Revealing the contrasting roles of soil moisture−atmosphere feedbacks in modulating the abrupt transition from drought to pluvial conditions

There is a significant increase in the occurrence of drought and pluvial transitions globally during 1980−2020. We find that the transitive causal chains induced by land−atmosphere feedbacks play an important role in triggering the occurrence of drought and pluvial transitions.
Published in Earth & Environment
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It’s hard to tune into the news these days without seeing multiple stories about extreme heat and drought in one place, followed by stories about destructive rain events and damaging flooding in another. These seemingly disparate events have a connection:  In many cases, it is these destructive rainstorms (pluvial events) that end long-term droughts.

For example, in 2011, the middle to low reaches of China’s Yangtze River was hit by an abrupt rain event that transitioned it out of drought. And in 2015, the growing season across the south-central United States faced an unusual sequence of events that flipped parts of the region from extreme drought conditions to pluvial downpours during spring. More recently, in January through March 2023, California experienced record flooding while back in December 2022 it was still in the midst of the worst drought in a millennium.

Such wild swings between water extremes–as extended periods of scorching heat and drought are broken by massive pluvial events—have a substantial influence on water resources and agricultural productivity. What’s more, our research has shown that these swings have become more prevalent in the last 40 years with the warming climate.

Certainly, factors such as atmospheric internal variability, ocean warming, ENSO, polar ocean ice loss, and anthropogenic influences all play a role. But given that destructive rain and punishing droughts and heatwaves all occur on land, questions remain as to what role the land itself plays in terms of soil moisture. Is it a passive response where the land is simply soaked by precipitation and then dried by evaporation, or is the land an active participant that contributes to the abrupt switch from droughts to deluges?

Understanding the land’s influence will help scientists make better predictions that can forecast the abrupt transition from drought to pluvial events on a global scale. These predictions are key to preventing and mitigating such natural disasters, and for providing meaningful information to increase society’s resilience to the sudden swing from drought to pluvial conditions.

Previous research mainly focuses on individual drought and pluvial events in terms of spatiotemporal characteristics in intensity and frequency, as well as the potential impacts of these events on the ecological environment. However, there remains a lack of understanding of the dynamics and underlying mechanisms behind the abrupt shift from drought to pluvial conditions, particularly from the perspective of land−atmosphere interactions and feedbacks. Therefore, in our study, we attempt to reveal the global pattern of drought and pluvial transitions and their spatiotemporal variations during the period of 1980−2020. We do this by using the observationally constrained soil moisture and precipitation data obtained from three reanalysis products. We also further explore the mechanisms causing the drought and pluvial transition occurrence from the perspective of soil moisture−atmosphere feedbacks.

We found that the drought and pluvial transitions became more frequent over eastern North America, Europe, East Asia, Southeast Asia, southern Australia, southern Africa, and southern South America. There was a significant increase in the frequency (0.24−1.03% year−1) of drought and pluvial transitions globally during our study period of 1980−2020. This suggests that recent decades have faced an increasing risk in abrupt shifts from drought to pluvial conditions.

Figure 1: Spatiotemporal pattern of the total number of drought and pluvial transitions during the period of 1980–2020. ad Spatial pattern of the total number of drought and pluvial transitions for each pixel during the period of 1980–2020 for different datasets. e Annual time series (solid lines) of globally averaged drought and pluvial transitions. f Box plots of the total number of drought and pluvial transitions for each pixel during the periods of 1981–2000 and 2001–2020.

This study uses the causal inference approach to detect the transitive causal chain among soil moisture, latent heat flux, moisture convergence, and precipitation to explain the drought and pluvial transitions. We found that the transitive causal chains play different roles in triggering the occurrence of drought and pluvial transitions in humid and arid regions. Specifically, the soil moisture−latent heat flux−precipitation causal chain is likely to trigger the rainfall following the dryness in humid regions where enhanced evaporation increases the actual atmospheric moisture favouring the pluvial occurrence. This chain is triggered by soil and vegetation drying out, which causes the air to become even hotter, which in turn enhances the evaporative demand, and thus, potentially enhances the surface latent heat flux, increasing atmospheric moisture content. Moreover, atmospheric water-holding capacity enhanced by rising temperature due to more partitioning of energy towards sensible heat allows air to hold more moisture under global warming. Thus, the increasing atmospheric moisture content along with enhanced atmospheric water-holding capacity potentially leads to the occurrence of rainfall, even heavy rainfall.

By contrast, although soil moisture limits evaporation and reduces moisture recycling for precipitation in arid regions, the decline in soil moisture can enhance moisture convergence which supplies water vapor for post-drought rainfall. Thus, the soil moisture−moisture convergence−precipitation causal chain enhances the post-drought rainfall in arid regions due to atmospheric circulation dynamics.

 

Figure 2: Proposed mechanism for explaining the shift from drought to pluvial conditions from the perspective of soil moisture−atmosphere feedbacks. a Soil moisture−latent heat flux−precipitation transitive chain for the drought and pluvial transitions. b Soil moisture−moisture convergence−precipitation transitive chain for the drought and pluvial transitions.

Our study identifies the critical regions of drought and pluvial transitions and reveals the underlying mechanisms triggering the abrupt transition from drought to pluvial conditions. Our findings provide meaningful information for policymakers and stakeholders to realize global hotspots that are expected to experience more drought and pluvial transitions in a changing climate. More importantly, revealing the contribution of soil moisture−atmosphere feedbacks to the drought and pluvial transitions enables us to pay more attention to the regions with strong soil moisture−atmosphere feedbacks, which will not only support forecasting efforts but also facilitate hazard preparedness and mitigation planning.

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