Recent increasing frequency of compound summer drought and heatwaves in Southeast Brazil

Abstract

An increase in the frequency of extremely hot and dry events has been experienced over the past few decades in South America, and particularly in Brazil. Regional climate change projections indicate a future aggravation of this trend. However, a comprehensive characterization of drought and heatwave compound events, as well as of the main land–atmosphere mechanisms involved, is still lacking for most of South America. This study aims to fill this gap, assessing for the first time the historical evolution of compound summer drought and heatwave events for the heavily populated region of Southeast Brazil and for the period of 1980–2018. The main goal is to undertake a detailed analysis of the surface and synoptic conditions, as well as of the land–atmosphere coupling processes that led to the occurrence of individual and compound dry and hot extremes. Our results confirm that the São Paulo, Rio de Janeiro and Minas Gerais states have recorded pronounced and statistically significant increases in the number of compound summer drought and heatwave episodes. In particular, the last decade was characterized by two austral summer seasons (2013/14 and 2014/15) with outstanding concurrent drought and heatwave conditions stemmed by severe precipitation deficits and a higher-than-average occurrence of blocking patterns. As result of these land and atmosphere conditions, a high coupling (water-limited) regime was imposed, promoting the re-amplification of hot spells that resulted in mega heatwave episodes. Our findings reveal a substantial contribution of persistent dry conditions to heatwave episodes, highlighting the vulnerability of the region to climate change.

Compound Hot and Dry Events in Europe: Variability and Large-Scale Drivers

“…In order to isolate T x values from the global warming sign, before the computation of HWDI, we removed the linear trend from the entire daily T x dataset, by applying a 1st degree polynomial regression technique. A similar approach has been used recently by Geirinhas et al (2021) in order to remove the global warming signal from the maximum temperature to compute heatwave events in southeast Brazil. Considering our definition of drought and daily heatwave episodes, a compound hot and dry event (CHD) is then defined as a combined index when a heatwave episode occurs during a period under drought conditions (i.e., a summer with an associated 3 month SPI value (SPI3 August) <-1).…”

“…Considering our definition of drought and daily heatwave episodes, a compound hot and dry event (CHD) is then defined as a combined index when a heatwave episode occurs during a period under drought conditions (i.e., a summer with an associated 3 month SPI value (SPI3 August) <-1). This metric has been successfully applied also for other regions (Russo et al, 2019;Geirinhas et al, 2021). For our analysis we used only the grid points which are satisfying the compound event criteria.…”

An open workflow for the study of unseen weather extremes

“…ECMWF, 2018). For example, the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 reanalysis (Hersbach et al, 2020) has been used to estimate rainfall intensity-duration-frequency (IDF) curves globally (Courty et al, 2019), trends in extremes (Faranda, 2020;Geirinhas et al, 2021;Kim et al, 2021), driving processes behind extreme events (Grazzini et al, 2020), and extreme weather indices (Kennedy-Asser et al, 2021;Wehner et al, 2020). Although reanalyses overcome spatial data scarcity, they can exhibit model deficiencies or inhomogeneities (Parker, 2016), and their typical length (~70years) may still be a limiting factor when studying extreme events.…”

The Study on Compound Drought and Heatwave Events in China Using Complex Networks

“…Heatwave events are mainly defined by relative thresholds (percentile thresholds), absolute thresholds, and durations [14,15]. In recent studies, the definition of CDHEs mainly refers to the combination of drought and heatwave event definitions mentioned above [4,[16][17][18]. There are also studies that directly describe the properties of CDHEs by corresponding metrics.…”