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  1. Streamflow often increases after fire, but the persistence of this effect and its importance to present and future regional water resources are unclear. This paper addresses these knowledge gaps for the western United States (WUS), where annual forest fire area increased by more than 1,100% during 1984 to 2020. Among 72 forested basins across the WUS that burned between 1984 and 2019, the multibasin mean streamflow was significantly elevated by 0.19 SDs ( P < 0.01) for an average of 6 water years postfire, compared to the range of results expected from climate alone. Significance is assessed by comparing prefire and postfire streamflow responses to climate and also to streamflow among 107 control basins that experienced little to no wildfire during the study period. The streamflow response scales with fire extent: among the 29 basins where >20% of forest area burned in a year, streamflow over the first 6 water years postfire increased by a multibasin average of 0.38 SDs, or 30%. Postfire streamflow increases were significant in all four seasons. Historical fire–climate relationships combined with climate model projections suggest that 2021 to 2050 will see repeated years when climate is more fire-conducive than in 2020, the year currently holdingmore »the modern record for WUS forest area burned. These findings center on relatively small, minimally managed basins, but our results suggest that burned areas will grow enough over the next 3 decades to enhance streamflow at regional scales. Wildfire is an emerging driver of runoff change that will increasingly alter climate impacts on water supplies and runoff-related risks.« less
  2. Free, publicly-accessible full text available November 1, 2023
  3. 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 mostmore »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.« less
  4. Abstract This work examines the effect of horizontal resolution and topography on the North American monsoon (NAM) in experiments with an atmospheric general circulation model. Observations are used to evaluate the fidelity of the representation of the monsoon in simulations from the Community Atmosphere Model version 5 (CAM5) with a standard 1.0° grid spacing and a high-resolution 0.25° grid spacing. The simulated monsoon has some realistic features, but both configurations also show precipitation biases. The default 1.0° grid spacing configuration simulates a monsoon with an annual cycle and intensity of precipitation within the observational range, but the monsoon begins and ends too gradually and does not reach far enough north. This study shows that the improved representation of topography in the high-resolution (0.25° grid spacing) configuration improves the regional circulation and therefore some aspects of the simulated monsoon compared to the 1.0° counterpart. At higher resolution, CAM5 simulates a stronger low pressure center over the American Southwest, with more realistic low-level wind flow than in the 1.0° configuration. As a result, the monsoon precipitation increases as does the amplitude of the annual cycle of precipitation. A moisture analysis sheds light on the monsoon dynamics, indicating that changes in the advectionmore »of enthalpy and moist static energy drive the differences between monsoon precipitation in CAM5 1.0° compared to the 0.25° configuration. Additional simulations confirm that these improvements are mainly due to the topographic influence on the low-level flow through the Gulf of California, and not only the increase in horizontal resolution.« less