The link between cut‐off lows and Rossby wave breaking in the Southern Hemisphere

Abstract The climatological link between cut‐off low (COL) pressure systems that occur from 20 to 50°S and Rossby wave breaking (RWB) in the Southern Hemisphere (SH) is examined for 1979–2008. It is shown that COLs are associated with either RWB events (89%) or with potential vorticity (PV) intrusions where there is north–south advection of high‐PV air (11%). In the vast majority of COLs, the RWB events occur upstream, on or before the day of the COL formation. The evolution of the PV, geopotential heights, static stability, absolute vorticity and temperature advection fields during the COLs are consistent with the formation of high‐PV anomalies that induce cyclonic circulations as predicted by theory. RWB plays a key role in producing the split flow associated with COLs in the SH, which in turn produces absolute vorticity anomalies by shear‐curvature vorticity conversion, and creates static stability anomalies. The COLs associated with RWB at 330 K are deeper and more persistent than those associated with 350 K RWB and surface processes differ depending on the isentropic surface on which the associated RWB occurs. The seasonality of the RWB and COLs are similar, and is linked to the seasonal march of the westerly jets. Copyright © 2010 Royal Meteorological Society