Types Of Heat Waves Research Articles

Characterizing heatwaves based on land surface energy budget

Heat extremes pose pronounced threats to social-ecological systems and are projected to become more intense, frequent, and longer. However, the mechanisms driving heatwaves vary across heatwave types and are not yet fully understood. Here we decompose perturbations in the surface energy budget to categorize global heatwave-days into four distinct types: sunny–humid (38%), sunny-dry (26%), advective (18%), and adiabatic (18%). Notably, sunny-dry heatwave-days decrease net ecosystem carbon uptake by 0.09 gC m−2 day−1 over harvested areas, while advective heatwave-days increase the thermal stress index by 6.20 K in populated regions. In addition, from 2000 to 2020, sunny-dry heatwaves have shown the most widespread increase compared to 1979 to 1999, with 67% of terrestrial areas experiencing a doubling in their occurrence. Our findings highlight the importance of classifying heatwave-days based on their underlying mechanisms, as this can enhance our understanding of heatwaves and improve strategies for heat adaptation.

Anthropogenic Climate Change and Urbanization Exacerbate Risk of Hybrid Heat Extremes in China

AbstractDry‐ and wet‐bulb temperature (Td and Tw) are usually to define heatwaves (HWs) which have been enhanced under anthropogenic climate change (ACC) and urbanization. However, responses of various types of HWs (i.e., dry HWs, only high Td; humid HWs, only high Tw; hybrid HWs, both high Td and Tw; total HWs, high Td or Tw), to ACC and urbanization remain unknown. In this study, both observations and simulations show significantly increasing occurrence probability of total HWs over China during 1971–2020, whereas this increase is mainly reflected in hybrid HWs, followed by dry HWs and humid HWs. 68.2%–93.0% of the observed increases in the above four types of HWs can be attributed to ACC; on the other hand, urbanization tends to suppress humid HWs but enhance dry HWs, as a result of contributing to the increase of hybrid HWs by 10.9%. Under future ACC, total HWs are projected to be more frequent as expected, which is mainly sourced from the increasing hybrid HWs because dry/humid HWs are projected to be steady/downward. As a consequence, urban population exposure to ACC‐induced total HWs would remarkably increase to 83.55 billion person‐days by the 2090s, 89.5% of which can be attributed to hybrid HWs. Urbanization would amplify this population exposure of ACC‐induced hybrid HWs from 74.79 billion person‐days to 110.9 billion person‐days. Our results underscore the importance of improving understanding of hybrid HWs in urban areas and developing targeted adaptation planning on a warmer planet.

Building and system design's impact on thermal resilience to overheating during heatwaves: An uncertainty and sensitivity analysis

Rising heatwaves necessitate thermally resilient buildings to mitigate occupant heat-stress and mortality, ensuring habitable conditions during thermal shocks. This study's aim and novelty is to identify key design parameters affecting three thermal resilience aspects -(i) shock impact or SET-Dh i.e., degree hours above standard effective temperature threshold of 28 °C (ii) absorptivity time (tabs) and (iii) recovery time (trec) across varying degrees of shock intensities (doS-low, medium and high). A reference Belgian apartment is simulated with varying design parameters (building orientation, envelope and glazing properties, glazing percentage, airtightness, operation and control of passive strategies, cooling systems) during three types of heatwaves. Uncertainty analysis quantified the impact of design variations on thermal resilience, while global sensitivity analysis pinpointed key parameters affecting resilience during different shocks. The study shows that while 87 % of design variations remain thermally resilient during shorter shocks, only 21 % do so during longer shocks and that WWR, cooling capacity, and passive strategies (operation of natural night ventilation and solar shading) have twice the impact on thermal resilience compared to building orientation and glazing properties. While tabs is affected by heat build-up factors (e.g., WWR, solar shading), trec is influenced by heat removal factors (e.g., cooling capacity, NNV). Parameters such as heavy thermal mass extend heat absorption but also delays heat dissipation during prolonged heatwaves. This study guides designers and architects to focus on interventions that enhance buildings' thermal resilience during both short and long heatwaves, aiding in climate adaptation and the ability to withstand future extreme heat periods.

Pattern-based forecasting enhances the prediction skill of European heatwaves into the sub-seasonal range

The prediction of European heatwaves at the subseasonal range is of key importance to mitigate their impact. This study builds on previous work which identifies five main European heatwave types based on their atmospheric circulation patterns (CPs). These CPs are potential predictors of heatwaves, as these patterns are connected with a high probability of 2-meter temperature exceeding the 90th percentile. Therefore, the aim of this study is to use these patterns to construct a pattern-based forecast method. The skill of this method to forecast extreme warm temperatures is then assessed and compared with the direct grid-point based forecast (using the direct 2-meter temperature forecast of the model). The extended (or subseasonal) range reforecast data from the European Centre for Medium-Range Weather Forecasts (ECMWF) is used for the skill evaluation. Firstly, the skill of the extended range model is assessed in predicting CPs. The pattern-based methodology is then compared with the direct prediction of extreme warm temperatures. The results show that the pattern-based methodology has a low skill at the short to medium range compared to the direct method, however it maintains skill for longer lead times, extending the forecast skill horizon significantly by up to six days over key heatwave regions. This improvement is localized over regions with the highest conditional probability of extreme warm temperatures. Furthermore, the prediction skill of 4-day periods of high temperatures using CPs lasting at least five days is also assessed. A similar improvement in forecast skill horizon is observed but the improvement is more modest and even more localized. This methodology provides skilful forecast at longer lead times to the end of the medium range and into the subseasonal range, which would be beneficial for early warnings of European heatwaves and therefore support the timely implementation of mitigation plans.

A novel quantitative method of heatwave classification for building resilience analysis

More frequent and extreme heatwaves result in an increased risk of overheating in residential buildings. The utilization of heatwave weather files is fundamental in evaluating the thermal resilience of buildings. However, limited studies have focused on quantitative classification methods that consider multiple heatwave characteristics and how buildings respond to different heatwave types. Therefore, this study aims to propose a quantitative method for classifying heatwaves by considering the duration and intensity, while also providing the thresholds for duration and intensity to identify different types of heatwaves. Subsequently, the impacts of different types of heatwaves on indoor overheating was quantified and compared through simulation of indoor thermal environment in three prototype residential buildings in China. The indoor overheating risk was then evaluated based on the indoor overheating degree. The results revealed four types of heatwaves: short and moderate, short and intense, long and moderate, and long and intense. The short and moderate heatwaves represented the majority. While the long and intense heatwaves had the highest influence on indoor overheating risks. Hence, these two types of heatwaves are suggested to be considered in building resilience analysis. The proposed method demonstrated its effectiveness in classifying heatwaves based on the impact on indoor environments.

Three-dimensional analysis reveals diverse heat wave types in Europe

Heat waves are among the most studied atmospheric hazards but commonly investigated near-surface temperature patterns provide only limited insight into their complex structure. Here we propose and evaluate a novel approach to the analysis of heat waves as three-dimensional (3D) phenomena, employing the ERA5 reanalysis in three European regions during 1979–2022. Four types of heat waves based on their vertical cross sections of temperature anomalies are introduced: near-surface, lower-tropospheric, higher-tropospheric, and omnipresent. The individual heat wave types differ in length, predominant occurrence within summer, and soil moisture preconditioning. While near-surface heat waves may persist for more than 2 weeks, those located mainly in higher troposphere are shortest (5 days at most). This demonstrates that warm advection must be accompanied by a downward propagation of positive temperature anomalies through air subsidence and diabatic heating to maintain long-lasting heat waves. We also show that soil-moisture preconditioning is crucial for near-surface heat waves only, thus pointing to different driving mechanisms for the individual 3D heat wave types.

Understanding the influence of soil moisture on heatwave characteristics in the contiguous United States

In the context of global climate change, heatwaves are becoming increasingly significant because of the adverse impacts on human health and ecosystems. However, the quantification of heatwaves relies on different temperature metrics, and little is known about how the different types of heatwaves are affected by soil moisture. Using a set of observational datasets during the period 1981–2020, this study investigates the characteristics of warm-season heatwaves over the contiguous United States (CONUS) derived from three different temperature metrics (temperature, wet-bulb temperature, and equivalent temperature), and examines how different types of heatwaves are associated with soil moisture. Increasing trends of all types of heatwaves are observed in most parts of CONUS except for the central US, posing potential risks to human health. Due to limited evaporative cooling over dry soil, there is a substantial negative relationship between soil moisture and temperature-only heatwaves across the CONUS. Meanwhile, in some regions of the western and central CONUS, there is an evident positive relationship between soil moisture and humidity-included heatwaves, which represent the combined effects of temperature and humidity. The event-based analysis in Nebraska emphasizes that temperature-only heatwaves occur over relatively dry soil conditions, while humid heatwaves tend to occur over somewhat wet soil. Our results highlight the importance of considering different types of heatwaves and their relationship with soil moisture from the land-atmosphere coupling perspective, offering valuable insights for local and regional climate planning and mitigation.

Are hotspots and frequencies of heat waves changing over time? Exploring causes of heat waves in a tropical country.

Heat waves significantly impact people's lives and livelihoods and are becoming very alarming and recognized as hot topics worldwide, including in Bangladesh. However, much less is understood regarding recent hotspots, the frequency of heat waves over time, and their underlying causes in Bangladesh. The objective of the study is to explore the current scenario and frequency of heat waves and their possible causes across Bangladesh. The Mann-Kendall and Sen's slope techniques were used to determine seasonal and annual temperature trend patterns of heat wave frequencies. Daily maximum temperature datasets collected from the Bangladesh Meteorological Department (BMD) during 1991-2021 are applied. The frequency of days with Tmax≥ 36°C as the threshold was used to compute different types of heat waves based on the BMD's operational definition. The results show that the mild heat wave (MHW) days followed the subsequent hotspot order: Rajshahi (103) > Chuadanga (79), Ishurdi (60), and Jessore (58), respectively. The frequency of days with Tmax≥36°C was persistence for many days in 2014, especially in the western part of Bangladesh compared to other parts. Similarly, the heat waves condition shown its deadliest event by increasing more days in 2021. The highest increasing trend was identified at the Patuakhali site, with a rate of 0.516 days/year, while the highest decreasing trend was noticed at the Chuadanga site, with a rate of -0.588 days/year. The frequency of days (Tmax≥36°C) is an increasing trend in the south-western part of Bangladesh. The synoptic condition in and around Bangladesh demonstrates that the entrance of heat waves in Bangladesh is due to the advection of higher temperatures from the south/southwest of the Bay of Bengal. The outcomes will guide the national appraisal of heatwave effects, shedding light on the primary causes of definite heatwave phenomena, which are crucial for developing practical adaptation tools.

Contrasting Intraurban Signatures of Humid and Dry Heatwaves over Southern California

Abstract Heatwaves in California manifest as both dry and humid events. While both forms have become more prevalent, recent studies have identified a shift toward more humid events. Understanding the complex interactions of each heatwave type with the urban heat island is crucial for impacts but remains understudied. Here, we address this gap by contrasting how dry versus humid heatwaves shape the intraurban heat of the greater Los Angeles area. We used a consecutive contrasting set of heatwaves from 2020 as a case study: a prolonged humid heatwave in August and an extremely dry heatwave in September. We used MERRA-2 reanalysis data to compare mesoscale dynamics, followed by high-resolution Weather Research and Forecasting modeling over urbanized Southern California. We employ moist thermodynamic variables to quantify heat stress and perform spatial clustering analysis to characterize the spatiotemporal intraurban variability. We find that, despite temperatures being 10° ± 3°C hotter in the September heatwave, the wet-bulb temperature, closely related to the risk of human heat stroke, was higher in August. While dry and humid heat display different spatial patterns, three distinct spatial clusters emerge based on nonheatwave local climates. Both types of heatwaves diminish the intraurban heat stress variability. Valley areas such as San Bernardino and Riverside experience the worst impacts, with up to 6° ± 0.5°C of additional heat stress during heatwave nights. Our results highlight the need to account for the disparity in small-scale heatwave patterns across urban neighborhoods in designing policies for equitable climate action. Significance Statement Heatwaves are the leading cause of morbidity and mortality among all environmental hazards. Moreover, their frequency and intensity are on the rise due to climate change. Southern California is no stranger to extreme heat, but persistently humid heatwaves still test the adaptability limits of its residents. We find that the set of two contrasting heatwaves that afflicted Los Angeles in the summer of 2020 forms the perfect testbed for characterizing the impacts of humid versus dry heatwaves on urban environment. Because climate model forecasts and long-term observational trends point to more humid heatwaves in the future for Southern California, our results underscore the importance of including moist heat in extreme heat warning frameworks.

Local mechanisms for global daytime, nighttime, and compound heatwaves

Heatwaves impose serious impacts on ecosystems, human health, agriculture, and energy consumption. Previous studies have classified heatwaves into independent daytime, independent nighttime, and compound daytime-nighttime types, and examined the long-term changes in the three types. However, the underlying mechanisms associated with the variations in different heatwave types remain poorly understood. Here we present the first investigation of the local physical processes associated with the daytime, nighttime, and compound heatwaves over the global land during 1979–2020. The results show that three heatwave types occur frequently and increasingly in most regions worldwide. Nighttime and compound heatwaves exhibit stronger increases in both frequency (the yearly number of the events) and fraction (the ratio of the yearly number of one heatwave type to the total yearly number of all types) than daytime heatwaves. Composite diagnostic analyses of local meteorological variables suggest that daytime heatwaves are associated with increased solar radiation under dry conditions and reduced cloud cover and humidity under a clear sky. In contrast, nighttime heatwaves are typically accompanied by moist conditions with increases in cloud fraction, humidity, and longwave radiation at night. These synoptic conditions for daytime and nighttime heatwaves are combined to contribute to compound heatwaves. Local divergences and moisture fluxes responsible for different heatwaves are further revealed. Positive moisture divergence anomalies are seen in most land areas for daytime and compound heatwaves, while they mainly appear in low latitudes for nighttime heatwaves. Our research provides a comprehensive understanding of the local mechanisms of different heatwave types, informing future risks and impact assessments.

Dynamics and Characteristics of Compound Heatwaves over Asia

The increasing frequency of heat waves in East Asia (EA) affects agriculture, water management, and people's livelihoods. In recent years, record-breaking heatwaves have occurred corresponding to extreme drought and are increasing in frequency. The two leading modes of heatwavesoverEA are closely related to dry conditions, but their temporal developments are somewhat different. The first major mode of heatwaves appears over northern EA, starting in early summer and lasting throughout the summer. The second mode of heatwaves occurs over central China and Korea and is closely related to negative precipitation anomalies during late summer and startsin July or August.This study investigated the quantitative feedback attribution of heatwave-related surface temperature anomalies using the coupled air-surface climate feedback-response analysis (CFRAM). The warming anomalies related tothe first mode of heatwaves are usually controlled by cloud, latent heat, and surface dynamics processes. It can be explained by reducing heat release from the surface to the atmosphere due to the lack of soil moisture under severe dry conditions.While surface warming related to the second mode of heatwaves is contributed by cloud feedback and atmospheric dynamic process. Reduction in cloud area associated with anticycloniccirculation anomalies induces increased insolation and it affects surface warming. However, the effect of humidity on high-temperature events has not yet been fully explored. Thus, this study identifiedcompound heatwaves that are described simultaneously with relative humidity conditions and suggested the future projections of two types of heatwaves over EA using phase six of the Coupled Model Intercomparison Project (CMIP6) model simulation. CMIP6 models projected intensification of dry heatwaves and increased moist heatwave days in response to projected increases in greenhouse gas concentrations.

European heatwaves: Link to large‐scale circulation patterns and intraseasonal drivers

AbstractThis study examines the European heatwaves' predictability at subseasonal timescales. Land surface feedbacks and tropical convection, due to their variability at intraseasonal timescales, are taken into consideration and their potential role in extending the predictability beyond the medium range (10 days) is explored. A classification of European heatwaves into five heatwave types is used to discriminate the effects of surface feedbacks and of tropical variability among the different heatwave types. The classification is computed in terms of circulation patterns. By inferring the near‐surface temperature through atmospheric circulation, we aim to identify the predictable component of the heatwave events. All five heatwave circulation patterns are characterized by persistent anticyclonic anomalies located over the region with maximum temperatures. We show that soil moisture deficit is not a required precondition for the occurrence of heatwaves over most of Europe. However, heatwave events over southern Europe exhibit some sensitivity to dry conditions. We use a simplified index to describe the dominant mode of tropical convection at intraseasonal timescales. The index, based on precipitation anomalies, represents the evolution of the Boreal Summer Intraseasonal Oscillation (BSISO). We find that episodes with strong BSISO amplitudes characterized by enhanced convection over India, Bay of Bengal and China sea favour the occurrence of heatwave events over Russia. The results highlight the role of tropical intraseasonal variability in enhancing the predictability of some extreme temperature events over Europe.

Significant Increases in Wet Nighttime and Daytime–Nighttime Compound Heat Waves in China from 1961 to 2020

Heat waves (HWs) with high humidity are dangerous to human health. However, existing studies on different types of HWs considering the effect of humidity are still limited. This study defines three types of wet summer HWs (that is, wet independent daytime and nighttime HWs and wet daytime-nighttime compound HWs) and investigates their spatial-temporal changes across China during 1961–2020. Results show significant upward trends of wet nighttime and compound HWs in terms of frequency, occurring days, duration, intensity and spatial extent, while changes for wet daytime HWs are weak and insignificant in nearly all sub-regions of China except for southwest and eastern northwest China. Compared with wet compound and daytime HWs, wet nighttime HWs accompanied by more elevated relative humidity exhibit larger growth rates in frequency, occurring days, duration and affected areas. Additionally, most wet nighttime and compound HWs with the longest duration and/or the maximum intensity are found to occur after the mid-1990s, compared to fewer than half for wet daytime HWs. Our findings emphasize the prominent intensifying trends in wet nighttime HWs across China for the last 60 years, and suggest more efforts on exploring humid HWs.

Mortality Risk Related to Heatwaves in Dezful City, Southwest of Iran.

We aimed to evaluate the impact of heatwaves on daily deaths due to non-accidental, cardiovascular and respiratory causes in the city of Dezful in Iran from 2013 to 2019. We collected daily ambient temperature and mortality and defined 2 types of heatwaves by combining daily temperature ⩾90th in each month of the study period or since 30 years with duration ⩾2 and 3 days. We used a distributed lag non-linear model to study the association between each type of heatwave definition, and deaths due to non-accidental, cardiovascular and respiratory causes with lags up to 13 days. There was no discernible correlation in this area, despite the fact that heatwaves raised the risk of death from cardiovascular causes and lowered the risk from respiratory causes. On the other hand, the risk of total non-accidental mortality on days with the heatwaves is significantly higher than normal days. In main effects, the heatwaves have a significant relationship with the risk of total non-accidental mortality (in the first heatwave definition, Cumulative Excess Risk (CER) in lag0-2 was 10.4 and in second heatwave definition, CER values in lag0, 0-2, and 0-6 were 12.4, 29.2, and 38.8 respectively). Also, in added effects, heatwaves have a significant relationship with the risk of total non-accidental mortality (in the first heatwave definition, CER in lag0 and 0-2 were 1.79 and 4.11 and in the second heatwave definition, CER values in lag0, 0-2, and 0-6 were 7.76, 18.35 and 24.87 respectively). In addition, heatwaves appeared to contribute to a cumulative excess risk of non-accidental death among the male group as well as the older adults. However, the results showed that heatwaves could have detrimental effects on health, even in populations accustomed to the extreme heat. Therefore, early warning systems which monitor heatwaves should provide the necessary warnings to the population, especially the most vulnerable groups.

On the atmospheric background for the occurrence of three heat wave types in East China

Compared with daytime (occurring only in daytime) and nighttime (occurring only in nighttime) heat waves (HWs), daytime-nighttime compound HWs (occurring simultaneously in daytime and nighttime) are highlighted to exert much severer impacts especially on human health. However, the physical mechanisms underlying compound HWs are poorly understood. Based on the observed maximum and minimum temperatures and NCEP/NCAR reanalysis data, this article addressed the physical processes for the occurrence of compound HWs in East China, where compound HWs occur most frequently across China. Comparisons with those related to daytime or nighttime HWs were also performed. The results indicate that the occurrences of three HW types are all associated with anticyclonic circulation anomalies from the upper troposphere to the lower troposphere, whereas their locations and intensities determine the configuration of atmospheric conditions for different categories of HWs. The resultant less (more) cloud cover and humidity as well as increased downward shortwave (longwave) radiation at the surface favor the warming of daytime (nighttime), conducive to the occurrence of daytime (nighttime) HWs. The combination of above conditions associated with daytime and nighttime HWs, which helps the persistence of high temperatures from daytime to nighttime, benefits the occurrence of compound HWs. In addition, nighttime and compound HWs occur with the northwestward extension of the western Pacific subtropical high (WPSH), while it stays in the climatological location for the occurrence of daytime HWs. Further investigation suggests that daytime (nighttime) HWs are accompanied with an upper-tropospheric meridional (zonal) wave train propagating downstream from western Siberia (the east to the Caspian Sea). In comparison, the wave train related to compound HWs shares the mixed features of daytime and nighttime HWs, characterized by a meridional wave propagation from the Scandinavian Peninsula to East China and then a zonal propagation toward the western Pacific.

Absence of mitochondrial responses in muscles of zebrafish exposed to several heat waves

Heat waves are extreme thermal events whose frequency and intensity will increase with global warming. As metabolic responses to temperature are time-dependent, we explored the effects of an exposure to several heat waves on the mitochondrial metabolism of zebrafish Danio rerio. For this purpose, zebrafish were acclimated at 26 °C or 31 °C for 4 weeks and some fish acclimated at 26 °C underwent 2 types of heat waves: 2 periods of 5 days at 31 °C or 10 days at 31 °C. After this acclimation period, mitochondrial respiration of red muscle fibres was measured at 26 °C and 31 °C for each fish, with the phosphorylation (OXPHOS) and basal (LEAK) respirations obtained with activation of complex I, complex II or complexes I and II. The respiratory control ratio (RCR) and the mitochondrial aerobic scope (CAS) were also calculated at both temperatures after the activation of complexes I and II. Under our conditions, heat waves did not result in variations in any mitochondrial parameters, suggesting a high tolerance of zebrafish to environmental temperature fluctuations. However, an acute in vitro warming led to an increase in the LEAK respiration together with a higher temperature effect on complex II than complex I, inducing a decrease of mitochondrial efficiency to produce energy at high temperatures. Increased interindividual variability for some parameters at 26 °C or 31 °C also suggests that each individual has its own ability to cope with temperature fluctuations.

Different mechanisms for daytime, nighttime, and compound heatwaves in southern China

Heatwaves often cause immense stress on human society and the natural environment. While heatwaves can be classified into daytime, nighttime, and compound daytime-nighttime types, the specific processes associated with different heatwave types remain poorly understood. In this paper, we identify different mechanisms operating in compound (i.e., extreme heat during both day and night) and independent daytime and nighttime heatwaves in southern China. Compound heatwaves generally exhibit stronger temperature increases than either daytime or nighttime types. Daytime heatwaves are accompanied by increased downward shortwave radiation under a clear sky with reduced cloud cover and moisture. Nighttime heatwaves are characterized by more cloudy and moist conditions, and increased downward longwave radiation at the surface at night. A combination of these conditions for daytime and nighttime heatwaves prevail during compound heatwaves. All three types of heatwaves are associated with strengthening and eastward extension of the South Asian high (SAH) in the upper troposphere, and strengthening and westward extension of the western North Pacific subtropical high (WNPSH) in the lower and middle troposphere. Further examinations suggest that compound heatwaves are accompanied by the strongest intensification of SAH and WNPSH. Compared with daytime heatwaves, nighttime events are associated with a stronger amplitude of SAH and WNPSH, and both highs tend to extend more southward when nighttime heatwaves occur. This southward extension induces an anomalous lower-level anticyclone that drives a southwesterly wind anomaly over southeastern China. This circulation feature transports warmer and more humid air towards southern China. The enhanced concentration of water vapor leads to increased absorption of outgoing longwave radiation, and increased re-emission of longwave radiation to the surface, thus resulting in surface warming at night.

Two spatial types of North China heatwaves and their possible links to Barents‐Kara Sea ice changes

AbstractHeatwaves of different spatial types can cause various impacts. Based on the spatial heatwave intensity, 47 North China heatwaves from 1961 to 2019 could be classified into two categories: the S‐type with a centre in the south and the N‐type in the north. There were more N‐type events (36) than the S‐type (11). Observational analysis indicated that under atmospheric circulations favourable to North China heatwaves (e.g. the enhanced continental high), there is a linkage between the sea ice concentration (SIC) changes in the Barents‐Kara Sea (BKS) and these two types of heatwaves. Relative to high BKS SIC, the reduction of SIC would strengthen the cyclonic circulation over the Siberian‐BKS, which could coordinate with the continental high to enhance the cyclonic over the Jianghuai region. A cyclonic‐anticyclonic‐cyclonic pattern is present over the BKS‐North China‐Jianghuai. The cyclonic circulation over the Jianghuai region would push the continental high northerly, resulting in the heatwave centre to the north. It would also cause favourable conditions for precipitation over southern North China, reducing temperatures and raising soil moisture, not conducive to the heatwave. Therefore, the heatwave intensity in southern North China would weaken and the heatwave centre would locate in the north, making the N‐type. Conversely, when the BKS SIC increases, the upper cyclonic over the Siberian‐BKS would not get strengthened, indicating entire North China would be controlled by continental high, and the heatwaves would be southerly relative to the N‐type, namely the S‐type. A series of numerical experiments using NCAR CAM5.3 confirm the above observational results, whereby if the BKS SIC reduces (increases), the North China heatwave is likely to be the N‐type (S‐type).

Identifying the evolving human imprint on heat wave trends over the United States and Mexico

Changes in frequency, duration and intensity of three heat wave (HW) types (compound, daytime, and nighttime) over the United States (U.S.) and Mexico during the second half of the 20th-century are investigated using the Community Earth System Model Large Ensemble (CESM-LE). The individual role of anthropogenic aerosols and greenhouse gases (GHGs), as well as the contribution from internal variability (IV), are identified and contrasted by means of the CESM-LE single forcing experiments during two periods: 1950–1975, when North American aerosol emissions peaked, and 1980–2005, when aerosol emissions declined. HW changes are strongly affected by anthropogenic forcing. During 1950–1975, aerosols, via both aerosol-radiation and aerosol-cloud interactions, dominate the decreasing trends in compound HWs over the central U.S., the daytime HWs in large parts of the domain and the nighttime HWs over Mexico. Conversely, all three HW types are considerably more frequent (2 HWs summer−1 decade−1), longer-lasting (with increases of up to 2 days HW−1 decade−1 in some regions) and more intense (e.g., C HW−1 decade−1 in compound HWs) across large regions of the domain during the 1980–2005 period. The results show that the decline in aerosol emissions and the continuous rise in GHGs lead to widespread warming and subsequent circulation adjustments, contributing to the positive HW trends. The contribution of IV is large during 1950–1975 (over 60% in most areas), and considerably reduced later on. This study provides a comprehensive picture of the role of anthropogenic forcing and IV on the marked HW changes in the recent decades and their underpinning physical mechanisms.

Sahelian Heat Wave Characterization From Observational Data Sets

AbstractThis paper makes use of spaceborne observational data sets in order to characterize radiative processes involved in spring time heat waves in the Sahel. Spring corresponds to the hottest period of the year, with a high aerosol load, a gradual moistening, and the presence of clouds contributing to greenhouse effect. Heat waves are defined as synoptic events that have a large spatial extent and a duration longer than 3 days. Two types of heat waves are studied: daytime heat waves, detected with the daily maximum temperature and nighttime heat waves, detected with the daily minimum temperature. Daytime heat waves correspond to situations where cloud optical thickness is lower than the climatology and a large number of these situations are also associated with a lower aerosol load and a drier atmosphere. Nighttime heat waves correspond to a moister atmosphere compared to the climatology. In a large fraction of them, an increase in aerosol loading is also observed. This study, only based on observational data sets, highlights the subtle but different radiative balance at play in both types of events.