlantic multidecadal oscillation (AMO). Rossby waves excited by the AMO modulated the atmospheric circulation across Eurasia, inducing an EU‐like teleconnection with alternating cyclonic and anticyclonic anomalies across Eurasia. The GMS was directly affected by the anticyclonic anomaly over East Asia. This anticyclonic anomaly had dual effects on the GMS precipitation. Firstly, it caused a descending anomaly over the northern GMS and generated easterly anomalies that impeded the moisture transport from the northern BOB. Secondly, it strengthened the East Asian winter monsoon (EAWM) that weakened the western North Pacific anticyclone (WNPAC) to facilitate moisture transport from South China Sea to the southern GMS. Consequently, the precipitation varied inversely between the southern and northern parts of GMS. Numerical experiments confirmed the causal link between the AMO and IDV of winter precipitation in the GMS. [ABSTRACT FROM AUTHOR]