The link between Rossby Wave Breaking and Cut off Low Pressure Systems in the Southern Hemisphere Upper Troposphere

Cut-off low (COL) pressure systems are synoptic scale weather phenomena that are associated with high potential vorticity anomalies, which are in turn considered to be residues of Rossby wave breaking (RWB). In this study, a climatological link between COLs at 200hPa and RWB near the subtropical and mid-latitude tropopause of the Southern Hemisphere (SH) is established. The combined inter-seasonal and longitudinal variability of the two processes based on NCEP/NCAR reanalysis data from 1979 to 2006 will be presented. It is found that there are many more RWB events than there are COLs and that both phenomena possess strong seasonality. For every COL identified in the SH a RWB event within a specified grid box is sought. A vast majority of COLs are found to have RWB events associated with them, with the largest percentage of the COLs having associations with RWB events that occurred the day before the formation of the former. Composite fields of these two processes and zonal winds are also formed to examine their spatial relationship. It is found that RWB brings about the split jet that is associated with COL formation in the SH.