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Extreme precipitation events (EPEs) have garnered considerable social concerns due to their hazardous and destructive nature. To identify the possible causes of EPEs in China, this paper presents an in-depth investigation of how EPEs coincide with atmospheric conditions, i.e., atmospheric instability, moisture availability and moisture convergence, as well as driving variables, i.e., vertical velocity, relative humidity and air temperature. Specifically, the classic coincidence probability is devised to explicitly relate 72-h EPEs to convective available potential energy (CAPE), precipitable water (PW), vertical velocity at 700 hPa (verV), relative humidity at 700 hPa (RH), average air temperature between 850 and 500 hPa (Tavg) as well as air temperature difference between 850 and 500 hPa (Tdiff). The results show that at the annual timescale, EPEs in Southeast and Southwest China are dominantly controlled by verV, in North and Central China by PW and in Northwest China by CAPE roughly. At the seasonal timescale, the spatial distributions of coincidence probability values and "competition" among atmospheric conditions and driving variables exhibit similar patterns as observed throughout the entire year except for December–January–February. Moreover, the diagnostic plots generated for three case study regions in China provide valuable insights into the temporal evolution of precipitation events, cumulative distribution function curves of influential factors and dominant controlling factors of EPEs. This paper contributes to understandings of the dominant controlling factors of EPEs for the whole of China. The spatial patterns of EPEs and their related atmospheric conditions and driving variables yield useful information for rainstorm forecasting and disaster risk management. Plain Language Summary: Atmospheric conditions, i.e., atmospheric instability, moisture availability and wind convergence, and driving variables, i.e., vertical velocity, relative humidity and air temperature, play important parts in the occurrence of extreme precipitation events (EPEs). In this paper, the 72-h EPEs in China are explicitly related to atmospheric conditions and driving variables by using the classic coincidence probability. Specifically, the relationships are measured by the ratio of the time when precipitation and the given factor both reaching at extreme to the whole time in EPEs. Assuming that the factor exhibiting the highest coincidence probability influences EPEs the most, it is observed that at the annual timescale, vertical velocity at 700 hPa has the greatest impact on EPEs in Southeast and Southwest China as well as part of Northeast China, precipitable water in North and Central China and convective available potential energy in Northwest China. Also, it is found that the results at the seasonal timescale are consistent with that throughout the entire year except for winter. Overall, the results of this paper can help us to understand the possible causes of EPEs and serve in rainstorm forecasting and risk management at grid cell level for the whole of China. [ABSTRACT FROM AUTHOR]

The prediction skill for precipitation anomalies in late spring and summer months—a significant component of extreme climate events—has remained stubbornly low for years. This paper presents a new idea that utilizes information on boreal spring land surface temperature/subsurface temperature (LST/SUBT) anomalies over the Tibetan Plateau (TP) to improve prediction of subsequent summer droughts/floods over several regions over the world, East Asia and North America in particular. The work was performed in the framework of the GEWEX/LS4P Phase I (LS4P-I) experiment, which focused on whether the TP LST/SUBT provides an additional source for subseasonal-to-seasonal (S2S) predictability. The summer 2003, when there were severe drought/flood over the southern/northern part of the Yangtze River basin, respectively, has been selected as the focus case. With the newly developed LST/SUBT initialization method, the observed surface temperature anomaly over the TP has been partially produced by the LS4P-I model ensemble mean, and 8 hotspot regions in the world were identified where June precipitation is significantly associated with anomalies of May TP land temperature. Consideration of the TP LST/SUBT effect has produced about 25–50% of observed precipitation anomalies in most hotspot regions. The multiple models have shown more consistency in the hotspot regions along the Tibetan Plateau-Rocky Mountain Circumglobal (TRC) wave train. The mechanisms for the LST/SUBT effect on the 2003 drought over the southern part of the Yangtze River Basin are discussed. For comparison, the global SST effect has also been tested and 6 regions with significant SST effects were identified in the 2003 case, explaining about 25–50% of precipitation anomalies over most of these regions. This study suggests that the TP LST/SUBT effect is a first-order source of S2S precipitation predictability, and hence it is comparable to that of the SST effect. With the completion of the LS4P-I, the LS4P-II has been launched and the LS4P-II protocol is briefly presented. [ABSTRACT FROM AUTHOR]

Dynamical prediction of monsoon rainfall has been an important topic and a long-standing issue in both research and operational community. This paper provides a comprehensive evaluation of the subseasonal-to-seasonal (S2S) prediction skill of the East Asian summer monsoon (EASM) rainfall using the hindcast record from the Beijing Climate Center Climate System Model, BCC CSM1.1m, during the period 1983–2019. The model exhibits reasonable skills for predicting the EASM rainfall at all lead times with the skill dropping dramatically from the shortest lead time of about 2 weeks (LM0) to 1-month lead (LM1), and then fluctuating remarkably throughout 2-month to 12-month lead times. Over the EASM domain, the rapid decline of the S2S rainfall prediction skill from LM0 to LM1 is mainly caused by the inferior skills over Central China in July and over Northeast China in August. Composite analysis based on hindcast records suggest that these inferior skills are directly tied to the model's difficulties in capturing above-normal precipitation over eastern Central China and Northeast China in the respective months, which are further shown to be associated with anomalous weakening and meridional movement of the Northwestern Pacific subtropical high and the activity of large-scale teleconnection pattern hard to be predicted over northeastern Asia in summer, respectively. These findings inform the intrinsic limits of the S2S predictability of the EASM rainfall by a dynamical model, and strongly suggest that the level of confidence placed upon S2S forecasts should be stratified by large-scale circulation anomalies known to significantly affect the prediction skill, e.g., the subtropical high and high-latitude teleconnection patterns for summer monsoon rainfall prediction in this region. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive analysis of the extreme Atlantic and Benguela Niño events that occurred during the boreal spring–summer of 2021. We conducted sensitivity experiments with a regional ocean–atmosphere coupled model of the tropical Atlantic to investigate the phenology of these interannual events, unravel their triggering mechanisms, and quantify the contributions of local and remote processes. The results revealed that both the 2021 Atlantic and Benguela Niños were triggered by anomalous atmospheric fluxes at the model southern boundary (32° S), leading to a significant and persistent weakening of the South Atlantic Anticyclone. The associated poleward anomalous coastal wind off Africa reduced coastal upwelling and evaporation south of 15° S, initiating the Benguela Niño. Then, the relaxation of the equatorial trade winds forced a downwelling equatorial Kelvin wave, which warmed the eastern equatorial region, marking the onset of the Atlantic Niño. The equatorial event reached full maturity in July 2021 through ENSO-like air-sea interactions in the equatorial basin, enhanced by the atmospheric connection associated with low-level winds converging toward the distant coastal warming. While air–sea interactions in the tropical Atlantic acted as a negative feedback for the coastal warming, the ocean connection with the equatorial variability through the propagation of equatorially-forced downwelling coastal waves intensified the coastal warming, peaking end of May 2021. Overall, this research provides valuable insights into the complex dynamics of Atlantic and Benguela Niños, emphasizing the interconnectedness between these two systems. This has important implications for improving Earth system models which currently struggle to simulate these extreme events. [ABSTRACT FROM AUTHOR]

Better understanding on the phases of Indian summer monsoon i.e., onset, progress and withdrawal helps in the decision making and proper management. In this paper, date of onset of monsoon (DOM) and date of withdrawal of monsoon (DWM) is estimated using the long-term daily rainfall data observed by the India Meteorological Department (IMD) and cumulative rainfall anomaly concept, as the onset/withdrawal represents a signature of rainfall transition. The results are promising with a better match between the estimated DOM and IMD announced DOM over Kerala. The length of monsoon season variability is also analysed. The concept of onset at grid point (OGP) in India is also evaluated and progress of monsoon is monitored by considering the latitudinal variation, persistent & strength of rainfall in the continental India. The zonal classification of the spread of monsoon in terms of the intensity and time (duration) is presented. Zone wise pentad analysis of onset clearly signifies the progress of monsoon and its dependency on various physical factors like SST, OLR, TTG, ITCZ, Energy budget and wind circulation. The impact of ENSO on the OGP is brought out, which clearly shows that DOM, DWM and progress is almost influenced by the largescale processes. The intensity–duration–frequency analysis quantified the actual ISM length, seasonal rainfall and rainy days at all locations in India. This study will help modellers in better understanding of onset and progress of monsoon dynamics in India and can be integrated with NWP for the accurate and advance prediction of DOM and the progress. [ABSTRACT FROM AUTHOR]

Previous studies have reported significant uncertainty in the relationship between El Niño-Southern Oscillation (ENSO) and the winter surface air temperature (SAT) over central North America (NA). This paper uses reanalysis data and statistical methods to investigate the possible modulation effect of the Victoria mode (VM) during the preceding spring on the winter ENSO–SAT relationship. The correlation between ENSO and the winter SAT over central NA was found to be significantly positive during the positive VM phase. However, this significant correlation is almost absent during the negative VM phase. Further analysis revealed that the zonal location of the sea surface temperature anomalies (SSTA) over the tropical Pacific is an essential aspect of this VM modulation. During the positive VM phase, the dominant positive SSTA in the central-eastern tropical Pacific contribute to the formation of the negative phase of the North Pacific Oscillation (NPO)-like teleconnection, which further enhances the anomalous southerly winds and SAT over central NA. In contrast, during the negative VM phase, the Aleutian low-like atmospheric response induced by the central tropical Pacific SSTA is limited to the central North Pacific, which has a limited effect on the variation of the central NA SAT. The VM modulation mechanism is shown to be well reproduced by historical simulations of the Coupled Model Intercomparison Project Phase 6. Our results suggest that, despite the uncertainty associated with the ENSO–SAT connection, ENSO may provide prediction skills for the winter SAT over central NA during the positive VM phase. [ABSTRACT FROM AUTHOR]

Tropical cyclones have resulted in casualties and economic losses in the areas surrounding the Bay of Bengal (BoB). Thus, a comprehensive investigation of these tropical cyclones holds vital implications for disaster preparedness and mitigation. This paper compares the occurrence of storms in the last two decades, i.e., 2002–2011 and 2012–2021, and results reveal that such storms exhibited predominantly a northwesterly track towards the northwestern BoB, with a severer intensity but equal total storm frequencies. Over the past decade, a southeast-northwest pathway (SNP) was identified, demonstrating a higher incidence of severe tropical cyclones (STC, with lifetime maximum intensity ≥ 64 knots) over the BoB. Further analysis of the changes in the environmental conditions between these two decades indicates that a southeasterly anomaly in the steering flow contributed to the formation of the SNP. During the same period, the more favorable oceanic conditions during the last decade, including higher sea surface temperatures, a greater upper ocean heat content, a thicker warm water layer, and a thicker barrier layer beneath the SNP, favored the development of these storms by providing more heat energy to the storms. The atmospheric conditions, including increased air-sea heat fluxes, moisture, and instability within the lower troposphere, as well as reduced vertical wind shear, facilitated the development of convection within these storm systems. These favorable conditions improved the potential for storm development into STCs and elevated the risk of the northwestern BoB being impacted by more destructive storms. [ABSTRACT FROM AUTHOR]

Meteorological processes over islands with complex orography could be better simulated by Convection Permitting Regional Climate Models (CP-RCMs) thanks to an improved representation of the orography, land–sea contrasts, the combination of coastal and orographic effects, and explicit deep convection. This paper evaluates the ability of the CP-RCM CNRM-AROME (2.5-km horizontal resolution) to simulate relevant meteorological characteristics of the Mediterranean island of Corsica for the 2000–2018 period. These hindcast simulations are compared to their driving Regional Climate Model (RCM) CNRM-ALADIN (12.5-km horizontal resolution and parameterised convection), weather stations for precipitation and wind and gridded precipitation datasets. The main benefits are found in the representation of (i) precipitation extremes resulting mainly from mesoscale convective systems affected by steep mountains during autumn and (ii) the formation of convection through thermally induced diurnal circulations and their interaction with the orography during summer. Simulations of hourly precipitation extremes, the diurnal cycle of precipitation, the distribution of precipitation intensities, the duration of precipitation events, and sea breezes are all improved in the 2.5-km simulations with respect to the RCM, confirming an added value. However, existing differences between model simulations and observations are difficult to explain as the main biases are related to the availability and quality of observations, particularly at high elevations. Overall, better results from the 2.5-km resolution, increase our confidence in CP-RCMs to investigate future climate projections for Corsica and islands with complex terrain. [ABSTRACT FROM AUTHOR]

The El Niño Southern Oscillation (ENSO) is an important interannual atmosphere–ocean interaction phenomenon in the tropical Pacific. To study ENSO, the atmosphere–ocean coupled models are frequently used to simulate ENSO behaviors and mechanisms. The capability of a single model is limited to simulate the temporal-spatial variations of ENSO; therefore, the simulations are required to be improved through the statistical ensemble method. Based on ENSO simulations of the Coupled Model Intercomparison Project Phase 5 (CMIP5), this paper applied the conditional multi-model ensemble (CMME) method, compared the MME method, investigating the ensemble results and physical reasons of improvement. The CMME can reproduce ENSO behaviors very well, including ENSO's interannual variability, temporal-spatial evolution, seasonal phase-locking, as well as the asymmetry of El Niño and La Niña. Besides, due to the reproductions of ENSO initiation, positive and negative feedback and nonlinear heating mechanism, we point out that the CMME capturing ENSO mechanisms is the reason why it can perform ENSO characteristics. Statistical results also support that this CMME method is effective, stable, and reliable. [ABSTRACT FROM AUTHOR]

In this paper, the influence of emerging low level jet (LLJ) core zone on the Indian summer monsoon rainfall (ISMR) examined during the period 1981–2020 using ERA5 reanalysis, Indian Monsoon Data Assimilation and Analysis (IMDAA) along with Indian Meteorology Department (IMD) rainfall. The analysis spans the entirety of India, encompassing various regions, including Central India (68°–87° E, 15° N–26° N), Northeast India (83°–98.5° E, 21°–30° N), Northwest India (69°–84° E, 22°–37° N), and the Southern Peninsula (74°–85° E, 7°–19° N). Our analysis found a significant correlation between the magnitude of LLJ core zone and ISMR across these regions, except for North-East India, in both data sets (0.5–0.8) at a 0.01 significance level. ERA5 and IMDAA, both data sets, reflect the spatial climatological aspects of LLJ at seasonal and sub-seasonal scales. However, compared to the ERA5 data, the amplitude of LLJ wind is lower in the IMDAA data over the evolving core zone. This study provides pivotal insights into the dynamics of the core LLJ zone and its relationship with ISMR, including its associations with varying Indian monsoon patterns and teleconnections with the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) phases. This study hold significant implications for various sectors reliant on accurate monsoon predictions and weather forecasting. By elucidating the evolving dynamics of the core zone of the LLJ and its connection with ISMR, the research provides a valuable framework for enhancing the precision of monsoon forecasting models. [ABSTRACT FROM AUTHOR]

The detection and the functioning of the mid-summer drought (MSD) represent valuable information due to its socio-economic implications for the Caribbean. Current methods using local-scale rainfall to define the MSD have some limitations. This paper presents a novel approach to detect MSD from regional-scale weather types (WT). Long sequences of a typical summertime anticyclonic WT allow the detection of the onset and demise dates of the MSD for most years in 1950–2021. The MSD defined with WT (MSD-WT) begins rather abruptly on June 13th and concludes on August 21st, on average. While the interannual variations of the MSD-WT onset are relatively weak, long-term trends since 1950 indicate progressive earlier onsets, longer durations, and weakening intensities of the MSD-WT. Our results do not show the anticipated westward shift of the North Atlantic subtropical high pressure typically related to MSD. Instead, they indicate a halt of its eastward shift close to the MSD-WT onset. The absence of significant correlations between MSD-WT onset and either the Tropical North Atlantic or the Eastern Tropical Pacific sea surface temperature (SST) in May–June also suggests a mostly intrinsic atmospheric mechanism locked to the annual cycle, thus limiting the seasonal predictability of the MSD-WT onset. Furthermore, our results indicate a lag between the timing of increased subsidence, acceleration of the Caribbean low-level jet (CLLJ), and intensification of the deep convection over Central America and Mexico. This leads us to develop a novel hypothesis putting forward the monsoon circulation over Meso-America as a potential precursor of the increased subsidence over the Caribbean basin and the synchronous acceleration of CLLJ. The MSD-WT demise is more gradual and corresponds to a decrease in regional-scale mean sea level pressure and a weakening of the CLLJ. MSD-WT demise dates exhibit also relatively larger interannual variations but, unlike its onset, do not show any significant long-term trend. Anomalously warm (cold) Tropical North Atlantic and cold (warm) East Pacific SST from early June are correlated with an earlier (later) MSD-WT demise than usual. MSD-WT demise seems to involve regional-scale air-sea couplings and exhibits thus more seasonal predictability. Finally, despite a consistent response to the CLLJ, distinct processes seem to produce the relatively dry conditions observed over the Pacific coast of Central America and the Caribbean basin during the MSD. [ABSTRACT FROM AUTHOR]

The onset of the South China Sea summer monsoon (SCSSM) implies the transition of East Asian monsoon circulation from winter to summer, which affects not only the local weather and climate near the South China Sea (SCS) but also other regions through remote teleconnection. In this paper, the interdecadal variability characteristics of the SCSSM onset are analyzed by using various datasets as well as its possible mechanisms. The results show that there is a significant interdecadal delay in the onset date of the SCSSM after 2010. The onset is early around May 13th during 1994–2009. However, the onset is significantly later during 2010–2020 with an average of May 30th. Besides, the air and ocean conditions in SCS in May also took a significant decadal shift around 2010 and manifested itself in atmospheric forcing on the ocean, thus the SCSSM onset is mainly remotely influenced by the tropical Ocean instead of local SST. It is found that the interdecadal variation of the SCSSM onset is mainly caused by the decadal warming/cooling of the tropical central Pacific SST. The SCSSM onset's relationship with the tropical central Pacific SST seems more binding and stable relative to that with the tropical eastern Pacific SST. Their correlation is stable and still reaches the significant test in recent years. Therefore, the warming of the tropical central Pacific SST during 2010–2020 tends to postpone the SCSSM onset by means of related anomalous anticyclones and weakened activity of low-frequency Oscillation and tropical cyclones. On the contrary, its cooling led to early SCSSM onset during 1994–2009. Therefore, more and special attention should be paid to the tropical central Pacific SST anomaly, it will help us to monitor and predict the SCSSM onset effectively, and improve the seasonal prediction of East Asia summer monsoon greatly. [ABSTRACT FROM AUTHOR]

The Tibetan Plateau vortices (TPVs) are mesoscale weather systems active at the near-surface of the Tibetan Plateau (TP), which are one of the major precipitation-producing systems over the TP and its surrounding areas. TPVs mainly occur in the warm season from May to September. In this paper, we investigated the interdecadal change of TPVs in the warm seasons of 1979–2020 with the six widely used reanalysis datasets. A significant change of the TPVs frequency appears around the mid-1990s, associated with less TPVs during 1979–1995 and more TPVs during 1996–2020, which is constant among the multiple reanalysis datasets. The abrupt change of TPVs is caused by a transition of the Atlantic Multi-decadal Oscillation (AMO) from a cold phase to a warm phase in the mid-1990s. The shift of AMO leads to a silk-road pattern-like wave train and a spatially asymmetric change of tropospheric temperature. It modifies the intensity of the subtropical westerly jet and the TP heating, leading to the interdecadal change of TPV activities. [ABSTRACT FROM AUTHOR]

Characteristic adjustment time scale τ is an important parameter in convective parameterization scheme. In this paper, we propose a dynamic calculation formula of τ in the Zhang–McFarlane scheme based on convective available potential energy (CAPE) in order to improve the simulation of precipitation. The performance of using the new formula is tested and evaluated using the Community Atmosphere Model (CAM5) for numerical experiments. The results indicate that the root mean square errors of mean precipitation amount over the Northern Hemisphere eastern equatorial Pacific Ocean, the central Indo-Pacific Warm Pool, the Tibetan Plateau, and northern South America are reduced by 20.15, 16.40, 12.56, and 6.45% in summer, respectively, after a dynamic τ is adopted. The dynamically computed τ improves the Pacific Walker circulation by strengthening the western ascending and eastern descending, and increases the intensity of deep convective precipitation in tropical areas by at least 1 mm/day. Dynamic τ also increases the frequency of large deep convective precipitation in the equatorial Pacific by almost 50%, and decreases the frequency of light deep convective precipitation in many other regions. The dynamic τ has positive effects in the areas with large CAPE and spatiotemporal variability, and also leads to an increase in deep convective precipitation and a significant decrease in large-scale precipitation over high-altitude regions of windward slopes. Therefore, the generation of exaggerated precipitation in the highlands is reduced in the CAM5. Overall, the dynamic calculation of characteristic adjustment time scale can improve precipitation simulation. [ABSTRACT FROM AUTHOR]

Air temperature variability and prediction are important considering the potential implications on human health, water resource management, agriculture and land-atmosphere interaction. In this paper, we investigate the delayed influence of El Niño on spring Surface Air Temperature (SAT) variability over India using observations and assess its predictability in coupled climate models. Analysis suggests that El Niño, at its decaying phase, exerts a strong influence on the spring SAT over Southern Peninsular India (SPI). This warming is mainly induced by an anomalous anticyclone, which extends from the Western North Pacific (WNP) region to the SPI. The anomalous anticyclone-induced downdraft reduces the cloud cover and increases incoming shortwave radiation at surface, which in turn causes a dry atmospheric column and dry soil moisture over this region. As a result of this decreased soil moisture, latent heat flux into atmosphere reduces and enhances sensible heat transfer, further contributing to dry soil moisture. Both dry soil moisture and anomalous anticyclone contributes to enhanced SAT over SPI region. It is found that 6 years out of the top 10 warmest years over the SPI region correspond to the decaying phase of El Niño, suggesting the importance of the present study. It is also found that the intense SAT warming over SPI aids in increasing frequencies of Discomfort Index hours (exceeding 28 and 30 °C in a day) and the Universal Thermal Climate Index during El Niño decaying spring years. Further, we have examined the predictability of the spring SAT during decaying El Niño years based on the Asia-Pacific Economic Cooperation Climate Center (APCC) models. It is noted that most of the APCC models reasonably predicted the spatial distribution of warm SAT over India during the El Niño decaying spring season at 1 and 3 month lead. This study highlights the predictability of spring SAT a season in advance over India, which would help in planning better adaptation and mitigation strategies. [ABSTRACT FROM AUTHOR]

As one of the most important greenhouse gases, water vapor in the upper troposphere and lower stratosphere (UTLS) has a significant impact on the global earth-atmosphere system. The Tibetan Plateau (TP) is an important high terrain which exerts a profound impact on the change of weather and climate, and mass exchange. Tropopause folds occur frequently over the TP due to the impact of the subtropical westerly jet, which affects water vapor transport between the stratosphere and the troposphere. In this paper, the spatial and temporal distribution characteristics of tropopause folds over the TP are examined by applying an improved three-dimensional (3D) labeling algorithm to the ERA5 reanalysis data (1979 to 2019). The effects of different fold depths in various regions over the TP on the variations of UTLS water vapor are further studied. The results of a case study (25 February 2008) suggest that there is a good continuity in identification of the fold depth for the same fold event using the improved 3D labeling algorithm. The fold depth and height are consistent with the results of radiosonde data and ERA5 reanalysis data. The fold frequency over the TP shows an increasing trend in the last 41 years, with slightly lower frequency of medium folds than that of shallow folds, and lowest frequency of deep folds. There is increasing water vapor in the UTLS over the TP due to tropopause folds. The results indicate that tropopause folds enhance the horizontal divergence of water vapor in the UTLS and increase the vertical water vapor flux in the UTLS region. The folding over the plateau leads to increased moisture in the UTLS. It is argued that vertical velocity anomalies in the vicinity of the fold and subgrid perturbations have a significant impact on the increase of UTLS water vapor over the TP. The results of this work provide a scientific basis for a better understanding of the stratosphere-troposphere exchanges due to tropopause folds over the TP. [ABSTRACT FROM AUTHOR]

The El Niño Southern Oscillation (ENSO), as one of the largest coupled climate modes, influences the livelihoods of millions of people and ecosystems survival. Thus, how ENSO is expected to behave under the influence of anthropogenic climate change is a substantial question to investigate. In this paper, we analyze future predictions of specific traits of ENSO, in combination with a subset of well-established precursors—the Trade Wind Charging and North Pacific Meridional Mode (TWC/NPMM). We study it across three sets of experiments from a protocol-driven ensemble from CMIP6—the High Resolution Model Intercomparison Project (HighResMIP). Namely, (1) experiments at constant 1950's radiative forcings, and (2) experiments of present (1950–2014) and (3) future (2015–2050) climate with prescribed increasing radiative forcings. We first investigate the current and predicted spatial characteristics of ENSO events, by calculating area, amplitude and longitude of the Center of Heat Index (CHI). We see that TWC/NPMM-charged events are consistently stronger, in both the presence and absence of external forcings; however, as anthropogenic forcings increase, the area of all ENSO events increases. Since the TWC/NPMM-ENSO relationship has been shown to affect the oscillatory behavior of ENSO, we analyze ENSO frequency by calculating CHI-analogous indicators on the Continuous Wavelet Transform (CWT) of its signal. With this new methodology, we show that across the ensemble, ENSO oscillates at different frequencies, and its oscillatory behavior shows different degrees of stochasticity, over time and across models. However, we see no consistent indication of future trends in the oscillatory behavior of ENSO and the TWC/NPMM-ENSO relationship. [ABSTRACT FROM AUTHOR]

Determining the underlying mechanisms leading to extreme events in dynamical systems is a challenging task. Under mild hypotheses, large deviations theory predicts that, as one increases the threshold defining an extreme, dynamical trajectories which reach the extreme look more and more like one another: they converge towards a typical, i.e. most probable, trajectory called the instanton. In this paper, we use a 2000-year simulation of the IPSL-CM6A-LR model under a stationary pre-industrial climate to test this prediction on the case of hot extremes. We investigate whether the physical mechanisms leading to extreme temperatures at four locations in Europe are more similar with increasing extreme temperatures. Our results show that most physical variables exhibit the expected convergence towards a most probable trajectory, with some geographical and temporal variations. In particular, we observe the presence of a cut-off low in some trajectories, which suggests the existence of multiple pathways leading to extreme temperatures. These findings confirm the relevance of instanton dynamics to understand the physical mechanisms driving extreme events in climate models. [ABSTRACT FROM AUTHOR]

Projections of West African Monsoon (WAM) precipitation are uncertain. To address this, an improved understanding of the mechanisms driving WAM precipitation change is needed to shed light on inter-model differences and aid model development. The full forcing of increased CO2 can be decomposed into different components such as the impact of ocean warming, or the direct radiative effect of increased CO2. This paper investigates such a decomposition, analysing the effect of a uniform 4K ocean warming whilst keeping atmospheric CO2 concentrations constant. The analysis highlights several mechanisms acting to decrease WAM precipitation over a range of timescales, from days after the abrupt ocean warming, to the long-term equilibrium response. The initial decrease in WAM precipitation is caused by warming and enhanced convection over the ocean, stabilising the atmosphere inland and disrupting the monsoon inflow at low levels. Later in the response (after about 5 days), the WAM precipitation is reduced through a strengthening of the shallow circulation over West Africa, associated with changes in the large-scale temperature gradients and a local warming of the atmosphere related to a soil moisture feedback mechanism over the Sahel. Finally, from around 20 days after the SST increase, the WAM precipitation is also reduced through changes in specific humidity gradients that lead to increased potency of dry air advection into the monsoon rainband. The analysis concludes by demonstrating that the processes affecting precipitation in the early stages of the response are also relevant to the long-term equilibrium response. [ABSTRACT FROM AUTHOR]

Both reliability and independence of global climate model (GCM) simulation are essential for model selection to generate a reasonable uncertainty range of dynamical downscaling simulations. In this study, we evaluate the performance and interdependency of 37 GCMs from the Coupled Model Intercomparison Project Phase 6 (CMIP6) in terms of seven key large-scale driving fields over 14 CORDEX domains. A multivariable integrated evaluation method is used to evaluate and rank the models' ability to simulate multiple variables in terms of their climatological mean and interannual variability. The results suggest that the model performance varies considerably with seasons, domains, and variables evaluated, and no model outperforms in all aspects. However, the multi-model ensemble mean performs much better than almost all models. Among 37 CMIP6 models, the MPI-ESM1-2-HR and FIO-ESM-2-0 rank top two due to their overall good performance across all domains. To measure the model interdependency in terms of multiple fields, we define the similarity of multivariate error fields between pairwise models. Our results indicate that the dependence exists between most of the CMIP6 models, and the models sharing the same idea or/and concept generally show less independence. Furthermore, we hierarchically cluster the top 15 models with good performance based on the similarity of multivariate error fields to identify relatively independent models. Our evaluation can provide useful guidance on the selection of CMIP6 models based on their performance and relative independence, which helps to generate a more reliable ensemble of dynamical downscaling simulations with reasonable inter-model spread. [ABSTRACT FROM AUTHOR]

Recent studies using convection-permitting (CP) climate simulations have demonstrated a step-change in the representation of heavy rainfall and rainfall characteristics (frequency-intensity) compared to coarser resolution Global and Regional climate models. The goal of this study is to better understand what explains the weaker frequency of precipitation in the CP ensemble by assessing the triggering process of precipitation in the different ensembles of regional climate simulations available over Europe. We focus on the statistical relationship between tropospheric temperature, humidity and precipitation to understand how the frequency of precipitation over Europe and the Mediterranean is impacted by model resolution and the representation of convection (parameterized vs. explicit). We employ a multi-model data-set with three different resolutions (0.44°, 0.11° and 0.0275°) produced in the context of the MED-CORDEX, EURO-CORDEX and the CORDEX Flagship Pilot Study "Convective Phenomena over Europe and the Mediterranean" (FPSCONV). The multi-variate approach is applied to all model ensembles, and to several surface stations where the integrated water vapor (IWV) is derived from Global Positioning System (GPS) measurements. The results show that all model ensembles capture the temperature dependence of the critical value of IWV (IWVcv), above which an increase in precipitation frequency occurs, but the differences between the models in terms of the value of IWVcv, and the probability of its being exceeded, can be large at higher temperatures. The lower frequency of precipitation in convection-permitting simulations is not only explained by higher temperatures but also by a higher IWVcv necessary to trigger precipitation at similar temperatures, and a lower probability to exceed this critical value. The spread between models in simulating IWVcv and the probability of exceeding IWVcv is reduced over land in the ensemble of models with explicit convection, especially at high temperatures, when the convective fraction of total precipitation becomes more important and the influence of the representation of entrainment in models thus becomes more important. Over lowlands, both model resolution and convection representation affect precipitation triggering while over mountainous areas, resolution has the highest impact due to orography-induced triggering processes. Over the sea, since lifting is produced by large-scale convergence, the probability to exceed IWVcv does not depend on temperature, and the model resolution does not have a clear impact on the results. [ABSTRACT FROM AUTHOR]

We present the first evaluation of the wind field from the ensemble of kilometer-scale simulations from the CORDEX-Flagship Pilot Study on convection, with focus on the Adriatic region. Kilometer-scale climate models, also known as convection-permitting models (CPMs), produce a good representation of small-scale topographic features and consequently a more detailed depiction of dynamical and thermal circulations. These enable a reliable view of climate characteristics of the wind field, especially in coastal regions and over complex terrain, such as the Adriatic region. We investigate the (potential) added value introduced by CPMs compared to classical "cumulus-parametrized" regional climate models (RCMs), reanalysis and station observations. For this purpose, wind components at 10 m level are used at 3-hourly frequency. All simulations cover a 10-year period, extending from 2000 to 2009. In terms of the standard statistical parameters such as correlation coefficient and temporal standard deviation, CPMs are very dependent on their parent RCM performance. However, the orographic forcing emphasizes the potential added value and CPMs contain some fine spatial scale variability (i.e., stronger extremes by 25% and more accurate wind direction) that is absent in coarser RCMs and reanalysis. The potential added value is higher in the cold season compared to the warm season due to the proportion of severe wind events. CPMs reproduce well the typical wind regimes along the Adriatic coast, namely Bora and Sirocco. The benefit of using CPMs is especially pronounced in simulating Bora maximum wind speeds in northern Adriatic and Sirocco frequencies in southern Adriatic. Based on our overall analysis, we conclude that CPMs provide added value compared to coarser models, especially in the complex coastal terrain. [ABSTRACT FROM AUTHOR]

Since a decade, convection-permitting regional climate models (CPRCM) have emerged showing promising results, especially in improving the simulation of precipitation extremes. In this article, the CPRCM CNRM-AROME developed at the Centre National de Recherches Météorologiques (CNRM) since a few years is described and evaluated using a 2.5-km 19-year long hindcast simulation over a large northwestern European domain using different observations through an added-value analysis in which a comparison with its driving 12-km RCM CNRM-ALADIN is performed. The evaluation is challenging due to the lack of high-quality observations at both high temporal and spatial resolutions. Thus, a high spatio-temporal observed gridded precipitation dataset was built from the collection of seven national datasets that helped the identification of added value in CNRM-AROME. The evaluation is based on a series of standard climatic features that include long-term means and mean annual cycles of precipitation and near-surface temperature where CNRM-AROME shows little improvements compared to CNRM-ALADIN. Additional indicators such as the summer diurnal cycle and indices of extreme precipitation show, on the contrary, a more realistic behaviour of the CNRM-AROME model. Moreover, the analysis of snow cover shows a clear added-value in the CNRM-AROME simulation, principally due to the improved description of the orography with the CPRCM high resolution. Additional analyses include the evaluation of incoming shortwave radiation, and cloud cover using satellite estimates. Overall, despite some systematic biases, the evaluation indicates that CNRM-AROME is a suitable CPRCM that is superior in many aspects to the RCM CNRM-ALADIN. [ABSTRACT FROM AUTHOR]

Regional Climate Models (RCMs) provide climate information required for evaluating vulnerability, impacts, and adaptation at finer scales than their global driving models. As they explicitly resolve the basic conservation and state equations, they solve physics with more detail, conserving teleconnection of larger scales provided by Global Climate Models (GCMs). In South America (SA), the regional simulations have been historically evaluated principally on climatological aspects, but the representativeness of extremes still needs a more profound assessment. This study aims to analyze three RCMs (RegCM4-7, REMO2015, and Eta) driven by different GCMs in SA, focusing on their capacity to reproduce extreme historical indices of daily precipitation and temperature. The indices of maximum consecutive 5 days precipitation (Rx5day), Consecutive Dry Days (CDD), daily maximum and minimum annual temperature (TXx and TNn, respectively) were evaluated regarding the historical spatio-temporal variability and trends. Furthermore, their projections for the 2071–2099 period, under the Representative Concentration Pathway 8.5 scenario, were analyzed. The historical behavior of RCMs (1981–2005) was compared with two gridded products: Climate Prediction Center (CPC) and agrometeorological indicators derived from the fifth generation of global reanalysis produced by the ECMWF (AgERA5), previously compared with records from meteorological stations to evaluate them. The results show that the highest differences within the gridded products and stations were observed in the regions with more scarce surface stations (North and West of SA) and with complex topography (The Andes Cordillera), being more pronounced in the precipitation-based indices. We found that RCMs generally show more agreement in the spatial variability than in the inter-annual variability for all the indices and SA regions. When analyzing the observed trends, all models better reproduced the long-term variability of extreme temperature indices than those of rainfall. More disagreement was observed for Rx5day and CDD indices trends, including substantial spatial heterogeneities in both magnitude and sign of tendency. Climate change projections exhibited significant agreement to warmer conditions in TXx and TNn, but precipitation signals differed between RCMs and the driving GCM within each regional model. Maximum dry spells are expected to increase in almost all SA regions, whereas the climate change signals in extreme precipitation events are more consistent over southeastern SA (northern and southwestern SA), with positive (negative) changes by the end of the century. [ABSTRACT FROM AUTHOR]

This study evaluates the fidelity of the latest high-resolution CORDEX-CORE individual model (i.e., REMO2015, COSMO-crCLIM-v1-1, and RegCM4.7) simulations and their multi-model mean (MMM) in simulating the spatio-temporal characteristics of the Indian summer monsoon (ISM) precipitation during 1980–2015. The present study focuses on the mean, extreme, onset date, and intraseasonal variability and its propagation characteristics of the ISM. In addition, the evaluation of the recent IMDAA reanalysis is also presented. The results show that the MMM produced a more realistic representation of the spatio-temporal distribution of seasonal mean precipitation and wet-day frequency/intensity than any individual CORDEX model. It is noted that all simulations (except for the REGCM model) largely captured the observed spatial patterns of extreme precipitation indices (i.e., R99p and Rx1day) with some variation in precipitation spatial variability. The skill of simulations in representing the observed frequency of most severe precipitation events is relatively low, although their performance for precipitation intensity is considerable. On the other hand, the IMDAA reanalysis has shown good skill with overestimation of observed precipitation over the regions of northeast India and Indo-Gangetic plains, which may be associated with the increased atmospheric instability due to orographic lift. Furthermore, the observed intraseasonal variability of the summer monsoon (i.e., active and break spells) and it's northward and eastward propagation characteristics have been well captured by the IMDAA as well as the CORDEX models (except for the REGCM). The REGCM has shown a northward rather than a westward stretch of monsoon precipitation during the active and break phases and hence the propagation characteristics are also unclear in the REGCM. It has been observed that the low-pressure system (with a slightly northward gradient) is confined to the region of central India in the REGCM which explains the northward shift of monsoon trough activity and precipitation. The observed variability of monsoon onset has been well reproduced by the IMDAA. Overall, the IMDAA has shown better performance than the CORDEX models in all cases/aspects, and the COSMO model has been found to be the best performing model among the three CORDEX models and is in line with the skills of IMDAA and MMM, although COSMO has an early onset of monsoon. [ABSTRACT FROM AUTHOR]

Recent observational and modeling studies have demonstrated the substantial influence of the Tibetan Plateau (TP) spring land surface temperature (LST) and subsurface temperature (SUBT) on downstream summer droughts/floods events in East Asia, highlighting the potential application of LST/SUBT on sub-seasonal to seasonal prediction (S2S). In this study, we employ the National Centers for Environment Prediction—Global Forecast System/Simplified Simple Biosphere model version 2 (GFS/SSiB2) to investigate the potential role of the late spring warm LST anomaly over the TP on the extraordinary June 1998 flood in the south of the Yangtze River region. Numerical experiments indicate that the warmer (above normal) May LST over the TP may contribute to the extreme flood of 1998 over the south of the Yangtze River region, with the LST reproducing about 57% and 64% of observed above-normal rainfall anomaly over the south of the Yangtze River region and southeastern China, respectively. Further analyses reveal a possible effect of springtime TP's LST on summer southern and eastern Asian rainfall and identify some hot spots, suggesting that the TP's spring LST effect is not only limited to the Yangtze River region, but to a much larger scale. The imposed warm LST/SUBT over the TP triggers a strong wave activities propagating eastward along the upper-level westerly jet, associated with an increase of the atmospheric baroclinic instability as well as a strengthening and southeastward movement of the South Asian high, leading to intensified moisture convergence and convective instability favorable to the excessive rainfall in the downstream region of East Asia. The results of the 1998 case have also been compared with the results from year of 2003, which had a very cold spring LST anomaly over the TP and a severe downstream June 2003 drought (flood) in southern (northern) of the Yangtze River Basin area. Simulation results provide further evidence of the great importance of the TP spring land surface temperature anomaly in regulating summer extreme hydroclimatic events (e.g. droughts and floods) in South and East Asia. The present study suggests that consideration of LST/SUBT anomalies has a strong potential for more skillful S2S prediction of extreme hydroclimatic events such as floods, droughts and heatwaves over both Southern and Eastern Asia. [ABSTRACT FROM AUTHOR]

This study applies three widely used land models (SSiB, CLM, and Noah-MP) coupled in a regional climate model to quantitatively assess their skill in preserving the imposed ± 5 °C anomalies on the initial land surface and subsurface temperature (LST/SUBT) and generating the 2-m air temperature (T2m) anomalies over Tibetan Plateau (TP) during May–August. The memory of the LST/SUBT initial anomalies (surface/soil memory) is defined as the first time when time series of the differences in daily LST/SUBT cross the zero line during the simulation, with the unit of days. The memory of the T2m anomalies (T2m memory) is defined in the same way. The ensemble results indicate that the simulated soil memory generally increases with soil depth, which is consistent with the results based on the observations with statistic methods. And the soil memory is found to change rapidly with depth above ~ 0.6–0.7 m and vary slowly below it. The land models have fairly long soil memories, with the regional mean 1.0-m soil memory generally longer than 60 days. However, they have short T2m memory, with the regional means generally below 20 days. This may bring a big challenge to use the LST/SUBT approach on the sub-seasonal to seasonal (S2S) prediction. Comparison between the three land models shows that CLM and Noah-MP have longer soil memory at the deeper layers (> ~ 0.05 m) while SSiB has longer T2m/surface memories and near-surface (≤ ~ 0.05 m) soil memory. As a result, it is difficult to say which land model is optimal for the application of the LST/SUBT approach on the S2S prediction. The T2m/surface/soil memories are various over TP, distinct among the land models, and different between the + 5 °C and − 5 °C experiment, which can be explained by both changes in the surface heat fluxes and variances in the hydrological processes over the plateau. [ABSTRACT FROM AUTHOR]

We investigate the interannual variability of surface air temperature in the twenty-first century using the Coupled Model Intercomparison Project Phase 6 (CMIP6) model data under three shared socioeconomic pathways (SSP1-2.6, SSP2-4.5, and SSP5-8.5). Relative to 1961−2014, the interannual variability of annual temperature is projected to increase by 6%, 6%, and 12% globally in the 2051–2100 period under the SSP1-2.6, SSP2-4.5, and SSP5-8.5 scenarios, respectively, with an average increase of 10–22% in low latitudes and decrease of 3–28% in high-latitude oceans. The spatial pattern of wintertime variability changes is mainly determined by variations in the meridional temperature gradient. Sea ice losses and variations in snow extent in the mid to high latitudes are both related with large variability decreases according to analysis of the underlying surface. In South America and South Africa, the variability enhancement is related to the adjustment of the wet–dry status, while that in South Asia and central Africa depends more on the positive longwave radiative effect due to the increase in clouds. In tropical oceans, the increases in sea surface temperature variabilities and air–sea interactions dominate the enhanced surface air temperature variability. [ABSTRACT FROM AUTHOR]

An increasing, but spatially inhomogeneous, trend in Tibetan Plateau (TP) precipitation, and especially the precipitation anomalies in southeastern TP (TPSE), has become the subject of major scientific concern. The Indian summer monsoon (ISM) is one of the TPSE's most important moisture sources, and its characteristics are therefore key for precipitation changes. Here we present the ISM activities (ISM onset/retreat date, ISM duration, and ISM intensity) and the percentages of varied moisture sources in TPSE, using OLR datasets and Hysplit modeling with gbl reanalysis. The recent precipitation anomalies in TPSE are analyzed with CMFD reanalysis. Major findings: (1) The ISM in TPSE generally begins later, retreats earlier, is of shorter duration, and weaker in intensity. (2) Uniformly decreasing (increasing) precipitation trends are found in TPSE for periods 1979 − 2018 and 1999 − 2018 (1979 − 1998). (3) Moistures along with the ISM are the primary controls of TPSE precipitation (~ 85% in summer); these are also verified by the generally same linear trends observed in ISM moistures and TPSE precipitation in summer. (4) The precipitation anomalies in TPSE are closely related with the ISM activities: the ISM onset date (retreat date) and its variabilities affect precipitation during May − June (September − October); precipitation during July − August correlates positively with the ISM duration and its intensity. The ISM activities impact the percentages of ISM moistures and finally affect the precipitation amount and their trends. These results will contribute to precipitation-related studies as hydrology, ecology, and paleoclimate reconstructions in TP. [ABSTRACT FROM AUTHOR]

Compound heat wave (CompoundHW) has attracted extensive attention for its prolonged extreme heat from daytime to nighttime during its process. However, the performance of identifying and characterizing CompoundHW across different datasets has not been systematically evaluated. Here, we compared the similarities and differences of the ERA5, Berkeley Earth, CHIRTS and CPC datasets in identifying and characterizing CompoundHW. Results showed that the match of CompoundHW identification between datasets was consistent in both temporal and spatial dimensions, with the highest match observed between the ERA5 and CHIRTS datasets. Match of CompoundHW identification exhibited significant correlation with the density of observation stations, with matching rates above 50% in regions with dense observation networks, but extremely low match in regions with sparse data coverage. The rising trends of the CompoundHW metrics were captured by all datasets, especially in parts of North America, Europe, western Russia and Asia. Despite differences in the amplitude of CompoundHW changes across the four datasets, over 42% of global regions concurred on the changes in CompoundHW frequency, duration, and magnitude, and more than 27% agreed on the changes in the proportion of CompoundHW occurrences. Inconsistencies of CompoundHW changes were predominantly observed in regions with low matching rates, indicating that precise identification of CompoundHW is the basis for characterizing the changes in CompoudHW characteristics accurately. This study highlights the importance of multiple datasets comparison in heat wave research, especially in metrics defined by multiple climate variables and regions with sparse observational data. [ABSTRACT FROM AUTHOR]

Analysis of observational data reveals the existence of a decadal subsurface spiciness mode that involves ocean–atmosphere coupling in the North Pacific. Specifically, the Aleutian Low, the dominant atmospheric forcing of the Pacific Decadal Oscillation (PDO), drives a dipole pattern of positive and negative spiciness anomalies in the eastern midlatitude and subtropics, respectively. These anomalies then propagate equatorward along a deflected route defined by the mean acceleration potential. The positive spiciness anomaly can be observed at 14° N after 7 years of propagation while the downstream negative anomaly can be tracked to 10° N after 3 years from its appearance. In addition, a negative spiciness anomaly appears in the midlatitude, followed by the formation of the positive spiciness anomaly 2 years later. It follows a similar pathway toward the tropics. Further analysis demonstrates a strong connection between equatorial sea surface temperature variability and extratropical spiciness anomalies. These processes, in turn, potentially lead to a decadal climate oscillation in the North Pacific involving extratropical-tropical interaction. The dominant physical processes responsible for subsurface spiciness variability differ significantly between the eastern midlatitude and subtropical North Pacific. In the midlatitude, isopycnal spiciness variability exhibits similar characteristics to temperature variations at around 50–120 m depth, mainly generated through subduction and reemergence processes. Conversely, interior subtropical spiciness variability follows the evolution of salinity anomalies at around 120–250 m and is mainly formed via subduction and spice injection. Furthermore, anomalous advection across mean spiciness gradients strengthens subsurface signals from the midlatitude to the subtropics. [ABSTRACT FROM AUTHOR]

Brazil's Pantanal wetland is one of the most threatened Brazilian ecosystems from direct anthropogenic pressures and climate change. In this study, the overarching research question is to explore whether compound drought-heat events (CDHEs) have become more recurrent, intense, and widespread over Brazil's Pantanal wetland in recent decades. For this, we purpose and tested two different approaches using validated long-term time series of monthly precipitation, temperature, and the satellite-based Vegetation Health Index (VHI) to characterize the spatiotemporal pattern of CDHEs over Pantanal. Firstly, we assessed global gridded precipitation and temperature data sets against ground measurements to choose an appropriate dataset for this study. Then, we calculated the Standardized Precipitation Index (SPI), Standardized Temperature Index (STI), and Standardized Precipitation Evapotranspiration Index (SPEI) from 1981 to 2021. The results showed that using both approaches (CDHE-M1 and CDHE-M2), the frequency of events is higher considering the moderate category, which is expected since the criteria are less restrictive. In addition, the highest frequency of CDHE events occurs between September and November (the end of the dry season). The results also indicated that CDHE events have been more recurrent and widespread since 2000 in Pantanal. Besides, considering all methods for identifying the CDHEs, the probability density function indicates a shift pattern to warmer and drier conditions in the last 40 years. The Mann–Kendall tests also confirmed the assumption that there is a significantly increasing trend in the compound drought-heat events in the Pantanal. Developing methodologies for monitoring compound climate events is crucial for assessing climate risks in a warming climate. Besides, it is expected that our results contribute to the convincing of the urgent need for environmental protection strategies and disaster risk reduction plans for the Pantanal. [ABSTRACT FROM AUTHOR]

Heat waves (HWs) can pose a serious threat to human health and natural ecosystems. Meanwhile, the mid-lower reaches of the Yangtze River are known as one of the most vulnerable regions suffering such disasters. At the mention of their potential drivers, the western North Pacific subtropical high (WNPSH) has been widely accepted as the key atmospheric circulation regulating these high-temperature extremes. Although considerable studies have proved the importance of the WNPSH, not enough work pays attention to the HWs which are not caused by the anomalous WNPSH. This study utilizes the instantaneous co-recurrence ratio α based on the Dynamical System (DS) method to quantify the forcing strength from the WNPSH to HWs, and classify HWs into four types according to their value of α and spatial pattern of the 500 hPa geopotential height, as high-α1, 2 and low-α1, 2. Results show that coverage by the WNPSH is not a necessary condition for the formation of HWs, which may also result from the anomalous South Asian high (SAH) and tropical cyclones (TCs), and the value of α can indicate whether the HWs are coupled with the WNPSH or not. Furthermore, the mechanisms behind these anomalous atmospheric systems are also different. The eastward-extended SAH in high-α HWs is associated with the latent heat of local condensation induced by the abnormal rainfall, while accompanied by an eastward-propagating wave train in low-α1. The intensification of the WNPSH in high-α HWs are attributed to both meridional and zonal wave trains originating from the maritime Continent and Western Europe, respectively, and the anomalous TCs in low-α2 are mainly due to the warmer sea surface temperature over the basin-wide equatorial Pacific, which may heat the bottom atmosphere and provide the convective instability. [ABSTRACT FROM AUTHOR]

Climate model evaluation presents a crucial pathway into the investigation of the simulation of future climate. It presents the only lens through which the future states of the climate of the planet can be explored. In this study, reference datasets including reanalysis products have been used to demonstrate the climatological annual migration of the West African Monsoon System (WAMS) in three components, namely, the West African Heat Low (WAHL), the West African Heat Band (WAHB) and the West African Rain Band (WARB). We have used 8 Global Climate Models (GCMs) from the Coupled Model Intercomparison Project Phase 6 (CMIP6) to assess how the models represent the climatological annual migration of these three features over the same domain for the same period. The results show close linkages between the WAHL, WAHB and the WARB as signature components of the WAMS in their climatological annual dynamics. All three features migrate from the south (with the minimum intensities) in January to the north (with the maximum intensities) in August over the region. The GCMs represent the climatological annual dynamics of all three features to some extent. However, in most of the models analysed the WAHL is too weak, and the WAHB and WARB are located too far south. Insights from this study can be beneficial for investigations of the future state of the WAMS. Also, development of models for the region should target the improved simulation of these features, specifically the intensity of the WAHL, and the locations of the WAHB and WARB. [ABSTRACT FROM AUTHOR]

Atmospheric rivers (ARs) reach High Mountain Asia (HMA) about 10 days per month during the winter and spring, resulting in about 20 mm day - 1 of precipitation. However, a few events may exceed 100 mm day - 1 , providing most of the total winter precipitation and increasing the risk of precipitation-triggered landslides and flooding, particularly when the height of the height of the 0 ∘ C isotherm, or freezing level is above-average. This study shows that from 1979 to 2015, integrated water vapor transport (IVT) during ARs that reach Western HMA has increased 16% while the freezing level has increased up to 35 m. HMA ARs that have an above-average freezing level result in 10–40% less frozen precipitation compared to ARs with a below-average freezing level. To evaluate the importance of these trends in the characteristics of ARs, we investigate mesoscale processes leading to orographic precipitation using Advanced Weather Research and Forecasting (ARW-WRF) simulations at 6.7 km spatial resolution. We contrast two above- and below- average freezing level AR events with otherwise broadly similar characteristics and show that with a 50–600 m increase in freezing level, the above-average AR resulted in 10–70% less frozen precipitation than the below-average event. This study contributes to a better understanding of climate change-related impacts within HMA's hydrological cycle and the associated hazards to vulnerable communities living in the region. [ABSTRACT FROM AUTHOR]

Heat waves, droughts, and compound drought and heat waves (CDHWs) have received extensive attention because of their disastrous impacts on agriculture, ecosystems, human health, and society. Here, we computed the heat wave magnitude index (HWMI), drought magnitude index (DMI), and compound drought and heat wave magnitude index (CDHMI) for Yangtze River Valley (YRV) from July to August during 1961–2022. We compared the large-scale atmospheric circulation characteristics of different extreme events based on these indexes. The results show that the positive center with sink motion in East Asia provides a favorable circulation background for heat wave events. Drought events are mainly affected by the zonal wave train dominated by a significant negative anomaly in Siberia and a high-pressure anomaly upstream, and a anticyclonic water vapor with strong divergence over the Yangtze River basin. During CDHW events, both anomalous systems that affect heat waves and droughts appear and strengthen simultaneously. Specifically, in the middle and upper troposphere, the positive height anomaly center in YRV expands abnormally, and the "+–+" wave train over the northern 50° N region of East Asia becomes more obvious. Therefore, the positive anomaly and water vapor anomaly brought by the two circulation patterns at different latitudes are superimposed over the YRV, leading to severe CDHWs. At the same time, the warm positive eddy center and cold negative eddy center in high latitudes exhibit more stable positive pressure features, which are conducive to the persistent development and strengthening of CDHWs. In addition, the anomalous warm sea surface temperature in western Pacific moderating the favorable circulation patterns may also promote the occurrence of CDHWs in the YRV during the same period. [ABSTRACT FROM AUTHOR]

Poleward heat (energy) transport plays a major role in shaping the Earth's climate. Its oceanic and atmospheric components carry heat from low to high latitudes thus reducing the equator-to-pole temperature contrast. In quasi-equilibrium climate states, changes in the top-of-the-atmosphere (TOA) energy fluxes and ocean heat content remain small. In such conditions, anomalies in oceanic and atmospheric heat transport must have the same magnitude but opposite signs. This phenomenon is known as the Bjerknes compensation (BJC). The BJC hypothesis is of high importance in climate, since it imposes a strong constraint on climate variability at sufficiently long timescales (on sub-decadal and shorter timescales TOA flux variations may become comparable to other heat budget terms, interfering with BJC). However, to which extent BJC operates in the climate system and the key mechanisms of the compensation remain poorly understood. Here we analyze BJC in the IPSL-CM6A-LR climate model, focusing on its timescale dependence, its links to the Atlantic Meridional Overturning Circulation (AMOC), and the connection to Intertropical Convergence Zone (ITCZ) shifts. We show that BJC occurs in the model at both multi-decadal and centennial timescales, but is stronger on centennial timescales than decadal. In both cases BJC is initiated by variations in ocean heat transport induced by AMOC variability that are partially or fully compensated by atmospheric heat transport. For decadal timescales, we find two regions of a strong BJC at latitudes associated with the storm track region and the marginal ice zone in the Northern Hemisphere. Finally, on centennial timescales we observe a Bjerknes-like interbasin compensation between the Atlantic and Indo-Pacific heat transports, which is also related to strong centennial AMOC fluctuations and involves Southern Ocean zonal heat transport. [ABSTRACT FROM AUTHOR]

Recent studies suggest that La Niña events can be classified into two categories: mega La Niña and equatorial La Niña. The understanding of the variations in boreal summer intraseasonal oscillation (BSISO) behaviors between such two conditions remains uncertain. Results in this work show during equatorial La Niña summers, in conjunction with the more adequate intraseasonal column-integrated moisture anomalies, the weaker intraseasonal outgoing longwave radiation anomalies are observed over the western North Pacific (WNP) at 3 pentads lag of the peak phase for the Maritime Continent (MC) BSISO events than during mega conditions. Such changes are closely linked with the different propagation features, specifically northwestward and northeastward propagations under mega and equatorial conditions respectively. The distinct propagations under these two conditions could be partly explained by the background column-integrated moisture anomalies. Under equatorial conditions, the less sufficient background moisture anomalies over the tropical western Pacific (WP), in comparison to mega conditions, suppress the activities of the BSISO and its northwestward propagation here. Meanwhile, the enhanced moisture anomalies over the northwestern MC and its surrounding area (NWMC) facilitate the northeastward propagation. Under mega conditions, the background moisture anomalies over the tropical WP are not significant. The southward moisture anomaly gradient over the NWMC hinders the meridional northward propagation and makes some BSISO activities move to the tropical WP region, performing the zonal westward propagation as a whole. The moisture budget and multi-scale interaction diagnoses also emphasize the significant role of the propagation change in the moisture tendency difference averaged over the WNP. Moreover, the extratropical circulation anomalies associated with the MC BSISO events are also discussed. These findings provide new insights into BSISO activity and offer potential improvements for subseasonal forecast. [ABSTRACT FROM AUTHOR]

The intraseasonal Pacific–Japan (PJ) pattern, characterized by a pronounced quasi-biweekly oscillation, is triggered by deep convection around the western North Pacific. Three possible dynamical mechanisms on multiple timescales responsible for the growth and decay of the quasi-biweekly PJ pattern are proposed in this study based on daily reanalysis data from the Japanese 55 year Reanalysis for the 1958‒2021 period. First, the eastward-propagating wave energy associated with the quasi-biweekly circumglobal teleconnection in the upstream region enters the mid-latitude North Pacific and induces the wavelike barotropic geopotential height anomalies, amplifying the magnitudes of three mid-latitude centers of the PJ pattern by about 40% through their linear constructive interference. Secondly, the barotropic feedback forcing of both high-frequency and low-frequency transient eddies triggered by the pronounced meridional SST gradient over the mid-latitude Pacific is beneficial to the development and persistence of the PJ-related centers to the east of Japan and around the Bering Strait, whereas it damps the PJ-related center in the Gulf of Alaska, increasing the amplitude difference between the former two centers and the latter center. Such feedback forcing also leads to the asymmetry of the positive and negative PJ events. Thirdly, the dry energy conversion from the background atmospheric circulation and the moist process due to the convective heating over the western North Pacific are both efficient enough to energize the PJ pattern in the developing and mature stages, indicating that the quasi-biweekly PJ pattern can be viewed as a convectively coupled dynamical mode. [ABSTRACT FROM AUTHOR]

Twenty years of IMERG precipitation estimates are used to evaluate the contributions of mesoscale convective system (MCS) rainfall to total rainfall in the Congo Basin. Studying these systems advances our basic understanding of Congo Basin rainfall on all time scales. The seasonality of MCS rainfall in the Congo Basin follows the seasonality of total rainfall with high rainfall in spring, summer, and fall and a winter dry season in each hemisphere. In the equinoctial seasons, MCS rainfall accounts for ≥ 80% of total rainfall within 5° of the equator with the highest rainfall rates occurring along the eastern and western boundaries of the basin. In boreal summer, MCS rainfall maxima occur near the Cameroon Highlands (9°–18°E) and in boreal winter, they occur along the eastern orography (22°–28°E). The 80% percent contribution is sustained in the continental interior (15°–25°E, 5°S–5°N) throughout the year. The diurnal cycle of MCS rainfall is similar to that of total rainfall. Diurnal cycles are unimodal in the equinoctial seasons but are regionally and seasonally inhomogeneous in the solstitial seasons. Regardless of modality, MCS rainfall is highest at 15Z (1600/1700 LT) and lowest at 10Z. MCS percent contribution changes little throughout the diurnal cycle but is highest (≥ 90%) at 04Z close to the continental interior. Larger MCSs contribute their greatest percentage of MCS rainfall (83–92%) between 04Z and 07Z, while more-intensely precipitating MCSs have no seasonally or regionally consistent diurnal cycle. Seasonal and diurnal MCS rainfall maxima are associated with unstable MSE profiles in the lower troposphere. Changes in moisture drive the seasonal cycle of MSE while changes in temperature drive its diurnal cycle. [ABSTRACT FROM AUTHOR]

Tropical convection plays a key role in regional and global climate variability, and future changes in tropical precipitation under anthropogenic global warming are critical for projecting future changes in regional climate. In this study, by analyzing Coupled Model Intercomparison Project (CMIP) phase 6 models, we show that changes in projected tropical precipitation in CMIP6 models vary among models, but they are largely associated with the model's warm pool intensity in the present-day (PD) climate. Models with stronger warm pools in the PD simulation tend to simulate an increase in precipitation in the central Pacific (CP) and a decrease in precipitation in the Maritime Continent (MC) under greenhouse warming. Significant differences in precipitation between the CP and MC regions induce low-level westerly anomalies over the west-central Pacific, favoring sea surface temperature warming in the CP region. This suggests that the associated air–sea interactions result in a particular tropical pattern in response to anthropogenic forcing. [ABSTRACT FROM AUTHOR]

Tropical cyclone (TC) rapid intensification (RI) in the nearshore region often poses a serious threat to populated coastal regions. Although several previous studies have revealed an increased probability of RI events just before landfall in North America under global warming, it is unknown whether a similar increase has occurred along China coast. Here, we examine the long-term change of TC RI events along China coast during 1981–2020. Results show that the RI events along China coast have experienced a statistically significant increase since 1981, particularly in the latest decade. Further analysis reveals that the local sea surface temperature, maximum potential intensity, upper-level divergence, low-level vorticity, and vertical wind shear along China coast become increasingly favorable for TC RI occurrence. Among these factors, the intensified maximum potential intensity caused by the significant warming along China coast is the most important factor, most likely contributed by global warming instead of internal climate variability. This result is further confirmed by the CMIP6 model simulations. [ABSTRACT FROM AUTHOR]

The preferred spring development of the Pacific Meridional Mode (PMM) is investigated through a combined observational analysis and modeling approach. A mixed-layer heat budget analysis shows that the PMM experiences its strongest growth two months prior to its mature phase, and the growth is primarily attributed to the surface latent heat flux (LHF) anomaly, via the wind-evaporation-SST (WES) feedback. The spring-fall difference in the LHF anomaly is caused by both the anomalous and seasonal mean wind fields. Idealized atmospheric general circulation model experiments (GCM) show that given the same PMM heating, atmospheric anomalous wind and LHF responses are much stronger in boreal spring than in boreal fall, which favors a greater WES feedback. Experiments with an intermediate air-sea coupled model with specified SST and surface wind mean state demonstrate that a PMM-like SSTA perturbation grows fastest in boreal spring among all seasons, indicating that the tropical mean wind and SST fields in spring are most favorable for the PMM development. Finally, we show that the atmospheric teleconnection between mid-latitude North Pacific and PMM is much more pronounced during the development of PMM in boreal spring than in boreal fall. [ABSTRACT FROM AUTHOR]

Global Climate Models (GCMs) depict a notable influence of climate change on southern South America (SSA). Future regional-to-local information for adaptation and mitigation policies can be obtained by downscaling over GCMs outputs, increasing the resolution of the climate projections. Current statistical downscaling approaches in the region [e.g., Generalised Linear Models (GLMs)] need to undergo "human-guided" feature selection, which is one of the main sources of uncertainty. Here, we explore the advantages and limitations of using Convolutional Neural Networks (CNNs) in SSA to downscale daily minimum and maximum temperatures. For this purpose, we elaborate three different experiments: a cross-validation (CV) in the present climate; downscaling the historical and RCP8.5 scenarios of the EC-Earth; a pseudo-reality experiment to measure the extrapolation skill. CV-experiment results show no remarkable differences between CNNs and GLMs, although the non-linearity of the CNNs improved the representation of the extreme aspects of temperatures. Additionally, we use eXplainable Artificial Intelligence to prove that co-linearities are better handled in CNNs. The pseudo-reality experiment shows a good extrapolation skill of CNNs, especially when the activation functions are linear. Overall, the automatic skill of CNNs to deal with co-linearities in predictor data—against conventional approaches—together with the plausible climate change projections obtained—verified with the pseudo-reality experiment—make them attractive to be used for downscaling beyond their non-linear nature. These results enforce the idea of incorporating CNNs into the battery of downscaling techniques over SSA and provide experimental guidelines with prospects to be utilised in climate change studies. [ABSTRACT FROM AUTHOR]

Westerly wind bursts (WWBs), aperiodic burst-like atmospheric disturbances in the tropics, play an important role during the development stage of El Niño–Southern Oscillation (ENSO) events and in turn can be strongly modulated by ENSO itself. Previous work argued that WWBs exhibit a pronounced seasonality at the equator during an El Niño evolution, i.e., being more frequent and vigorous during the developing boreal autumn compared to the decaying spring. Here we show that this seasonal difference in WWB activity at the equator is a segmentary manifestation of a meridional migration of WWB activity, which is closely linked to the southward displacement of the El Niño-related deep convection from the developing autumn to decaying spring. The highest climatological SSTs over the western-central Pacific are located mainly to the north of the equator in boreal autumn and south of it during boreal spring; hence the equatorially quasi-symmetric El Niño-related SST anomalies tend to induce deep convection and generate more WWBs in the Northern Hemisphere during the developing autumn and more in the Southern Hemisphere during the decaying spring. Despite some differences in the zonal distribution and intensity, there appears a close resemblance between the seasonal-varying features of WWBs during eastern Pacific (EP) and central Pacific (CP) El Niño events regarding the north–south displacement, due to similar meridional shifts of deep convection anomalies. This study provides a more comprehensive picture of the spatiotemporal characteristics of WWBs during El Niño events and improves our understanding of the relationship between ENSO and high-frequency atmospheric variability. [ABSTRACT FROM AUTHOR]

In this study we analyse the physical processes causing the weakening of the tropical circulation in the Coupled Model Intercomparison Project 6 (CMIP6). We apply a diagnostic model for the large-scale tropical circulation (vertical motion) based on the moist static energy for the first baroclinic mode (MSEB) and evaluate the sensitivity of the tropical circulation changes to the changes in advection of moisture and heat, the net radiation, the moist static stability, the baroclinic mode, and to the height of the tropopause. Based on the CMIP6 model simulations we find that the tropical circulation weakens by about 10-15% over the twenty-first century. The analysis of the MSEB model suggests that the primary cause for this weakening of the tropical circulation is the lifting of the tropopause height. This effect is fairly uniform throughout the tropics and present in all model simulations. The tropopause height increase shifts the first baroclinic mode away from lower levels with unstable air masses into high levels of the troposphere where stable air masses lead to a stabilisation of the large-scale circulation. Other factors such as changes in the advection of moisture and heat, increased net heating or increased instability of the lower tropospheric gross moist stability do have strong regional differences, and mostly increase tropical circulations, counteracting the weakening caused by the lifting of the tropopause. [ABSTRACT FROM AUTHOR]

The relationship between Arctic Oscillation (AO) and surface air temperature (SAT) over the Tibetan Plateau (TP) has been long debated, particularly in boreal summer. Based on the statistical analysis, this study reveals a significant interdecadal shift in the relationship between summer AO and the first mode of the summer SAT (SAT_EOF1) over the eastern TP in recent decades. Specifically, during 1979–1998, the correlation between summer AO and SAT_EOF1 was weak and insignificant, but during 1999–2018, it was strong and significant. Changes in the AO-related regional atmospheric circulations are responsible for this enhanced AO-SAT_EOF1 connection. During the latter period, the anomalous anticyclone related to AO has extended further westward from eastern China to the eastern TP, leading to the increased water vapor transport and prevalent descending over the eastern TP. As a result, there has been a decrease in cloud cover over the eastern TP, allowing more solar radiation to reach the ground surface and thus contributing to the enhanced AO-SAT_EOF1 linkage. Further analysis indicates that the phase transition of the Pacific decadal oscillation (PDO) is a crucial factor in regulating the AO-SAT_EOF1 linkage. Since the late 1990s, the PDO has transitioned from a positive to a negative phase, playing a role in the westward extension of the AO-related anomalous anticyclone over the eastern side of the TP through the Pacific-East Asian (PEA) teleconnection. Finally, the model simulation results provide conclusive evidence that the phase transition of the PDO contributes to the unstable AO-SAT_EOF1 connection. [ABSTRACT FROM AUTHOR]

The ERA5 dataset from the European Center for Medium-Range Weather Foreceasts is the first global reanalysis to reach a horizontal resolution of 31 km and thus provides a unique opportunity to look at tropical cyclones (TC), and in particular at the 3D fields associated with observed TCs. To that end, a specifically calibrated TC tracking scheme is applied on ERA5 along with a track pairing algorithm to match the detected tracks with the IBTrACS catalog in order to investigate how well TCs are represented in the reanalysis. After tuning of the tracking scheme and the application of a dynamic mid-latitude system filtering technique, it is shown that the majority of IBTrACS TCs are detected in ERA5 and that the amount of false alarms is kept reasonably low in most regions. By comparing detected tracks with their IBTrACS counterparts, it is found that TC intensity is still strongly underestimated in ERA5 but that the minimum sea-level pressure distribution is better represented than maximum wind speed. The comparison between the life cycles from both datasets highlights key differences between ERA5 and the best-track catalog, showing in particular that the delay with which TCs from ERA5 reach their peak intensity compared to IBTrACS increases significantly with real TC intensity increase. Finally, the internal structure of TCs in the reanalysis for each intensity class are analyzed and reveal distinct intensification patterns up to Category 3. [ABSTRACT FROM AUTHOR]

The present study analyses the climate change response of Alpine-Mediterranean heavy precipitation events (HPEs), often associated with hazardous flooding. We investigate HPE properties describing their propagation, scale and intensity, by applying a storm tracking algorithm in an ensemble of convection permitting regional climate models (cpRCMs) under the CMIP5-RCP85 forcing scenario. The climate change response of HPE properties we derive by comparing their mean values for periods nearfuture [2040, 2050] and farfuture [2090, 2100] against those of a present-day period historical [1996, 2005]. By the end of the 21st century the model ensemble projects the region's surface temperatures to increase by 4.0 ∘ C. In this warmer climate HPEs are found to propagate farther and faster. They last longer by 5%, their area increases by 15% and their precipitation volume by 35%. Their maximum precipitation rate increases by 13% and an estimate of severity is found increased by 20%. By differentiating specific HPE categories, regions and seasons we find that landfalling and orographically forced HPEs occurring in fall are most intense, heavy and severe. Most importantly we find that these events are also projected to respond most strongly to climate warming, meaning that they will be even more intense, heavy and severe. Consequently in Mediterranean regions the most dramatic changes must be expected, in particular in Southern Italy. But also in regions north of the Alps, where HPEs are generally weaker than to the south, HPEs are found to occur twice as often in wintertime accompanied with an increase in precipitation volume of about 50%. Our analysis also confirms that even though the Mediterranean climate is generally drier by the year 2100, precipitation amounts associated with HPEs are found to increase by 52%. Further HPEs are projected to be 12% more frequent overall, although their occurrence frequency decreases in summer. Eventually we show that the characteristic HPE properties scale with surface warming, meaning that the warmer the local climate is, the more frequent, intense, heavy and severe are HPEs. Notably statistical tests show that the ten-member cpRCM ensemble provides robust climate change responses of all HPE properties investigated. The projected changes of HPE properties infer an increase of flooding risk for communities of the Alpine-Mediterranean area and thus the results presented here may encourage local authorities in adapting their flood protection measurements to climate change. [ABSTRACT FROM AUTHOR]

Extreme cold events exert enormous impacts on the ecosystems and human health. A quantitative attribution analysis of the surface temperature anomalies associated with extreme cold events in China is conducted using an updated version of the coupled atmosphere-surface climate feedback response analysis method. For both extreme warm and cold years, the effects of anomalous surface albedo, water vapor, atmospheric dynamics, and aerosols are the major positive contributors, and the processes associated with surface dynamics provide a major negative effect. Specifically, the effect of surface albedo (water vapor) is directly linked to the change in local snow cover (precipitable water) over China. More local snow cover (precipitable water) provides a cooling (warming) effect and vice versa. The process associated with atmospheric dynamics exhibits an asymmetric contribution for the extreme warm and cold years. It provides a warming effect over most of China in the extreme warm years, which is possibly related to the nation-wide warm air advection. On the other hand, the effect of atmospheric dynamics displays large regional and year-to-year variations for the cold cases. In addition, a considerable impact of anthropogenic aerosols is detected over central-southern China, which is possibly related to the emission reduction policies in China since 2013. Although clouds show only a moderate impact on the national-averaged temperature anomalies, they provide a considerable effect over southern China. [ABSTRACT FROM AUTHOR]