Post-tropical cyclones (PTCs) extend many hazards associated with tropical cyclones (TCs) to the midlatitudes. Despite recent high-impact cases affecting Europe such as Ophelia, little research has been done to characterize the risk of PTCs. Here we compare the climatologies and intensity distributions of midlatitude cyclones (MLCs) and PTCs in the North Atlantic and Europe by tracking cyclones in the ERA5 reanalysis. Considering hurricane season cyclones impacting Northern Europe, PTCs show a significantly higher mean maximum intensity than MLCs, but make only a small contribution to total windstorm risk. Our results show that a disproportionately large fraction of high-intensity cyclones impacting Europe during hurricane season are PTCs. The fraction of PTCs impacting N Europe with storm force (>25 m s −1) winds is~10 times higher than that for MLCs. Less than 1% of cyclones impacting Northern Europe are identified to be PTCs. This rises to 8.8% when considering cyclones which impact with storm force winds. Plain Language Summary Ex-hurricanes (post-tropical cyclones; PTCs) can bring hazardous weather such as damaging winds and extreme precipitation to the midlatitudes. The importance of these cyclones for European wind and flood risk is still an open question. By tracking cyclones through 39 years of data, we show that on average, the maximum intensity of PTCs over Europe is significantly higher than that for European windstorms (midlatitude cyclones, MLCs). The difference between the maximum intensity of PTCs and MLCs is larger across Northern Europe than Southern Europe. Our results show that a disproportionately large fraction of high-intensity cyclones impacting Northern Europe during hurricane season are PTCs. The fraction of PTCs impacting Northern Europe with storm force winds is~10 times greater than for MLCs. Less than 1% of cyclones impacting Northern Europe during the North Atlantic hurricane season are PTCs. This rises to 8.8% when only considering cyclones which impact Northern Europe with storm force winds.

Recurving tropical cyclones (TCs) can cause extensive damage along the US East Coast and later in their life cycle over Europe as post-tropical cyclones. While the existing literature attempts to understand the drivers of basin-wide and regional TC variability, less work has been undertaken looking at recurving TCs. The roles played by the interannual variabilities of TC frequency and the steering flow in governing recurving TC interannual variability are investigated in this study. Using a track-matching algorithm, we identify observed TC tracks from the HURricane DATabase version 2 (HURDAT2) in the ERA5 and MERRA2 reanalyses. This allows for detailed analysis of the post-tropical stages of the tracks in the observational TC record, enabling robust identification and separation of TCs that recurve. We show that over 75% of the interannual variance in annual recurving TC frequency can be explained by just two predictors – the frequency of TCs forming in the subtropical Atlantic, and hurricanes (TCs with wind speeds > 33ms−1) forming in the Main Development Region. An index describing the seasonal mean meridional steering flow shows a weak, non-significant relationship with recurving TC frequency, supported by composite analysis. These results show that the interannual variability in recurving TC frequency is primarily driven by the seasonal TC activity of the MDR and the subtropical Atlantic, with seasonal anomalies in the steering flow playing a much smaller, secondary role. These results help to quantify the extent to which skillful seasonal forecasts of Atlantic hurricane activity benefit regions vulnerable to recurving TCs.

Post-tropical cyclones (PTCs) can bring high winds and extreme precipitation to Europe. Although the structure and intensity of observed Europe-impacting PTCs has been investigated in previous studies, a quantitative understanding of the factors important for PTCs to reach Europe has not been established. By tracking and identifying the full life cycle of tropical cyclones (TCs) in the ERA5 reanalysis, we investigate why some PTCs impact Europe and why others do not, using a composite analysis. We show that PTCs that impact Europe are typically ∼4–6 m s−1 stronger at their lifetime maximum intensity and throughout the extratropical transition process. They are also twice as likely to reintensify in the midlatitudes. During ET, the Europe-impacting PTCs interact more strongly with an upstream upper-level trough in a significantly more baroclinic environment. The Europe-impacting PTCs are steered on a more poleward trajectory across a midlatitude jet streak. It is during the crossing of the jet that these cyclones often undergo their reintensification. Using contingency table analysis, TC lifetime maximum intensity, and whether post-ET reintensification occurs are shown to be significantly associated with the odds that a PTC reaches Europe. This supports our composite analysis and further indicates that TC intensity and reintensification both modulate the likelihood that a PTC will impact Europe. Significance Statement Some post-tropical cyclones (PTCs) reach Europe, often associated with extreme precipitation and high winds. It is currently unclear what factors allow this to occur. In this study, we track cyclones in two reanalyses using a feature tracking scheme and identify the PTCs by matching (in space and time) reanalysis tracks with observed tracks. Using a composite analysis, we show that 1) tropical cyclones (TCs) that are more intense, and 2) TCs that reintensify after extratropical transition, are more likely to reach Europe. TCs that reintensify interact strongly with an upper-level upstream trough and cross a midlatitude jet streak. Reintensification occurs as the cyclones cross this jet streak.

Abstract. Post-tropical cyclones (PTCs) can cause extensive damage across Europe through extreme winds and heavy precipitation. With increasing sea surface temperatures, tropical cyclones (TCs) may form and travel further poleward and eastward than observed historically. Recent work has suggested that the frequency of intense Europe-impacting PTCs may increase substantially in the future. Using an objective feature-tracking scheme and TC identification method, we track and identify the full life cycle of TCs in the North Atlantic in five CMIP6 climate models in the historical (1984–2014) period and in the future under the SSP5-85 scenario (2069–2099). These five models are selected based on their ability to simulate TC frequency similar to observed in the North Atlantic, although model deficiencies remain. We find no robust changes in Europe-impacting PTC frequency or intensity in the future. This is because two competing factors – a significant decrease in TC frequency of 30 %–60 % and an increase in the proportion of TCs reaching Europe – are approximately the same size. The projected increase in the proportion of TCs reaching Europe is largely driven by an increase in the likelihood of recurvature and is consistent with projected decreases in vertical wind shear and increases in potential intensity along the US East Coast in the future. The projected increased likelihood of recurvature is also associated with a shift in TC genesis away from the main development region, where model biases cause very few TCs to recurve. This study indicates that large uncertainties surround future Europe-impacting PTCs and provides a framework for evaluating PTCs in future generations of climate models.

<p>Post-tropical cyclones (PTCs) are often associated with high winds and extreme precipitation over Europe. For example, ex-hurricanes Debbie (1961) and Ophelia (2017) were both responsible for national wind speed records in Ireland, and further east across Europe, ex-hurricane Debby (1982) caused significant wind damage over Finland. In previous work, we show that despite comprising only 1% of European impacting cyclones during hurricane season, almost 10% of those cyclones with storm force (>25ms<sup>-1</sup>) are PTCs, indicating that PTCs are disproportionately responsible for European windstorm risk.</p><p>By tracking and identifying observed TCs in two reanalyses, we explore the physical drivers for recurving TCs impacting Europe. Our methods of cyclone tracking and TC identification allow for a detailed analysis of the post-tropical stage of the TCs in the observational record, allowing us to separate the recurving TCs based on whether they impact Europe.</p><p>Using a composite analysis, we show that recurving TCs which impact Europe are significantly stronger at their lifetime maximum intensity, and for several days during and after extratropical transition. They are also 65% more likely to reintensify in the midlatitudes after completing extratropical transition. The Europe impacting recurving TCs interact more favourably with an upstream upper-level trough, which steers the TCs on a more poleward trajectory across a midlatitude jet streak. It is during the jet streak interaction that extratropical reintensification often occurs.</p><p>We show that TC lifetime maximum intensity and whether extratropical reintensification occurs both modulate the likelihood that a recurving TC will impact Europe as a PTC. Our results highlight the challenges of projecting PTC impacts over Europe in a future climate. Some climate model projections indicate a poleward shift in the jet, possibly indicating less opportunity for recurving TCs to interact with the jet and reintensify. However, sea surface temperatures are projected to warm, and lifetime maximum intensity may therefore increase. If the change in TC intensity outweighs any poleward shift in the jet, then a larger proportion of recurving TCs could reach Europe in the future.</p>