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The Tarim Basin is a large inland arid basin in the arid region of northwest China and has been experiencing significant "warming and wetting" since 1987. As a result, the purpose of this paper is to determine whether the climate transition phenomenon occurred in the Tarim Basin as well as the role of atmospheric circulation in this process. We use meteorological data and atmospheric circulation indexes to study the seasonal trends of climate change in this region from 1987 to 2020 to understand how they are affected by atmospheric circulation. The findings show that, from 1987 to 2020, the Tarim Basin experienced significant warming and wetting; with the exception of the winter scale, all other seasonal scales exhibited a clear warming and wetting trend. From the perspective of spatial distribution, most of the areas showed a significant warming trend, and the warming amplitude around the basin is greater than that in the central area of the basin. However, there are significant regional differences in precipitation change rates. Meanwhile, wavelet analysis shows that there is a significant oscillation period of 17–20 years between climate change and the atmospheric circulation index during 1987–2020. The correlation analysis shows that the Pacific decadal oscillation (PDO) and El Niño-Southern Oscillation (ENSO) are the main influencing factors of climate change in the Tarim Basin at different seasonal scales, while the teleconnection of the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) is low and the PDO dominates the summer and autumn temperature changes in the Tarim Basin. The research results of this paper show that, despite the warming and wetting trends since 1987 in the Tarim Basin, the climate type did not change. From 1987 to 2020, the main teleconnection factors of climate change in the Tarim Basin were PDO and ENSO. [ABSTRACT FROM AUTHOR]

Rainfall, with its intensity, duration, and seasonal distribution, is among the main factors causing soil erosion, which is a widespread environmental problem in Bulgaria. Rainfall erosivity shows the potential of precipitation to generate erosion processes and is an essential indicator of the climate vulnerability of a region. This paper aims to evaluate rainfall erosivity in a part of the Eastern Rhodopes Mountains, an area that is characterised by high-intensity erosion processes and high erosion risk. Local peculiarities of rainfall erosivity were revealed by the calculation of some precipitation indices based on the monthly precipitation for the period 2000–2021, such as the precipitation concentration index (PCI), Angot precipitation index, Fournier index (FI), and modified Fournier index (MFI). The analysis of the extremely wet and extremely dry months at the annual and seasonal (October–March and April–September) levels was performed to evaluate the susceptibility to erosion. The results from the study show that rainfall erosivity in the studied area varies from low to moderate in the northern part of the study area and from high to very high in the south. According to the MFI, high and very high erosivities have been observed mainly since 2012. The erosivity increases from north to south, to the area with a complex relief, where the combination of orography and atmospheric circulation make favourable conditions for the occurrence of extreme precipitation. The analyses of the calculated indices show that the precipitations in most of the studied area generally have from a low to a moderate erosivity, but this does not exclude the occurrence of cases with high and very high erosivities, which are characteristic of recent years and are related to the increase in annual precipitations and extreme precipitation months. The results of this study can contribute to the development and implementation of measures and preventive activities for the reduction and possible elimination of the negative impacts of extreme precipitation. [ABSTRACT FROM AUTHOR]

The Tarim Basin is a large inland arid basin in the arid region of northwest China and has been experiencing significant “warming and wetting” since 1987. As a result, the purpose of this paper is to determine whether the climate transition phenomenon occurred in the Tarim Basin as well as the role of atmospheric circulation in this process. We use meteorological data and atmospheric circulation indexes to study the seasonal trends of climate change in this region from 1987 to 2020 to understand how they are affected by atmospheric circulation. The findings show that, from 1987 to 2020, the Tarim Basin experienced significant warming and wetting; with the exception of the winter scale, all other seasonal scales exhibited a clear warming and wetting trend. From the perspective of spatial distribution, most of the areas showed a significant warming trend, and the warming amplitude around the basin is greater than that in the central area of the basin. However, there are significant regional differences in precipitation change rates. Meanwhile, wavelet analysis shows that there is a significant oscillation period of 17–20 years between climate change and the atmospheric circulation index during 1987–2020. The correlation analysis shows that the Pacific decadal oscillation (PDO) and El Niño-Southern Oscillation (ENSO) are the main influencing factors of climate change in the Tarim Basin at different seasonal scales, while the teleconnection of the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) is low and the PDO dominates the summer and autumn temperature changes in the Tarim Basin. The research results of this paper show that, despite the warming and wetting trends since 1987 in the Tarim Basin, the climate type did not change. From 1987 to 2020, the main teleconnection factors of climate change in the Tarim Basin were PDO and ENSO.

Co-occurring extreme heat, drought, and moisture events are increasing under global warming and pose serious threats to ecosystem and food security. However, how to effectively link compound agrometeorological disasters (CADs) with climate change has not been well assessed. In this study, we focus on the comprehensive influence of large-scale climate factors on CADs rather than extreme meteorological elements. The results indicate that there are two main CADs of spring maize in Shenyang, Northeast China (NEC), including concurrent drought and cold damage (DC) and drought in multiple growth periods (MD). The related circulation anomalies at mid–high latitudes are identified as four patterns, namely, the Northeast Asia Low (NEAL) and Ural High (UH) patterns affecting DC, the Baikal High and Okhotsk Low (BHOL), and the Northeast Asia High (NEAH) patterns leading to MD. The vertical profile and water vapor transport anomalies further demonstrate the influence mechanism of large-scale circulation on compound heat–moisture stresses. This study highlights the role of atmospheric circulation, which can provide effective predictors for these synergistic agrometeorological disasters. [ABSTRACT FROM AUTHOR]

This paper reviews the current state and development of different numerical model classes that are used to simulate the global atmospheric system, particularly Earth's climate and climate-chemistry connections. The focus is on Chemistry-Climate Models. In general, these serve to examine dynamical and chemical processes in the Earth atmosphere, their feedback, and interaction with climate. Such models have been established as helpful tools in addition to analyses of observational data. Definitions of the global model classes are given and their capabilities as well as weaknesses are discussed. Examples of scientific studies indicate how numerical exercises contribute to an improved understanding of atmospheric behavior. There, the focus is on synergistic investigations combining observations and model results. The possible future developments and challenges are presented, not only from the scientific point of view but also regarding the computer technology and respective consequences for numerical modeling of atmospheric processes. In the future, a stronger cross-linkage of subject-specific scientists is necessary, to tackle the looming challenges. It should link the specialist discipline and applied computer science. [ABSTRACT FROM AUTHOR]

To improve how extreme events and climate variations are managed, there is a need to foster a deeper understanding of their interconnections. Consistent with this objective, this paper describes how precipitation extremes change both temporally and spatially in the Inner Mongolian Plateau (IMP), China and explains their causal factors. The paper refers to data collected from 43 meteorological stations in IMP and describes how precipitation extremes formed and how they influence agriculture. Data gathered and presented in this paper may be useful in understanding the extent to which the IMP is being influenced by global environmental change. This study reveals that the eleven precipitation extremes indices, except the number of precipitation days with over 0.5 mm (R0.5), number of heavy precipitation days (R10), and total precipitation in wet days (PRCPTOT), decreased in the IMP between 1959 and 2014, and most of them were non-significant in temporal. But the dry index has a larger magnitude decreasing trend than that of the wet indices, which can indicate that the dry situation was alleviated in IMP during the study interval. This study also indicated that precipitation extremes have strong relationships with elevation, latitude, and longitude. Atmospheric circulation and topography may be further primary reasons which result in the spatial variation characteristics in precipitation extremes over the IMP. Decreases in precipitation extremes, together with human activities such as livestock improvement and ecological restoration programs, has a positive effect in gross output value of agriculture and animal husbandry in the IMP. The results contribute to a deeper insight on the possible impacts of precipitation extremes and support the development of appropriate adaptation and mitigation strategies to cope with climate extremes. This paper further proposes science-based policies for grassland protection, agriculture, and animal husbandry on the national or regional and herdsman scales. [ABSTRACT FROM AUTHOR]

Linking sinuosity, a fairly recently developed metric, with high temperature extremes (HTEs) can be both useful and insightful to better understand the physical mechanisms behind HTEs. However, it is not clear whether there exists a relationship between the sinuosity changes and HTE changes in present and future climate conditions over southeastern China. In this paper, the anomalous characteristics of the atmospheric circulation are quantified by sinuosity. Three sinuosity metrics are used in this study: individual sinuosity (SIN), aggregate sinuosity (ASIN), and comprehensive sinuosity (CSIN). Furthermore, we examine the relationship between sinuosity changes and HTE changes in present and future climate conditions. ASIN is strongly correlated with surface air temperature (SAT). We find that the influence of individual sinuosity (SIN) at different latitudes on the SAT of southeastern China is different. The SIN of low (middle) latitude isohypses has significant positive (negative) correlations with the SAT of southeastern China. The SIN of high-latitude isohypses is rather limited and can therefore be ignored. The projected relationship between the sinuosity changes and HTE changes in the late 21st century suggests similar results. The change in SAT is related to the changes in climate variables over southeastern China in the future, and these changes increase with the increase in Z500 or V850 and the decrease in U500. Moreover, the frequencies of large (small) comprehensive sinuosity (CSIN) values at low (mid) latitudes will increase. At the end of the 21st century, Z500 isohypses at different latitudes will have an obvious poleward shift. Our results indicate that measuring the aggregate waviness of the midtropospheric flow (via sinuosity) can provide insight regarding HTEs, and the climate model output can be used to examine the future likelihood of increased HTE. [ABSTRACT FROM AUTHOR]

To study the dynamical mechanism by which Arctic amplification affects extreme weather events in mid-latitude, we investigated the local and remote circulation response to pan-Arctic and regional Arctic thermal forcing. A comprehensive atmospheric GCM (General Circulation Model) coupled to a slab mixed-layer ocean model is used for the experiment. With the increasing thermal forcing in the pan-Arctic configuration, the mid-latitude jet tends to shift equatorward, mainly due to the southward shift of the convergence zone of eddy-heat flux and eddy-momentum flux. From the regional Arctic forced experiments, zonal mean response is similar to the response from the pan-Arctic configuration. The non-zonal response is characterized by the 300 hPa circumpolar zonal wind of wavenumber-1 structure, which establishes an enhanced wavier mid-latitude jet. In the polar region at 300 hPa, regional thermal forcing drives a distinct east–west dipole circulation pattern, in which anticyclonic circulation is located to the west of the thermal forcing, and cyclonic circulation is located to the east. The lower-level circulation shows the opposite pattern to the upper-level circulation in the polar region. While the strength of circulation increases with gradual thermal forcing, the overall dipole pattern is unchanged. In regional warming simulation, compared to the pan-Arctic warming, increasing residual heat flux in a dipole pattern causes enhanced heat advection to mid-latitude. [ABSTRACT FROM AUTHOR]

Atmospheric ducting is an anomalous atmospheric structure that affects electromagnetic wave propagation. In the context of global warming, the navigation capacity of the Arctic is increased, and the atmospheric duct can affect communication and navigation in the Arctic. In this study, based on the European Centre for Medium-Range Weather Forecasts reanalysis data (ERA-interim), the climate characteristics and their variations of atmospheric ducts over the Arctic polar region (north of 60° N) from 1989 to 2018 were analyzed, including the occurrence frequency, spatial distribution, thickness and intensity of the atmospheric ducts. The results show that the overall frequency of atmospheric ducts in the Arctic is low, with the average frequency of all types of ducts being less than 10% throughout the year. The frequency of surface ducts is 2~3 times that of elevated ducts. More than 90% of the atmospheric ducts in the Arctic have a trapped layer with a thickness of less than 100 m, and the average thickness of surface ducts is higher than that of the elevated ducts. The intensity of the Arctic surface ducts is stronger than that of the elevated ducts, with an average intensity of 2.1 M (±2.3 M) to 4.5 M (±4.5 M) for the surface ducts and 1.7 M (±2 M) to 2.5 M (±2.9 M) for the elevated ducts. There is a positive correlation between the ducts' trapped layer thickness and duct intensity. The variation in atmospheric ducts is responsive to the changes in atmospheric circulation and the sea ice extent. This anomalous circulation changes surface wind in the Arctic, which affects the formation and maintenance of the ducts. The trends of ducts in the Arctic Ocean are consistent with those of the Arctic Sea ice extent, while the Arctic continental and coastal ducts show the opposite trend. [ABSTRACT FROM AUTHOR]

The increase in the frequency and intensity of hazardous hydrometeorological phenomena is one of the most dangerous consequences of climate instability. In this study, we summarize the data on severe weather phenomena using the data from 23 meteorological stations in Crimea from 1976 to 2020. Particular attention was paid to the precipitation phenomena descriptions. For the last 45 years, a significant positive trend of interannual variability of the annual occurrence of severe weather phenomena was estimated to be +2.7 cases per decade. The trend for severe precipitation phenomena was estimated to be +1.3 cases per decade. The probable maximum annual daily precipitation as a quantitative indicator of hazardous events was estimated for each station by using both the stationary and the non-stationary GEV models. For at least half of the meteorological stations, a non-stationary GEV model was more appropriate for the estimation of the precipitation extremes. An analysis of the main synoptic processes that drive severe weather phenomena of precipitation was carried out. The greatest contribution to the formation of severe precipitation was made by Mediterranean–Black Sea cyclones. At the same time, half of all of the cases of extreme precipitation were caused by cyclones generated over the Black Sea only, in all seasons apart from winter. In the mid-troposphere, four types of meridional circulation were identified depending on the location of troughs and ridges, with respect to the Black Sea region. More than 42% of severe precipitation phenomena were accompanied by an isolated high-altitude cyclone in the mid-troposphere over the Black Sea region. The main recommendation that can be drawn from this study is that long-term climatic non-stationarity should be taken into account whenever the risk assessment or hazard analysis is to be carried out. The results can also favor the designing of drainage and sewerage systems in urban areas. The findings of atmospheric patterns can be used for the improvement of extreme precipitation forecasts. [ABSTRACT FROM AUTHOR]

The effect of global climate change on the distribution of snow water is a great concern. Thus, it is important to clarify the characteristics of winter precipitation variability, including mountain precipitation, together with climate indices. In this study, regional snowfall characteristics were investigated with the daily gridded precipitation over Sapporo City (located on the Japan Sea side of Hokkaido in northern Japan), which was quantified by the APHRODITE method and by adding local precipitation observation data. We found places of showing large interannual variability that is different from that of daily precipitation variability. Applying an EOF analysis to the daily grid precipitation, we defined four local precipitation types. The occurrence of each precipitation type and associated atmospheric circulation was analyzed, and the results revealed that (except for the Super El Niño winter of 1997/1998) more snow fell in the southwestern mountains and inland areas during El Niño winters, and more snow fell in the northeastern plains and along the sea during La Niña winters. Continued development and evaluation of the precise data that incorporate local precipitation network is needed. [ABSTRACT FROM AUTHOR]

Precipitations are one of the most important factors affecting water resources in the transboundary Polish–Saxon region. The main goal of the research was to examine the multiannual changes in precipitations in the April–September period in 1971–2018, depending on circulation conditions, based on Ojrzyńska's classification. The analysis was carried out based on meteorological data from Polish and German meteorological stations. The results showed that most of precipitation totals and intensive precipitations were observed under SW-A and SW-C circulation, whereas the anticyclonic types of NE-A, NW-A and SW-A were mainly responsible for dry days occurrence. In terms of multiannual changes, most of the stations were characterized by insignificant trends for the considered indices. Some positive trends were observed for intensive precipitations in the lower hypsometric zones. In the mountains, a decreasing tendency dominated for both precipitation totals and intensive precipitations, especially for the northern types of circulation. Furthermore, a significant increase was reported throughout the region for most of the indices for the SW-A type, including precipitation totals, strong precipitations and dry days. Considering the observed trends, floods related to heavy rains can intensify in the lowlands, while a potential increase in the anticyclonic circulation can significantly limit water resources in the region. [ABSTRACT FROM AUTHOR]

Based on newly-available meteorological reanalysis, we compile and present extended seasonal series of the North Atlantic Oscillation (NAO) and Greenland Blocking indices spanning 1800–2020, which we analyse for evidence of significant trends. This represents a major backward extension of the previously available instrumental-/reanalysis-based Azores–Iceland and principal component-based NAO indices, and allows us to evaluate the potential effect of natural climate perturbations, especially the 1809 and 1815 major volcanic eruptions and ~1790s–1830 Dalton solar minimum, on North Atlantic atmospheric circulation. We find that winters 1809/10 and 1816/17 mark positive NAO peaks, relative to several years before and afterwards, which is in accordance with the theory of volcanic forcing of climate. However, there is little evidence of a summer NAO volcanic signature. Overall, based on the significantly longer new reanalysis time series, the new series presented here corroborate and extend our previous results of: (1) a significantly more variable year-to-year NAO with a recent exceptional clustering of extreme events since 2000 for winter; (2) a significant increasing trend in blocking over Greenland in summer. These trends have major repercussions for the probability of the occurrence of extreme weather events over northwest Europe and for the sensitivity and response of the Greenland Ice Sheet to global warming, especially if they continue as an integral part of anthropogenic climate change. [ABSTRACT FROM AUTHOR]

During the second half of the 20th century, in the littoral and pre-littoral areas of the Valencia region, the diurnal temperature range (DTR) registered a significant drop in summer (July and August). Meanwhile, in the same period in the lower Ebro Valley (Ebro Observatory), to the north of the Valencia region, the maximum temperature and DTR increased steeply. In order to explain the DTR drop in the coastal areas of the Valencia region, some studies have proposed an increase in the summer sea-surface temperature (SST) and others have attributed it to the urban heat island effect. Nevertheless, this drop occurred well before the current climate change was evident and therefore, before the rise of the SST. Furthermore, regarding the second proposed explanation, the drop in the DTR does not disappear when working with selected stations away from heavily urbanized areas. In this work, we propose that both the DTR rise at the Ebro Observatory and the DTR drop in the Valencia region are linked to the same process: changes in atmospheric circulation on a synoptic-scale in the North Atlantic that occurred in the late 1960s. [ABSTRACT FROM AUTHOR]

This paper reviews the current state and development of different numerical model classes that are used to simulate the global atmospheric system, particularly Earth’s climate and climate-chemistry connections. The focus is on Chemistry-Climate Models. In general, these serve to examine dynamical and chemical processes in the Earth atmosphere, their feedback, and interaction with climate. Such models have been established as helpful tools in addition to analyses of observational data. Definitions of the global model classes are given and their capabilities as well as weaknesses are discussed. Examples of scientific studies indicate how numerical exercises contribute to an improved understanding of atmospheric behavior. There, the focus is on synergistic investigations combining observations and model results. The possible future developments and challenges are presented, not only from the scientific point of view but also regarding the computer technology and respective consequences for numerical modeling of atmospheric processes. In the future, a stronger cross-linkage of subject-specific scientists is necessary, to tackle the looming challenges. It should link the specialist discipline and applied computer science.

As one of the largest arid and semi-arid regions in the world, central Asia (CA) is very sensitive to changes in regional climate. However, because of the poor continuity of daily observational precipitation records in CA, the spatial and temporal variations of extreme precipitation in recent decades remain unclear. Considering their good spatial and temporal continuity, gridded data, such as Climate Prediction Center (CPC) global precipitation, and reanalysis data, such as ERA-Interim (ERA), are helpful for exploring the spatial–temporal variations of extreme precipitation. This study evaluates how well CPC and ERA can represent observed precipitation extremes by comparing the differences in eight extreme precipitation indices and observation data at 84 meteorological stations. The results indicate that the CPC (except for 1979–1981) is more suitable for depicting changes in precipitation extremes. Based on the CPC data for the period 1982–2020, we found that seven indices of precipitation extremes, including consecutive wet days (CWD), max1-day precipitation amount (Rx1day), max5-day precipitation amount (Rx5day), number of heavy precipitation days (R10), very wet days (R95p), annual total precipitation in wet days (PRCPTOT), and simple precipitation intensity index (SDII) have increased by 0.2 d/10a, 0.9 mm/10a, 1.8 mm/10a, 0.3 d/10, 8.4 mm/10a, 14.3 mm/10a and 0.1 mm/d/10a, respectively, and the consecutive dry days (CDDs) have decreased by −3.10 d/10a. It is notable that CDDs decreased significantly in the north of Xinjiang (XJ) but increased in Kyrgyzstan (KG), Tajikistan (TI), and eastern Turkmenistan (TX). The other indices increased clearly in the west of XJ, north of Kazakhstan (KZ), and east of KG but decreased in the south of KG, TI, and parts of XJ. For most indices, the largest change occurred in spring, the main season of precipitation in CA. Therefore, the large-scale atmospheric circulation in April is analyzed to contrast between the most and least precipitation years for the region. A typical circulation pattern in April for those extremely wet years includes an abnormal low-pressure center at 850 hpa to the east of the Caspian Sea, which enhances the southerly winds from the Indian Ocean and hence the transportation of water vapor required for precipitation into CA. This abnormal circulation pattern occurred more frequently after 2001 than before, thus partly explaining the recent increasing trends of precipitation extremes in CA. [ABSTRACT FROM AUTHOR]

The Pacific Walker circulation (PWC) is one of the most important components of large-scale tropical atmospheric circulations. The PWC and its influences have been studied extensively by numerical models and reanalysis. The newly released ERA5 and NCEP2 are the most widely used reanalysis datasets and serve as benchmarks for evaluation of model simulations. If the results of these datasets differ significantly, this could lead to a bias in projected long-term climate knowledge. For better understanding of future climate change, it is necessary to evaluate PWC reanalysis productions. As a result, we compared the PWC structures between the ERA5 and NCEP2 datasets from month to seasonal time scales. We used the zonal mass streamfunction (ZMS) over the equatorial Pacific to indicate the strength of the PWC. The PWC's average monthly or seasonal cycle peaks around July. From February to June, the NCEP2 shows a higher PWC intensity, whereas the ERA5 shows greater intensity from July to December. The circulation center in the NCEP2 is generally stronger and wider than in the ERA5. The ERA5, however, revealed that the PWC's west edge (zero line of ZMS over the western Pacific) had moved 10 degrees westward in comparison to the NCEP2. In addition, we compared the PWC mean state in the reanalysis and CMIP6 models; the mean state vertical structures of the tropical PWC in the CMIP6 multi-model ensemble (MME) are similar to those of the reanalyses in structure but weaker and wider than in the two reanalysis datasets. The PWC is broader in CMIP6, and the western boundary is 7 and 17 degrees farther west than in the ERA5 and NCEP2, respectively. This study suggests that, when using reanalysis datasets to evaluate PWC structural changes in intensity and western edge, extreme caution should be exercised. [ABSTRACT FROM AUTHOR]

Tropical cloud clusters (TCCs) are embryos of tropical cyclones (TCs) and may have the potential to develop into TCs. The genesis productivity (GP) of TCCs is used to quantify the proportion of TCCs that can evolve into TCs. Recent studies have revealed a decrease in GP of western North Pacific (WNP) TCCs during the extended boreal summer (July–October) since 1998. Here, we show that the changing tendencies in GP of WNP TCCs have obvious seasonality. Although most months could see recent decreases in GP of WNP TCCs, with October experiencing the strongest decreasing trend, May is the only month with a significant recent increasing trend. The opposite changing tendencies in May and October could be attributed to different changes in low-level atmospheric circulation anomalies triggered by different sea surface temperature (SST) configurations across the tropical oceans. In May, stronger SST warming in the tropical western Pacific could prompt increased anomalous westerlies associated with anomalous cyclonic circulation, accompanied by the weakening of the WNP subtropical high and the strengthening of the WNP monsoon. Such changes in background atmospheric circulations could favor the enhancement of atmospheric eddy kinetic energy and barotropic energy conversions, resulting in a recent intensified GP of WNP TCCs in May. In October, stronger SST warming in the tropical Atlantic and Indian Oceans contributed to anomalous easterlies over the tropical WNP associated with anomalous anticyclonic circulation, giving rise to the suppressed atmospheric eddy kinetic energy and recent weakened GP of WNP TCCs. These results highlight the seasonality in recent changing tendencies in the GP of WNP TCCs and associated large-scale atmospheric-oceanic conditions. [ABSTRACT FROM AUTHOR]

Drought is one of the most complex climate-related phenomena and is expected to progressively affect our lives by causing very serious environmental and socioeconomic damage by the end of the 21st century. In this study, we have extracted a dataset of exceptional meteorological drought events between 1975 and 2019 at the country and subregional scales. Each drought event was described by its start and end date, intensity, severity, duration, areal extent, peak month and peak area. To define such drought events and their characteristics, separate analyses based on three drought indices were performed at 12-month timescale: the Standardized Precipitation Index (SPI), the Standardized Precipitation Evapotranspiration Index (SPEI), and the Reconnaissance Drought Index (RDI). A multivariate combined drought index (DXI) was developed by merging the previous three indices for more understanding of droughts' features at the country and subregional levels. Principal component analysis (PCA) was used to identify five different drought subregions based on DXI-12 values for 312 Mediterranean stations and a new special score was defined to classify the multi-subregional exceptional drought events across the Mediterranean Basin (MED). The results indicated that extensive drought events occurred more frequently since the late 1990s, showing several drought hotspots in the last decades in the southeastern Mediterranean and northwest Africa. In addition, the results showed that the most severe events were more detected when more than single drought index was used. The highest percentage area under drought was also observed through combining the variations of three drought indices. Furthermore, the drought area in both dry and humid areas in the MED has also experienced a remarkable increase since the late 1990s. Based on a comparison of the drought events during the two periods—1975–1996 and 1997–2019—we find that the current dry conditions in the MED are more severe, intense, and frequent than the earlier period; moreover, the strongest dry conditions occurred in last two decades. The SPEI-12 and RDI-12 have a higher capacity in providing a more comprehensive description of the dry conditions because of the inclusion of temperature or atmospheric evaporative demand in their scheme. A complex range of atmospheric circulation patterns, particularly the Western Mediterranean Oscillation (WeMO) and East Atlantic/West Russia (EATL/WRUS), appear to play an important role in severe, intense and region-wide droughts, including the two most severe droughts, 1999–2001 and 2007–2012, with lesser influence of the NAO, ULMO and SCAND. [ABSTRACT FROM AUTHOR]

Fu et al. [[1]] examine two influential modes of stratospheric variability - the quasi-biennial oscillation (QBO) and sudden stratospheric warmings (SSWs) in the Last Glacial Maximum (LGM) - using the well-suited Whole Atmosphere Community Climate Model version 6 (WACCM6). Mewes and Jacobi [[3]] examined patterns of temperature flux in RCP8.5 warming projections in CMIP5. 10.3390/atmos11060664 3 Mewes D., Jacobi C. Horizontal Temperature Fluxes in the Arctic in CMIP5 Model Results Analyzed with Self-Organizing Maps. [Extracted from the article]

In semi-arid regions of the world, knowledge about the long-term hydroclimate variability is essential to analyze and evaluate the impact of current climate change on ecosystems. We present the first tree-ring δ18O based hydroclimatic reconstruction for northern semi-arid Iran spanning the period 1515–2015. A highly significant correlation between tree-ring δ18O variations of juniper trees and spring (April–June) precipitation reveals a major influence of spring water availability during the early growing season. The driest period of the past 501 years occurred in the 16th century while the 18th century was the wettest, during which the overall highest frequency of wet year events occurred. A gradual decline in spring precipitation is evident from the beginning of the 19th century, pointing to even drier climate conditions. The analysis of dry/wet events indicates that the frequency of years with relatively dry spring increased over the last three centuries, while the number of wet events decreased. Our findings are in accordance with historical Persian disaster records (e.g., the severe droughts of 1870–1872, 1917–1919; severe flooding of 1867, the 1930s, and 1950). Correlation analyses between the reconstruction and different atmospheric circulation indices revealed no significant influence of large-scale drivers on spring precipitation in northern Iran. [ABSTRACT FROM AUTHOR]

Stable water isotopologues in paleoclimate archives ( δ 18 O ) have been widely used as an indicator to derive past climate variations. The modern observed spatial δ 18 O -temperature relation in the middle and high latitudes has been used to infer the paleotemperatures changes from ice core data. However, various studies have shown that the spatial slope is larger than the temporal slope at the drill site by a factor of 2. Physically, the different spatial and temporal slope has been suggested to result from the amplified local surface air temperature cooling in the polar region at Last Glacial Maximum (LGM), according to the slope ratio equation derived in our previous study. To explicitly confirm the "polar amplification" effect in understanding the differences between temporal and spatial isotope–temperature relations, here we use the same isotope-enabled atmospheric general circulation model with a moisture-tracing module embedded to quantitatively estimate the contributions of different sources to the precipitated heavy oxygen isotopes in the middle and high latitudes. Our results show that the major sources of δ 18 O in precipitation over middle and high latitudes are from oceans where the sea surface temperature cooling at Last Glacial Maximum (LGM) is less than −2   ° C , while the local moisture sources with a higher cooling can be also relevant for polar regions, such as north Greenland. Additionally, the neglect of the strengthened local inversion layer strength at LGM could be the main cause for the overestimated source temperature cooling by the slope ratio equation, especially for the polar regions in the Northern Hemisphere. [ABSTRACT FROM AUTHOR]

The climate of the Arabian Peninsula is characterized by significant spatial and temporal variations, due to its complex topography and the large-scale atmospheric circulation. Furthermore, the role of dust in the formation of regional climate is considered to be crucial. In this work, the regional climatology for the Arabian Peninsula has been studied by employing a high resolution state of the art atmospheric model that included sophisticated physical parameterization schemes and online treatment of natural aerosol particles. The simulations covered a 30-year period (1986–2015) with a temporal resolution of 3 h and a spatial distance of 9 km. The main focus was given to the spatial and temporal variations of mean temperature and temperature extremes, wind speed and direction, and relative humidity. The results were evaluated using in situ measurements indicating a good agreement. An examination of possible climatic changes during the present climate was also performed through a comprehensive analysis of the trends of mean temperature and temperature extremes. The statistical significant trend values were overall positive and increased over the northwestern parts of the examined area. Similar spatial distributions were found for the daily minimum and maximum temperatures. Higher positive values emerged for the daily maxima. [ABSTRACT FROM AUTHOR]

This study investigates the changes in West African monsoon features during warm years using the Regional Climate Model version 4.5 (RegCM4.5). The analysis uses 30 years of datasets of rainfall, surface temperature and wind parameters (from 1980 to 2009). We performed a simulation at a spatial resolution of 50 km with the RegCM4.5 model driven by ERA-Interim reanalysis. The rainfall amount is weaker over the Sahel (western and central) and the Guinea region for the warmest years compared to the coldest ones. The analysis of heat fluxes show that the sensible (latent) heat flux is stronger (weaker) during the warmest (coldest) years. When considering the rainfall events, there is a decrease of the number of rainy days over the Guinea Coast (in the South of Cote d'Ivoire, of Ghana and of Benin) and the western and eastern Sahel during warm years. The maximum length of consecutive wet days decreases over the western and eastern Sahel, while the consecutive dry days increase mainly over the Sahel band during the warm years. The percentage of very warm days and warm nights increase mainly over the Sahel domain and the Guinea region. The model also simulates an increase of the warm spell duration index in the whole Sahel domain and over the Guinea Coast in warm years. The analysis of the wind dynamic exhibits during warm years a weakening of the monsoon flow in the lower levels, a strengthening in the magnitude of the African Easterly Jet (AEJ) in the mid-troposphere and a slight increase of the Tropical Easterly Jet (TEJ) in the upper levels of the atmosphere during warm years. [ABSTRACT FROM AUTHOR]

The main goal of the study is to determine the pressure conditions that cause waves of hot nights in Central Europe. The goal was implemented on the basis of data from 1966 to 2015, made available by the Institute of Meteorology and Water Management—National Research Institute, Deutscher Wetterdienst and the National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR). A hot night was defined as a day with the minimum air temperature above 18 °C. In the analysed years, an increase in the number of hot nights was found, which was predominantly statistically significant within the studied area. The study shows that the occurrence of waves of hot nights in Central Europe was associated on average with the ridge of high pressure, within which a local high-pressure area developed. During the waves of hot nights, there were positive anomalies of heights of isobaric surfaces over the study area with a maximum in the upper troposphere. [ABSTRACT FROM AUTHOR]