A climatology of cut-off lows at 200 hPa in the Northern Hemisphere, 1990–1994

A statistical study of the size, temporal and spatial distribution of cut-off low (COL) systems on the 200 hPa pressure surface has been conducted for the Northern Hemisphere. The 200 hPa surface is of relevance for stratosphere–troposphere exchange processes. The study covered a 5-year period and recorded information (duration, size, position, movement) for all COLs that lasted for 2 days or more. Cut-off lows form more often in summer than winter and in preferred geographical regions. The majority of COLs lasted 2–3 days and very few lasted more than 10 days. Cut-off lows were found over a wide range of sizes, but the majority were between 200 and 1200 km (max diameter in latitude). Approximately 50% of the COLs that lasted more than 3 days moved considerable (>600 km) distances. Although their motion was, as a rule, rather irregular, it was seen that COL systems would characteristically move northwards, or eastwards–northwards as they began to decay. Finally, there was some degree of interannual variability in COL distribution.