Heat Wave Characteristics Research Articles

Heatwaves in Mozambique 1983–2016: Characteristics, trends and city-level summaries using high-resolution CHIRTS-daily

The intensity, frequency, and duration of heatwaves are increasing worldwide. Still, heatwaves are systematically underreported and underresearched across the African continent. This study examines heatwaves across Mozambique, a country highly vulnerable to a variety of climate risks yet where heatwaves have received little to no attention. A spatio-temporal analysis was conducted for five heatwave characteristics (heatwave number, frequency, duration, amplitude, and magnitude) and corresponding trends from 1983 to 2016. This was done using the remotely sensed CHIRTS-daily, which presents one of the most accurate and highest resolution (5 × 5 km) daily temperature product currently available, especially for data-scarce regions. Three heatwave definitions were analyzed and compared, which are based on (1) the 90th percentile of daily maximum temperature (TX90), (2) the 90th percentile of daily minimum temperature (TN90), and (3) the Excess Heat Factor (EHF). Results were overlayed with high-resolution population data to obtain heatwave exposure and likely potential implications. Our findings show that Mozambique has experienced many heatwaves over the past decades. On average, 2–18.6 annual heatwave days (HWF) were recorded with the longest heatwaves (HWD) lasting X- 11.5 days. More and longer heatwaves were observed in the North and along the coast of Mozambique. Heatwave magnitude (HWM) ranged from 0.3 to 6.8 °C and amplitude (HWA) from 0.8 to 11.7 °C, with highest values in South and Central Mozambique. Heatwave events, days, and duration were found to be significantly increasing (p < 0.05) for many populated regions, yet trends for heatwave magnitude and amplitude were largely insignificant. A total of 13.6 million inhabitants (48% of the population) were found to be exposed to a significant increase in heatwave days, with most people exposed in Zambezia, followed by Nampula and Maputo province. City-level summaries of Maputo, Beira, Nampula, and Tete showed that these cities have been exposed to 50+ heatwave events since 1983. Overall, this study is one of the first to analyze historical heatwave events and trends on a high-resolution scale, both on country- and city-level scale, urgently required to increase awareness and spur action to reduce the current and future risk of extreme heat.

Changes in the associations between heatwaves and human mortality during two extreme hot summers in Shanghai, China

Heatwaves are projected to increase in frequency, intensity, and duration as the climate warms. However, it is unclear whether human mortality from heatwaves is changing in frequency with time. We used Quasi-Poisson regression with a distributed non-linear model (DLNM) to examined associations of heatwaves and their characteristics (intensity, duration, and timing) with human mortality due to different diseases and total non-accidental diseases (TND) for different sociodemographic subgroups between 2002 and 2004 and 2012–2014 in Shanghai, China. We found that heatwaves showed a significant association with cause-specific mortality and TND for socio-demographic subgroups during the two study periods. Relative risks (RR) of mortality decreased for most demographic subgroups from 2002 to 2004 to 2012–2014, while RR of respiratory diseases (RD) increased over time. The association between heatwave characteristics and human mortality changed over time. RRs of heatwaves on mortality were higher for females, the elderly, and low- and middle-educational level populations than for males, younger and highly educated counterparts, respectively. Overall, heatwaves had a stronger association with the mortality of RD from 2002 to 2004 to 2012–2014. Heatwave duration also had an enhanced association with all subgroups over time. Our research findings could provide insights into the design of sustainable cities and society.

Coupling Relationship Between Heat Waves and Air Pollution: A Case Study of Central Tianjin

With the increasing contradiction between urban construction and ecological and societal resources, the impact of heat wave and air pollution on cities has become increasingly conspicuous. How to create a comfortable and cool urban environment has become a hot topic that the whole society attaches great importance to. By analyzing the spatio-temporal distribution of heat wave and air pollution, the coupling characteristics of heat wave and air pollution are obtained.

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.

Compound and successive events of extreme precipitation and extreme runoff under heatwaves based on CMIP6 models

Global warming accelerates the rate of interregional hydrological cycles, thus leading to a significant increase in the frequency and intensity of global extreme events. An extreme event that causes other extreme events within a short period of time is a successive event. Compound and successive extreme events are more harmful than single extreme events. Therefore, this study revealed the evolution characteristics of compound heatwave and extreme precipitation/runoff events (CHP/CHR), successive heatwave and extreme precipitation/runoff events (SHP/SHR). The population exposure of the four compound events was assessed in the future. The results are as follows: (1) the frequencies of CHP, CHR, SHP, and SHR have all shown a significant upward trend since the Industrial Revolution, especially at low and high latitudes. Under the future SSP585 scenario, CHP and CHR had the largest change rates from 2065 to 2099 at 2.01 events/decade and 1.86 events/decade, respectively. (2) The proportion of severe and extreme events increased significantly in various regions from 1970 to 2014. SHP and SHR have the largest proportion of severe/extreme events in 2015–2039/2065–2099. (3) The CHP and CHR changes in the historical period mainly occurred at high latitudes, while SHP and SHR had the largest change rates in low latitudes. The temperature was dominant compound and successive events in the future. The intensity of the compound event was much larger than that of its corresponding successive event under the high-emission scenario. (4) Climate effect had the most obvious impact on the change of population exposure. Compared with the SSP126 scenario, the population exposure change of the compound event increased by 3.1 times and 3.2 times under the SSP585 scenario during 2065–2099 and 2015–2039, respectively.

Characteristics of Heat Waves in Mainland China since 1961 Based on Absolute and Relative Methods

Based on gridded temperature data from the China Meteorological Administration (CMA), two types of methods, i.e., absolute methods and relative methods, respectively, were used to identify heat waves in Mainland China. Four statistical indicators, including the occurrence frequency, duration days, earliest occurrence date, and latest extinction date, were constructed to analyze the spatial-temporal characteristics of heat waves, especially on the annual and decadal change trends. Firstly, we found that both the frequency and the duration of heat waves decreased in the period from 1960 to 1989 but increased in the 1990s and increased significantly from the early 2000s to the 2010s. Spatially, the frequency and the duration obtained by each type of method are significantly different among different regions when considering different facts, such as different regions that have different degrees of tolerance to heat waves. Secondly, the decadal distribution characteristics of the earliest occurrence date and the latest extinction date of heat waves well capture the hot summer, the stronger sensitivity of winter to warming than other seasons, and the gradually increasing intensity of heat waves. It provides a multidimensional reference for the cause analysis and prediction of extreme heat waves in China.

Marine heatwaves characteristics in the Mediterranean Sea: Case study the 2019 heatwave events

Marine Heatwaves (MHWs) are considered one of the main consequences of global warming. MHWs negatively affect ecosystems, threaten economies, and intensify storms. In this work, we investigated the main characteristics of MHWs in the Mediterranean Sea from 1982 to 2020 and examined the relationship between their frequency and teleconnection patterns (e.g., East Atlantic Pattern (EAP) and East Atlantic/Western Russian Pattern (EATL/WRUS)). We then focused on the most intense MHW events that occurred in 2019 in the western Mediterranean Sea (WMB) and eastern Mediterranean Sea (EMB) by examining the link between the SST anomaly (SSTA) and various atmospheric forcings during these events. Our results showed that MHWs were more frequent and intense in the WMB than in the EMB on temporal and spatial scales, while the duration of MHWs was longer in the EMB. The trend of MHW frequency and duration in the Mediterranean Sea between 1982 and 2020 was about 1.3 ± 0.25 events/decade and 3.6 ± 1.16 days/decade, respectively. More than half of all MHW events in the Mediterranean Sea were recorded in the last decade (2011-2020). The results also showed that the EAP plays an important role in modulating MHW frequency in the Mediterranean Sea, with a strong positive correlation of 0.74, while the EATL/WRUS was strongly negatively correlated with MHW frequency in the EMB, with a correlation of about -0.60. In 2019, six MHW events were observed in the WMB, three of which were classified as strong events (SST exceeded two times the climatological threshold), while two events were detected and classified as strong events in the EMB. In the WMB, the 2019 MHWs extended to a depth of about 20 meters into the water column, while the MHWs in the EMB extended to greater depths of over 50 meters. The strong MHW events in the WMB were associated with a large positive heat flux anomaly and a shallow mixed layer. In the EMB, the high SSTA associated with MHW events caused heat loss from the ocean to the atmosphere and was associated with a shallow mixed layer and anomalously low mean sea level pressure. Finally, a negative relationship between the SSTA and surface Chl-a concentrations was observed during the 2019 MHW events. This negative influence of MHWs on Chl-a was more pronounced in the WMB than in the EMB, suggesting that the WMB MHW events were intense enough to infer a response to chlorophyll-a concentrations.

Heat waves and mortality in the Brazilian Amazon: Effect modification by heat wave characteristics, population subgroup, and cause of death.

The Brazilian Amazon faces overlapping socio-environmental, sanitary, and climate challenges, and is a hotspot of concern due to projected increases in temperature and in the frequency of heat waves. Understanding the effects of extreme events on health is a central issue for developing climate policies focused on the population's health. We investigated the effects of heat waves on mortality in the Brazilian Amazon, examining effect modification according to various heat wave definitions, population subgroups, and causes of death. We included all 32 Amazonian municipalities with more than 100,000 inhabitants. The study period was from 2000 to 2018. We obtained mortality data from the Information Technology Department of the Brazilian Public Healthcare System, and meteorological data were derived from the ERA5-Land reanalysis dataset. Heat waves were defined according to their intensity (90th; 92.5th; 95th; 97.5th and 99th temperature percentiles) and duration (≥2, ≥3, and ≥4 days). In each city, we used a time-stratified case-crossover study to estimate the effects of each heat wave definition on mortality, according to population subgroup and cause of death. The lagged effects of heat waves were estimated using conditional Poisson regression combined with distributed lag non-linear models. Models were adjusted for specific humidity and public holidays. Risk ratios were pooled for the Brazilian Amazon using a univariate random-effects meta-analysis. The pooled relative risks (RR) for mortality from total non-external causes varied between 1.03 (95% CI: 1.01-1.06), for the less stringent heat wave definition, and 1.18 (95% CI: 1.04-1.33) for the more stringent definition. The mortality risk rose as the heat wave intensity increased, although the increase from 2 to 3, and 3-4 days was small. Although not statistically different, our results suggest a higher mortality risk for the elderly, this was also higher for women than men, and for cardiovascular causes than for non-external or respiratory ones. Heat waves were associated with a higher risk of mortality from non-external causes and cardiovascular diseases. Heat wave intensity played a more important role than duration in determining this risk. Suggestive evidence indicated that the elderly and women were more vulnerable to the effects of heat waves on mortality.

Projecting the Changes in Multifaceted Characteristics of Heatwave Events Across China

AbstractUnderstanding future variations and trends of heatwave events has critical implications for the ecosystem and human health. However, the diverse metrics of heatwave affect the comparable assessment of heatwave evolution at regional scales. The inadequate consideration of the enhanced warming trend and local antecedent heat conditions at different climate zones undermines the comprehensive understanding of future heatwave changes. Here, we systematically assess variations and trends in duration, frequency, and intensity of heatwave events in China from 1961 to 2100, using historical observations and climate model simulations from Coupled Model Intercomparison Project Phase 6. The increased rates and trends in the duration and frequency are more evident than those in intensity and severe heatwave days. Regionally, the northern and western regions are projected to suffer longer and more frequent consecutive heatwaves, while southern regions are likely at greater risk of severe heatwave days. A comparison among four scenarios shows that the future heatwave characteristics projected by the high forcing Shared Socioeconomic Pathway (SSP5‐8.5) exhibit substantial intensification than those in other three experiments, imposing intractable dangers to numerous organisms and ecosystems. Under the SSP1‐2.6, the acceleration of all heatwave characteristics is projected to slow down in all regions after 2040. In addition to maximum temperature, temperature advections are projected to contribute to heatwave intensification in western regions. Our results provide a comprehensive assessment of future variations and trends in heatwave events. The comparable future changes across unevenly developed regions are necessary for improving regional adaptive capacity to extreme heat risk.

A Study on the Segmentation of Heat Wave Warning Zones using Cluster Analysis

A heat wave countermeasure period was selected and operated to minimize the damage from heat waves in Korea. However, to this date, there are no clear ideas and methodology for selecting a heat wave countermeasure period, resulting in continuous budget expenditure incurred for its preparation and response activities. This requires an efficient selection of countermeasure periods. This study analyzed the time series data trend of heat index using Mann-Kendall and Pettitt test for cities and counties in Gyeongsangbuk-do, and it confirmed the current characteristics of increasing heat waves compared to- those in the past. In addition, K-means cluster analysis was applied to derive the city-wise heat wave characteristics. The cities and counties were classified into three based on the heatwave characteristics and starting point and ending point of each heat wave characteristic representing the years 2004 to 2021 between July 21 to August 11, July 16 to August 15, and July 11 to August 19. It was also confirmed that there was a significant difference from the heat wave countermeasure period (May 20~Sep 30) currently selected. Hence, in the future, economic losses may be minimized in terms of disaster preparedness and response through the selection of the heatwave countermeasure period reflecting temporal and spatial characteristics.

Fine-resolution analysis of the spatiotemporal characteristics of heatwaves in the Maloti-Drakensberg region, southern Africa: 1979–2021

Consistent with global patterns, heatwaves have increased in frequency, duration and intensity across southern Africa; under enhanced global warming this is projected to worsen. Heatwaves have many adverse impacts, but in biologically unique mountainous regions, like the Maloti-Drakensberg region, impacts on ecological processes and hydrological cycles are particularly dire given their highly sensitive nature. Despite this, extreme temperature changes and interannual variability patterns remain understudied in mountainous regions, owing largely to remoteness and inaccessibility. This is especially true for the Maloti-Drakensberg region. Hence, using heatwave indices developed by the World Meteorological Organization (WMO) Expert Team on Sector-Specific Climate Indices (ET-SCI), we explored spatiotemporal patterns for the seasonal average number, length and magnitude of summer (November-March) heatwaves for 1979/80–2020/21 using the AgERA5 reanalysis. Although higher elevation regions typically experienced more frequent, longer-lasting heatwave events with higher interannual variability levels, on average the Maloti-Drakensberg region experienced 1.4 heatwave events, lasting for 6.5 days with a magnitude of 3.5°C2. Interannually, the El Niño-Southern Oscillation influenced variability of heatwave characteristics, with positive (negative) heatwave number and length anomalies predominantly detected during El Niño (La Niña) phases, while for heatwave magnitudes no clear pattern was evident. These heatwave aspects were predominantly characterised by increasing trends, however, few regions exhibited statistically significant trends. Overall, trends averaged 0.003 events/year, 0.03 days/year and 0.02°C2/year for the mean number, length and magnitude of heatwaves, respectively. These results highlight that the Drakensberg-Maloti region is vulnerable to an increasing frequency, duration and magnitude of heatwaves, however, implications thereof require further study.

Heatwave Variability and Structure in South Africa during Summer Drought

Pronounced subsidence leading to summer drought over southern Africa causes warmer than average surface air temperatures or even heatwave (HW) conditions. We investigated the occurrence of HWs during the summer drought over South Africa based on station data and the ECMWF ERA5 reanalyses. Temperature observations from the South African Weather Service were analyzed for seasonality and long-term trends (1981–2020) as background to the occurrence and variability of HWs. We focused on three severe El Niño Southern Oscillation (ENSO)-induced drought seasons, i.e., 1982/83, 1991/92, and 2015/16, to investigate HW characteristics. While 1997/98 was among the strongest El Niño seasons, the impacts were not as severe because it coincided with an intense Angola low, which allowed for rain-bearing cloud bands to form. Results showed that the hottest months were spread across the austral summer season from December to February. Regions experiencing high mean maximum temperatures and high HW frequencies exhibited a strong ENSO signal, with record HWs occurring during 2015/16. The establishment and persistence of a middle-level high-pressure system over Botswana/Namibia (Botswana High) appears to trigger the longest-lasting HWs during drought seasons. The Botswana high is usually coupled with a near-surface continental heat low and/or tropical warm air advection towards the affected region. It was also found that intense ENSO-induced drought events coincided with high HW frequency over South Africa, such as during 1982/83, 1991/92, and the recent 2015/16 events. The results of this study contribute to understanding drought and heat wave dynamics in a region experiencing rapid warming as a result of climate change.

Seasonal Diversity of El Niño‐Induced Marine Heatwave Increases in the Bay of Bengal

AbstractThis study explored the characteristics of marine heatwaves (MHWs) and the related dynamics in Bay of Bengal (BOB) associated with El Niño events during 1982–2020. This showed that El Niño can induce more MHW days, which displays remarkable seasonality. The strong influences of El Niño were found in the southern BOB at its peak phase in winter and the following spring, characterized by an increase in MHW days by 106% in winter and 83% in spring. In the proceeding summer, its influences weakened and shifted to the central bay. The more MHW days during winter and spring are primarily caused by the enhanced solar radiation associated with El Niño and partly contributed by the thick barrier layer in winter. In contrast, besides atmospheric heating, the deepening thermocline in the central bay induced by equatorial and local winds also plays an important role in the increase in MHWs during summer.

More frequent, persistent, and deadly heat waves in the 21st century over the Eastern Mediterranean

Heat waves are extreme events characterized by sweltering weather over an extended period. Skillful projections of heat waves and their impacts on human mortality can help develop appropriate adaptation strategies. Here, we provide nuanced projections of heat wave characteristics and their effect on human mortality over the Eastern Mediterranean based on ERA5 reanalysis and CORDEX ensemble simulations. Heat waves were identified according to the 90th percentile threshold of the Climatic Stress Index (CSI), specifically tailored for the summer conditions in this region.We provide evidence that heat waves in the region are projected to occur seven times more often and last three times longer by the end of the 21st century (RCP8.5). We find that heat waves will become more persistent in a warmer world. Finally, we offer a conservative estimate of excess mortality in Israel based on a simple linear model. The projected changes in heat stress intensity and frequency may result in ~330 excess deaths per summer at the end of the 21st century (RCP8.5) compared to the historical baseline of ~30 heat-related deaths, particularly pronounced in the elderly (65+ years).We conclude that heat waves increasingly threaten society in the vulnerable Eastern Mediterranean. We also emphasize that true interdisciplinary regional collaborations are required to achieve adequate public health adaptation to extreme weather events in a changing climate.

Indian heatwaves in a future climate with varying hazard thresholds

India has experienced remarkable changes in temperature extremes in recent decades due to rapid global warming leading to extreme heat events with disastrous societal impacts. In response to continuing global warming, this study investigates summertime (March–June) heatwave characteristics over India in the present and future climate. During 1951–2020, India Meteorological Department observational data show rising trends in heatwave characteristics such as frequency, intensity, duration, and season length, mainly over India’s northwest, central, and south peninsular regions. Further, the present study explores the changes in future heatwave characteristics using the state-of-the-art statistically downscaled bias-corrected climate models data from Coupled Model Intercomparison Project Phase 6 (CMIP6) of the Shared Socioeconomic Pathway scenario. This study uses varying hazard thresholds, namely fixed (time-invariant historical climatological threshold) and decadal moving thresholds (time-varying future climatological threshold), to define heatwaves and examine the future changes in heatwave characteristics over India. Results show a significant increase in mean summertime heatwaves defined using fixed thresholds in terms of their frequency, duration, number, amplitude, cumulative magnitude, and season length in the near future (NF) (2025–2054) and the far future (FF) (2065–2094) compared to the baseline period (1985–2014) over much of India, with the most substantial increases seen in the FF. However, heatwaves defined using the decadal moving thresholds show no significant changes in their characteristics during the NF but a substantial change in the FF over many parts of India. This work is the first attempt to use bias-corrected CMIP6 models data to project heatwave characteristics utilising the concept of the varying hazard thresholds across India. Overall, this study provides a comprehensive assessment of climate change’s impact on Indian heatwaves, which can help in planning better adaptation and mitigation strategies.

Spatiotemporal Characteristics and Influencing Factors of Summer Heatwaves in Hexi Oasis during 1962–2020

Based on the daily gridded (0.5° × 0.5°) maximum temperature data during 1962–2020, the spatiotemporal characteristics of heatwaves in Hexi Oasis, Gansu Province, China and their influencing factors are investigated. The results showed that for the last 59 years, the overall trends of high-temperature heatwaves in Hexi Oasis were prolonged duration (0.276 d/10a), increased frequency (0.007 times/10a), and decreased intensity (−0.072 °C/10a). In terms of spatial variation, there was a gradually decreasing trend from northwest to southeast for both the duration and frequency of heatwaves. In the contrary, heatwaves with higher intensity were mainly distributed over the southeastern and central parts of Hexi Oasis. The Mann–Kendall (M-K) analysis demonstrated that the mutation years of the duration and intensity of heatwaves are 2009 and 1992, respectively, while the frequency remained nearly constant for the last 59a. In addition, the cycles for the duration (2.6a and 7.2a), frequency (2.8a and 7.6a), and intensity (2.6a) of heatwaves agree well with those of atmospheric circulation and El Niño events, indicating that the above events have a great impact on the heatwaves. The influencing factors analyzation implies that the heatwaves are mainly influenced by Asian zone polar vortex area index (APVAI), East Asia major trough (EAT), Qinghai-Tibetan Plateau index (TPI), and carbon dioxide emissions (CDE). Additionally, it is concluded that the intensity of heatwaves was negatively correlated with the size of the subtropical high-pressure area in the western Pacific Ocean.

DYNAMICS OF THE WARM SEASON EXTREME TEMPERATURE EVENTS IN IZHEVSK IN THE PERIOD OF 1961-2020

Modern climate changes present us the task of constant monitoring and updating of data on the occurrence of temperature extreme events that may pose danger to several areas of the economy. To identify the extreme temperature events, the 90 and 95 percentiles of daily maximum temperature during the warm season months of 1961-2020 in Izhevsk were calculated. Then the number of days with extreme temperature conditions was determined for every decade of the research period. The number, overall and average length, and intensity of the registered heat waves in 1961-2020 were also evaluated, and their dynamics analyzed. The results suggest the increase in number for both days with extreme temperature and heat waves. Other heat wave characteristics demonstrate more complex patterns of change, but nevertheless point on climate becoming more extreme during the last decades. None of the linear trends are statistically significant.

Spatio-Temporal Analysis of Heatwaves Characteristics in Greece from 1950 to 2020

Heatwave events are of major concern in the global context, since they can significantly impact ecosystems, economies and societies. For this reason, more detailed analyses of the characteristics and trends of heatwaves represent a priority that cannot be neglected. In this study, the interannual and decadal variability of seven indices of heatwaves were investigated during the warmest period of the year (June–August) by using an enhanced resolution reanalysis model (ERA5-Land) over a 71-year period (1950–2020) for the area of Greece. Heatwaves were defined as periods where two thresholds, based on a modified version of the Excess Heat Factor index (EHF) and the 95th percentile of the maximum daily temperature, were exceeded for at least three consecutive days. Greece experiences almost yearly 0.7 heatwaves on average during the whole period of study, while this value has increased by ~80% since 1990. Trend analysis revealed that heatwaves have become more frequent, longer, and more intense since 1950. The percentage of the land area that experiences at least one heatwave per year was almost doubled in the examined period. An increasing trend in the number of heatwaves that occurred in June was identified.

Representation and evaluation of southern Africa's seasonal mean and extreme temperatures in the ERA5-based reanalysis products

Over data-sparse regions such as southern Africa, reanalysis products represent valuable proxies for observed weather data. These products are, however, associated with a range of strengths and weaknesses which cannot be overlooked. Hence, for the period 1979–2021, we explored the performance of three ERA5-based reanalysis products (i.e. AgERA5, ERA5 and ERA5-Land) for their spatiotemporal representation of mean summer (November–March) and winter (May–September) temperatures, and coldwave and heatwave characteristics of the seasonal average number of events and days, and magnitude across southern Africa. Compared to the National Oceanic and Atmospheric Administration Climatology Prediction Centre (NOAA CPC) gridded observation-based temperature reference dataset, the reanalysis datasets adequately reproduced spatiotemporal characteristics of mean daily temperatures, with high area average correlations (r > 0.8). Summer (winter) temperatures were, however, typically under-estimated (over-estimated), with area average biases ranging from −0.22 °C (ERA5) to −0.11 °C (AgERA5) for summer and 0.31 °C (ERA5) to 0.51 °C (ERA5-Land) for winter. These biases were reflected in the general tendency for under-estimation (over-estimation) of the average coldwave (heatwave) magnitudes, with biases ranging from an average of 1.04 °C2 (AgERA5) to 1.76 °C2 (ERA5-Land) for coldwave magnitudes and −0.03 °C2 (AgERA5 and ERA5-Land) to −0.09 °C2 (ERA5) for heatwave magnitudes. Of the coldwave and heatwave indices investigated, the ERA5-based coldwave and heatwave seasonally averaged magnitudes were associated with the poorest performance, with the weakest correlations and most statistically significant biases, especially for regions outside South Africa which have substantially fewer observed temperature records. While the reanalysis products were spatiotemporally consistent in their performance for the seasonally averaged number of coldwave and heatwave days and events, they typically over-estimated the number of days, by almost two days for both indices, contributing to these events, while they generally under-estimated (over-estimated) the number of coldwave (heatwave) events by less than one event for both indices. On average, seasonally averaged heatwave characteristics were somewhat better represented than that for coldwaves. Despite this, the ERA5-based reanalysis products performed relatively well overall and are valuable to apply in further research considering coldwaves and heatwaves across southern Africa, provided their limitations are acknowledged. Impact-based studies exploring heatwave and coldwave influences on human health, crop yields, livestock thermal comfort, and water availability, for instance, will, however, likely require higher grid resolutions offered by the AgERA5 and ERA5-Land products.

European heatwaves in present and future climate simulations: a Lagrangian analysis

Abstract. Heatwaves are prolonged periods of anomalously high temperatures that can have devastating impacts on the environment, society and economy. In recent history, heatwaves have become more intense and more numerous over most continental areas, and it is expected that this trend will continue due to the ongoing global temperature rise. This general intensification may be modified by changes also in the underlying thermodynamical and dynamical processes. In order to study potential changes in heatwave characteristics and dynamics, we compare Lagrangian backward trajectories of airstreams associated with historic (1991–2000) and future (2091–2100) heatwaves in six different European regions. We use a percentile-based method (Heat Wave Magnitude Index daily) to identify heatwaves in a large ensemble of climate simulations (Community Earth System Model Large Ensemble (CESM-LE) with 35 members). The simulations have been forced by historical representative concentration pathways (RCPs) up to 2005 and by the RCP8.5 scenario afterwards. In general, we find that air parcels associated with heatwaves are located to the east or inside the respective regions 3 d prior to the events. For future heatwaves, the model projects a north-/northeastward shift of the origin of the air masses in most study regions. Compared to climatological values, airstreams associated with heatwaves show a larger temperature increase along their trajectory, which is connected to stronger descent and/or stronger diabatic heating when the air parcels enter the boundary layer. We find stronger descent associated with adiabatic warming in the northern, more continental regions and increased diabatic heating in all regions (except of the British Isles) in the simulated future climate. The enhanced diabatic heating is even more pronounced for heatwaves over continental regions. Diabatic temperature changes of near-surface air are driven by sensible heat fluxes, which are stronger over dry soils. The amplified diabatic heating associated with future heatwaves may thus be explained by an additional drying of the land surface.