Climatological Features of Cutoff Low Systems in the Northern Hemisphere

Abstract

This study presents the first multidecadal climatology of cutoff low systems in the Northern Hemisphere. The climatology was constructed by using 41 yr of NCEP-NCAR reanalysis data and identifying cutoff lows by means of an objective method based on imposing the three main physical characteristics of the conceptual model of cutoff low (the 200-hPa geopotential minimum, cutoff circulation, and the specific structure of both equivalent thickness and thermal front parameter fields).Several results were confirmed and climatologically validated: 1) the existence of three preferred areas of cutoff low occurrence (the first one extends through southern Europe and the eastern Atlantic coast, the second one is the eastern North Pacific, and the third one is the northern China-Siberian region extending to the northwestern Pacific coast; the European area is the most favored region); 2) the known seasonal cycle, with cutoff lows forming much more frequently in summer than in winter; 3) the short lifetime of cutoff lows, most cutoff lows lasted 2-3 days and very few lasted more than 5 days; and 4) the mobility of the system, with few cutoff lows being stationary. Furthermore, the long study period has made it possible (i) to find a bimodal distribution in the geographical density of cutoff lows for the European sector in all the seasons (with the exception of winter), a summer displacement to the ocean in the American region, and a summer extension to the continent in the Asian region, and (ii) to detect northward and westward motion especially in the transitions from the second to third day of occurrence and from the third to fourth day of occurrence.The long-term cutoff low database built in this study is appropriate to study the interannual variability of cutoff low occurrence and the links between cutoff lows and jet stream systems, blocking, or major modes of climate variability as well as the global importance of cutoff low in the stratosphere-troposphere exchange mechanism, which will be the focus of a subsequent paper.

The link between cut-off lows and Rossby wave breaking in the Southern Hemisphere

“…In the Northern Hemisphere (NH), Kentarchos and Davies (1998) found, using a subjective approach, that COLs tend to occur more often in summer than in winter at 200 hPa. This was confirmed by Nieto et al (2005) in an objective hemisphericwide study using a much longer dataset. Both studies suggest that the seasonal variability of COLs is related to that of jet streams; that is, COLs tend to form when the jets are weaker.…”

“…Both processes have been established to have durations longer than one day (e.g. Peters and Waugh 1996;Fuenzalida et al, 2005;Nieto et al, 2005;Reboita et al, 2009). Thus for our purposes it suffices to use daily averaged reanalysis data (from four times each day).…”

“…The methodology used to define COLs is very similar to that of Nieto et al (2005) and Reboita et al (2009), with a more explicit imposition of the cold-core condition similar to that in Fuenzalida et al (2005). The algorithm is a multiple step process, and has been applied to identify COLs on different pressure surfaces.…”

“…In their study they also addressed the regional interseasonal variability of these systems. More recently, Reboita et al (2009) extended the work of Nieto et al (2005) to the SH and produced a hemispheric climatology of COLs at 200 hPa. These two SH climatologies show that there are significant differences in the interseasonal variability of COLs between levels: Fuenzalida et al (2005) found higher frequencies during winter, in contrast to the summer maximum found by Reboita et al (2009).…”

Resolution Dependency of Numerically Simulated Stratosphere-to-Troposphere Transport Associated with Mid-Latitude Closed Cyclones in Early Spring around Japan

“…The definition of the STT-producing cyclone in this study meets the cut-off low condition for the deepening of an upper trough with a well-developed closed circulation. However, it does not include the condition for detaching from the basic westerly current to be located on the equatorward side of this current (e.g., Nieto et al 2005). We converted the numerical data to the same horizontal resolution (~1° × 1° longitude-latitude resolution) before event detection.…”