Decay of the Northern Hemisphere stratospheric polar vortex and the occurrence of cut-off low systems: An exploratory study

The authors perform an exploratory analysis on the effect of the timing of the stratospheric vortex breakup in the occurrence of cut-off low systems (COLs) in the Northern Hemisphere. The first multidecadal Northern Hemisphere COLs database (Nieto et al, 2005) covering a 41 year-long period (1958-1998) was used in the analysis. The dates of stratospheric vortex breakup were obtained using two different approaches recently purposed in literature based in potential vorticity and zonal winds. An analysis of differences of COLs occurrences for the five earlier (later) breakup years showed that, at latitudes lower than 45 degrees CeN, COLs are more frequent for earlier vortex years during the following spring and summer. The monthly analysis showed that, in general, the significant differences start in May lasting until September, being especially relevant for the European sector, the area with the highest rates of COLs occurrence in the Northern Hemisphere.