Interannual variability in the number of Northern Hemisphere Cut-off low systems.

Cut-off low-pressure systems-COLS- are usually closed circulations at middle and upper troposphere developed from a deep trough in the westerlies. The importance of their study is due to both the convective severe events that can occur if they are over warm ocean and because they are important mechanisms of Stratosphere-troposphere exchange- STE-. However few is known about their interannual variability, due to the limited duration of the study (five years) of previous global climatologies. In this study we identify COLs systems in the Northern Hemisphere for a 41-year period (1958 to 1998) using an approach based in imposing the three main physical characteristics of the conceptual model of COL (a. closed circulation and minimum of geopotential, minimum of equivalent thickness, and two baroclinic zones, one in front of the low and the other behind the low). Data from NCAR-NCEP reanalysis were used. The aim of the study is to detect trends and to identify associations both with blocking events and major modes of climate variability. Results show that 1) in the Asian sector both less intense and more intense COLs had a significant positive trend whereas in the Pacific and the Atlantic sectors only less intense COLs had a significant positive trend, 2) Most of COLs were associated with blocking events, 3) During positive ENSO phases the number of less intense COLs in the Pacific were lower than during negative ENSO phases and 4) During positive Northern Annular Mode (NAM) phases the number of less intense COLs in the Atlantic were higher than during negative NAM phases.