Explore the vast range of titles available.

Arctic amplification refers to the greater surface warming of the Arctic than of other regions during recent decades. A similar phenomenon occurs in the troposphere and is termed “tropospheric Arctic amplification” (TAA). The poleward eddy heat flux and eddy moisture flux are critical to Arctic warming. In this study, we investigate the synoptic transient eddy activity over the North Pacific associated with TAA and its relationship with the subtropical jet stream, and propose the following mechanism. A poleward shift of the subtropical jet axis results in anomalies of the meridional gradient of zonal wind over the North Pacific, which drive a meridional dipole pattern of synoptic transient wave intensity over the North Pacific, referred to as the North Pacific Synoptic Transient wave intensity Dipole (NPSTD). The NPSTD index underwent an interdecadal shift in the late 1990s accompanying that of the subtropical jet stream. During the positive phase of the NPSTD index, synoptic eddy heat flux transports more heat to the Arctic Circle, and the eddy heat flux diverges, increasing Arctic temperature. This mechanism highlights the need to consider synoptic transient eddy activity over the North Pacific as the link between the mean state of the North Pacific subtropical upper jet and TAA.

The asymmetric impacts of El Niño and La Niña on the Pacific–North American teleconnection pattern in boreal winter have important implications for the surface air temperature and precipitation anomalies in North America. Previous studies have shown that the varying tropical convective heating contributes to the zonal shift of the teleconnection pattern during different El Niño/Southern Oscillation phases. In this study, using reanalysis, atmospheric general circulation model (AGCM) simulations, and a linear baroclinic model, we further present that the discrepancy of the subtropical jet stream (STJ) during El Niño and La Niña also contributes to the asymmetry. The atmospheric anomalies readily extract kinetic energy and effectively develop at the exit of the STJ. During El Niño (La Niña) years, as the central-eastern tropical Pacific warms up (cools down), the meridional temperature gradient in central subtropical Pacific increases (decreases), leading to the eastward (westward) shift of the STJ. The movement of the STJ leads to the shift of the location where disturbance develops most efficiently, ultimately contributing to the asymmetry of the teleconnection pattern.

The Northern Hemisphere transient atmospheric response to Arctic sea decline is investigated in autumn and winter, using sensitivity experiments performed with the CNRM-CM6-1 high-top climate model. Arctic sea ice albedo is reduced to the ocean value, yielding ice-free conditions during summer and a more moderate sea ice reduction during the following months. A strong amplification of temperatures over the Arctic is induced by sea ice loss, with values reaching up to 25°C near the surface in autumn. Significant surface temperature anomalies are also found over the midlatitudes, with a warming reaching 1°C over North America and Europe, and a cooling reaching 1°C over central Asia. Using a dynamical adjustment method based on a regional reconstruction of circulation analogs, we show that the warming over North America and Europe can be explained both by changes in the atmospheric circulation and by the advection of warmer oceanic air by the climatological flow. In contrast, we demonstrate that the sea ice–induced cooling over central Asia is solely due to dynamical changes, involving an intensification of the Siberian high and a cyclonic anomaly over the Sea of Okhotsk. In the troposphere, the abrupt Arctic sea ice decline favors a narrowing of the subtropical jet stream and a slight weakening of the lower part of the polar vortex that is explained by a weak enhancement of upward wave activity toward the stratosphere. We further show that reduced Arctic sea ice in our experiments is mainly associated with less severe cold extremes in the midlatitudes.

Given the significant importance of spring precipitation for agricultural production in China and the presence of the spring predictability barrier, scientists have dedicated extensive efforts to understand the factors influencing spring precipitation variability and explore new predictors. However, the effects of Arctic stratospheric ozone (ASO) on precipitation in China during boreal spring, if any, and the underlying mechanisms remain unclear. We found the robust influences of March ASO on the differences in the precipitation and evaporation in April over Eastern China during 1980–2020. When ASO decreases in March, it tends to result in a higher and colder tropopause in the polar, a stronger subtropical jet stream, an intensified local Hadley circulation accompanied by anomalous downward motion over Eastern China, and consequently, drying in this region, and vice versa. These findings suggest that the likelihood of April moistening over East Asia may be potentially predicted by employing the ASO index. Plain Language Summary Food production in East Asia, which is home to a quarter of the world's population, holds immense importance. The spring season in this region marks the crop planting period, making the precipitation during this time crucial for agricultural production. However, it is challenging to predict spring moistening/drying over East Asia. Therefore, there is a need for new predictors to enhance our understanding of spring precipitation variability. Whether a connection exists between Arctic stratospheric ozone (ASO) and spring precipitation over EC has remained unknown. Here, we have highlighted a strong relationship between ASO in March and moistening over EC in April, particularly in the middle and lower reaches of the Yangtze River (YRB). Specifically, an increase (decrease) in March ASO corresponds to moistening (drying) over YRB in April. The implications of these findings are significant for forecasting spring precipitation over East Asia, which is crucial for agricultural planning and production. Key Points There are robust influences of March Arctic stratospheric ozone (ASO) on the precipitation in April over Eastern China (EC) during 1980–2020 Depletion of ASO in March tends to result in decreased precipitation over EC in April Decreased ASO results in the higher but warmer tropopause, stronger subtropical jet, and anomalous downward motion over EC

Previous research has extensively explored the “stilling” and “reversal” phenomena in annual near‐surface wind speed (NSWS). However, the variations in the strengths of these phenomena between different months remain unclear. Here the monthly changes in observed NSWS from 769 stations across China during 1979–2020 were analyzed. The analysis reveals a consistent decline in NSWS that ceased around 2011, followed by an increasing trend detected in all months except March, where a distinct hiatus is observed. The hiatus in March NSWS is primarily attributed to a significant reduction in NSWS over North and Northwest China. This reduction can be linked to the southward shift of the East Asian subtropical jet (EASJ), which resulted in a decreased meridional temperature gradient and weakened transient eddy activities across northern China. These findings emphasize the importance of considering changes in the EASJ to gain a comprehensive understanding of NSWS changes at regional scale. Plain Language Summary Understanding how near‐surface wind has changed and identifying the factors driving these changes are crucial. This can help in developing adaptation strategies to increase society's resilience to possible future climate, such as understanding the future revenues of electricity production from wind farms. By analyzing wind observations from 769 stations across China since 1979, we confirmed a general decrease (stilling) that ceased around 2011, followed by a general significant increasing tendency (reversal) in all months but March. Indeed, March's wind series after 2011 showed a pause (i.e., hiatus) from the 1979–2011 slowdown. This hiatus was mainly caused by the general wind reduction across northern China since 2011, which differs from the wind increase observed in other regions. The slowdown in March from 2011 to 2020 is related to the southward shift of East Asian subtropical jet streams, which are fast‐flowing, narrow, and meandering air currents in the upper atmosphere. Jet streams play an important role in shaping both upper and lower air circulation and influence surface wind by transporting high and low‐pressure systems. Key Points March near‐surface wind speed (NSWS) over China experienced a hiatus after 2011, distinct from other months The observed hiatus in March NSWS was primarily caused by a significant reduction in NSWS over North and Northwest China A southward shift of the East Asian subtropical jet may have contributed to the detected hiatus

Analyses of observation data from 1961 to 2014 by using the empirical orthogonal function (EOF) method indicate that the primary mode (a monosign pattern) of heavy snowfall over northern China in winter shows evident variations from a negative polarity to a positive polarity in the mid-1990s. Associated with this decadal change, the southward displacement of the polar front jet stream and northward shift of the subtropical jet stream in the upper troposphere are apparent. Accordingly, a negative height anomaly dominates the region from Lake Balkhash to Lake Baikal and a positive height anomaly occupies the midlatitudes of the North Pacific in the middle troposphere. Such anomalous patterns in the middle and high troposphere correspond approximately to the northern mode of the East Asian winter monsoon (EAWM) and may favor the interaction of cold air with moist airflows over northern China, which helps increase local heavy snowfall. Further investigation shows that the shift in the Atlantic multidecadal oscillation (AMO) from a cold phase to a warm phase in the 1990s may also play a role, through its linkage to the above atmospheric circulations with the aid of a downstream propagation of wave train that emanates from the Atlantic Ocean.

Addressing aviation turbulence not only enhances passenger comfort but also help to reduce fuel consumption and environmental impact, supporting aviation sustainability. By using ERA5 reanalysis data we explore how changing atmospheric circulation, monitored via sea level pressure trends, impacts aviation turbulence over Europe. Our results show coherent climate anomalies, with rising turbulence intensity over the UK and Northern Europe where most events involve clear air turbulence, which occurs unexpectedly and without warning, particularly at flight altitudes. We also highlight a clear seasonal patterns in moderate‐or‐greater turbulence encounters, most frequent and intense in winter, with a key role of wind shears due to the sub‐tropical jet position over Southern Mediterranean. Our approach adds to previous studies on the same topic by analyzing individual atmospheric circulation pattern changes and their effects on turbulence‐related factors. This offers insight into how climate change affecting atmospheric dynamics may contribute to increased aviation turbulence. Plain Language Summary This study investigates how climate change can affects aviation turbulence over Europe, which is crucial for improving passenger comfort, reducing fuel consumption, and promoting sustainable aviation. Our findings indicate an increase in turbulence, particularly over the UK and Northern Europe with a significant portion of this increase involving clear air turbulence. This is particularly hazardous as it occurs without warning and cannot be detected visually, especially at cruising altitudes. We also found that turbulence is more frequent and intense during winter with a seasonal pattern influenced by wind shears associated with the position of the subtropical jet stream over the Southern Mediterranean. Finding ways to face with turbulence could make flights smoother, safer, and more environmentally friendly. Key Points Changing atmospheric circulation due to climate change increases turbulence over Europe Turbulence peaks in winter, linked to wind shears from the subtropical jet stream over the Southern Mediterranean Most increasing episodes are related to clear air turbulence, occurring unexpectedly at flight cruise altitudes

A set of three-dimensional jet indices (jet speed index, jet pressure index, jet latitude index) has been proposed in previous literature to describe the variation of jet streams in both the horizontal and vertical direction. We refer to these indices at the ‘AC’ indices, after the names of the researchers involved. However, the physical meaning of the AC indices and the relationship between AC indices and climate systems are not fully understood. Further study is still needed for applying the indices in East Asia (70°–140° E). In this study, based on the understanding of the physical meaning of the AC indices, latitudinal ranges of East Asian jet streams are determined, and a set of modified AC indices is proposed. Based on the modified AC indices, the linear trends in East Asian jet streams are studied, and the relationship between East Asian jet streams and the climate is researched. The results show that the jet speed index corresponds to the meridional temperature gradient (MTG) of the middle to upper troposphere (500–200 hPa); the jet pressure index corresponds to the pressure level at which the MTG equals zero; and the jet latitude reflects the meridional MTG distribution. The latitudinal ranges of jet streams are determined based on the meridional profiles of climatological zonal-mean zonal winds. Within such a latitudinal range, the climatological zonal-mean zonal winds between 400 and 100 hPa are only westerly, and the maximum wind speed in the vertical direction at every latitude appears between 400 and 100 hPa. The jet streams can be further classified according to the features of the profiles. For East Asia (70°–140° E), jet streams can be classified into winter subtropical jet streams (15°–47.5° N), summer subtropical jet streams (27.5°–60° N), and summer polar front jet streams (60°–87.5° N). The classification of jet streams can be supported by their correspondence to the distribution of tropospheric baroclinicity. A set of modified AC indices can be acquired by using the new ranges of East Asian jet streams in the definition of the original AC indices. Descriptions of jet streams using the modified AC indices are more in accordance with the distributional features of the climatological zonal winds over East Asia, and the physical meanings of the modified AC indices are more definite than the original indices. Using the modified AC indices, we find a significant weakening trend in the strength of the summer subtropical jet stream (−0.13 m/s/10 yr) and a significant northward shift of the winter subtropical jet stream (0.22°/10 yr), and the possible reasons for these trends are studied. Finally, the relationships of East Asian jet streams in winter and summer with atmospheric circulation, temperature, and precipitation are also investigated in this study.

Changes in the Tibetan Plateau upper‐level westerlies (TPUW) and El Niño‐Southern Oscillation (ENSO) are crucial in regulating local hydrological cycles and large‐range climate. Here we report a gradual disappearance of the cross‐seasonal ENSO‐TPUW teleconnection since the early‐2000s, with correlation decreasing from 0.75 to near zero. Before the early‐2000s, the anomalous western North Pacific (WNP) anticyclone (WNPAC) induced by ENSO can last from winter to summer under the maintenance of Indo‐western Pacific surface seawater temperature gradient (IWPSSTG), prompting the WNP subtropical high to expand and strengthen toward South Asia, attracting the Tibet Plateau subtropical high southward, thereby causing the subtropical westerly jet to move southward and forming a cross‐seasonal ENSO‐TPUW teleconnection. However, given the prominent tropical WNP warming since the early‐2000s, the early‐summer outage of IWPSSTG and WNPAC disrupts the ENSO‐TPUW chain. This discontinuity poses severe challenges to the improvement of short‐term climate predictions in the Tibetan Plateau and surrounding areas. Plain Language Summary It is well known that the Tibetan Plateau upper‐level westerlies (TPUW) play a crucial role in shaping regional and even global weather and climate. In the past, the TPUW variations during high summer were strongly linked to preceding winter El Niño‐Southern Oscillation (ENSO). We find that this cross‐seasonal teleconnection became much weaker since the early 2000s. The reason for this change is closely related to the evolution of Indo‐western Pacific surface seawater temperature (SST) gradient and western North Pacific anticyclone (WNPAC) during the ENSO decaying. Acting as a relay baton, the WNPAC helps extend ENSO's far‐reaching impact to TPUW by drawing the Tibetan Plateau High‐pressure and subtropical jet stream southward. However, given the prominent SST warming in the tropical western North Pacific since the early 2000s, such Indo‐western Pacific SST gradient is weakened and thereby cannot sustain the WNPAC from winter to summer, breaking this chain of influence. Our results provide some new insights into the ongoing hot debate on the linkages between ENSO and Tibetan Plateau and are useful for deepening the understanding of the ENSO‐TPUW interplay and for improving relevant coupled models to certain extent. Key Points Impact of preceding‐winter El Niño‐Southern Oscillation (ENSO) on the Tibetan Plateau upper‐level westerlies (TPUW) variations during following high‐summer has weakened since the early 2000s In the past, ENSO affects TPUW mainly via the western North Pacific anticyclone (WNPAC)‐induced southward shifts of the Tibetan Plateau subtropical high and westerly jet stream Disappearing of the chain from ENSO to TPUW via WNPAC can be attributed to the early‐summer outage of Indo‐western Pacific SST gradient

Atmospheric angular momentum (AAM) is used to study the variability of Earth’s atmospheric circulation during the past 45 years, a time of considerable climate change. Using global AAM, two interdecadal states are defined covering the periods 1977–98 (hereinafter P1) and 1999–2022 (P2). Global AAM decreased from P1 to P2 and was accompanied by weakened subtropical jet streams in both hemispheres, strong convection around the northern Maritime Continent, and a strengthened sea surface temperature (SST) gradient across the tropical Pacific Ocean. The period differences project onto 1) internal interdecadal Pacific variability (IPV), 2) a postulated transient ocean thermostat response to greenhouse gas and aerosol emissions, and 3) circulation anomalies related to the ozone hole. During 1977–2023, the first two processes are forcing the climate toward larger Pacific Ocean SST gradients and a poleward expansion of the Indo-Pacific warm pool (IPWP), especially into the Northern Hemisphere. The ozone hole produces its own distinct pattern of anomalies in the Southern Hemisphere that tend to become persistent in the early 1990s. The zonal and vertical mean AAM variations during P1 have frequent westerly wind anomalies between 40°N and 40°S with poleward propagation on interannual time scales. During P2, the circulation is dominated by subtropical easterly wind anomalies, poleward-shifted jets, and weaker propagation. Locally, the zonal mean anomalies manifest as midlatitude ridges that lead to continental droughts. Case studies illustrate the weakened subtropical jet streams of P2 and examine the factors behind a transition to La Niña in early 2020 that maintains the P2 pattern.