Relationships between cut-off lows and the semiannual and southern oscillations

The characteristics of Southern Cut-off Lows (CoLs) are studied for the period 1979-2008. The systematic identification of CoLs is realized by applying an original automated scheme using mean daily geopotential height and air temperature at 500 hPa NCEP-DOE II Reanalysis data. From closed lows' trajectories established from the Equator to the polar jet stream, extratropical lows are analyzed and the stage of cut-off is defined as a secluded low presenting a cold core. From 4,843 cases the general CoL features are presented and confirm several previous results such as the geographic distribution which shows that they are more frequent in the latitudinal band contained between 20 degrees S and 45 degrees S and in close proximity to the continents. On a seasonal time scale, CoLs are more numerous from late summer to autumn, with a maximum of frequency in March/April. In winter (June-July-August), they are fewer but deeper than during the rest of the year. In the median domain (similar to 32.5 degrees S), the annual cycle of the frequency is bimodal and present two peaks during transitional seasons. In this zone, the seasonal cycle varies in accordance with the Semiannual Oscillation. Thereby, when the meridional gradient of temperature/pressure is reinforced between mid and high latitudes, CoLs are more frequent in the median domain. Over the period 1979-2008, the annual CoLs' frequency exhibits a positive trend of about 25%. This increase is associated with a widening of the latitudinal domain of occurrence equatorward as well as poleward. The trend is linked with an abrupt positive shift in the number of CoLs' generation between 1998 and 1999. The geographical distribution of CoLs frequency varies significantly in accordance with El Nino Southern Oscillation with more CoL's at lower (higher) latitudes during La Nina (El Nino) events, principally in the Southern Pacific.