Structure and evolution of intense austral cut‐off lows

Abstract

AbstractThis 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.