A Midlatitude Climatology and Interannual Variability of 200- and 500-hPa Cut-Off Lows

Abstract

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 of 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 shift 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 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.

Tropospheric Ozone Variability Over Hong Kong Based on Recent 20 years (2000–2019) Ozonesonde Observation

“…Various dynamic processes such as cut‐off low and tropopause fold can trigger stratospheric ozone intrusion over the abovementioned region (Li et al., 2014; Lin et al., 2014; Ma et al., 2014; Zhao et al., 2019a). While cut‐off low events occur more frequently in the northern midlatitude region (e.g., Northeast Asia) (Munoz et al., 2020), tropopause folds occur more frequently in the southern midlatitude region (e.g., Yangtze River Basin) (Skerlak et al., 2015). If atmospheric waves associated with tropopause folds have large amplitudes, they could break, resulting in the mixing of tropical and extratropical air (Pan et al., 2009) and contributing to the exchange of trace gases, such as ozone, between the tropics and extratropics.…”

Structure and evolution of intense austral cut‐off lows

“…Our results show that the tropopause intersects in the COL centre inside the cold core (just above the coldest anomalies) within a region of sharp temperature gradient, also called frontal or baroclinic zone. However, there have been many studies on COLs using methods based on criteria that search for cold cores at very high levels (for example, between 200 and 300 hPa) in both the Southern and Northern Hemispheres (Nieto et al, 2005;Gimeno et al, 2007;Porcù et al, 2007;Reboita et al, 2010;Muñoz et al, 2019) which may result in differences between studies, as recently discussed by Pinheiro et al (2019). Unfortunately the lack of studies focusing on the structure of COLs and the difference between hemispheres does not allow us to determine whether the difference in interpretation is a result of the regional peculiarities.…”

Synoptic causes of torrential rainfall in South-eastern Spain (1941–2017)

“…The acronym DANA refers to an isolated high-altitude depression and has come to replace, with some nuance, the popular expression 'gota fría', or cut-off low (Palmén, 1949). A DANA results from the strangulation of a polar jet stream trough at latitudes similar to those of southern Europe, which gives rise to a cyclonic vortex in the middle and upper troposphere with the corresponding injection of cold air (Palmén, 1949;Martín-Vide, 1984;Quereda, 1989;Martín-Vide, 1989;Llasat, 1991;Gil Olcina and Olcina, 1999;Martín León, 2003;Nieto et al, 2005;Llasat et al, 2007;Munoz et al, 2019;Alshouhani, 2020).…”