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Tropical cyclone (TC) translation speed (TCTS) can affect the duration of TC-related disasters, which is critical to coastal and inland areas. The long-term variation of TCTS and its relationship to the variability of the midlatitude jet stream and storm migration is discussed here for storms near the North Atlantic coast during 1948–2019. Our results reveal the prominent seasonality in the long-term variation of TCTS, which can be largely explained by the seasonality in the covariations of the midlatitude jet stream and storm locations. Specifically, significant increases of TCTS occur in June and October during the past decades, which may result from the equatorward displacement of the jet stream and poleward migration of storm locations. Prominent slowdown of TCTS is found in August, which is related to the weakened jet strength and equatorward storm migration. In September, the effects of poleward displacement and weakening of the jet stream on TCTS are largely compensated by the poleward storm migration, and therefore no significant change in TCTS is observed. Meanwhile, the multidecadal variability of the Atlantic may contribute to the multidecadal variability of TCTS. Our findings emphasize the significance in taking a seasonality view in discussing the variability and trends of near-coast Atlantic TCTS under climate change.

The Antarctic Oscillation (AAO) is the dominant mode of the southern extratropical atmospheric mass variability which has potential influences on the Northern Hemisphere (NH). This study reveals a significantly negative correlation between the September–October (SO) AAO index (AAOI) and the occurrence rate of following January–February (JF) wet-cold weather (the latter is quantified by a Precipitation-Temperature (PT) Index) in Southern China (SC) especially the Middle and Lower Reaches of Yangtze River Basin (MLRY). JF PT of SC (SCPT) is modulated by both northerly air flow in the lower troposphere and southerly air flow in the middle-lower troposphere. The SO AAO stimulates Southern Ocean Dipole (SOD) pattern-like sea surface temperature anomalies (SST anomalies: SSTA), which induces a Northern Atlantic Oscillation (NAO)-like atmospheric response along with the tropical northern Atlantic (NA) precipitation anomalies in the following JF through ocean-air interaction. As for wet-cold conditions, there exists an eastward propagational wave train from the tropical-subtropical NA to the East Asian atmospheric patterns, in turn influencing the intensity of East Asian Winter Monsoon (EAWM) and Middle East Jet Stream (MEJS), subsequently the penetration of northerly cold flow and southerly wet flow into MLRY. The variation of JF SOD also regulates the local meridional-vertical cells, in turn influencing the intensity of precipitation over the tropical NA and SC, accompanied with the penetration of wet-warm flow into SC and the adjacent regions. In addition to the tropospheric processes, the stratospheric Quasi Biennial Oscillation (QBO) may serve as the stratospheric pathway for the impact of SOD on NH climate, inducing/reducing the JF SCPT response to SOD SST. To summarize, SO AAO affects the JF SCPT by modulating both dry-cold northerly air flow and wet-warm southerly air flow through tropospheric and stratospheric pathway.

Three striking and impactful extreme cold weather events successively occurred across East Asia and North America during the mid-winter of 2020/21. These events open a new window to detect possible underlying physical processes. The analysis here indicates that the occurrences of the three events resulted from integrated effects of a concurrence of anomalous thermal conditions in three oceans and interactive Arctic-lower latitude atmospheric circulation processes, which were linked and influenced by one major sudden stratospheric warming (SSW). The North Atlantic warm blob initiated an increased poleward transient eddy heat flux, reducing the Barents-Kara seas sea ice over a warmed ocean and disrupting the stratospheric polar vortex (SPV) to induce the major SSW. The Rossby wave trains excited by the North Atlantic warm blob and the tropical Pacific La Nina interacted with the Arctic tropospheric circulation anomalies or the tropospheric polar vortex to provide dynamic settings, steering cold polar air outbreaks. The long memory of the retreated sea ice with the underlying warm ocean and the amplified tropospheric blocking highs from the midlatitudes to the Arctic intermittently fueled the increased transient eddy heat flux to sustain the SSW over a long time period. The displaced or split SPV centers associated with the SSW played crucial roles in substantially intensifying the tropospheric circulation anomalies and moving the jet stream to the far south to cause cold air outbreaks to a rarely observed extreme state. The results have significant implications for increasing prediction skill and improving policy decision making to enhance resilience in “One Health, One Future”.

Plumes from the boreal spring biomass burning (BB) in northern peninsular Southeast Asia (nPSEA) are lifted into the subtropical jet stream and transported and deposited across nPSEA, South China, Taiwan and even the western North Pacific Ocean. This paper as part of the Seven SouthEast Asian Studies (7-SEAS) project effort attempts to improve the chemical weather prediction capability of the Weather Research and Forecasting coupled with the Community Multiscale for Air Quality (WRF–CMAQ) model over a vast region, from the mountainous near-source burning sites at nPSEA to its downwind region. Several sensitivity analyses of plume rise are compared in the paper, and it is discovered that the initial vertical allocation profile of BB plumes and the plume rise module (PLMRIM) are the main reasons causing the inaccuracies of the WRF–CMAQ simulations. The smoldering emission from the Western Regional Air Partnership (WRAP) empirical algorithm included has improved the accuracies of PM10, O3 and CO at the source. The best performance at the downwind sites is achieved with the inline PLMRIM, which accounts for the atmospheric stratification at the mountainous source region with the FINN burning emission dataset. Such a setup greatly improves not only the BB aerosol concentration prediction over near-source and receptor ground-based measurement sites but also the aerosol vertical distribution and column aerosol optical depth of the BB aerosol along the transport route. The BB aerosols from nPSEA are carried by the subtropical westerlies in the free troposphere to the western North Pacific, while BB aerosol has been found to interact with the local pollutants in the Taiwan region through three conditions: (a) overpassing western Taiwan and entering the central mountain area, (b) mixing down to western Taiwan, (c) transport of local pollutants upwards and mixing with a BB plume on higher ground. The second condition, which involves the prevailing high-pressure system from Asian cold surge, is able to impact most of the population in Taiwan.

The El Niño–Southern Oscillation (ENSO) modulates the lightning flash density (LFD) variability over India during premonsoon, monsoon and postmonsoon seasons. This study intends to shed light on the impact of ENSO phases on the LFD over the Indian subcontinent using the data obtained from Optical Transient Detector (OTD) and Lightning Imaging Sensors (LIS) onboard the Tropical Rainfall Measuring Mission (TRMM) satellite. Results suggest the LFD over northeast India (NEI) and southern peninsular India (SPI) strengthened (weakened) during the warm (cold) phase of ENSO in the premonsoon season. During monsoon season, NNWI (north of northwest India) shows above (below) normal LFD in the cold (warm) ENSO phase. It is striking to note that there are three hot spots of LFD over the Indian land region which became more prominent during the monsoon seasons of the last decade. A widespread increase in LFD is observed all over India during the warm phase of ENSO in the postmonsoon season. A robust rise in graupel/snow concentration is found during the postmonsoon season over SPI in the ENSO warm phase, with the lowest fluctuations over the NEI and NNWI regions. The subtropical westerly jet stream is shifted south in association with the warm phase, accompanied by an increase in geopotential height (GPH) all over India for the same period. This exciting remark may explain the indirect influences of ENSO's warm phase on LFD during the postmonsoon season by pushing the mean position of the subtropical westerly towards southern latitudes. However, the marked increase in LFD is confined mostly over the NNWI in the cold ENSO phase.

Persistent hot–dry or cold–wet summer weather can have significant impacts on agriculture, health, and the environment. For northwestern Europe, these weather regimes are typically linked to, respectively, blocked or zonal jet stream states. The fundamental dynamics underlying these circulation states are still poorly understood. Edward Lorenz postulated that summer circulation may be either fully or almost intransitive, implying that part of the phase space (capturing circulation variability) cannot be reached within one specific summer. If true, this would have major implications for the predictability of summer weather and our understanding of the drivers of interannual variability of summer weather. Here, we test the two Lorenz hypotheses (i.e., fully or almost intransitive) for European summer circulation, capitalizing on a newly available very large ensemble (2000 years) of present-day climate data in the fully coupled global climate model EC-Earth. Using self-organizing maps, we quantify the phase space of summer circulation and the trajectories through phase space in unprecedented detail. We show that, based on Markov assumptions, the summer circulation is strongly dependent on its initial state in early summer with the atmospheric memory ranging from 28 days up to ~45 days. The memory is particularly long if the initial state is either a blocked or a zonal flow state. Furthermore, we identify two groups of summers that are characterized by distinctly different trajectories through phase space, and that prefer either a blocked or zonal circulation state, respectively. These results suggest that intransitivity is indeed a fundamental property of the atmosphere and an important driver of interannual variability.

Statistical analyses of the seasonal and temporal distributions of turbulence events in the Santiago (Chile)–Mendoza (Argentina) route region are constructed using automated in situ turbulence observation measurements of the vertical acceleration in flights from LATAM Airlines aircraft. The VRTG database contains 2485 records over 22 months, from March 2018 to December 2019. Although winter in the Southern Hemisphere (SH) was the period with the greatest occurrence of turbulence in the Andes, severe turbulence was more frequent in the summer and early autumn in SH. Through synoptic-scale analysis generated by the post-processing of the Global Forecasting System, it was possible to determine that the unusual severe turbulence events are not related to either convectively induced turbulence or mountain wave turbulence, but to the upper-level atmospheric mechanisms that caused severe clear air turbulence by an interaction tripod between the Bolivian high, upper-level jet stream, and topography.

One of the significant mountain region running parallel to India's west coast is the Western Ghats. Normally, this area receives three times rainfall because it is nearly vertical to the low-level jet stream. But this rainfall is affected in recent years due to the aerosol particles. Aerosols, tiny particles present in the atmosphere, play crucial roles in weather and climate. It is generated primarily as solid particles, for example, Asian dust, Saharan dust, sea salt, or soot. Aerosols can have a variety of impacts (e.g., direct and indirect effects) on the atmosphere, which can lead to the earth's radiation budget. It has a strong impact on the formation of clouds and precipitation processes also. The aerosol change-based influences are considered as the main factor related to air pollution controlling and environmental regulations. Hence, this article mainly concentrates on climate influences related to global aerosol variation. In this paper, recent aerosol behaviors and their impacts on the Western coastal (WG) region of India from 2014 to 2017 are analyzed. From the analysis, we observed that the precipitation is increased in the West Coast region.

The thermal effect of the Tibetan Plateau (TP) on the northern hemisphere climate has long been a hot topic of scientific research. However, the global effects of the TP heat source are still unclear. We investigate the teleconnection patterns coincident with the TP heat source in boreal summer using both observational data and numerical models including a linearized baroclinic model and an atmospheric general circulation model. The western TP shows the most intense variability in atmospheric heating and the most active connection to atmospheric circulations. The surface sensible heating component of the western TP heat source is associated with a high-latitude wave train propagating from North Japan to central North America through the Bering Sea and Canada. The radiative heating component is accompanied by a wavenumber-4 wave train over Eurasia. We focus on the global zonally-oriented pattern that is connected with the latent heat release from the western TP, referred to here as the TP–circumglobal teleconnection (TP-CGT). The TP-CGT pattern is triggered by the western TP latent heating in two parts starting from the TP: an eastward-propagating wave train trapped in the westerly jet stream and a westward Rossby wave response. The TP-CGT accounts for above 18% of the total variance of the circumglobal teleconnection pattern and modulates mid-latitude precipitation by superimposition. The western TP is the key region in which diabatic heating can initiate the two atmospheric responses concurrently, and the heating over northeastern Asia or the Indian Peninsula is unable to induce the circumglobal pattern directly. The unique geographical location and strong tropospheric heating also make the western TP as a “transit area” of transferring the indirect impact of the Indian summer monsoon (ISM) to the TP-CGT. These results enhance our understanding of the relationship between the circumglobal teleconnection and the ISM and is helpful for improving the prediction of the circumglobal teleconnection variability.

Torrential rains cause many losses in city infrastructure, crops, and deaths in several regions of the world including Iraq as in the case that we will discuss in this work, on January 28 and 29, 2019. Torrential rain caused the flow of torrents in several areas of Iraq and the neighboring areas. This research work aims to identify the synoptic characteristics of torrential rains and the causes of this case. This will be done by analyzing and interpreting the weather maps at different pressure levels with focusing on the troughs and fronts locations, relative vorticity, polar jet stream effect as well as the moisture flux. The Geographic Information System (GIS) was used to analyze the satellite images in order to calculate the Normalized Difference Water Index (NDWI) to confirm the heavy rain case. The weather maps were obtained from the Modern-Era Retrospective analysis for Research and Applications Version 2 (MERRA-2). As for the satellite images we used the satellite imagery from Sentinel-2 and EMUTSAT.

Turbulence is a major factor affecting flight safety and its study is of great importance in order to improve air transports quality. Aviation-scale turbulence is recorded in situ by commercial aircraft through the Aircraft Meteorological Data Relay (AMDAR) programme. In this study, we analyze a plethora of AMDAR records for an 11-year period (2008–2018) over Europe. The available indicator of turbulence is the Derived Equivalent Vertical Dust Velocity (DEVG), which is an aircraft-independent metric. We focus on two flight level layers, below 5000-ft MSL and over 20,000-ft MSL in order to study the low-level and upper-level heavy-severe (H–S) turbulence, respectively. The seasonal variability is discussed, and the impact of possible turbulence sources is analyzed. Moreover, the vertical distribution of H–S turbulence records is presented. We found a strong impact of the jet stream in the records of the upper-level turbulence events over Europe and an increased number of low-level turbulence events in the southern regions. Low-level H–S turbulence encounter frequencies present a more distinct cycle with a maximum during late spring and early summer and a minimum during autumn months. This seasonal cycle seems to shift a few months later in the upper-level turbulence frequencies with the maximum occurring during late summer and autumn. Moreover, the diurnal variability of the low-level turbulence revealed a maximum before sunrise and a maximum over the continental areas during the midday hours of spring and summer.

Global closed basins, occupying almost one fifth of the world’s land area, spatially coincide with arid and semiarid areas. Paleoclimatic proxies can indicate basin-wide environmental change and human activity. However, previous studies have not approached the use of proxies in the same way to reconstruct natural and anthropogenic processes at regional and global scales. Here we present a regional study to investigate the basic processes of paleoclimatic proxies, from a typical closed-basin system in arid China. We use multiple paleoclimatic proxies of surface samples and sediments, as well as groundwater and sediment ages to study environmental change and human activity. We then establish a dataset for paleoclimatic proxies from global closed basins and do a numerical analysis on it. Regional studies verify that human activity greatly impacts paleoclimatic proxies, especially with regard to surface samples, as well as groundwater age, but Holocene sediments are less affected. Results from global studies indicate that the major changing trend of the wet/dry status of closed basins is associated with the movement of the westerly jet streams controlled by long-term changes in winter insolation. There is an abrupt change between 1800 AD and 1900 AD, according to a numerical synthesis of paleoclimatic proxies from global closed basins, which can be linked to human impact. We suggest this time period can be considered as a start point for the Anthropocene based on the sedimentary evidence of closed basins, globally.

Migratory cyclones and anticyclones account for most of the day-to-day weather variability in the extratropics. These transient eddies act to maintain the midlatitude jet streams by systematically transporting westerly momentum and heat. Yet, little is known about the separate contributions of cyclones and anticyclones to their interaction with the westerlies. Here, using a novel methodology for identifying cyclonic and anticyclonic vortices based on curvature, we quantify their separate contributions to atmospheric energetics and their feedback on the westerly jet streams as represented in Eulerian statistics. We show that climatological westerly acceleration by cyclonic vortices acts to dominantly reinforce the wintertime eddy-driven near-surface westerlies and associated cyclonic shear. Though less baroclinic and energetic, anticyclones still play an important role in transporting westerly momentum toward midlatitudes from the upper-tropospheric thermally driven jet core and carrying eddy energy downstream. These new findings have uncovered essential characteristics of atmospheric energetics, storm track dynamics and eddy-mean flow interaction.

The Sahel region, located between the tropical rainforests of Africa and the Sahara Desert, has rainfall that varies widely from year to year, associated with extremely deep convection. This deep convection, strongly heated by water vapor condensation, suggests the possibility of exerting a remote influence on mid- and high-latitude climate similar to the well-known influences of tropical oceanic convection on global climate. Here we investigate the possibility that deep convection over the Sahel initiates a semi-circumglobal teleconnection extending to eastern Eurasia. Statistical analysis and numerical experiments support the possible existence of this teleconnection at an interannual time scale. We propose that the anomalous heat source due to deep convection over the Sahel in the late monsoon season influences meandering of the mid-latitude jet stream over Europe through the combination of a Matsuno-Gill response and advection of absolute vorticity. This subtropical jet meander may in turn drive an eastward propagation of a Rossby wave across Eurasia as far as East Asia. Because deep convection over other subtropical land areas may exert a similar remote influence upon extratropical extreme weather, further studies of the influence of overland convection may provide us with an expanded comprehension of teleconnections.

Northern hemisphere mid-latitudeweather is strongly influenced by the polar jet stream (PJS), which dictates the position of storm tracks; this influence also extends to weather patterns conducive to the ignition and growth of large wildfires. We examined the role of the PJS on extreme wildfire events in North America (NA) between 40° and 70°N latitude, using fire spread events (FSEs) for 2002—2016. Climatologies of the 300-hPa wind components and derived quantities show that the PJS weakens and moves northward in the Boreal summer coincident with the fire season. We use spatio-temporal compositing of 300-hPa winds and 500-hPa geopotential height anomalies to show that FSEs are associated with an upper-level ridge and high centered over events, except eastern Canada where patterns are displaced westward. Ridge patterns persist longer for FSEs in western NA compared with Eastern NA, as well as for May to August compared with April, September and October. These tropospheric patterns also occur concomitantly with surface weather drivers of fire spread including positive daily mean temperature and vapor pressure deficit anomalies and negative precipitation anomalies. Distributions of maximum and minimum latitudinal jet stream peak, ridge and trough positions, relative to FSEs, confirm that events occur predominantly southward of the jet stream core and near a ridge for low split flow configurations but not necessarily for high split flow configurations. These findings have wide-reaching implications for better understanding NA fire regimes and potential fire management strategies (eg. resource pre-positioning and tactical suppression) through improved forecasting of fire weather.

Extratropical cyclones (ETCs) over East Asia and Northwest Pacific are identified and tracked by applying an objective algorithm to the 850-hPa relative vorticity fields from the ERA-Interim reanalysis. A total of 2866 ETCs originating at the western side of the date line have been identified in the extended November–March winters from 1979 to 2018. The ETC tracks are counted and visualized using a hexagonal tessellation rather than the regular longitude–latitude grids. Two generalized linear models (GLMs), Poisson regression model and Gamma regression model, are firstly applied to investigate the associations of wintertime ETCs with three atmospheric teleconnection patterns. The West Pacific (WP) pattern and the Pacific/North American (PNA) pattern are more responsible for the meridional variability of ETC activities in the North Pacific, while the influence of the Polar/Eurasia pattern on ETC activities is negligible. Results of composite analysis are qualitatively consistent with that of regression analysis. Composite maps of differences of jet stream, thermal gradient and mid-tropospheric baroclinicity in the positive and negative phases of teleconnection patterns also support the close associations of ETC activities with WP and PNA teleconnection patterns.

To know the reasons for changing the daily rainfall amount, cases study were chosen for Baghdad station by specifying the maximum and minimum daily rain for the months of the rainy season for each year of the study period (2007-2018). The behavior of daily rainfall for the rainy season varying temporally with maximum (89 mm) and minimum (0.1mm). Synoptic analysis of weather chart for pressure levels (300, 500, 700, 850, and surface) was performed. Visible images of satellites were used to determine the quantities of the cloud for the selected cases. The results showed that synoptic analysis in all cases occurs due to the presence of a jet stream in the upper atmosphere, accompanied by the presence of a basin at the level of 300 hectopascals with an enhancement of relative humidity exceeding 70% at the level of 700 hectopascals, but the presence of the front makes the amount of rain greater in the absence of it. At the level of 850 hectopascals and the surface, the depressions that affect Iraq also differ in their severity and the amount of moisture loaded.

This study presents results from the Polar Amplification Multimodel Intercomparison Project (PAMIP) single-year time-slice experiments that aim to isolate the atmospheric response to Arctic sea ice loss at global warming levels of +2°C. Using two general circulation models (GCMs), the ensemble size is increased up to 300 ensemble members, beyond the recommended 100 members. After partitioning the response in groups of 100 ensemble members, the reproducibility of the results is evaluated, with a focus on the response of the midlatitude jet streams in the North Atlantic and North Pacific. Both atmosphere-only and coupled ocean–atmosphere PAMIP experiments are analyzed. Substantial differences in the midlatitude response are found among the different experiment subsets, suggesting that 100-member ensembles are still significantly influenced by internal variability, which can mislead conclusions. Despite an overall stronger response, the coupled ocean–atmosphere runs exhibit greater spread due to additional ENSO-related internal variability when the ocean is interactive. The lack of consistency in the response is true for anomalies that are statistically significant according to Student’s t and false discovery rate tests. This is problematic for the multimodel assessment of the response, as some of the spread may be attributed to different model sensitivities whereas it is due to internal variability. We propose a method to overcome this consistency issue that allows for more robust conclusions when only 100 ensemble members are used.

This thesis analyses the sea surface temperature variations around Ireland’s coastline from 2002-2020, using time series analysis of the one inactive and five currently active marine buoys. In depth analysis of the annual and seasonal sea surface temperature variations were assessed as well as spells of maximum and minimum sea surface temperature for each buoy. Both short term and long-term causations were identified as contributors to the variations in sea surface temperature experienced around Ireland’s coastline. Particular focus on the Atlantic Multidecadal Variability, The NAO Index, the North Atlantic hurricane season and the position of the jet stream were assessed and correlated with sea surface temperature changes around Ireland’s coastline from 2002-2020. These processes are interconnected and display a proportional relationship to each other as there is a high correlation between changes in these processes and a domino effect of other atmospheric and oceanic processes that influence sea surface temperatures around Ireland’s coastline. There were variations in the annual average SST between the buoys. The buoys M4 and M5 both recorded an overall increase in mean annual SST from 2002-2020. Buoys M2, M3 and M6 recorded an overall decrease in the mean annual SST from 2002-2020. No clear connection between the different annual SST trends between the buoys were identified, however their individual location in relation to the continental shelf was a local factor that may have contributed to the variations between the buoys. In depth analysis of the long- and short-term contributors to variations in SST around Ireland’s coastline showed that the most significant influencing factors of SST variations around Ireland’s coastline were the North Atlantic hurricane season track positions and the varying position of the jet stream. Both of these factors displayed a significant impact on the annual SST variations and the spells of either maximum or minimum SST for each buoy. These factors also contributed to changes in the NAO Index, which further influenced SST variations around Ireland’s coastline. The hurricane events contribute to the mixing of waters around Ireland’s coastline, causing unstable conditions and a high level of mixing waters during negative NAO Index occurrences. The AMV signal impacts on air temperature, causing an increase in the effect and frequency of hurricane events in the North Atlantic which influence SST around Ireland’s coastline. Assessment of ongoing changes in the position of the jet stream would be a critical factor in evaluation of future SST variations around Ireland’s coastline.

In recent years much attention has been devoted to the investigation of the impact of increasing the horizontal resolution of global climate models. In the present work, a set of atmosphere-only idealized sensitivity simulations with EC-Earth3 have been designed to disentangle the relative roles of increasing the resolution of the resolved orography and of the atmospheric grid. Focusing on the winter Northern Hemisphere, it is shown that if the grid is refined while keeping the resolved orography unchanged, model biases are reduced only in some specific occasions. Conversely, increasing the resolved (or mean) orography is found to clearly reduce several important systematic model errors, including synoptic transient eddies, the North Atlantic jet stream variability and atmospheric blocking frequency and duration. From an analysis of the radiation budget it is concluded that the large changes in radiative fluxes caused by the resolution increase – something commonly observed in climate models – have a relevant impact on the atmospheric circulation, partially offsetting the benefits obtained from the increase in orographic resolution. These findings point to the necessity of always tuning climate models to fully exploit the benefits of high horizontal resolution.

China experienced significant flooding in the summer of 2020 and multiple extreme cold surges during the winter of 2020/21. Additionally, the 2020 typhoon season had below average activity with especially quiet activity during the first half of the season in the western North Pacific (WNP). Sea surface temperature changes in the Pacific, Indian, and Atlantic Oceans all contributed to the heavy rainfall in China, but the Atlantic and Indian Oceans seem to have played dominant roles. Enhancement and movement of the Siberian High caused a wavier pattern in the jet stream that allowed cold polar air to reach southward, inducing cold surges in China. Large vertical wind shear and low humidity in the WNP were responsible for fewer typhoons in the first half of the typhoon season. Although it is known that global warming can increase the frequency of extreme weather and climate events, its influences on individual events still need to be quantified. Additionally, the extreme cold surge during 16–18 February 2021 in the United States shares similar mechanisms with the winter 2020/21 extreme cold surges in China.

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 mechanisms of the propagation and maintenance of the South Asian jet wave train are investigated in this study by examining the eddy kinetic energy (EKE) balance along the wave train. The intensity and evolution of the disturbances along the wave train can be well represented by the variation of EKE centers. The east-to-west discrepancy in the intensity of EKE centers indicates the wave train is not only maintained by the waveguide effect of the jet stream, but also fueled and damped by other physical processes along the wave train. It is found that both baroclinic and barotropic conversions are key energy sources during the lifespan of the wave train, characterized by regional differences. They evolve with the eddies and are offset mainly by energy exported by the ageostrophic geopotential fluxes after the peak of the eddies. They are strong upstream of the wave train, as extremely strong cooling/warming advection appears behind/ahead of the trough over the eastern Mediterranean Sea-Middle East where abnormal descent/ascent occurs, and potential energy is transformed into kinetic energy. As this trough tilts southwestward, kinetic energy can be converted from the mean flow to eddies on the north flank of the South Asian jet. Strong dissipation occurs over the Middle East, which might be responsible for the relatively weaker EKE centers downstream. Over China, the eddy is fueled by strong barotropic conversion again, as a result of the northward shift of the jet axis.

Europe’s recent summer droughts have had devastating ecological and economic consequences, but the severity and cause of these extremes remain unclear. Here we present 27,080 annually resolved and absolutely dated measurements of tree-ring stable carbon and oxygen (δ13C and δ18O) isotopes from 21 living and 126 relict oaks (Quercus spp.) used to reconstruct central European summer hydroclimate from 75 bce to 2018 ce. We find that the combined inverse δ13C and δ18O values correlate with the June–August Palmer Drought Severity Index from 1901–2018 at 0.73 (P

In this study, we analyse the influence of North Atlantic midwinter storm track suppressions on European synoptic temperature and precipitation anomalies to determine the large-scale conditions relevant for the so-called Christmas thaw. We diagnose this relation in daily ERA5 reanalysis data in the spatial resolution of 0.25∘ between 1979 and 2018. To access synoptic time scales, a 3–10-day band-pass filter is applied. An index for the suppression is defined by the upper tropospheric Eddy Kinetic Energy (EKE) anomalies in the North Atlantic. We define the strong jet stream years as the year exceeding the 75% of the winter seasonal values at 250 hPa. In winters with strong jet activity, the storm track suppression is found, in agreement with the barotropic governor mechanism. Composites of European surface temperature and precipitation for low index values reveal weakly warmer conditions during winter (DJF) in Central Europe and the British Isles and a distinct cooling in Northern Europe. In the 1-month interval during December 15 to January 15, the warming is more pronounced. The clearest signal is the precipitation increase with a magnitude of 1 mm/day in the Mediterranean region.

In this paper we describe a technique for measuring the tropospheric wind profile using a 35-GHz Doppler profiler. The parameters of the equipment and the accuracy of measurements of the wind parameters are given. An algorithm for estimating wind pulsations at various altitudes is described. The measurement results are compared with the data obtained on the meteorological mast. Examples of measurements under various meteorological conditions, including the registration of jet streams, are given. Differences between experimental and theoretical results when measuring the radar reflectivity of clear sky signals are discussed. Statistical data on the probability of registering radar reflections in various seasons of the year are presented.

To understand structural changes and forecast error, a case study of binary typhoons in the western North Pacific (WNP) of 2018 was investigated using best track and reanalysis data. Soulik was generated on August 16 and Cimaron was generated on August 18, respectively. The 19th typhoon Soulik and 20th typhoon Cimaron co-existed from August 18 to 24 and approached each other. Soulik was located on the western side and Cimaron was located on the eastern side of the WNP. They were located approximately 1300 km from each other at 00 UTC August 22. The Soulik structure began changing around August 22 and became weak and slow, while Cimaron maintained its intensity, size, and moving speed. This observational evidence is likely caused by the binary interaction between two typhoons within a certain distance and environmental steering flow, such as the location of the North Pacific high and strong jet stream of the northern flank of the North Pacific high. Soulik was initially forecasted to make landfall and reach Seoul; however, its track changed from northward to northeastward from August 21 to 23 according to both official guidance and unified model (UM). Four global numerical weather prediction models forecasted different tracks of Soulik. UM and JGSM forecasted a northward track whereas ECMWF and GFS showed a northeastward track for 12 UTC August 21 through 12 UTC August 24. The latter models were similar to the best track. The track forecast error and spread of Soulik were larger than those of Cimaron. The mean absolute error of the maximum wind speed of Soulik was similar to the average of total typhoons in 2018.

In this study, we demonstrate a new mechanism, on how the warm phase of El Nino and Southern Oscillation (ENSO) delays the Indian Summer Monsoon onset through coupled ocean-atmospheric processes. Strong basin-wide warming is prominent over North Indian Ocean (NIO) during the El Nino years. The warming is intense over the South West Indian Ocean (SWIO) due to the westward propagation of the equatorial Rossby waves from the Pacific Ocean. It increases the convective activities over the southern tropical Indian Ocean (0–10° S), adjacent to the SWIO region. The warming over the SWIO and the NIO strengthens the divergent wind from the Indian Ocean to the sub-tropics via a wind-evaporation-SST feedback mechanism, which causes more upper-level convergence within 30° N latitudes. Besides, a warmer Indian Ocean enhances the upper-level diabatic heating over the southern Arabian Peninsula and Eastern Indian landmass. These factors strengthen but shift the local Hadley circulation over the Indian Ocean more southward, with an ascending branch centered over the SWIO region. The shifting of the local Hadley circulation during the El Nino years causes the Sub Tropical Jet (STJ) to migrate more southward and centered more over the Indian subcontinent. This southward movement of the STJ over the Indian subcontinent in response to the El Nino condition inhibits the establishment and propagation of the tropical easterly jet during the monsoon season, which subsequently hinders the monsoon circulation, thus delays its onset.

This study investigates the concurrent location and intensity changes of the East Asian polar front jet (EAPJ) and subtropical jet (EASJ) on decadal time scale using NCEP/NCAR, ERA5 and JRA-55 reanalysis data, along with the relationship with the mid-high latitude circulation changes. Results show that EAPJ was intensified accompanied by the weakening of EASJ after 1985, and shifted equatorward in conjunction with the poleward migration of EASJ after 1999. The three-dimensional structures of the atmospheric circulation over East Asia exhibited distinct changes corresponding to the two regime shifts. Dipole anomalous sea level pressure (SLP) between the Arctic and the North Pacific, a NAM-like geopotential height anomalous pattern, and a deepened polar vortex occurred after 1985, accompanied with the out-of-phase intensity changes of the two jets. In comparison, dipole SLP anomalies between the mid-high latitudes and Tibetan Plateau, a zonal wave train over Eurasia accompanied with a meridional wave train over East Asia, and a polar vortex shifting toward the Siberia were observed after 1999, when the two jets approached each other. These circulation changes exerted pronounced effects on the East Asian climate anomalies. After the mid-1980s, the positive SLP anomaly over the northern North Pacific led to decreased land-sea SLP contrast, and the positive mid-tropospheric geopotential height center near Lake Baikal implied weakened East Asian trough (EAT). Such changes favored the warming of southern East Asia and the weakening of the southern mode of East Asian winter monsoon (EAWM). In contrast, the high latitude positive SLP anomalies denoted enhanced Siberian high (SH) and the East Asian meridional wave train resulted in tilted EAT after the late 1990s, benefitting the cold anomalies over northern Eurasia with the northern mode of EAWM intensified. Possible mechanisms for the decadal changes of the jets are discussed from the perspective of the dynamic forcing of the synoptic-scale transient eddy activities (STEA), and the thermal forcings from the East Asian landmass, the North Pacific sea surface and the Arctic sea ice cover.

Summer monsoonal rainfall over East Asia is dominated by precipitation associated with the East Asian summer monsoonal front (EASMF). A Community Atmospheric Model (CAM5.1) with a high horizontal resolution of 50 km is employed in this study to investigate the interannual variability as well as projected future trends in the EASMF under the Representative Concentration Pathway 8.5 scenario. Seasonal march of the EASMF is reproduced reasonably well in the model’s present-day simulation despite a northward shift of the simulated front from its observed position. Based upon a suite of objectively-defined daily indices of the EASMF, we show that the EASMF in the late twenty-first century will be more intense and displaced eastward and southward from its present-day mean location. Moreover, EASMF events will exhibit a wider meridional expansion and a longer duration. Monsoonal precipitation over East Asia is particularly sensitive to the meridional displacements of EASMF. In conjunction with the projected southward shift of EASMF, an enhanced rain band is seen to extend northeastward from southern China to the northwestern Pacific south of Japan. This precipitation feature is associated with strengthened and southward-shifted westerly jet streams at 250 and 700 hPa, which are respectively linked to tropical warming in the upper troposphere and warming over the South China Sea in the lower troposphere during the twenty-first century. Within the latitudinal “gap” south of the upper-level jet and north of the lower-level jet, the local vorticity tendencies are maintained by upper-level divergence and lower-level convergence, thus accompanied by enhanced upward motion and precipitation. The site at which this “jet stream-precipitation” relationship prevails is notably modulated by long-term trends in the temperature and circulation patterns associated with climate change.

The Asian subtropical westerly jet (AWJ) exerts crucial influences on Eurasian continent weather and climate. This paper analyzes the advantages and limitations of CRA-40, which is China’s first generation 40-yr (1979–2018) glo-bal atmosphere and land reanalysis product, in describing the characteristics of AWJ, compared with the ECMWF Reanalysis version 5 (ERA5) and NCEP Climate Forecast System Reanalysis (CFSR). The results show a close agreement across the three reanalyses on the whole. (1) In terms of climatology, overall differences of 200-hPa zonal wind across the three reanalyses are within ± 0.5 m s−1 (i.e., ± 2%). Large differences with maxima of ± 2 m s−1 (± 5%) appear over the Iranian Plateau and south of the Tibetan Plateau in the mid–upper troposphere in winter. (2) For seasonal cycle, the position and intensity of the AWJ centers in the three reanalyses are highly consistent, with correlation coefficient over 0.98. But there are some discrepancies in the zonal shift of the western AWJ center during the transition season. (3) On the interannual timescale, intensity of all AWJ centers varies consistently among the three reanalyses, while larger differences appear in their meridional displacement, especially in the eastern AWJ center. (4) For long-term variations, the three reanalyses all present a significant northward movement of the westerly jet axis in winter, and a southward displacement over central Asia (40°–80°E) and a northward migration over East Asia (80°–110°E) in summer. Thus, this study has provided confidence that CRA-40 has comparable performance with ERA5 and CFSR in depicting the characteristics of AWJ.

The environments of 53 severe squalls and tornadoes that caused large-scale windthrows in the forest zone of European Russia and the Ural in 1989–2019 are analyzed. The CFSR and ERA-5 reanalyses and sounding data were used to estimate characteristics of the environments including convective instability indices. It was found that the substantial temperature gradient on the atmospheric front (9.6°C/500 km on average) and the jet stream presence in the lower or middle troposphere oriented along the frontal zone are important factors to estimate environments of the formation of severe squalls and tornadoes. In most cases, squalls and tornadoes require a combination of high precipitable water content (40 mm on average), moderate or high convective instability ( $$\mathrm{CAPE}> 1000$$ J/kg), and moderate or strong wind shear. High precipitable water content and strong convective instability are important for the formation of squalls, while low-level wind shear plays a principal role for the tornado generation.