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1. ENSO-DRIVEN PRECIPITATION EVENTS RECORDED IN LAKE SEDIMENTS AND ALLUVIAL DEPOSITS IN THE SOUTHERN ECUADORIAN ANDES

   https://doi.org/10.1130/abs/2023AM-395659  

   Rodbell, Donald; Newcomer, Nikola; Mark, Samuel; Delgado, Grace; Caplow, Nancy (January 2023, Geologic Society of America)

   The El Niño Southern Oscillation (ENSO) is a major source of global interannual climate variability, however our ability to predict the response of ENSO to changes in the mean state of climate is limited in part by a paucity of long-term records of ENSO. The sediment record from Laguna Pallcacocha in El Cajas National Park, southern Ecuador (4060 masl; 2°46’S; 79°14’W) records El Niño floods spanning the Holocene (Rodbell et al., 1999; Moy et al., 2002; Mark et al., 2022). The sediment record is unusual for the nearly continuous dark- and light-colored laminations (0.1-2.0 cm thick) that comprise the Holocene section. Light laminae represent deposition during periods of increased precipitation, mobilization of unvegetated sediment above the lake, and increased stream discharge, all of which generate density-driven undercurrents. Conversely, dark laminae are deposited relatively slowly by sedimentation of organic matter. To date, no other lake has yielded such a high-resolution record of rainfall events, and here we review sediment cores from the western-most lakes in the region that are most likely to also be influenced by convective driven precipitation during coastal (Pacific) El Niño events. All lakes in this region contain multiple distal tephra 0.1-1.0 cm thick that enable precise correlation among records. Cores from Laguna Pampiada, Pampiada Bog, and Laguna Narigüiña are all located between 3500 and 4000 masl, all are located within steep catchments, and cores from these lakes reveal high-resolution records of clastic sediment delivery as recorded in bulk magnetic susceptibility. The stratigraphy of flood plain alluvium in the catchment of lakes studied provides an independent record of the geomorphic response to intense rainfall events. Buried soil A-horizons are clear indicators of major flooding events, and radiocarbon dates from charcoal in the uppermost sections of these A horizons can provide age estimates for large magnitude rain events and resultant floods that are comparable to the record from lake sediment cores. A 3-year record of atmospheric pressure and temperature from a data logger located at 4143 masl in El Cajas National Park provides the basis for comparing atmospheric conditions in the region of the studied lakes with those at sea level on the Ecuador coast, ~50 km to the west. 

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2. A Nonstationary Stochastic Rainfall Generator Conditioned on Global Climate Models for Design Flood Analyses in the Mississippi and Other Large River Basins

   https://doi.org/10.1029/2023WR036826  

   Liu, Yuan; Wright, Daniel B; Lorenz, David J (May 2024, Water Resources Research)

   Abstract Existing stochastic rainfall generators (SRGs) are typically limited to relatively small domains due to spatial stationarity assumptions, hindering their usefulness for flood studies in large basins. This study proposes StormLab, an SRG that simulates precipitation events at 6‐hr and 0.03° resolution in the Mississippi River Basin (MRB). The model focuses on winter and spring storms caused by water vapor transport from the Gulf of Mexico—the key flood‐generating storm type in the basin. The model generates anisotropic spatiotemporal noise fields that replicate local precipitation structures from observed data. The noise is transformed into precipitation through parametric distributions conditioned on large‐scale atmospheric fields from a climate model, reflecting spatial and temporal nonstationarity. StormLab can produce multiple realizations that reflect the uncertainty in fine‐scale precipitation arising from a specific large‐scale atmospheric environment. Model parameters were fitted monthly from December–May, based on storms identified from 1979 to 2021 ERA5 reanalysis data and Analysis of Record for Calibration (AORC) precipitation. StormLab then generated 1,000 synthetic years of precipitation events based on 10 CESM2 ensemble simulations. Empirical return levels of simulated annual maxima agree well with AORC data and show an overall increase in 1‐ to 500‐year events in the future period (2022–2050). To our knowledge, this is the first SRG simulating nonstationary, anisotropic high‐resolution precipitation over continental‐scale river basins, demonstrating the value of conditioning such stochastic models on large‐scale atmospheric variables. StormLab provides a wide range of extreme precipitation scenarios for design floods in the MRB and can be further extended to other large river basins. 

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3. Connecting Hydrometeorological Processes to Low‐Probability Floods in the Mountainous Colorado Front Range

   https://doi.org/10.1029/2021WR029768  

   Yu, Guo; Wright, Daniel B.; Holman, Kathleen D. (April 2021, Water Resources Research)

   Abstract Estimating the probabilities of rare floods in mountainous watersheds is challenging due to the hydrometeorological complexity of seasonally varying snowmelt and soil moisture dynamics, as well as spatiotemporal variability in extreme precipitation. Design storm methods and statistical flood frequency analyses often overlook these complexities and how they shape the probabilities of rare floods. This study presents a process‐based approach that combines gridded precipitation, stochastic storm transposition (SST), and physics‐based distributed rainfall‐runoff modeling to simulate flood peak and volume distributions up to the 10,000‐year recurrence interval and to provide insights into the hydrometeorological drivers of those events. The approach is applied to a small mountainous watershed in the Colorado Front Range in the United States. We show that storm transposition in the Front Range can be justified under existing definitions of regional precipitation homogeneity. The process‐based results show close agreement with a statistically based mixture distribution that considers underlying flood drivers. We further demonstrate that antecedent conditions and snowmelt drive frequent peak discharges and rarer flood volumes, while the upper tail of the flood peak distribution appears to be controlled by heavy rainfall and rain‐on‐snow. In particular, we highlight the important role of early fall extreme rainfall in controlling rare flood peaks (but not volumes), despite only one such event having been observed in recent decades. Notwithstanding issues related to the accuracy of gridded precipitation datasets, these findings highlight the potential of SST and process‐based modeling to help understand the relationships between flood drivers and flood frequencies. 

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4. Contrasting Ocean–Atmosphere Dynamics Mediate Flood Hazard across the Mississippi River Basin

   https://doi.org/10.1175/EI-D-22-0015.1  

   Muñoz, Samuel E.; Hamilton, Brynnydd; Parazin, B. (January 2023, Earth Interactions)

   Abstract The Mississippi River basin drains nearly one-half of the contiguous United States, and its rivers serve as economic corridors that facilitate trade and transportation. Flooding remains a perennial hazard on the major tributaries of the Mississippi River basin, and reducing the economic and humanitarian consequences of these events depends on improving their seasonal predictability. Here, we use climate reanalysis and river gauge data to document the evolution of floods on the Missouri and Ohio Rivers—the two largest tributaries of the Mississippi River—and how they are influenced by major modes of climate variability centered in the Pacific and Atlantic Oceans. We show that the largest floods on these tributaries are preceded by the advection and convergence of moisture from the Gulf of Mexico following distinct atmospheric mechanisms, where Missouri River floods are associated with heavy spring and summer precipitation events delivered by the Great Plains low-level jet, whereas Ohio River floods are associated with frontal precipitation events in winter when the North Atlantic subtropical high is anomalously strong. Further, we demonstrate that the El Niño–Southern Oscillation can serve as a precursor for floods on these rivers by mediating antecedent soil moisture, with Missouri River floods often preceded by a warm eastern tropical Pacific (El Niño) and Ohio River floods often preceded by a cool eastern tropical Pacific (La Niña) in the months leading up peak discharge. We also use recent floods in 2019 and 2021 to demonstrate how linking flood hazard to sea surface temperature anomalies holds potential to improve seasonal predictability of hydrologic extremes on these rivers. 

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5. Extreme Precipitation Events in Alaska: Historical Trends and Projected Changes

   https://doi.org/10.3390/atmos13030388  

   Bachand, Claire L.; Walsh, John E. (March 2022, Atmosphere)

   While recent increases in heavy precipitation events in some midlatitude regions are consistent with climate model simulations, evidence of such increases in high latitudes is more tenuous, partly because of data limitations. The present study evaluates historical and future changes in extreme precipitation events in Alaska. Using the ERA5 reanalysis, station data, and output from two downscaled global climate models, we examine precipitation-driven flood events at five diverse locations in Alaska where major historical floods provide benchmarks: Fairbanks (August 1967), Seward (October 1986), Allakaket/Bettles (August 1994), Kivalina (August 2012), and Haines (December 2020). We place these precipitation events into a framework of historical trends and end-of-century (2065–2100) model projections. In all but one of the flood events, the amount of rainfall was the highest on record for the event duration, and precipitation events of this magnitude are generally projected by the models to remain infrequent. All of the cases had subtropical or tropical moisture sources. None of the locations show statistically significant historical trends in the magnitude of extreme precipitation events. However, the frequencies of heavy precipitation events are projected to increase at most of the locations. The frequency of events with 2 year and 5 year historical return intervals is projected to become more frequent, especially in the Interior, and in some cases increase to several times per year. Decreases are projected only for Seward along Alaska’s southern coast. 

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