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Creators/Authors contains: "Michaelis, Allison C."

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  1. Abstract

    Tropical cyclones (TCs) undergoing extratropical transition (ET) can develop into intense cyclonic systems accompanied by high-impact weather in areas far removed from the original TC. This study presents an analysis of multiseasonal global simulations representative of present-day and projected future climates using the Model for Prediction Across Scales–Atmosphere (MPAS-A), with high resolution (15-km grid) throughout the Northern Hemisphere. TCs are tracked as minima in sea level pressure (SLP) accompanied by a warm core, and TC tracks are extended into the extratropical phase based on local minima in SLP and use of a cyclone phase space method. The present-day simulations adequately represent observed ET characteristics such as frequency, location, and seasonal cycles throughout the Northern Hemisphere. The most significant changes in future ET events occur in the North Atlantic (NATL) basin. Here, a more favorable background environment, a shift toward stronger TC warm cores in the lower troposphere, and a significant poleward shift in TC location lead to a ~40% increase in the number of NATL ET events and a ~6% increase in the fraction of TCs undergoing ET. This equates to approximately 1–2 additional ET events per year in this region. In the future simulations, ET in the NATLmore »occurs markedly farther north by ~4°–5°N, and the resultant extratropical cyclones are stronger by ~6 hPa. These changes hold potentially important implications for areas directly affected by ET events, such as eastern North America, as well as for regions indirectly impacted by downstream effects, including western Europe.

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  2. Abstract

    Successive atmospheric river (AR) events—known as AR families—can result in prolonged and elevated hydrological impacts relative to single ARs due to the lack of recovery time between periods of precipitation. Despite the outsized societal impacts that often stem from AR families, the large-scale environments and mechanisms associated with these compound events remain poorly understood. In this work, a new reanalysis-based 39-yr catalog of 248 AR family events affecting California between 1981 and 2019 is introduced. Nearly all (94%) of the interannual variability in AR frequency is driven by AR family versus single events. Usingk-means clustering on the 500-hPa geopotential height field, six distinct clusters of large-scale patterns associated with AR families are identified. Two clusters are of particular interest due to their strong relationship with phases of El Niño–Southern Oscillation (ENSO). One of these clusters is characterized by a strong ridge in the Bering Sea and Rossby wave propagation, most frequently occurs during La Niña and neutral ENSO years, and is associated with the highest cluster-average precipitation across California. The other cluster, characterized by a zonal elongation of lower geopotential heights across the Pacific basin and an extended North Pacific jet, most frequently occurs during El Niño years andmore »is associated with lower cluster-average precipitation across California but with a longer duration. In contrast, single AR events do not show obvious clustering of spatial patterns. This difference suggests that the potential predictability of AR families may be enhanced relative to single AR events, especially on subseasonal to seasonal time scales.

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