A midlatitude climatology and interannual variability of 200- and 500-hPa cut-off lows

A climatology of midlatitude 200- and 500-hPa cut-off low systems in the Northern and Southern Hemispheres is constructed from the NCEP–NCAR reanalysis by detecting and tracking, under one consistent method, all of the systems that persisted for more than 36 h for the 58 years 1960–2017. This method identifies a cut-off low as a cold-core geopotential height minimum that is isolated from the main westerlies and with a strong temperature gradient on its eastern flank. The obtained spatial and seasonal distributions show preferred regions of occurrence and that within these regions there is a level-dependent seasonality of cut-off lows. Whereas 200-hPa systems are more frequent in summer and autumn, 500-hPa systems are more evenly distributed throughout the seasons. Within each region and at each level, the annual number of cut-off lows has been increasing over time, trends that are consistent with documented signals of climate change such as a weakening and poleward shifting of the subtropical jets and an increase in blocking frequency. These trends explain as much as 64% of the variance in the annual number of cut-off lows. The contribution of the annular modes and the El Niño–Southern Oscillation to the interannual variability of the number of cut-off lows per season in each hemisphere is also investigated. Only the Northern Hemisphere annular mode has a statistically significant negative correlation throughout all seasons that explains 18–45% of the variance in the yearly number of Northern Hemisphere 500-hPa cut-off lows.