Two different mechanisms cause severe hot droughts in the western Amazon

Abstract. In the last few decades, the Amazon basin has experienced several periods of simultaneous extreme drought and heat. It is crucial to gain a better causal understanding of these compound events, which can severely damage the rain forest ecosystem. Here, we study the role of atmospheric mass, moisture and heat transport to the western Amazon, an area where pristine tropical rain forests can still be found. To investigate how anomalous the atmospheric transport during extreme events is compared to the climatological mean, we use a Lagrangian reanalysis dataset created with the particle dispersion model FLEXPART driven with meteorological input data from the ERA5 reanalysis. For the period 1979–2021, air masses over the western Amazon are selected every three hours and traced 10 days backward in time. We compared the overall transport climatology for the end of the dry season (August–September) with the transport for 21 extreme compound events that were selected for their extremity in terms of high air temperature and low soil water content. We find that extreme events over the western Amazon that happen during El Niño events have very different causes from those that take place under La Niña or neutral ENSO (El Niño Southern Oscillation) conditions. For the five extreme events that occurred under El Niño conditions, around 50 % of the air, as compared to 30 % for the climatology, is located over the tropical Atlantic Ocean 10 days prior to arrival. Air that is already anomalously dry and warm is transported from this region towards the western Amazon, highlighting the role of long-range transport for these extremes. The other 16 extreme events occurred under La Niña or neutral ENSO conditions and show similar transport patterns to the overall transport climatology until 3 days prior to arrival. During the last 3 days, however, air is preferentially transported over the southern Amazon, where already dry soil conditions cause a malfunctioning of moisture recycling, resulting in the propagation of extreme conditions to the western Amazon. Air arriving in the western Amazon during these events is travelling over areas heavily affected by deforestation. Therefore, we expect that landuse changes over the southern Amazon have a stronger impact on compound drought and heat extremes in the western Amazon under La Niña or neutral ENSO conditions than during El Niño events.