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1. Dryland hydroclimatic response to large tropical volcanic eruptions during the last millennium

   https://doi.org/10.1038/s41612-024-00636-y  

   Zhou, Shangrong; Liu, Fei; Dai, Aiguo; Zhao, Tianbao (December 2024, npj Climate and Atmospheric Science)

   Abstract Drylands are highly vulnerable to climate change due to their fragile ecosystems and limited ability to adapt. In contrast to the global drying after tropical volcanic eruptions shown previously, we demonstrate that large tropical volcanic eruptions can induce significant two-year hydroclimatic wetting over drylands by employing the last millennium simulations. During this wetting period, which extends from the first to the third boreal winter after the eruption, several hydroclimatic indicators, such as self-calibrating Palmer Drought Severity Index based on the Penman-Monteith equation for potential evapotranspiration (scPDSIpm), standard precipitation evapotranspiration index (SPEI), aridity index (AI), top-10cm soil moisture (SM_10cm), and leaf area index (LAI), show significant positive anomalies over most drylands. The primary contribution to the wetting response is the potential evapotranspiration (PET) reduction resulting from dryland surface cooling and reduced solar radiation, as well as a weak contribution from increased precipitation. The latter is due to the wind convergence into drylands caused by slower tropical cooling compared to drylands. The wetting response of drylands to volcanic eruptions also demonstrates some benefits over the global hydrological slowdown resulting from stratospheric aerosol injection, which replicates the cooling effects of volcanic eruptions to address global warming. 

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2. Gross discrepancies between observed and simulated 20th-21st-century precipitation trends in Southeastern South America

   https://doi.org/10.1175/JCLI-D-20-0746.1  

   Varuolo-Clarke, Arianna M.; Smerdon, Jason E.; Williams, A. Park; Seager, Richard (May 2021, Journal of Climate)

   null (Ed.)

   Abstract Southeastern South America (SESA; encompassing Paraguay, Southern Brazil, Uruguay, and northern Argentina) experienced a 27% increase in austral summer precipitation from 1902-2019, one of the largest observed trends in seasonal precipitation globally. Previous research identifies Atlantic Multidecadal Variability and anthropogenic forcing from stratospheric ozone depletion and greenhouse gas emissions as key factors contributing to the positive precipitation trends in SESA. We analyze multi-model ensemble simulations from Phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP) and find that not only do Earth System Models simulate positive SESA precipitation trends that are much weaker over the historical interval, but some models persistently simulate negative SESA precipitation trends under historical forcings. Similarly, 16-member ensembles from two atmospheric models forced with observed historical sea surface temperatures never simulate precipitation trends that even reach the lower bound of the observed trend’s range of uncertainty. Moreover, while future 21 st -century projections from CMIP6 yield positive ensemble mean precipitation trends over SESA that grow with increasing greenhouse-gas emissions, the mean forced response never exceeds the observed historical trend. Pre-industrial control runs from CMIP6 indicate that some models do occasionally simulate centennial-scale trends in SESA that fall within the observational range, but most models do not. Results point to significant uncertainties in the attribution of anthropogenically forced influences on the observed increases in precipitation over SESA, while also suggesting that internal decadal-to-centennial variability of unknown origin and not present in state-of-the-art models may have also played a large role in generating the 20 th -21 st -century SESA precipitation trend. 

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3. Hydroclimatic trends during 1950–2018 over global land

   https://doi.org/10.1007/s00382-021-05684-1  

   Dai, Aiguo (February 2021, Climate Dynamics)

   null (Ed.)

   Global hydroclimatic changes from 1950 to 2018 are analyzed using updated data of land precipitation, streamfow, and an improved form of the Palmer Drought Severity Index. The historical changes are then compared with climate model-simulated response to external forcing to determine how much of the recent change is forced response. It is found that precipitation has increased from 1950 to 2018 over mid-high latitude Eurasia, most North America, Southeast South America, and Northwest Australia, while it has decreased over most Africa, eastern Australia, the Mediterranean region, the Middle East, and parts of East Asia, central South America, and the Pacifc coasts of Canada. Streamfow records largely confrm these precipitation changes. The wetting trend over Northwest Australia and Southeast South America is most pronounced in austral summer while the drying over Africa and wetting trend over mid-high latitude Eurasia are seen in all seasons. Coupled with the drying caused by rising surface temperatures, these precipitation changes have greatly increased the risk of drought over Africa, southern Europe, East Asia, eastern Australia, Northwest Canada, and southern Brazil. Global land precipitation and continental freshwater discharge show large interannual and inter-decadal variations, with negative anomalies during El Niño and following major volcanic eruptions in 1963, 1982, and 1991; whereas their decadal variations are correlated with the Interdecadal Pacifc Oscillation (IPO) with IPO’s warm phase associated with low land precipitation and continental discharge. The IPO and Atlantic multidecadal variability also dominate multidecadal variations in land aridity, accounting for 90% of the multidecadal variance. CMIP5 multi-model ensemble mean shows decreased precipitation and runoff and increased risk of drought during 1950–2018 over Southwest North America, Central America, northern and central South America (including the Amazon), southern and West Africa, the Mediterranean region, and Southeast Asia; while the northern mid-high latitudes, Southeast South America, and Northwest Australia see increased precipitation and runoff. The consistent spatial patterns between the observed changes and the model-simulated response suggest that many of the observed drying and wetting trends since 1950 may have resulted at least partly from historical external forcing. However, the drying over Southeast Asia and wetting over Northwest Australia are absent in the 21st century projections. 

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4. Paleoclimate Constraints on the Spatiotemporal Character of Past and Future Droughts

   https://doi.org/10.1175/JCLI-D-20-0004.1  

   Coats, S.; Smerdon, J. E.; Stevenson, S.; Fasullo, J. T.; Otto-Bliesner, B.; Ault, T. R. (November 2020, Journal of Climate)

   null (Ed.)

   Abstract Machine-learning-based methods that identify drought in three-dimensional space–time are applied to climate model simulations and tree-ring-based reconstructions of hydroclimate over the Northern Hemisphere extratropics for the past 1000 years, as well as twenty-first-century projections. Analyzing reconstructed and simulated drought in this context provides a paleoclimate constraint on the spatiotemporal characteristics of simulated droughts. Climate models project that there will be large increases in the persistence and severity of droughts over the coming century, but with little change in their spatial extent. Nevertheless, climate models exhibit biases in the spatiotemporal characteristics of persistent and severe droughts over parts of the Northern Hemisphere. We use the paleoclimate record and results from a linear inverse modeling-based framework to conclude that climate models underestimate the range of potential future hydroclimate states. Complicating this picture, however, are divergent changes in the characteristics of persistent and severe droughts when quantified using different hydroclimate metrics. Collectively our results imply that these divergent responses and the aforementioned biases must be better understood if we are to increase confidence in future hydroclimate projections. Importantly, the novel framework presented herein can be applied to other climate features to robustly describe their spatiotemporal characteristics and provide constraints on future changes to those characteristics. 

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5. Coherent Mechanistic Patterns of Tropical Land Hydroclimate Changes

   https://doi.org/10.1029/2022GL102285  

   Duan, Suqin_Q; Findell, Kirsten_L; Fueglistaler, Stephan_A (April 2023, Geophysical Research Letters)

   Abstract Changes in tropical (30 S–30 N) land hydroclimate following CO₂‐induced global warming are organized according to climatological aridity index (AI) and daily soil moisture (SM) percentiles. The transform from geographical space to this novel process‐oriented phase space allows for interpretation of local, daily mechanistic relationships between key hydroclimatic variables in the context of time‐mean and/or global‐mean energetic constraints and the wet‐get‐wetter/dry‐get‐drier paradigm. Results from 16 CMIP models show coherent patterns of change in the AI/SM phase space that are aligned with the established soil‐moisture/evapotranspiration regimes. We introduce an active‐rain regime as a special case of the energy‐limited regime. Rainfall shifts toward larger rain totals in this active‐rain regime, with less rain on other days, resulting in an overall SM reduction. Consequently, the regimes where SM constrains evapotranspiration become more frequently occupied, and corresponding hydroclimatic changes align with the position of the critical SM value in the AI/SM phase space. 

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