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.

According to the latest global and regional circulation models, the probability of occurrence of large heat waves (HWs), such as the 2003 European or the 2010 Russian events will increase significantly in the following decades under most climate scenarios. Currently, there are numerous studies for the Northern Hemisphere characterizing HWs and evaluating their impacts in several areas such as public health, economy, and agriculture. However, over South America, and in particular for Brazil, similar analysis is lacking despite its large geographical extension and numerous population potentially affected. Here we perform an assessment of HW events and characteristics recorded in six large Brazilian cities during the last five decades. The performed analysis reveals the existence of positive and significant trends in HW frequency since the 1980s, particularly for the cities of São Paulo, Manaus, and Recife. Over the last decades, Brasília was the city that recorded the highest number of days per year under a HW regime, contrasting with Rio de Janeiro that recorded the lowest value. The assessment of the large‐scale atmospheric circulation patterns associated with summer HWs, indicated for Porto Alegre, São Paulo, Rio de Janeiro, and Brasília the presence of well‐marked concentric and positive 500 hPa geopotential height anomalies followed by positive 850 hPa temperature anomalies. These anomalies are likely associated with quasi‐stationary anticyclonic systems promoted by anomalous westward displacements of the South Atlantic Subtropical High System which are related to a weakening of other transients (and non‐transient) systems such the Intertropical Convergence Zone (ITCZ) and the South Atlantic Convergence Zone. For Manaus, the identified anomalies are linked to a northward displacement of the ITCZ. This configuration is compatible with an increase in solar radiative pattern and decreased soil moisture, enhancing surface temperature values, possibly associated with positive feedback mechanisms between soil and the atmosphere.

The year 2020 had the most catastrophic fire season over the last two decades in the Pantanal, which led to outstanding environmental impacts. Indeed, much of the Pantanal has been affected by severe dry conditions since 2019, with evidence of the 2020’s drought being the most extreme and widespread ever recorded in the last 70 years. Although it is unquestionable that this mega-drought contributed significantly to the increase of fire risk, so far, the 2020’s fire season has been analyzed at the univariate level of a single climate event, not considering the co-occurrence of extreme and persistent temperatures with soil dryness conditions. Here, we show that similarly to other areas of the globe, the influence of land-atmosphere feedbacks contributed decisively to the simultaneous occurrence of dry and hot spells (HPs), exacerbating fire risk. The ideal synoptic conditions for strong atmospheric heating and large evaporation rates were present, in particular during the HPs, when the maximum temperature was, on average, 6 ºC above the normal. The short span of the period during those compound drought-heatwave (CDHW) events accounted for 55% of the burned area of 2020. The vulnerability in the northern forested areas was higher than in the other areas, revealing a synergistic effect between fuel availability and weather-hydrological conditions. Accordingly, where fuel is not a limiting factor, fire activity tends to be more modelled by CDHW events. Our work advances beyond an isolated event-level basis towards a compound and cascading natural hazards approach, simultaneously estimating the contribution of drought and heatwaves to fuelling extreme fire outbreaks in the Pantanal such as those in 2020. Thus, these findings are relevant within a broader context, as the driving mechanisms apply across other ecosystems, implying higher flammability conditions and further efforts for monitoring and predicting such extreme events.

No abstract

The 2013/2014 summer in Southeast Brazil was marked by historical unprecedented compound dry and hot (CDH) conditions with profound socio-economic impacts. The synoptic drivers for this event have already been analyzed, and its occurrence within the context of the increasing trend of CDH conditions in the area evaluated. However, so far, the causes for these record temperatures remain poorly understood. Here, a detailed characterization of the 2013/2014 austral summer season over Southeast Brazil is proposed, emphasizing the role played by land–atmosphere interactions in temperature escalation. We demonstrate that a strong soil moisture–temperature coupling regime promoted record-breaking temperatures levels exceeding almost 5 °C over the previous highest record, and played a key role in triggering an outstanding ‘mega-heatwave’ that lasted for a period of around 20 days. This pronounced soil desiccation occurred within a current climate change trend defined by drier and hotter conditions in the region. The soil dry-out, coupled with strong radiative processes and low entrainment of cooler air masses through mesoscale sea-breeze circulation processes, led to a water-limited regime and to an enhancement of sensible heat fluxes that, ultimately, resulted in a sharp increase of surface temperatures.