across the Andes all year. As a consequence of using vorticity for the tracking method a longer lifetime of COLs is detected than in other studies, but this does not affect the total frequency of occurrence. Comparisons with other studies suggest that the differences in seasonality are due to uncertainties in the reanalyses and the methods used to identify COLs.

Cut-off Low (COLs) are often associated with heavy precipitation and strong wind events, but there are still uncertainties on how their identification affect the numbers and seasonality. This paper aims to determine the sensitivity of identifying Southern Hemisphere COLs in the ERA-Interim reanalysis to different types of identification criteria. Upper-level cyclones are initially tracked on the 300 hPa level using relative vorticity and geopotential in order to present different perspectives. This reveals significant differences between the numbers and length of the identified tracks for the two fields. To identify the COLs different post-tracking filters are applied which are divided into two steps. Firstly, three filters are considered to separate cut-off lows from open troughs by sampling winds at different offset radial distances from the upper-level cyclonic centres. Secondly, potential vorticity and temperature criteria are imposed to determine how these conditions affect the identified COLs in terms of numbers, seasonality and intensity. It was found that methods based on multiple criteria restrict the COL identification by imposing specific characteristics, while methods based on simpler schemes (e.g. using only winds) can detect larger samples of COLs observed visually in the geopotential maps. Although it is difficult to say which method is more accurate in identifying COLs, because of the subjective aspect of observer’s assessment, a scheme using only winds should be more representative of reality as this simply imposes on the detection system a cyclonic circulation appearance regardless of the physical and dynamical characteristics. Therefore, this type of method could be considered as a standard method for identifying COLs that can be used for either operational or research purposes.

This study examines the three‐dimensional structure and evolution of the 200 most intense Cut‐off Lows (COLs) in the Southern Hemisphere (SH). This is done using feature tracking and cyclone‐centred compositing based on the ERA‐Interim reanalysis. Composites confirm the existence of a well‐defined tropospheric moist cold core co‐located with warm dry air in the lower stratosphere. Such cores are surrounded by regions of strong temperature gradients (frontal zones) which move downstream throughout the life cycle. The stratospheric air intrusion into the troposphere is identified in vertical cross‐sections of potential vorticity and ozone, a process referred to as tropopause folding. Precipitation occurs ahead of the COLs because of the low (high)‐level convergence (divergence) and strong upward motion. The maximum precipitation is observed during decay, indicating a possible link between COLs and surface cyclones. Composites conditioned on relative vorticity and precipitable water suggest these variables may be related to precipitation. The COLs exhibit a westward tilt during their early stages but they change to a barotropic state in the mature stage. Finally, the main characteristics of the COLs are summarised by categories which discriminate different intensities, indicating there are differences in the structure of COLs with consequences for precipitation. These efforts aim to provide new insights into the development of COLs in the SH which could aid in identifying and forecasting their various types and associated precipitation patterns.

Cut‐Off Lows (COLs) in the Southern Hemisphere and the mechanisms involved in their development are investigated in detail using the eddy kinetic energy (EKE) budget applied to data from the ERA‐Interim reanalysis. This approach considers the most important processes that are typical for the evolution of midlatitude disturbances such as the baroclinic (BRC) and barotropic (BRT) conversions, and the ageostrophic flux convergence (AFC), known as downstream development. Composites of the volume‐integrated EKE and its components are evaluated based on the 200 most intense SH COLs (>98th percentile) observed in a 36‐year period. Results show that the AFC together with the BRC conversion are the most important contributors to the EKE growth for the COLs, characterizing the downstream baroclinic development. The AFC plays an important role in genesis and intensification phases of the COLs, while the BRC conversion is important for the system maintenance. The dissipation of the COLs occurs due to dispersive fluxes (ageostrophic flux divergence) together with other processes not directly computed in the EKE equation, such as friction and latent heat release which are problematic in reanalysis datasets. The BRT conversion contributes negatively to the COL development by transferring EKE to the zonal flow kinetic energy, though this is not enough to dampen the intensification. Regional differences were found in the energetics, indicating that COLs originating upstream of the continents are clearly dominated by ageostrophic fluxes, while the systems over the Australian region are mostly driven by baroclinic processes.

Four recent reanalysis products ERA-Interim, NCEP-CFSR, MERRA-2 and JRA-55 are evaluated and compared to an older reanalysis JRA-25, to quantify their confidence in representing Cutoff lows (COLs) in the Southern Hemisphere. The climatology of COLs based on the minima of 300-hPa vorticity (300) and 300-hPa geopotential (Z 300) provides different perspectives of COLs and contributes to the understanding of the discrepancies observed in the literature regarding their numbers and seasonality. The COLs compare better among the newest reanalyses than compared to the older reanalysis JRA-25. The difference in number between the latest reanalyses are generally small for both 300 and, with more COLs identified in 300 than in Z 300 for all reanalyses. The spatial differences observed between the newest reanalyses are mainly due to differences in the track lengths, which is larger in ERA-Interim and JRA-55 than in NCEP-CFSR and MERRA-2, resulting in disparities in the track density. This is likely due to the difference in the assimilation data system used in each reanalysis product. The largest differences in intensities occur in the 300 , because this field is very sensitive to the reanalysis resolution. The mean separation distance of the COLs that match between the latest reanalyses are generally small, while the older JRA-25 has a broader distribution and larger number of matches with relatively large distances, indicating larger uncertainties in location of COLs. The results show significant improvements for the most recent reanalyses compared to the older JRA-25 reanalysis, indicating a progress in representing the COL properties.