Search

The Southwest Vortex (SWV) is a mesoscale closed low-pressure system in Southwest China that significantly influences regional heavy rainfall. Understanding SWV characteristics and formation mechanisms is crucial for research in this field, with effective identification as the foundation. Currently, Chinese researchers lack a unified standard for SWV identification. Despite the availability of high spatiotemporal resolution reanalysis data, which provides a superior foundation for studying SWVs, existing methods struggle to fully exploit these datasets. Therefore, this study proposes PVSIA (Geopotential-Height-Field and Vector-Wind-Field based SWV Identification Algorithm), a method that integrates the geopotential height field and wind field characteristics. The PVSIA employs an innovative calculation method for wind vector rotation properties, enhancing identification speed and overcoming challenges posed by complex terrain. Furthermore, this study utilizes the ERA5 reanalysis dataset from 2019 to 2021 to systematically compare the performance of the PVSIA method with traditional geopotential height field methods in identifying SWVs, employing key evaluation metrics such as hit ratio, false alarm rate, and missing report rate. During this period, the PVSIA method achieved a hit rate of 94.25% in identifying SWVs, with a false alarm rate of 5.20% and a missing report rate of 5.74%. In contrast, the geopotential height field method, while achieving a higher hit rate of 98.85%, exhibited a significantly higher false alarm rate of 70.54%. In contrast, the PVSIA method maintains a high hit rate while significantly reducing the false alarm rate, demonstrating a more stable and reliable identification performance. [ABSTRACT FROM AUTHOR]

This study investigates the large-scale circulation anomalies induced by straight-moving tropical cyclones (TCs) over the western North Pacific (WNP) during winter. Corresponding to the straight-moving TCs, quasi-stationary wave trains appear as alternative geopotential height anomalies in the upper troposphere stretching from East Asia to the North Pacific. Specifically, the anomalous anticyclones are initially formed to the south of Japan and then lead to the subsequent anomalies over the Sea of Okhotsk and the Gulf of Alaska, respectively. Further analysis reveals that the upper-level anticyclonic anomalies are excited by negative Rossby wave sources, which are mainly attributed to the poleward vorticity advection by anomalous divergence relevant to TCs. In addition, the diagnosis indicates that the generation of wave source is caused by the product of the TC-induced divergent flows and the prominent meridional vorticity gradient in association with East Asian upper-tropospheric westerly jet. These findings imply that the tropical disturbances over the WNP, such as straight-moving TCs, can remotely affect weather over the extratropics, and thus have implications for improving the weather forecast over the extratropics through improving tropical disturbance forecast. [ABSTRACT FROM AUTHOR]

Extreme rainfall in southwestern Iran annually leads to considerable financial and human losses. Reports over several years show different levels of damage caused by these events. This study aims to identify and investigate changes in the spatial patterns of systems that generate extreme rainfall in southwestern Iran. We utilized the maximum 24-hr precipitation from seven synoptic stations in southwestern Iran from 1971 to 2020. The principal component analysis technique was utilized on geopotential height data at 500 hpa to identify spatial patterns of precipitation. The data was collected from 5° to 60°N and 10° to 70°E that were received from the NCEP/NCAR reanalysis dataset. The study period was divided into two based on the identified change point by the Pettitt test, and according to the results, the first climatic period in southwestern Iran had nine sources or components of extreme rainfall. In comparison, in the second period, they had eight sources of extreme rainfall. The most important source of extreme rainfall for both periods and the first source of the second period are located at 15° north latitude over the Indian Ocean, the Arabian Sea, the southern Red Sea, the Gulf of Aden, and certain areas in the eastern tropical regions of Africa. Research highlights: As a result, the statistical period was divided into two periods of 21 and 29 years. The first period, which includes 82 extreme rainfall events, spans from 1971 to 1991, and the second period, which provides 96 extreme rainfall events, spans from 1992 to 2020. The most important source of extreme rainfall for both periods and the first source of the second period are located at 15º north latitude over the Indian Ocean, the Arabian Sea, the southern Red Sea, the Gulf of Aden, and certain areas in the eastern tropical regions of Africa. This component accounts for 18.8 and 20.8% of the total variance in the data for the first and second climatic periods, respectively. It has the most significant impact on changes in the topography of the 500 hpa geopotential height. As a result, changes in the topography of the 500 hpa geopotential height in these regions have played a crucial role in forming systems that create extreme rainfall in southwestern Iran. This source facilitates the advection of moisture in the upper and middle levels of the atmosphere towards the studied area due to the circulation of high pressure at the 500 hpa level. In addition, the formation of low-pressure thermal centers and the displacement of their tongues towards the studied area are crucial in extreme rainfall in the study area. The location of sources that create extreme rainfall systems did not see significant changes between the two climatic periods, but their importance in forming these systems varied. Additionally, there were no significant spatial changes in the sources of extreme rainfall formation in southwestern Iran during the two climatic periods, but there were changes in their intensity. For instance, the seventh source exhibited a weaker influence during the first climatic period (PC7) but strengthened during the second climatic period (PC5). In this source, the presence of a ridge over Russia strengthens the Siberian high-pressure system, causing it to expand over Iran. As these high-pressure ridges expand and pass over the Oman Sea and the Persian Gulf, they transport moisture toward the study area. Conversely, the eighth source demonstrated a stronger impact during the first climatic period (PC8), whereas it exhibited a weaker effect during the second climatic period (PC4). The changes in the 500 hpa level in this geographic region are mainly associated with forming different blocking systems, accompanied by the breaking of Rossby waves. The southern branch of these systems affects the study area. [ABSTRACT FROM AUTHOR]

Climate change is one of the most urgent and challenging issues in Canada. In this study, high-resolution climate projections over Canada have been developed through the WRF model. The spatial and temporal variations of Canada's temperature and precipitation in the 21st century have been comprehensively analyzed. It is found that the annual mean temperature over Canada is projected to increase by [1.53, 1.98], [2.51, 3.86], and [2.94, 6.19] °C in the 2030s, the 2050s, and the 2080s under RCP4.5 and RCP8.5 respectively, with the largest increase in winter. The annual total precipitation is projected to increase by [16.33, 68.96], [64.80, 121.62], and [123.62, 184.33] mm in three future periods under RCP4.5 and RCP8.5 respectively. It is also found that sea surface temperatures (SST) anomalies and 500 hPa geopotential height (GPH) have strong impacts on Canada's annual and seasonal precipitations. The results can provide valuable information for mitigation and adaptation of climatic changes in a Canadian context. [ABSTRACT FROM AUTHOR]

This study attempts to predict the maximum temperature, along with the number of heat wave days (HWDs) at lead times of 7 and 15 days over Rajasthan, i.e., a semiarid region, using four machine learning (ML) algorithms, namely, artificial neural networks (ANN), support vector regression (SVR), random forest (RF), and eXtreme gradient boosting (XGBoost). It uses five key atmospheric variables, i.e., air temperature, geopotential height, relative humidity, U-wind, and V-wind, to predict the daily maximum temperatures for the months of April, May, and June, for the period from 1991 to 2020. The ML models are developed by using spatially averaged atmospheric variables and daily maximum temperature. The study demonstrates decent accuracy in forecasting the total annual count of HWDs and daily maximum temperature for Rajasthan for the 7-day lead time. However, as the lead time extends to 15 days, the model performances experience a decline. While comparing the performances of the four models, the SVR outperforms ANN, RF, and XGBoost in prediction. The findings of the current study indicate the potential utility of spatiotemporal dynamics in meteorological variables for long-term heat wave prediction. Moreover, the successful performances of the considered models, i.e., ANN, SVR, RF, and XGBoost, exhibit substantial future potential for reliable application in this context. [ABSTRACT FROM AUTHOR]

Different from the general heatwaves that mainly occur in southeastern China, the super heatwaves in China are concentrated mostly in the mid–lower reaches of the Yangtze River basin. Daily maximum temperature data over land from CPC/NOAA revealed that the basin has become the centre with the highest frequency increase of super heatwaves in the East Asian monsoon regions in the 21st century. Further analyses also indicted that the extent of super heatwaves in the basin has a much higher increasing rate than that of general heatwaves by using running thresholds. The westward extension of the western Pacific subtropical high (WPSH) plays the most dominant role. Statistical results suggested that a 10‐gpm increment of the geopotential height (GPH) over the basin leads to a 0.43°C increase in the regionally averaged maximum temperature. Additionally, spatial extent of the super heatwave may expand by approximately 4%. In contrast to the stable eastern boundary of the North Africa high, the western boundary of the WPSH has significantly expanded westward in the 21st century. This expansion has led to the enhancement of the GPH over the Yangtze River basin, resulting in the super heatwave centre due to the heat‐dome effect. Projections from 29 Coupled Model Intercomparison Project Phase 6 (CMIP6) models under moderate greenhouse gas emission scenario (SSP2‐4.5) suggest that the GPH over the Yangtze River basin will continue to strengthen throughout the 21st century. This implies that the Yangtze River basin will continue to be the centre of super heatwaves in East Asia monsoon region. [ABSTRACT FROM AUTHOR]

In Inner Eurasia, the monthly mean 850‐hPa geopotential height anomaly around the Ural Mountains exhibits a large and irregular interannual variability. Its intrinsic mechanism remains unclear. Based on a nonlinear theoretical model and reanalysis data sets during 1979–2023, we find similar qualitative and quantitative relationships, termed pitchfork‐like (PL) relationships, between the 850‐hPa Ural geopotential height anomaly in June, July, and January, and local soil moisture, snow depth water equivalent, and snow cover in the preceding months, respectively. These months are near the crest and trough of the annual cycle when surface forcing factors vary slowly. These nonlinear PL relationships reveal that the interannual Ural atmospheric variability in June and July (January) is intrinsically a transition between three (two) quasi‐equilibrium states. Moreover, when each forcing factor exceeds its critical threshold, the positive and negative anomaly quasi‐equilibrium states may occur with equal probability, implying a low predictability. Plain Language Summary: Over the Ural region, Inner Eurasia, the monthly mean 850‐hPa geopotential height anomaly, which indicates the state of the atmosphere, shows large and irregular fluctuations from year to year. It often induces high‐impact weather and climate events such as cold waves in Eurasia. We uncover the intrinsic mechanism of such atmospheric fluctuation based on a nonlinear theoretical model. Just like one may see six quasi‐equilibrium states of a dice when the dice rolls slowly, we find that when local surface forcing factors including soil moisture, snow depth water equivalent and snow cover vary slowly in certain months, approaching their annual maximum or minimum, the subsequent 850‐hPa Ural geopotential height anomaly in June and July (January) emerge three (two) quasi‐equilibrium states, which are revealed by three (two) point clusters that form pitchfork‐like patterns in the scatter diagrams. Particularly, the two quasi‐equilibrium states representing positive and negative geopotential height anomalies may occur with equal probability, implying a low predictability. Key Points: The 850‐hPa Ural geopotential height shows nonlinear lead‐lag relationships with soil moisture, snow depth water equivalent and snow coverSuch relationships appear in the months near the crest and trough of the annual cycle when surface forcing factors vary slowly over timeSuch relationships reveal three quasi‐equilibrium states of the atmosphere, two anomaly states of them occurring with equal probability [ABSTRACT FROM AUTHOR]

Understanding sub‐daily precipitation extremes (SPEs) can provide scientific insights for taking effective measures to mitigate climate risks. Leveraging gauge observations at hourly precipitation in 2,312 meteorological stations and extreme sub‐daily precipitation indices (ESPIs), we investigate the changes of SPEs in flooding season of 1971–2022 in China. On country scale, the occurrences and intensity of SPEs have significantly increased and even accelerated since the 21st century, suggesting increases in 2010s by 15%–38% compared with that in 1970s. The SPE risks for 20‐year and 50‐year return‐period increased by 2–4 and 8–20 times in 2001–2022 compared with that in 1971–2000, respectively. Over 80% stations are found to have positive trends in all ESPIs. On regional scale, seven sub‐regions experienced significant increases in ESPIs with larger magnitudes in the East China. The enlarged 500‐hPa geopotential height, 700‐hPa pseudoequivalent potential temperature, 700‐hPa specific humidity, saturated vapor pressure and urbanization ratio may be bonded to more SPEs. Plain Language Summary: Short‐term precipitation extremes have serious impacts on human safety, agriculture, energy, and infrastructure. It is very urgent to understand their variations and underlying mechanisms for policy‐makers to take effective approaches to mitigate extreme precipitation‐related risks. Here, we investigate the spatio‐temporal changes of sub‐daily precipitation extremes in the flooding season of 1971–2022 in China. We examine six metrics of extreme sub‐daily precipitation based on hourly observation data. We find that the occurrences and intensity of SPEs have significantly increased during the recent 52 years with acceleration since the 21st century on both country and regional level. The once‐in‐20‐year and once‐in‐50‐year events in 2001–2022 increased by 2–4 and 8–20 times compared with that in 1971–2000, respectively. The larger upward magnitudes of ESPIs are mainly located in the East China. In the era of rapid global warming, thermodynamic effects of abnormal atmospheric circulation and rapid urbanization possibly facilitate the occurrences of more SPEs in China. Key Points: Sub‐daily precipitation extremes have substantially increased over China during 1971–2022, even accelerating since the 21st centuryMost of China exhibits consistent upward trends in all extreme sub‐daily precipitation indices with lager magnitudes in the East ChinaThermodynamic effects of abnormal atmospheric circulation and urbanization are possibly boned to increased sub‐daily precipitation extremes [ABSTRACT FROM AUTHOR]

The Tibetan Plateau (TP) has experienced "south drying‐north wetting" extreme precipitation changes over the past half century. The effects of water vapor transport at different timescales on TP extreme precipitation changes remain unexplored. Here, we utilize the reanalysis data sets to quantify the contributions of stationary and transient processes of water vapor transport to the long‐term changes in the extreme precipitation (R95p) during wet season (Jun‐Jul‐Aug‐Sep) over the TP and surrounding regions. We find that the daily scale transient processes dominate the dipole trend of extreme precipitation with a contribution of 55.1% in the northern and 79.5% in the southern TP, respectively, whereas the contribution of monthly scale stationary processes is of 19.0% and 20.5%. The long‐term changes in extreme precipitation are dominated by the transient dynamic component. We identified the synoptic circulation patterns affecting the changes of R95p over the northern and southern TP by using k‐means clustering. The patterns featured with a 500 hPa trough, 200 hPa wind divergence and low transient geopotential height are identified. The frequency of the dominant circulation patterns increases in the northern TP and decreases in the southern TP, which leads to the dipolar changes of extreme precipitation over the TP and surrounding regions. Plain Language Summary: The Tibetan Plateau (TP), known as the "Asian water tower," is facing a crisis of water resources imbalance. Extreme precipitation has attracted much attention because of its large amount and disastrous nature. A "south drying‐north wetting" pattern of extreme precipitation changes over the TP and surrounding regions is seen in recent decades. Understanding the contribution of water vapor transport at different timescales to this dipolar pattern is of vital importance for the water resources changes therein. Based on the state‐of‐the‐art reanalysis data sets, we show that the "south drying‐north wetting" of extreme precipitation over the TP and surrounding regions is dominated by the long‐term changes of transient processes, whereas stationary processes have the same effect but are smaller in magnitude. We also stress the dominant role of changes in the frequency of synoptic circulation patterns on the "south drying‐north wetting" pattern of extreme precipitation changes over the TP and surrounding regions. Our results highlight the strong link between changes in the frequency of synoptic‐scale processes and long‐term changes in extreme precipitation over the TP and surrounding regions. Key Points: The transient processes dominate the extreme precipitation changes over the TP with contributions of 55.1% (north) and 79.5% (south)The frequency changes of the synoptic circulation patterns are responsible for the extreme precipitation changes over the TP [ABSTRACT FROM AUTHOR]

The spring‐high temperature events (SHTE) over northeast China (NEC) and the related local atmospheric factors including cloud cover, radiation flux, and soil moisture exhibited an increased interannual variability after 1992/93. Correlation analyses reveal that both the North Atlantic quadrupole sea surface temperature anomalies (NAQSSTA) and the spring Siberian snow depth (SSSD) had a stronger linkage with SHTE since the early 1990s. Additionally, the interannual variability of SSSD also showed an interdecadal increase, which is a key factor in the changes of interannual variability of SHTE. Further analyses showed that the NAQSSTA could excite Rossby wave via increasing low‐level atmospheric baroclinicity during 1993–2017. One branch of the wave trains propagated eastward and the other branch propagated northeastward through the Siberian high latitudes, which eventually reached NEC together and resulted in local anomalous anticyclonic circulation and sinking motion. The latter branch could contribute to the low geopotential height and cyclonic anomalies over the Siberian high latitudes, which further favored the increase of snow depth there. Subsequently, the positive snow anomalies facilitate the more occurrences of SHTE by exacerbating the meridional thickness gradient between the polar region and mid‐latitudes and then limiting the Arctic cold air to invade into the south. Meanwhile, the positive polar‐Eurasian pattern (POL) associated with higher SSSD could guide the Rossby wave train originating from the North Atlantic to propagate northeastward, which is consistent with the SHTE‐related wave train path after 1992/93. Model results further reproduce the physical processes that linking the NAQSSTA with SHTE. Plain Language Summary: The existing studies regarding the high temperatures in northeast China (NEC) in summer are relatively sufficient, but less attention has been paid to the spring high‐temperature events (SHTE). In this study, we find that the interannual variability of SHTE over NEC exhibited an evident amplification after the early 1990s. The linkage between the North Atlantic quadrupole sea surface temperature anomalies (NAQSSTA), spring Siberian snow depth (SSSD), and SHTE became much stronger after 1992/93 and the interannual variability of SSSD also showed an interdecadal increase. During 1993–2017, the NAQSSTA exerted influences on the SHTE by exciting Rossby wave trains. One branch of the wave trains propagated eastward and the other branch propagated northeastward through the Siberian high latitudes, which ultimately reached NEC together and resulted in local high pressure and sinking motion anomalies. The latter wave train had promoting effects on the increase of snow depth over the Siberian high latitudes. Positive snow anomalies then provided favorable conditions for SHTE by limiting the Arctic cold air to invade into the south. Our study compensates for the lack of researches on spring high temperatures over NEC and can provide a certain guidance for the agricultural management there. Key Points: The interannual variability of spring high‐temperature events (SHTE) over northeast China (NEC) increased since the early 1990sThe interdecadal changes of local meteorological factors are responsible for the increase in interannual variability of SHTEThe anomalies of North Atlantic SST and Siberian snow depth played combined effects on SHTE after 1992/93 [ABSTRACT FROM AUTHOR]

A weather regimes approach is applied to study large-scale circulation patterns over the North Eurasian sector (NE, defined as 0–180° E and 40–80° N domain) during the 1940–2022 period using ERA5 reanalysis data. We identified four NE weather regimes (WRs) both for boreal winter and summer seasons using a k-means standard cluster analysis method for daily geopotential height fields (z500). Whereas the winter NE WRs do not have any significant long-term changes in occurrence, three summer NE WRs are found to have statistically significant linear trends of occurrence over the studied period, the most notable of which is a regime associated with anticyclonic activity centered over the Ural Mountains called Summer Ural High, which has a statistically significant trend of +2.4 days of seasonal occurrence per decade. Loops of statistically significant likely transitions are found for the NE WRs in both seasons, showing that the NE WRs evolution follows preferred paths and is not purely random. NE WRs' seasonal occurrence is found to change depending on the phase of the El Nino oscillation and Northern Hemisphere sea-ice area anomalies in preceding seasons. El Nino events in autumn are associated with an increased occurrence of the Winter Scandinavian Blocking regime and a decreased occurrence of the Winter North Eastern High regime. Negative sea-ice area anomalies in autumn are associated with an increased occurrence of the Winter Ural High and a decreased occurrence of the Winter Scandinavian Low regime. NE WR's are associated with considerable, up to several times, changes in probability of temperature and precipitation extremes over the NE region. [ABSTRACT FROM AUTHOR]

The devastating effects of rain-induced attenuation on communication links operating above 10 GHz during rainy events can significantly degrade signal quality, leading to interruptions in service and reduced data throughput. Understanding the spatial and seasonal distribution of rain heights is crucial for predicting these attenuation effects and for network performance optimization. This study utilized ten years of atmospheric temperature and geopotential height data at seven pressure levels (1000, 850, 700, 500, 300, 200, and 100 hPa) obtained from the Copernicus Climate Data Store (CDS) to deduce rain heights across nine stations in South Africa. The kernel density estimation (KDE) method was applied to estimate the temporal variation of rain height. A comparison of the measured and estimated rain heights shows a correlation coefficient of 0.997 with a maximum percentage difference of 5.3%. The results show that rain height ranges from a minimum of 3.5 km during winter in Cape Town to a maximum of about 5.27 km during the summer in Polokwane. The spatial variation shows a location-dependent seasonal trend, with peak rain heights prevailing at the low-latitude stations. The seasonal variability indicates that higher rain heights dominate in the regions (Polokwane, Pretoria, Nelspruit, Mahikeng) where there is frequent occurrence of rainfall during the winter season and vice versa. Contour maps of rain heights over the four seasons (autumn, spring, winter, and summer) were also developed for South Africa. The estimated seasonal rain heights show that rain-induced attenuations were grossly underestimated by the International Telecommunication Union (ITU) recommended rain heights at most of the stations during autumn, spring, and summer but fairly overestimated during winter. Durban had a peak attenuation of 15.9 dB during the summer, while Upington recorded the smallest attenuation of about 7.7 dB during winter at a 0.01% time exceedance. Future system planning and adjustments of existing infrastructure in the study stations could be improved by integrating these localized, seasonal radio propagation data in link budget design. [ABSTRACT FROM AUTHOR]

This article examines whether the atmospheric fields selected after correlation tests with observed temperature data can reproduce, through classification, the types of atmospheric circulations observed during the summer on the Côte d'Azur, especially those associated with heat waves. The robustness of this classification of the reanalysis fields needs to be verified by comparing it with a classification based on observed temperatures, specifically targeting synoptic classes that favor very hot days. These data were also used for historical analyzes in order to observe their evolution in the study area. Finally, using observations (temperature, specific and relative humidity) measured by the Nice Airport station, the aim was to identify trends for 73 years (1949–2021). The reanalysis variables (temperature and specific humidity at 850 hPa, geopotential height at 500 hPa and meridional component V at 850 hPa) retained via correlations realized with the observed temperatures proved effective in their representations of the atmospheric conditions in place during the summertime. It seems that over the temporal range considered, the number of days within the synoptic class relating to heat waves is increasing, and those in the synoptic class relating to temperatures evaluated as low for the season are decreasing. After producing climatic means (1950–2021) from which ten-year means were then deduced, it can be observed that the anomaly compared to the climatic mean for the temperature at 850 hPa, and the geopotential at 500 hPa is intensified over the last two decades (2000–2009 and 2010–2019). The specific humidity at 850 hPa remains stable, the percentage of relative humidity experiences a drop compared to the climatic average and this is observed from the 1980–1989 decade, continuing its slight decrease over the following ten-year means. Concerning the evolution for the Nice-Airport station, the temperature has increased significantly and it seems that this increase is accentuated at night, the specific humidity is stagnant, while the relative humidity is significantly decreasing. [ABSTRACT FROM AUTHOR]

Large-scale atmospheric circulations are a significant driver of rainfall extremes. However, little attention has so far been devoted to understanding how large-scale circulation patterns influence sub-daily rainfall extremes. Using a gauge-based sub-daily rainfall dataset, we investigate the relationship between large-scale circulations and 3-hour extremes (defined here as ≥ 40 mm rainfall in 3 h) for western Europe. A set of 30 weather patterns (WPs) developed by the UK Met Office and reanalysis data of geopotential height at 500 hPa (z500) are used to represent large-scale atmospheric conditions. Strong associations with 3-hour extremes are found for a small number of WPs: over 50% of 3-hour rainfall extremes across Western Europe occur with just 5 WPs. Composites of z500 reveal the WPs resulting in southerly or south-westerly flow along the leading edge of a trough, accompanied by a ridge to the east or northeast, are most favourable for sub-daily rainfall extremes, with a statistically significant difference between the atmospheric conditions on WP days with a 3-hour extreme rainfall event compared to WP non-event days. Given that large-scale circulations are predictable much further in advance than individual extreme rainfall events, these identified relationships could therefore have important implications for forecasting, aiding in the early identification of periods with increased risk of short-duration rainfall extremes. [ABSTRACT FROM AUTHOR]

The solar cycle is believed to have an important effect on the stratosphere with some degree of modulation by the quasi-biennial oscillation (QBO). To examine the likely effects of solar cycle and their modulation by QBO, in this study, the JRA-55 reanalysis data with a spatial resolution of 1.25 degrees in both the zonal and meridional directions were used for the period of 1958 to 2017 (60 years in total). Also, the solar flux at a wavelength of 10.7 cm, called F10.7, was extracted from the LISIRD website data for the study period. The monthly average temperature, geopotential height and potential vorticity (PV) at 30 hPa level as well as the solar flux were determined for the 60-year duration of the study. The months of the study period were classified into easterly and westerly phases of QBO called EQBO and WQBO, respectively. A similar classificaltion was used for the set of years in which solar activity was greater (lower) than normal, called HS (LS) for high (low) solar. Using the FUB website data, the correlations of temperature, geopotential height and PV at 30 hPa with F10.7 in the Northern Hemisphere summer (July), early winter (November and December) and late winter (January, February and March) were calculated for the whole set of data and the two subsets obtained for the two phases of QBO. Besides, the changes in temperature and geopotential height between HS and LS phases were investigated. The main results of this research in terms of solar and QBO phases are as follows. For the temperature and geopotential height at 30 hPa level, the correlations with solar cycle in July are stronger in the subtropics, and larger and spatially wider for the set of EQBO than both the set of all years and the set of WQBO. The correlation of PV with F10.7 is generally small in July, exhibiting no clear pattern during QBO phases. In early and late winter, for the temperature, geopotential height and PV of 30 hPa, the correlations with F10.7 are small for the set of all years and the set of WQBO, but attain a relatively large value for the set of EQBO with maximum values found in the tropical regions of both hemispheres. The corresponding correlation pattern for PV is distributed over many regions of both hemispheres with stronger values for the set of EQBO than the set of WQBO. Also, while the temperature and geopotential height difference between HS and LS for the set of WQBO reach their peak in the Southern Hemisphere, there is a spatially wider correlation pattern for the set of EQBO in the tropical regions of both hemispheres. Results also point to a possible impact of solar cycle on the Brewer–Dobson meridional circulation, a topic which needs a separate study. [ABSTRACT FROM AUTHOR]

This study focuses on the primary synoptic‐scale patterns and precursors of extreme and non‐extreme precipitation over the Tibetan Plateau (TP). Atmospheric circulation anomalies and their precursors associated with regional extreme precipitation events (REPE) demonstrate distinct precursor wave train and heightened intensity than regional non‐extreme precipitation events (non‐REPE). Specifically, REPE over the northwestern TP (NWTP) exhibits a geopotential height anomaly induced by a latitudinal propagating Rossby wave train along 40°N. In contrast, over the southeastern TP (SETP), REPE is characterized by a geopotential height anomaly caused by a northwest‐southeastward propagating Rossby wave train. The ascending motion anomalies of REPE over both NWTP and SETP are primarily attributed to the geostrophic zonal temperature advection, which is significantly stronger during REPE compared to non‐REPE. This finding provides valuable insights for forecasting summer extreme precipitation over TP. Plain Language Summary: Tibetan Plateau (TP) is experiencing warming and increased humidity in the context of climate change, leading to a rise in extreme precipitation events. This study focuses on analyzing the circulation patterns and precursors associated with summer extreme precipitation over TP, aiming to identify differences between extreme and non‐extreme precipitation events, as well as explaining the unique characteristics of summer extreme precipitation over TP to answer the question of what distinguishes summer extreme precipitation from non‐extreme precipitation over TP. The findings suggest that summer regional extreme precipitation events (REPE) over the northwest and southeast TP (NWTP and SETP, respectively) display distinct characteristics in terms of atmospheric circulation anomalies and their precursors, which are significantly stronger compared to regional non‐extreme precipitation events (non‐REPE) of varying intensities observed in these two regions. The ascending motion (ω) anomalies responsible for TP's REPE are primarily influenced by the Laplacian of geostrophic zonal temperature advection (term B‐x). Key Points: The extreme precipitation characteristics of the NWTP and SETP, as well as the atmospheric circulation patterns influencing them, exhibit significant disparitiesCirculation anomalies associated with extreme precipitation exhibit a stronger and more distinct precursor wave train, compared to non‐extreme precipitationThe dominant factor driving ascending motion for extreme precipitation over TP is the geostrophic zonal temperature advection [ABSTRACT FROM AUTHOR]

Forecasts of sea surface temperature anomalies (SSTAs) provide essential information to stakeholders of marine resources in coastal ecosystems, such as the California Current Large Marine Ecosystem (CCLME), at management-relevant monthly-to-annual time scales. Diagnosing dynamical sources of predictability and the mechanisms differentiating skill among forecasts is required for verification and improvement in operational forecasting systems. Using retrospective forecasts (1982–2020) from a four-member subset of the North American Multi-Model Ensemble (NMME), we evaluate the conditional skill of SSTA forecasts in the CCLME at monthly resolution for lead times up to 10.5 months. Forecasts from ensemble members with relatively small SSTA errors at shorter lead times retain higher skill at longer lead times, with the most substantial and long-lasting increases for forecasts initialized in the fall and early spring. The "best" low-error SSTA forecasts are characterized by increased skill in the prediction of North Pacific atmospheric circulation [sea level pressure (SLP) and 200-hPa geopotential height] the month prior to the evaluation of SSTA errors in the CCLME and exhibit more realistic progressions of anomalous SLP. The Pacific meridional mode (PMM) emerges as a diagnostic of skillful North Pacific atmosphere–ocean coupling, as forecasts that correctly simulate the PMM and its associated SLP variability increase the SSTA prediction skill in the CCLME in the fall through spring. Predictable coupled ocean–atmosphere modes provide a target for enhancing predictability with early detection of the onset of a deterministic progression emerging from stochastic atmospheric variability. Significance Statement: Global forecast systems provide near-term climate predictions that inform the management of marine resources, such as those of the California Current Large Marine Ecosystem. In this study, we probe the processes which lead forecasts to succeed or fail at predicting sea surface temperatures in the California Current at seasonal time scales among retrospective forecasts from the North American Multimodel Ensemble. We demonstrate that forecasts which best simulate sea surface temperatures at the earliest lead times sustain advantages in forecast skill and find that correctly simulating extratropical atmospheric circulation increases the predictive skill of sea surface temperatures in the northeast Pacific in the following lead times. Our results offer North Pacific atmospheric circulation as a target for forecast model improvement that would additionally enhance ocean forecasts. [ABSTRACT FROM AUTHOR]

This study investigated variations in the Madden–Julian oscillation (MJO) behavior between two types of La Niña winters: mega and equatorial La Niñas. Results of this work show that in contrast to mega conditions, accompanied by more abundant intraseasonal column-integrated moisture anomalies over the planetary boundary layer, intensities of intraseasonal outgoing longwave radiation anomalies are stronger over the tropical western Pacific (WP) at MJO phases 5–6 under equatorial conditions. The occurrence of the moisture anomaly change averaged over the tropical WP is supported by a distinct moisture tendency difference. Moisture budget and multiscale interaction diagnoses underscore the pivotal effect of an area-averaged vertical gradient change in low-frequency background moisture in driving such tendency change. Considering notable MJO convection variations, the teleconnections associated with MJO phases 5–6 over East Asia (EA) were also explored, including warmer surface air temperature anomalies and stronger positive geopotential height anomalies at 200 hPa on the intraseasonal time scale under equatorial conditions. Results from a linear baroclinic model demonstrate that such teleconnection changes could be effectively explained by the linear response to the MJO's diabatic heating anomaly. Roles of mean state and MJO itself variations in the linear response change are also discussed. These findings provide novel insights into MJO activity and offer potential improvements for subseasonal forecasting in EA. Significance Statement: The relationship between El Niño–Southern Oscillation (ENSO) and MJO has been extensively explored recently. However, variations in the MJO behavior and its climate effects under mega and equatorial La Niña winters have not been thoroughly investigated. In this work, we utilized reanalysis datasets and an idealized linear baroclinic model to address these questions. We observed more robust and abundant intraseasonal convection activity and planetary boundary layer–integrated moisture anomaly averaged over the tropical WP at MJO phases 5–6 under equatorial conditions than mega conditions, which is closely linked with the vertical gradient change in low-frequency background moisture. Additionally, MJO-related teleconnections also exhibit some changes. This work may provide a new perspective on understanding the relationship between the MJO and ENSO and offer potential improvements for the subseasonal forecast. [ABSTRACT FROM AUTHOR]

An unprecedented heavy rainfall event in China ("21.7" extreme rainfall event) was simulated using the global variable‐resolution model (MPAS‐Atmosphere) across the scales (4, 8, 16 and 50 km). Although almost all experiments at different resolutions reproduce the spatiotemporal characteristics of precipitation, the simulated precipitation intensity from high to low is 16, 8, 50, and 4 km, with the 16 km simulation being closest to the observations. Precipitation magnitude is prominently influenced by the difference in simulated large‐scale circulation across a range of grid spacings. Further analysis revealed that the differences in latent heating across scales affect the geopotential height and wind field by altering temperature. The latent heating in 4 km simulation is the minimum while the 16 km simulation is maximum. More latent heating release leads to the low‐level pressure depression, amplifies the water vapor flux convergence, produces stronger upward motion and more clouds, and ultimately results in stronger precipitation. The sensitivity experiments for turning off latent heating tendencies during the event showed that the latent heat release has positive feedback on the "21.7" heavy rainfall event. This study highlights the importance of scale‐awareness of latent heat at different resolutions and suggests that the difference in simulated latent heat release during the event is the main reason for simulated different atmospheric circulation and precipitation across scales. Plain Language Summary: Due to the advancements in the numerical computing power, numerical weather prediction models (NWP) are operating at horizontal grid spacing ranging from 1 to 10 km. This study evaluates the simulation performance of the global variable‐resolution model MPAS‐A for a heavy precipitation event and investigates the influence of scale‐aware convective parameterizations on modeling rainfall at horizontal resolutions across the scales (4, 8, 16 and 50 km). The results show that 4 km simulation exhibits the weakest precipitation while the simulated precipitation at 16 km is the strongest. The difference in precipitation simulated by four experiments across scales derives from the variations in large‐scale circulation simulated with different refined resolutions. Further analysis revealed that the differences in latent heat at different resolutions ultimately lead to the changes in geopotential height and large‐scale wind field by altering temperature. 4 km experiment simulates less latent heat than other lower‐resolution experiments. Owing to the positive feedback of the latent heat release on precipitation, 4 km experiment simulates the minimum precipitation. This work highlights that latent heat plays a vital role in simulation of synoptic condition and precipitation. Key Points: Precipitation is influenced by the difference in simulated atmospheric circulation across a range of grid spacingsThe differences in simulated latent heat release across scales affect the geopotential height and wind field by altering temperatureThe latent heat release has positive feedback impact on the "21.7" heavy precipitation event [ABSTRACT FROM AUTHOR]

Seasonal‐range predictability of summer climate in northwestern Europe is generally considered to be low. This is an increasing issue given the worsening impact of summer heatwaves, droughts and intense convective rainfall in a rapidly changing climate. In wintertime, predictive skill in the region is derived from a variety of sources, not least teleconnections with the El Niño‐Southern Oscillation (ENSO). Summer ENSO teleconnections, however, are often considered to be negligible. In this paper, we revisit the topic of summer teleconnections between ENSO and the North Atlantic‐European region. We build on previous work identifying upper tropospheric responses to tropical forcing, since dynamical teleconnections are most apparent at this level. Our results confirm that significantly increased geopotential heights are found stretching over the North‐Atlantic region and into western Europe when La Niña conditions are prevalent during summer. This pattern is part of the previously identified 'circumglobal' pattern of wider northern‐hemisphere height changes. We then look for these responses in a range of climate models used in operational seasonal prediction. While parts of the circumglobal pattern are weakly present, none of them produce the response seen over the North Atlantic, even when the effect of sampling on the observed teleconnection is accounted for. We additionally estimate the contribution of the previous (wintertime) phase of ENSO on the following summer. We find a significant delayed response, particularly in heights, to the earlier phase. The combination of the delayed and current responses gives height anomalies that are larger, on average, when ENSO changes phase from winter to summer. Finally, we show that a modest level of regional prediction skill from ENSO does exist. There is a contribution to skill in heights from the previous ENSO phase, but the equivalent contribution to the skill of zonal winds is smaller. [ABSTRACT FROM AUTHOR]

Heatwaves in southeastern Australia have characteristic weather patterns that are well understood but are outnumbered by days with similar synoptic‐scale patterns that are not heatwaves. Accordingly, the aim of this study is to identify the key differences between heatwave and non‐heatwave days from 40 years of reanalysis data. A synoptic climatology of seven weather states was constructed by k$$ k $$‐means cluster analysis. Four of these states account for more than 80% of heatwave days across south‐and‐central eastern Australia. Moreover, the spatial maxima in the frequency of the heatwave days are distinct and geographically separated. Heatwave days have a stronger upper anticyclone or ridge that has propagated further equatorward in comparison with non‐heatwave days. The air upstream of the ridge is more humid on heatwave days, whereas downstream of the ridge the air is much drier. These dry anomalies are co‐located with midtropospheric subsidence and the moist anomalies with ascent, and their respective spatial distributions are consistent with regions of adiabatic warming and latent heating identified in recent studies of southeast Australian heatwaves. The corresponding vertical motion on non‐heatwave days is weaker and shifted further poleward. Southeast Queensland heatwave days exhibit increased baroclinicity over the Australian Subtropics and reduced rainfall over Queensland. Further south and west, heatwave days are associated with more amplified Rossby waves and increased rainfall over the Australian Tropics. Anticyclonic Rossby wave breaking is greatly enhanced on heatwave days south of 30°S. For every day in each of these four weather states, the 3‐day‐mean maximum temperature in the region of peak heatwave day frequency is positively correlated with 500 hPa geopotential height anomalies on the equatorward flank of the cluster‐mean upper ridge. These findings underline the importance of equatorward Rossby wave propagation in the dynamics of southeast Australian heatwaves. [ABSTRACT FROM AUTHOR]

Flash droughts (FDs) are distinguished by a rapid development associated with strong precipitation deficits and/or increases in atmospheric evaporative demand in the short‐term, but little is known about the atmospheric conditions underlying these events. In this study, we analyse for the first time the atmospheric dynamics involved in the development of FDs in Spain over the period 1961–2018. FDs are related to large‐scale atmospheric circulation patterns affecting the region, in particular with the positive phase of the North Atlantic Oscillation (NAO). The NAO is the main atmospheric driver of FDs in winter and autumn, and it is essential in explaining FD development in spring. We also found that FDs are typically linked to strong positive anomalies in 500 hPa geopotential height and sea level pressure over the region during the weeks prior to the onset. At the synoptic scale, the most common weather types (WTs) recorded during the development of FDs are Anticyclonic Western (ANT_W_AD), East (E_AD) and Northeast (NE_AD) advection, and Anticyclonic (ANTICYC). In particular, ANTICYC WT is the main atmospheric driver of FDs in summer. Ridging conditions occur frequently during FDs in all seasons, being the most important factor controlling FD development in spring. Likewise, we noted that some of the FDs recorded in summer are related to and/or exacerbated by Saharan air intrusions associated with pronounced ridges. The results of this research have important implications for the understanding, monitoring and prediction of FDs in Spain, providing a detailed assessment of the main atmospheric dynamics involved in FD triggering at different spatial scales. [ABSTRACT FROM AUTHOR]

The Asian summer monsoon anticyclone (ASMA) plays a vital role in the transport and redistribution of the tracers in the upper troposphere and lower stratosphere during transient monsoon conditions. The ASMA can be identified using various diagnostics such as winds and divergence, potential vorticity, the Montgomery stream function or geopotential height. Its representation based on the commonly‐used existing method that takes a climatological range of the geopotential height (GPH) at a given pressure level is not robust and fails in several circumstances. In this study, we propose a new GPH method to define the ASMA for the active and break phases of the monsoon based on the GPH values at the tropical easterly jet (TEJ) and subtropical westerly jet (STJ) locations. The proposed method compares the GPH values at the locations corresponding to the wind speed maxima of TEJ and STJ at 100 hPa with the maximum GPH at 100 hPa and selects the one with the minimum difference to define the ASMA extent. The same procedures were applied to obtain the ASMA extent at 150 hPa. The ASMA extents are obtained using the method proposed here and the existing fixed GPH method for the 30 active and 27 break phases from 2005 to 2023. The fixed GPH method provides a larger ASMA extent for 24 active and 21 break phases, which appears to be an unrealistic estimation compared to the proposed method. Our proposed method identifies the ASMA extent that is larger and centred around the Iranian side during the active phases while it is smaller and centred around the Tibetan side during the break phases. Furthermore, the minimum concentration of ozone (O3), and maximum concentrations of carbon monoxide (CO) and water vapour (H2O) are also found to be centred near the Iranian side during the active phases and on the Tibetan side during the break phases. The tracer concentrations are generally higher for CO, H2O and O3 during the active phase than in the break phase. Tracer concentrations within the ASMA extent using the fixed GPH method are higher for O3 while lesser for CO and H2O when compared to the proposed method. [ABSTRACT FROM AUTHOR]

This study investigated the influence of interannual variations in tropical Indian Ocean tripole (IOT) on the surface air temperature (SAT) over the western Tibetan Plateau (TP) during boreal summer. During the positive phase of the IOT, two northward cross-equatorial airflows are induced over the tropical eastern and western Indian Ocean. These airflows reinforce the ascending motion over the southern tropical Asia (15°–25°N, 80°–125°E), increasing local precipitation, as confirmed by observations and simulations by the Community Atmosphere Model. The upper-level Asian Continental Meridional Teleconnection (ACMT) pattern is excited by the latent heat released from precipitation and transmits signals from the southern tropical Asia to the western TP, leading to the positive geopotential height anomalies and anomalous anticyclones over there. Upper-level circulation anomalies over the western TP enhance atmospheric thickness through adiabatic processes, consequently elevating local SAT. The ACMT associated with precipitation anomalies thus serves as an atmospheric bridge connecting the IOT and the SAT variations over the western TP. [ABSTRACT FROM AUTHOR]

Extreme cold temperatures were observed in July and August 2023, coinciding with the WINFLY (winter fly-in) period of mid to late August into September 2023, meaning aircraft operations into McMurdo Station and Phoenix Airfield were adversely impacted. Specifically, with temperatures below −50°C, safe flight operation was not possible because of the risk of failing hydraulics and fuel turning to gel onboard the aircraft. The cold temperatures were measured across a broad area of the Antarctic, from East Antarctica toward the Ross Ice Shelf, and stretching across West Antarctica to the Antarctic Peninsula. A review of automatic weather station measurements and staffed station observations revealed a series of sites recording new record low temperatures. Four separate cold phases were identified, each a few days in duration and occurring from mid-July to the end of August 2023. A brief analysis of 500-hPa geopotential height anomalies shows how the mid-tropospheric atmospheric environment evolves in relation to these extreme cold temperatures. The monthly 500-hPa geopotential height anomalies show strong negative anomalies in August. Examination of composite geopotential height anomalies during each of the four cold phases suggests various factors leading to cold temperatures, including both southerly off-content flow and calm atmospheric conditions. Understanding the atmospheric environment that leads to such extreme cold temperatures can improve prediction of such events and benefit Antarctic operations and the study of Antarctic meteorology and climatology. [ABSTRACT FROM AUTHOR]

The meteorology of cutoff lows over South Africa is characterized by statistical analysis of daily field data in the period 1970–2023. An index is formulated by subtracting 500 hPa geopotential height in the mid-latitudes from the subtropics. Cutoff lows (COL) are identified by positive values, mostly in autumn and spring. Statistics indicate that climate forcing is seasonal: La Nina/El Nino favors COL in March–May/September–November. Hemispheric regressions reveal anomalous highs across the southern mid-latitudes when COL are frequent over South Africa. A 14-case composite was formed from the most intense daily COL events in autumn and spring. The composite shows a NW- tilted Rossby wave and jet stream loop around the COL. Maritime easterlies induce a warm east—cool west SST pattern, but composite moist inflows are shallow, so stormy weather hugs the coastal plains. Overturning circulations meet in an upper-level "saddle" over South Africa. 500 hPa sinking motions to the Southwest are of similar strength to rising motions to the Northeast. A COL case study exhibited hourly rain rates >10 mm at Port Alfred 18–20 October 2012 fed by tropical inflow. New insights emerged from this study via composite height sections over South Africa. [ABSTRACT FROM AUTHOR]

Weather forecasts at subseasonal-to-seasonal (S2S) timescales have little forecast skill in the troposphere: individual ensemble members are mostly uncorrelated and span a range of atmospheric evolutions that are possible for the given set of external forcings. The uncertainty of such a probabilistic forecast is then determined by this range of possible evolutions – often quantified in terms of ensemble spread. Various dynamical processes can affect the ensemble spread within a given region, including extreme events simulated in individual members. For forecasts of geopotential height at 1000 hPa (Z1000) over Europe, such extremes are mainly comprised of synoptic storms propagating along the North Atlantic storm track. We use ECMWF reforecasts from the S2S database to investigate the connection between different storm characteristics and ensemble spread in more detail. We find that the presence of storms in individual ensemble members at S2S timescales contributes about 20 % to the total Z1000 forecast uncertainty over northern Europe. Furthermore, certain atmospheric conditions associated with substantial anomalies in the North Atlantic storm track show reduced Z1000 ensemble spread over northern Europe. For example, during periods with a weak stratospheric polar vortex, the genesis frequency of Euro-Atlantic storms is reduced and their tracks are shifted equatorwards. As a result, we find weaker storm magnitudes and lower storm counts, and hence anomalously low subseasonal ensemble spread, over northern Europe. [ABSTRACT FROM AUTHOR]

This paper shows the first systematic dataset of flood episodes referring to the entire Pyrenees massif, named PIRAGUA_flood, which covers the period 1981–2015 (available at http://hdl.handle.net/10261/270351 , last access: 21 July 2024). First, the structure of the database is detailed so that it can be reproduced anywhere else in the world, adapting to the specific nature of each situation. Subsequently, the paper addresses the spatial and temporal distribution of flood episodes and events (including trends) that affected the Pyrenees regions of Spain (Catalonia, Aragon, Navarre, the Basque Country), France (Nouvelle-Aquitaine, Occitanie) and Andorra, as well as the massif as a whole, for a given period of time. In the case of the Spanish regions, it was also possible to analyse the compensation payments by the Spanish Insurance Compensation Consortium and the number of deceased. The weather types associated with flood episodes were also classified based on sea level pressure and 500 hPa geopotential height from ERA5. The results show 181 flood events and 154 fatalities, some of which affected more than one region. In the Spanish part of the Pyrenees, between 1996 and 2015, there was a total compensation payout amounting to EUR 142.5 million (2015). The eastern part of the area records more flood events than the western one, with Catalonia being the community that registered the highest number of events, followed by Andorra and Occitanie. Associated weather types are dominated by the southern component flow over the Pyrenees region, with a talweg on the Iberian Peninsula and a depression in the vicinity, either in the Atlantic or in the Mediterranean. In terms of the entire massif, there is a slight positive trend of 0.84 eventsperdecade , driven by the evolution of ordinary and extraordinary floods but not significant at 95 %. At a regional level, flood behaviour is more heterogeneous, although not significant for the most part. Nouvelle-Aquitaine is the only region that shows a positive and significant trend of 0.34 eventsperdecade. [ABSTRACT FROM AUTHOR]

Copyright of Plateau Meteorology is the property of Plateau Meteorology Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The mid‐latitude extreme weather disasters are often associated with explosively developing cyclones (ECs). Based on different vertical development characteristics, 4,608 ECs identified over the North Pacific in the cold season of 44 years of NCEP‐CFSR reanalyzes are divided into four types of upward development and four types of downward development categories. ECs with vertical upward (downward) development follow a northeastward (nearly eastward) path, mainly explosively developing over the Northwest Pacific (Asia continent and Pacific). Furthermore, utilizing the piecewise potential vorticity inversion method reveals the synergetic forcing of the turbulent heat transport and baroclinicity in the lower troposphere, the latent heat release in the middle levels, the upper‐level jet stream, and the downward intrusion of stratospheric potential vorticity on the ECs. Different configurations of these influences from the troposphere to the stratosphere result in the occurrences of eight types of ECs in the cold season over the North Pacific. Plain Language Summary: Since the late twentieth century, there has been a significant variation in the frequency of explosively developing cyclones (ECs), which bring extreme weather disasters to the mid‐latitudes. This study classifies winter North Pacific ECs into four upward and four downward developing categories. The upward developing ECs include those enhanced only in the lower troposphere (UL‐EC), those developing upward to the middle troposphere (UM‐EC), those developing upward to the upper troposphere (UU‐EC), and those developing upward into the stratosphere (US‐EC). In contrast, the downward developing ECs can be classified as those intensifying only in the upper troposphere (DU‐EC), those developing downward to the middle troposphere (DM‐EC), those developing downward to the lower troposphere (DL‐EC), and those developing downward from the stratosphere (DS‐EC) to the lower troposphere. Furthermore, the piecewise potential vorticity inversion results show that the explosive development of UL‐EC and UM‐EC are dominated by the thermodynamical processes in the middle‐to‐lower troposphere, exhibiting a baroclinic structure with opposite anomalous geopotential height between the troposphere and stratosphere. However, the other types have a consistent stratospheric‐to‐tropospheric negative anomalies, which is determined by the upper‐layer dynamical processes. The downward intrusion of stratospheric potential vorticity dominates the explosive development for the US‐EC and DS‐EC. Key Points: The North Pacific explosively developing cyclones during cold season are divided into eight up‐ and down‐ward vertically developing typesThe explosively developing cyclones are proved to be an important link between the stratosphere and troposphere in the mid‐high latitudesUsing the piecewise potential vorticity inversion method, the quantitative forcings on the explosively developing cyclones are explored [ABSTRACT FROM AUTHOR]

This study investigates the dust aerosol optical depth (DAOD) variability over the Arabian Peninsula (AP) in the spring season, a region profoundly affected by dust activity due to its desert terrain. Employing the MERRA-2 DAOD reanalysis dataset for the period 1981–2022, a significant trend in DAOD is noted in the spring season compared to the other seasons. The leading Empirical Orthogonal Function (EOF) explains 67% of the total DAOD variance during the spring season, particularly over the central and northeastern parts of AP. The analysis reveals the strengthening of upper-level divergence over the western Pacific, favoring mid-tropospheric positive geopotential height anomalies over the AP, leading to warm and drier surface conditions and increased DAOD. A statistically significant negative relationship (correlation = −0.32, at 95% confidence level) is noted between DAOD over AP and the El Niño-Southern Oscillation (ENSO), suggesting that La Niña conditions may favor higher dust concentrations over the AP region and vice versa during El Niño phase. The high (low) DAOD over the region corresponds to mid-tropospheric positive (negative) geopotential height anomalies through strengthening (weakening) of the upper-level divergence (convergence) over the western Pacific during the La Niña (El Niño) phase. This study shows that ENSO could be a possible precursor to predicting dust variability on a seasonal time scale. [ABSTRACT FROM AUTHOR]

The primary aim of this study is to examine the characteristics of atmospheric circulation patterns at various geopotential levels and their relationship with precipitation in southern Romania during the period from 1961 to 2020. Daily geopotential heights (1000 hPa, 850 hPa, 700 hPa and 500 hPa) were utilized in an automatic updated atmospheric circulation scheme for the creation of daily calendars of 12 circulation types (5 anticyclonic and 7 cyclonic) as well as daily time series derived from five stations over the domain of interest. To assess the influence of the atmospheric circulation on precipitation, correlations and time trends were explored between the rainfall totals and the different circulation types. The findings reveal a rising trend in anticyclonic circulation types across the region, while cyclonic types exhibit a consisted decrease. Precipitation and number of rain days percentages associated with specific cyclonic types depend on the geopotential levels, while annual and seasonal precipitation linked to cyclonic types decreases progressively from higher to lower levels. The strongest correlations in circulation type frequencies are observed between adjacent circulation types. Taylor diagram analysis indicates that the relationships between circulation types and precipitation vary both seasonally and across different atmospheric levels. Notably, the two rainiest circulation types are more accurately simulated at higher atmospheric levels (700 hPa and 500 hPa). [ABSTRACT FROM AUTHOR]

The aim of this paper was to characterize the warm and dry compound events (WD days) in Poland during the period of 1966–2023, focusing on assessing the frequency and intensity of such events and their spatial and temporal variability, as well as on the driving factors of warm and dry compound events. WD days are those days that have a maximum temperature equal to or higher than the 90th percentile and the precipitation on that day and the 14 preceding days are equal to or less than the 25 percentile. During 1966–2023, the frequency of WD days increased significantly, mainly in April, the summer months, and December. Higher temperatures favored the occurrence of WD days from March to November, but, in winter months, the heat did not favor the occurrence of WD days. The exception was December, when high temperatures in the first part of the analyzed period did not favor the occurrence of a dry day, whereas, in the second part, it did. The strongest influence on the frequency of WD days had the East Atlantic pattern, where air flowed over Poland from the southwest. Warm and humid air flowing from the Atlantic Ocean must overcome the mountain barrier; therefore, it flows to Poland as warm and dry air. From spring to autumn, the WD days were related to an increase in the geopotential height in central Eastern Europe, and, in the winter, they were related with blocking over the Balkans. [ABSTRACT FROM AUTHOR]

Variability of the Surface Air Temperature (SAT) over the Western South Asia (WSA) region leads to frequent heatwaves during the early summer (May-June) season. The present study uses the European Centre for Medium-Range Weather Forecast's fifth-generation seasonal prediction system, SEAS5, from 1981 to 2022 based on April initial conditions (1-month lead) to assess the SAT predictability during early summer season. The goal is to evaluate the SEAS5's ability to predict the El Niño-Southern Oscillation (ENSO) related interannual variability and predictability of the SAT over WSA, which is mediated through upper-level (200-hPa) geopotential height anomalies. This teleconnection leads to anomalously warm surface conditions over the region during the negative ENSO phase, as observed in the reanalysis and SEAS5. We evaluate SEAS5 prediction skill against two observations and three reanalyses datasets. The SEAS5 SAT prediction skill is higher with high spatial resolution observations and reanalysis datasets compared to the ones with low-resolution. Overall, SEAS5 shows reasonable skill in predicting SAT and its variability over the WSA region. Moreover, the predictability of SAT during La Niña is comparable to El Niño years over the WSA region. [ABSTRACT FROM AUTHOR]

The onset of the rainy season in Vietnam displays distinct characteristics compared to other Asian summer monsoon (ASM) regions due to its elongated shape running from south to north, coupled with its complex terrain and the influences of multiple weather systems. Determining the onset is challenging, although numerous empirical indices exist. Here, we proposed an alternative approach to determine the onset by examining the large-scale weather patterns associated with the abrupt increase in rainfall in the country. In this new approach, the evolutions of the ASM system were interpreted as consecutive occurrences of multiple weather patterns, detected by a clustering algorithm Self Organizing Maps (SOMs). Specifically, SOMs were applied on outgoing longwave radiation, 850 hPa wind, and geopotential height over the ASM regions from April to July. The primary features of these atmospheric variables were generalized to 16 patterns. A high-resolution Vietnam Gridded Precipitation (VnGP) dataset was employed to investigate the linkage between these large-scale patterns and local-scale rainfall. The large-scale patterns associated with the onset were distinguished, which are characterized by the development of westerly winds and large-scale convections. Then, the onset of summer rainfall in Vietnam is identified based on the initial appearance of wet-condition weather patterns. This approach offers several advantages: it is objective and does not require additional thresholds of rain or wind to determine the monsoon onset. It also more effectively captures the characteristics of large-scale circulation and deep convection compared to relying on predefined monsoon indices alone. Additionally, this approach helped to elucidate two types of weather patterns associated with the onset: those associated with the northeastward development of deep convection and westerly wind from the Indian Ocean to the Indochina Peninsula and activities of the Boreal Summer Intraseasonal Oscillation. [ABSTRACT FROM AUTHOR]

This study evaluates the prediction of MJO teleconnections in two versions of the NOAA Unified Forecast System (UFS): prototype 5 (UFS5) and prototype 6 (UFS6). The differences between the two prototypes in the number of vertical layers (64 in UFS5 vs. 127 in UFS6) and the model top (54 km in UFS5 vs. 80 km in UFS6) can potentially impact the prediction of MJO teleconnections. With respect to ERA-Interim, the global teleconnections of the MJO to the Northern Hemisphere show similar biases in 500 hPa geopotential height over the North Atlantic and European sectors in both prototypes, whereas UFS6 has slightly smaller biases over the North Pacific region. Both prototypes capture the extratropical cyclone activity occurring in weeks 3–4 over the North Atlantic after the MJO phases 6–7 and over the North Pacific and North America after MJO phases 4–5. Both prototypes successfully forecast the sign and approximate locations of 2-m temperature anomalies over the mid-to-high latitude continents occurring in weeks 3–4 after MJO phase 3 but fail to capture the sign reversal of anomalies over North America between weeks 3 and 4 after MJO phase 7. Overall, the two prototypes show similar performance in simulating the tropospheric basic state as well as prediction skill of the MJO and MJO teleconnections, suggesting that the increase in model vertical resolution and model top does not strongly improve the prediction of MJO teleconnections in the troposphere in UFS. [ABSTRACT FROM AUTHOR]

The dynamical characteristics of the zonal wavenumber 1 (s = 1) Rossby-gravity (RG) wave are examined using recently available reanalysis data for the whole neutral atmosphere over 16 years. An isolated peak is detected in the two-dimensional zonal wavenumber-frequency spectra that likely corresponds to the theoretically-expected s = 1 RG mode at heights of z = 30, 50, 65, and 80 km. The wave period of the spectral peak is approximately 1.3 days, which is close to one day. The s = 1 RG wave is successfully extracted using a band-pass filter after removing the diurnal tide with quite large amplitudes. The s = 1 RG wave exhibits a characteristic seasonal variation: the geopotential height (GPH) amplitudes are largest in the winter hemisphere in the stratosphere and lower mesosphere while enhancement is observed in both the winter and summer hemispheres in the upper mesosphere. Phase structures are examined in detail for a strong case. The horizontal phase structure at each height is consistent with the normal mode theory. The vertical phase structure is approximately barotropic from the lower stratosphere to the upper mesosphere at 30°N and 30°S where the amplitudes are large. [ABSTRACT FROM AUTHOR]

In order to quantify the influence of external forcings on the predictability limit using observational data, the author introduced an algorithm of the conditional nonlinear local Lyapunov exponent (CNLLE) method. The effectiveness of this algorithm is validated and compared with the nonlinear local Lyapunov exponent (NLLE) and signal-to-noise ratio methods using a coupled Lorenz model. The results show that the CNLLE method is able to capture the slow error growth constrained by external forcings, therefore, it can quantify the predictability limit induced by the external forcings. On this basis, a preliminary attempt was made to apply this method to measure the influence of ENSO on the predictability limit for both atmospheric and oceanic variable fields. The spatial distribution of the predictability limit induced by ENSO is similar to that arising from the initial conditions calculated by the NLLE method. This similarity supports ENSO as the major predictable signal for weather and climate prediction. In addition, a ratio of predictability limit (RPL) calculated by the CNLLE method to that calculated by the NLLE method was proposed. The RPL larger than 1 indicates that the external forcings can significantly benefit the long-term predictability limit. For instance, ENSO can effectively extend the predictability limit arising from the initial conditions of sea surface temperature over the tropical Indian Ocean by approximately four months, as well as the predictability limit of sea level pressure over the eastern and western Pacific Ocean. Moreover, the impact of ENSO on the geopotential height predictability limit is primarily confined to the troposphere. [ABSTRACT FROM AUTHOR]

Cold surges (CSs) often occur in the mid-latitude regions of the Northern Hemisphere and have enormous effects on socioeconomic development. We report that the occurrences of CSs and persistent CSs (PCSs) have rebounded since the 1990s, but the trends related to the frequencies of strong CSs (SCSs) and extreme CSs (ECSs) changed from increasing to decreasing after 2000. The highest-ranked model ensemble approach was used to project the occurrences of various CSs under the SSP1-2.6, SSP2-4.5, and SSP5-8.5 scenarios. The frequencies of the total CSs show overall decreasing trends. However, under the SSP1-2.6 scenario, slight increasing trends are noted for SCSs and ECSs in China. Atmospheric circulations that are characterized by an anomalous anticyclonic circulation with a significantly positive 500-hPa geopotential height (Z500) anomaly at high latitudes along with significant negative anomalies in China were favorable for cold air intrusions into China. In addition, the frequencies of all CS types under the SPP5-8.5 scenario greatly decreased in the long term (2071–2100), a finding which is thought to be related to negative SST anomalies in the central and western North Pacific, differences in sea level pressure (SLP) between high- and mid-latitude regions, and a weaker East Asian trough. In terms of ECSs, the decreasing trends observed during the historical period were maintained until 2024 under the SSP1-2.6 scenario. Compared to the SSP1-2.6 scenario, the Z500 pattern showed a trend of strengthened ridges over the Ural region and northern East Asia and weakened troughs over Siberia (60°–90°E) under the SSP2-4.5 and SSP5-8.5 scenarios, contributing to the shift to increasing trends of ECSs after 2014. [ABSTRACT FROM AUTHOR]

The ever-increasing complexity and data volumes of numerical weather prediction demand innovations in the analysis and synthesis of operational forecast data. Here we show how dynamical thinking can offer directly applicable forecast information, taking as a case study the extreme northern Italy flooding of May 2023. We compare this event with long-lasting historical northern Italy rainfall events in order to determine (a) why it was so extreme, (b) how well it was predicted, and (c) how we may improve our predictions of such extremes. Lagrangian analysis shows, in line with previous work, that 48-hourly extreme rainfall in Italy can be caused by moist air masses originating from the North Atlantic; North Africa; and, to a lesser extent, eastern Europe, with compounding moisture contributions from all three regions driving the May 2023 event. We identify the large-scale precursors of typical northern Italy rainfall extremes based on geopotential height and integrated vapour transport fields. We show in European Centre for Medium-Range Weather Forecasts (ECMWF) operational forecasts that a precursor perspective was able to identify the growing possibility of the Emilia-Romagna extreme event 8 d beforehand – 4 d earlier than the direct precipitation forecast. Such dynamical precursors prove to be well suited for identifying and interpreting predictability barriers and could help build forecasters' understanding of unfolding extreme scenarios in the medium range. We conclude by discussing the broader implications and operational potential of dynamically rooted metrics for understanding and predicting extreme events, both in retrospect and in real time. [ABSTRACT FROM AUTHOR]

The influence of northern polar vortex in the stratosphere (SPV) in December‐January on Asia's surface air temperature (SAT) in February has been examined using reanalysis data sets and a barotropic model. An out‐of‐phase interannual linkage between the SPV in December‐January and SAT in February during 1979–2022 has been observed, that is, a strong (weak) SPV corresponds to a cooling (warming) over Asia. Approximately 25% of the SAT over Asia in February can be explained by the SPV in December‐January. This relationship between the SPV and SAT is independent of the Arctic Oscillation. The influence of the SPV on SAT over Asia cannot be solely explained by radiative processes, but is instead related to circulation anomalies in the troposphere. A stronger SPV tends to result in negative geopotential height anomalies with cyclonic circulation over Asia. The SPV‐related geopotential height over Asia is accompanied by a weakened teleconnection pattern between the North Atlantic and Asia, with three centers from the northeastern Atlantic‐eastern Europe‐Asia, and fewer stationary waves propagated from North Atlantic into Asia. These anomalous circulation patterns and anomalous northerly wind over Central Asia in February are beneficial to the colder air transportation from the higher latitudes to Asia, facilitating a surface cooling over Asia. Our results shed light on the interannual linkage between SPV and SAT over Asia, suggesting that the SPV in December‐January could be considered as a new predicator of SAT in February over Asia. Plain Language Summary: Scientists and the public have become increasingly aware of the significant influences of the stratosphere on tropospheric weather and climate. While numerous studies have focused on the crucial roles of stratospheric anomalies in shaping surface air temperature (SAT) anomalies over Europe and North America, relatively little attention has been given to understanding the potential impacts of stratospheric polar vortex over Arctic (SPV) on SAT over Asia. In this study, we found a robust and out‐of‐phase linkage between the SPV in December‐January and SAT in February over Asia during 1979–2022. A stronger SPV tends to induce an anomalous cyclonic circulation characterized by negative geopotential height, facilitating the transport of colder air from higher latitudes to Asia, thereby contributing a cooling effect on the surface. Our findings suggest that the December‐January SPV can serve as a predictor for SAT in February over Asia. Key Points: A strong (weak) SPV in December‐January tends to result in a cooling (warming) over Asia in February on interannual timescalesThe out‐of‐phase linkage between December‐January SPV and February surface air temperature (SAT) is independent of AO in FebruaryThe influence of SPV on Asia SAT is related to the tropospheric circulation anomalies via dynamic process, rather than radiative process [ABSTRACT FROM AUTHOR]

Antarctic warm extremes impact the cryosphere, with very warm extremes driving surface melt on ice shelves. Here, we analyze temperatures exceeding the 90th percentile and the associated circulation patterns and radiation anomalies. ERA5 reanalysis data show positive geopotential height anomalies related to the occurrence of warm extremes. The highest temperature during warm extremes appears on the western periphery of high‐pressure systems, consistent with anticyclonic advection. Temperature anomalies during warm extremes are strongest in winter due to the transport of warm and moist air and a strong meridional temperature gradient. In summer, the weak meridional gradients of top‐of‐atmosphere downward solar radiation flux and surface air temperature contribute to weak temperature anomalies. Warm extremes are associated with positive longwave radiation anomalies in all seasons, but with negative shortwave radiation anomalies at the surface except during polar night. These relationships are verified by station observations. Our results confirm that Antarctic warm extremes are mostly driven by meridional advection of warm air, and suggest that these warm air masses are predominantly moist and cloudy. Plain Language Summary: Changes in Antarctic climate will alter the amount of water stored in the kilometer‐thick Antarctic ice cover, and thereby global sea level. So far, only some regions of the continent have experienced substantial warming under climate change, while others remain largely unaffected. In this study, we analyze the relatively warmest Antarctic temperatures in each place and season. We find that these warm extremes depart more strongly from seasonal mean temperatures in winter than in summer. Warm extremes are often a consequence of warmer air being transported from further North and coincide with moist, probably cloudy air that emits more infrared radiation to the surface than dry, cold air. Key Points: Warm extremes accompanied by high‐pressure systems are driven by advectionWinter experiences strongest temperature anomalies during warm extremesLongwave radiation anomalies during warm extremes are positive and contribute to warm extremes, shortwave radiation anomalies are negative [ABSTRACT FROM AUTHOR]

A method for joint statistical processing of the forecast fields of geopotential height and temperature over the Northern Hemisphere with a lead time up to 72 hours is described. These forecast fields were obtained from several operational hydrodynamic models. The approach reduces essentially the forecast errors for both meteorological fields for all lead times at standard pressure levels in the troposphere and lower stratosphere as compared to the best hydrodynamic schemes. Along with statistical methods, the variational adjustment of the vertical profiles of geopotential height and temperature is applied and reduces the forecast error when using the approximation of the profiles by cubic splines. The temperature forecast error increases approximately linearly with lead time, and the geopotential height error grows quadratically. [ABSTRACT FROM AUTHOR]

Destructive extreme high-temperature events have attacked northern China (NC) more frequently in recent decades, with regional discrepancies. However, to what extent the distribution of extreme high temperature days (EHDs) over NC is predictable remains unexplored. To address this issue, this study investigates the predictability sources and builds physics-based empirical (P-E) models to predict the spatial distribution of the June–July mean number of EHDs over NC and estimate its predictability via Predictable Mode Analysis. Two observed major empirical orthogonal function (EOF) modes of EHDs over NC are extracted that explain 64% of the total variance during the period 1980–2009. The first mode is a homogeneous mode of EHDs dominated by a barotropic high-pressure anomaly over NC. The second mode is a southwest–northeast dipole pattern of EHDs modulated by a dipole pattern of geopotential height anomalies centered over Northwest China and Northeast Asia. Tracking back the origins of boundary anomalies, physically meaningful predictors are selected to establish a set of P-E models to predict the first two principal components (PCs). The causative links between the predictors and corresponding PCs are discussed using lead–lag correlation analysis. The P-E models can predict the first two modes well, with significant skill, and thus the first two modes can be regarded as the predictable modes. If the predictable modes can be perfectly predicted, 64% of the total observed variability of EHDs over NC is potentially predictable. By reconstructing the pattern of the predictable modes and corresponding predicted PCs, the pattern of EHD anomalies over NC can be predicted. The temporal mean pattern correlation coefficient (areal mean temporal correlation coefficient) for the prediction is 0.26 (0.27) during the independent forecast period (2010–2019), providing a reference for the actual predictability. [ABSTRACT FROM AUTHOR]

Extreme high-temperature events pose huge threats on human health and ecological environment. Central Asia (CA), located in an arid region, experiences frequent occurrences of extreme high-temperature events with regional discrepancy. However, it is unknown to what extent the distribution of summer extreme high-temperature events over CA can be predicted. This study aims to investigate the dynamic origins of summer distribution of extreme high-temperature days (EHDs) over CA and estimate the predictability using Predictable Mode Analysis (PMA). Based on daily maximum temperature data from 1980 to 2010, two major EOF (Empirical Orthogonal Function) modes of EHDs over CA are identified. The first mode exhibits a homogeneous positive pattern, which is associated with a barotropic anticyclonic anomaly that covers the entire CA. The second mode features a meridional dipole pattern, corresponding to a north to south see-saw geopotential height anomaly pattern in CA. Based on the understanding of the simultaneous physical factors and tracing lower boundary anomalous forcing in the previous season, two physical predictors are selected for each principal component (PC), and a set of Physics-based Empirical (P-E) models is established. The temporal correlation coefficient (TCC) skill between observed and predicted PC1 (PC2) is 0.60 (0.74) during the independent forecast period (2010–2021). According to the criteria of PMA, the first two modes can be considered as predictable modes. If predictable modes can be perfectly predicted, 64.8% of the total observed variability of EHDs over CA is potentially predictable. Using the predicted values of the first two PCs and the corresponding observed EOF patterns, the predicted distribution of EHDs can be reconstructed. During the independent forecast period, the areal averaged TCC skill can reach 0.44, providing a reference for actual predictability. [ABSTRACT FROM AUTHOR]

The variation of synoptic systems impacting Iran's precipitation climatology can have significant climatic consequences. Among the cyclones contributing to the occasionally widespread precipitation in Iran are the coincident Mediterranean-Red Sea cyclones (CMRSC). This research aims to elucidate the long-term associated with the frequency and intensity of CMRSC by examining geopotential height (GH) and geopotential height gradient (GHG), along with CMRSC patterns, as influential factors on Iran's precipitation climatology. To achieve this, 4-daily GH data at 1000 hPa from the European Centre for Medium-Range Weather Forecasts (ECMWF), ERA-Interim, spanning from 1979 through 2018, were utilized. Throughout this period, a total of 97 CMRSC events accompanied precipitation in Iran. Methodologically, the non-parametric Chi-square test, alongside the Standard Normal Homogeneity Test (SNHT), were employed to assess variations in the cyclones' frequency and strength. The Chi-square non-parametric statistic was harnessed to discern trends in GH and GHG, while linear regression was applied to ascertain long-term trends. The findings indicate that the number of CMRSC did not display statistically significant changes over the study period when comparing successive decades and two successive halves of the time series. Notwithstanding, a more detailed examination of shorter timescales, particularly towards the end of the study period, disclosed statistically significant changes in two- and four-year averages. Additionally, the increase in GH at the formation site of Mediterranean cyclones and the decrease in GHG likely contributed to reduced atmospheric instability and precipitation in the affected regions. A pronounced GHG jump in the Mediterranean Sea in 1996 divided the time series into two distinguishable periods. The results demonstrate an upward trend in both periods; however, the second period exhibited a more gradual increase compared to the preceding period. [ABSTRACT FROM AUTHOR]

By exploring the multifaceted physical mechanisms underlying hot extreme events, the way for informed strategies to mitigate their impacts and enhance the nation's resilience in the wake of a changing climate can be paved. The long-term spatio-temporal variability of hot extremes analysis found an increasing trend (0.001 °C) of DMXT over whole Indian region except the eastern parts (Indo-Gangetic plains). The hot day is noted when DMXT > 40 °C, DMXT > 37 °C, DMXT > 35 °C and also DMXT is above the 90th percentile at each grid point in the plains, hilly, coastal region of India. The temperature tendency equation has been computed to identify the role of various physical processes driving the local extreme temperature changes in India. The analysis showed that the surface adiabatic warming found to be one of the prominent drivers of hot extreme events over India. The impact of El Niño-Southern Oscillation (ENSO) phases on the frequency of hot extreme days at local scale in India are quantified. The linear trend of hot days has been found increasing at a rate of 0.14/year, 0.08/year and 0.06/year in El Niño, La Niña and neutral years respectively. Similarly, the DMXT anomaly found to be 0.4 °C higher in El Niño phase as compared to La Niña phase. The results infers that the chances of hot days occurrence are increased (decreased) by 38% (16% and 8%) in El Niño (La Niña and neutral) period. The advection of hot air by westerlies into the Indian region leads to an increase in temperature. The composite elevated (dropped) geopotential height at 500 hPa showed the presence of high (low) pressure systems which leads to hot days occurrence over western, southern and central (northern, north western) India, where winds are westerlies (easterlies) in El Niño (La Niña) phase of ENSO. This reversal of wind is linked with the movement of warm sea surface temperature from western to eastern Pacific Ocean. Comprehending the dynamics of hot extremes in the India is vital for the development of strategies to mitigate the adverse impacts on various sectors, including agriculture, health, water resources, and infrastructure. [ABSTRACT FROM AUTHOR]

The thermal and dynamic effects of the special topography of the Qinghai–Tibet Plateau have a significant impact on rainfall in China. Utilizing NCEP/NCAR monthly reanalysis data alongside precipitation observations from 1936 monitoring stations across China spanning from 1966 to 2022, this study establishes a location index for the thermal low-pressure center situated over the Qinghai–Tibet Plateau. Temporal variations in the location index and summer (July) precipitation patterns in China were studied. Over the past six decades, thermal low-pressure centers have been predominantly positioned near 90° E and 32.5° N within a geopotential height of 4360 gpm, with their distribution extending from east to west rather than from south to north. The longitudinal and latitudinal position indices showed the same linear trend, with a negative trend before the 21st century, and then began to turn positive. Mutation analysis highlights pronounced weakening mutations occurring in 1981 and 1973, with the longitudinal index transitioning from an interannual cycle of approximately 6–8 years, while the latitudinal index displays quasi-cyclic oscillations of 5 and 8 and 12–14 years. Strong negative correlations are evident between the location indices and precipitation along the southeastern edge of the Qinghai–Tibet Plateau and in southern China, contrasting with the positive correlations observed in the central-eastern plateau, northwest, north, and the Huang-Huai region of China. The center of the thermal low is located to the east and north, corresponding to the deeper surface thermal low in most areas east of China, and the stronger transport of warm and wet air from the southwest wind, leading to greater convergence of southwest wind and northwest wind in China's northern region. The south of the Yangtze River is controlled by the strengthening West Pacific subtropical high and South Asia high, resulting in a significant decrease in precipitation, and the warm and humid air from the southwest on the west side of the West Pacific subtropical high is also transported to the north, increasing the precipitation in most parts of the north. [ABSTRACT FROM AUTHOR]

This study investigates the origins and the atmospheric circulation patterns that led to the occurrence of Wet-Cold compound events (WCCEs) recorded by the Hellenic National Meteorological Service (HNMS) between 1980 and 2004. The study employed two methods to provide insights into the issue. The first method involved identifying clusters from backward trajectories (BTs) at three heights above the arrival point (500, 1500 and 5500 m). The second method aimed at detecting weather patterns (WPs) that result from the clustering of atmospheric pressure at sea level (SLP) and the geopotential height (GH500) at 500 hPa using ERA5 reanalysis data. To detect clusters for both methods, k-means clustering was applied. The analysis of backward trajectories produced seven clusters of BTs at each height level. Most of these clusters originated from the northwest or north, with medium, short, and long-distance clusters observed at the heights of 500 m, 1500 m and 5500 m, respectively. Despite analysing the temporal data, no clear connection was established between the clusters and months. From the cluster of GH500 and SLP variables for 94 different dates, we derived three main weather patterns. All weather patterns showed high GH500 values in Western Europe and lower GH500 values in Eastern Europe. Although SLP values differed significantly among the clusters, they helped to identify distinct weather patterns. Finally, we found that on 7 out of a total of 94 different dates with WCCEs, large amounts of moisture are transported through the atmosphere to Greece over long distances by atmospheric rivers (ARs). [ABSTRACT FROM AUTHOR]

Understanding trends in hydroclimatic variables is crucial for linking local climatic drivers with regional water use practices, particularly in a vulnerable Haor basin in tropical country like Bangladesh. This study evaluated the spatiotemporal trends in hydroclimatic variables at annual and seasonal scales using advanced statistical methods, including the Modified Mann–Kendall (MK) test, Sen's slope, Sequential Mann-Kendal, Pettitt test, and linear regression model. Additionally, Detrended Fluctuation Analysis (DFA) and Morlet Wavelet Analysis (MWA) were utilized to analyze historical periodic cycles and predict future trends. Results show a significant decrease in annual and seasonal surface water levels (SWL) and rainfall, except for the monsoon, while both maximum and minimum temperatures simultaneously increased. The decline in annual SWL at a rate of 1.18 m/year was influenced by an increase in maximum temperature at a rate of 0.03 °C/year and a decrease in annual total rainfall at a rate of 5.25 mm/year. DFA analysis suggests long-term correlations among these variables, predicting future increases in temperature but continued decreases in rainfall and SWL. Periodic cycles with various frequencies were observed in rainfall, maximum, and minimum temperatures. ECMWF ERA5 reanalysis datasets attribute these changes to higher pre-monsoon geopotential heights, lower relative humidity, and higher monsoon rainfall associated with lower surface pressure. The findings of the study will help develop targeted climate adaptation strategies to mitigate the adverse effects on agriculture, biodiversity, and freshwater availability in the region. The overall study provides essential data that can inform water resource management strategies. [ABSTRACT FROM AUTHOR]