Search

In recent decades, the winter surface air temperature (SAT) anomaly in the Northern Hemisphere has exhibited a warm Arctic‐cold Eurasia (WACE) dipole pattern and this has undergone significant decadal variation. In this paper, the physical cause of the decadal variability of the WACE pattern is explored, and it is shown to be mediated by the phases of the Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO). Although the negative PDO (PDO−) or positive AMO (AMO+) favors the WACE pattern, the meridional structure of the pattern is significantly influenced by whether PDO− or AMO+ dominates. During PDO− (AMO+) phase, the winter‐mean WACE pattern shows an asymmetric dipole with weak (strong) warm anomaly over the Barents‐Kara Seas or BKS and strong (weak) cold anomaly over central Eurasia or Siberia, which corresponds to Ural blocking (UB) concurring with the negative (positive) phase of North Atlantic Oscillation in a winter‐mean field. The winter asymmetric WACE pattern is more strongly related to PDO− than AMO+. It is further found that an asymmetric sub‐seasonal WACE dipole pattern with strong (weak) warm anomaly over BKS and weak (strong) cold anomaly over Siberia is usually formed during the UB episode due to favored (suppressed) sub‐seasonal downward infrared radiation and turbulent heat flux over BKS during AMO+ (PDO−), which leads to a strong asymmetric dipole of the winter WACE pattern. Plain Language Summary: The most prominent climate change in the Northern Hemispheric mid‐high latitudes in recent decades is that the winter surface air temperature anomaly has exhibited a warm Arctic‐cold Eurasia (WACE) dipole pattern. However, it is still unknown what causes the decadal variability of the winter WACE pattern and its meridional asymmetry. In this paper, we address this issue by examining the different effects of Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO) on the winter WACE pattern. It is shown that PDO− (AMO+) plays an important role in the occurrence of the asymmetric WACE dipole with weak (strong) warm anomaly over the Barents‐Kara Seas and strong (weak) cold anomaly over central Eurasia or Siberia through modulating Ural blocking and associated sub‐seasonal cold anomaly. It is also found that the asymmetric WACE dipole is more strongly dependent on PDO− than AMO+. Our findings are of great significance for understanding the physical cause of the decadal variability of the WACE dipole pattern in spatial structure. Key Points: Ural Blocking plays an important role in the occurrence of the sub‐seasonal warm Arctic‐cold Eurasia (WACE) dipole pattern in winterUral blocking can amplify the winter‐mean Warm Arctic‐cold Eurasia (WACE) patternWarm Arctic‐cold Eurasia (WACE) is linked to the Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO) [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]

This study investigates the intermodel spread of changes in the Eurasian winter surface air temperature (SAT) based on Coupled Model Intercomparison Project 6 (CMIP6) and explains it from the perspective of circulation under shared socioeconomic pathway 2–4.5. Results show that the leading intermodel spread of Eurasian SAT change, derived from the intermodel empirical orthogonal function analysis (EOF), is characterized by a warming pattern of SAT change. This warming pattern is associated with the changes in the weakened Siberian High and Aleutian Low, the weakened subtropical jet, the poleward polar jet, and the tilting trough. The combination of these circulations has prevented the intrusion of the cold air to the south and therefore results in the warming SAT change pattern. Their relative contribution weights of the weakened Siberian High and Aleutian Low, the tilting East Asian trough, the weakened subtropical jet, and the poleward polar jet are 12.73%, 20.43%, 30.24%, 20.96%, and 15.64%, respectively. All these changes may be traced to the warm sea surface temperature (SST) changes in the western North Pacific and tropical Atlantic Ocean, via the meridional temperature gradient and wave trains, respectively. These warmings would be the potential metrics for reducing the intermodel uncertainties of the winter SAT projection at the end of the 21st century over the Eurasian continent. Plain Language Summary: There are still large intermodel uncertainties in the projection of the winter SAT over the Eurasian continent. In this study, the intermodel EOF is utilized to derive the intermodel spread in the winter SAT change at the end of the 21st century over the Eurasian continent. The leading mode is a warming pattern over Eurasia, which is closely related to the large‐scale circulations, including the changes in the weakened Siberian High and Aleutian Low, the tilt of the East Asian trough, the weakened western subtropical jet, and the northward shift of the eastern polar front jet. All these changes may be traced to the warming in the western North Pacific and the tropical Atlantic. Our findings provide a perspective to improve the projection of Eurasian winter SAT. Key Points: The leading intermodel spread in winter surface air temperature change at the end of the 21st century over the Eurasian continent is a warming pattern from the CMIP6 modelsThe large‐circulation changes in the sea‐level pressure, the tilting trough, and jets are highly associated with this intermodel spreadAll these changes might be traced to the warm SST changes in the western North Pacific and the tropical Atlantic [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]

A zonal dipole pattern of the interannual variability of Eurasian snow cover during late autumn has attracted growing interest due to its extensive influences on large‐scale atmospheric circulations and climate anomalies, but it remains unknown what influences the formation of this pattern. This study revealed the potential drivers and the related physical processes of the dipole mode of the snow cover variability during 1979–2018. Results showed that the atmospheric circulation anomalies play a crucial role in forming the dipole mode, which could be further attributed to the influences of the atmospheric teleconnection patterns and the variations of sea surface temperature (SST) and sea ice concentration. Specifically, the anomalous circulations related to the teleconnections of the Eastern Atlantic pattern, East Atlantic–Western Russia pattern, and Scandinavian pattern could lead to the anomalies of air temperature and snowfall that directly shape the variability of snow cover. In addition, a dipole structure of the SST anomalies over the North Atlantic exerts an important effect on the snow cover variability through exciting an eastward propagating Rossby wave train prevailing over Eurasia. Moreover, the snow cover variability is closely linked to the sea ice variations over the Barents Sea that can regulate the anomalous circulations over Eurasia by altering the atmospheric dynamic and thermodynamic conditions. These influencing factors jointly account for about 66% of variances in the dipole mode of snow cover variability, suggesting that our findings can substantially improve the understanding of the Eurasian snow cover variations. Plain Language Summary: This study investigated the possible influencing factors responsible for the formation of the west‐east dipole structure of Eurasian snow cover variability during late autumn that has notable impacts on the atmospheric circulations and climate anomalies in the following season. Results suggested that the anomalous circulations associated with the atmospheric teleconnections of the Eastern Atlantic pattern, East Atlantic‐Western Russia pattern, and Scandinavian pattern can effectively regulate the snow cover variability related to the dipole pattern through inducing the anomalies of surface air temperature and snowfall. In addition, the North Atlantic sea surface temperature anomalies play a considerable role in forming the dipole mode by exciting a wave train type anomalous circulation over Eurasia. Furthermore, the variations of sea ice concentration in the Barents Sea could lead to the snow cover variability through modulating the temperature gradient over mid‐high latitudes and the resultant circulation anomalies over Eurasia. The synergetic effect of these influencing factors explains about two‐thirds of the snow cover variability related to the dipole mode, indicating the important contribution of our results to understanding the Eurasian autumn snow cover anomaly. Key Points: The west‐east dipole pattern is the intrinsic mode of the Eurasian snow cover variability during late autumnThe dipole mode is closely associated with the Eastern Atlantic pattern, East Atlantic‐Western Russia pattern, and Scandinavian patternAnomalies of North Atlantic sea surface temperature and Barents sea ice can exert considerable effects on the dipole mode [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]

This study investigates the interdecadal variation of the intensity of the interannual variability (IIV) of surface air temperature (SAT) over mid‐ and high‐latitude Asia (MHLA) in winter months (December–February) during 1980–2015 and the role of the Arctic sea‐ice. Results show only the IIV of February SAT is significantly enhanced around the late‐1990s, which partly results from enhanced IIV of the frequency of extreme warm and cold events (FEWE and FECE). The influence of the sea‐ice in the Barents Sea from December to February on the February SAT, FEWE and FECE over MHLA is remarkably enhanced around the late‐1990s. During 1998–2015, the sea‐ice anomalies contribute to the enhanced IIV of February SAT over MHLA by modulating the monthly and synoptic tripole patterns over northern Eurasia, the positive phase of which exhibits an anomalous low over Eurasia north of 60°N and anomalous highs over southern Europe and MHLA. On the monthly average, the increased sea‐ice induces Rossby waves and then a positive phase of the monthly tripole pattern, inducing weakened East Asian trough and Siberian high, enhanced westerlies over 50°–70°N, and resultant decreased blocking high frequency over Eurasia north of 60°N. These anomalies favor increased February SAT over MHLA. On synoptic timescales, the increased blocking over the north side of the Ural Mountains favors more (fewer) extreme positive (negative) phases of the synoptic tripole pattern and hence increased FEWE (decreased FECE) over MHLA, further strengthening the increased SAT. Numerical experiments basically confirm the mechanisms above. Key Points: Intensity of interannual variability of February surface air temperature over mid‐ and high‐latitude Asia reinforces after 1998Intensity of interannual variability of frequency of February extreme warm and cold events over the region is enhanced after 1998Sea‐ice anomalies in the Barents Sea play an important role via modulating monthly and synoptic circulation over northern Eurasia [ABSTRACT FROM AUTHOR]

Based on statistical analysis using observations and idealized model simulations, previous studies have revealed the potential response of early‐winter atmospheric circulation and temperature anomalies to November Ural blocking (UB) anomalies. Using a large number of coupled simulations, this study found that the response is sensitive to the intensity of November mid‐latitude westerly jet over Eurasia. Stronger‐than‐normal November UB without a significantly weakened westerly jet could not cause significant atmospheric response in early‐winter. By contrast, stronger‐than‐normal November UB with a significantly weakened jet would be followed by a warmer Arctic‐colder Eurasia (WACE) pattern in December. The significantly weakened westerly jet favors stronger upward propagation of planetary waves, which causes stronger weakening and longer persistence of the stratospheric polar vortex. This stratospheric response persists into December and propagates downward into the troposphere interfering with planetary waves (especially wavenumber‐1). The lead‐lag UB‐WACE linkage modulated by mid‐latitude jet may have implications for sub‐seasonal predictability. Plain Language Summary: Observations and idealized model simulations have shown that strong Ural blocking (UB) anomalies in November can induce the "warmer Arctic, colder Eurasia" anomaly in the following December. In this study, a large number of coupled simulations are employed. The results show that the impact of November UB on early‐winter Arctic‐Eurasian temperature is sensitive to the intensity of November mid‐latitude (around 60°N) westerly jet over Eurasia. When November UB anomalies are accompanied by a significantly weakened mid‐latitude westerly jet, early‐winter would experience a warmer Arctic‐colder Eurasia pattern. The significantly weakened westerly wind waveguide is favorable for stronger upward propagation of planetary waves caused by UB anomalies, which leads to persistent atmospheric response in the following December. Key Points: Early‐winter atmospheric response to November Ural blocking (UB) anomalies is sensitive to the background state of westerly jetStrong UB with sufficiently weakened westerly jet in November can induce a warmer Arctic‐colder Eurasia pattern in DecemberThe weakened westerly wind waveguide promotes stronger upward propagation of planetary waves forced by UB anomalies [ABSTRACT FROM AUTHOR]

In this study, the single value decomposition method was used to analyze the covariation between the winter (December, January, and February) North Atlantic sea surface temperature (SST) and spring (April and May) normalized difference vegetation index (NDVI) over mid‐high‐latitude Eurasia for the period 1982–2015. The results show that the first paired mode explains 32.8% of the total squared covariance, and the correlation of time coefficients is +0.67 (p < 0.01), suggesting that the meridional tripole structure of the winter North Atlantic SST is significantly associated with the three zonally anomalous centers of the spring mid‐high‐latitude Eurasian NDVI. Surface air temperature (T2m) is the most important factor related to spring NDVI changes. The tripole pattern of the winter North Atlantic SST can persist until the following spring, triggering an eastward anomalous Rossby wave train in the middle troposphere, which features two negative geopotential height anomalies centers near northwestern Europe and eastern Siberia and one positive center over central Eurasia. Surface temperature anomalies are well consistent with these anomalous pressure centers. This implies that the winter North Atlantic SST could be a robust driver and potential precursor for spring vegetation prediction over the mid‐high latitudes of Eurasia. Key Points: There is a significant covarying mode between the tripole pattern of the winter North Atlantic sea surface temperature (SST) and spring normalized difference vegetation index (NDVI) anomaly over EurasiaT2m is the main intermediate factor linking SST and large‐scale NDVIThe tripole pattern can persist until the following spring, which modulates T2m over Eurasia through a Rossby wave train [ABSTRACT FROM AUTHOR]

The impact of soil freezing‐thawing processes on August rainfall in Southern China (SC) during 1979–2008 and related physical mechanisms are investigated using the coupled land‐atmosphere model Grid‐Point Atmospheric Model version 2.0—Community Land Model version 2.0 (GAMIL2.0‐CLM2.0). The moisture‐budget analysis is employed to quantify the contributions of different factors to the change of precipitation in SC. The results indicate that the vertical moisture advection and its dynamic component play an essential role in the long‐term trends of August rainfall. The possible physical mechanism is that the model which includes the supercooled water simulates much higher soil/air temperature in August, especially in the north of 40°N, which is not only weakened the meridional thermal contrast but also induced an anomaly "north high–south low" circulation pattern in the middle and high latitude of Eurasia. Both resulted in the 200 hPa zonal winds weakening, and 850 hpa northerly wind southward moved. Meanwhile, a convergence anomaly cyclone is initiated in the lower troposphere over SC regions, accompanied by enhanced ascending and strong moisture flux convergence, which is beneficial to the increased precipitation. This study provides a new interpretation of the "southern flooding" during 1979–2008 from the point of frozen soil changing. Key Points: The coupled land‐atmosphere model (Grid‐Point Atmospheric Model version 2.0‐Community Land Model version 2.0) with the consideration of supercooled water reproduces the trend of the August rainfall in Southern ChinaThe dynamic processes of the vertical moisture advection play a vital role in the long‐term trend of the August rainfallThe decreasing soil ice content has contributed to the wetter of Southern China in August over 1979–2008 [ABSTRACT FROM AUTHOR]