Cutoff Lows, Moisture Plumes, and Their Influence on Extreme-Precipitation Days in Central Chile

Abstract

A study of 500-hPa cut-off lows in central Chile during 1979–2017 was conducted to contrast cut-off lows associated with the lowest quartile of daily-precipitation amounts (LOW25) with cut-off lows associated with the highest quartile (HIGH25). To understand the differences between low- and high-precipitation cut-off lows, daily-precipitation records, radiosonde observations, and reanalyses were used to analyze the three ingredients necessary for deep moist convection (instability, lift, moisture) at the eastern and equatorial edge of these lows. Instability was generally small, if any, and showed no major differences between LOW25 and HIGH25 events. Synoptic-scale ascent associated with Q-vector convergence also showed little difference between LOW25 and HIGH25 events. In contrast, the moisture distribution around LOW25 and HIGH25 cut-off lows was different, with a moisture plume more defined and more intense equatorward of HIGH25 cut-off lows compared to LOW25 cut-off lows where the moisture plume occurred poleward of the cut-off low. The presence of the moisture plume equatorward of HIGH25 cut-off lows may have contributed to the shorter persistence of HIGH25 events by providing a source for latent-heat release when the moisture plume reached the windward side of the Andes. Indeed, whereas 48% of LOW25 cut-off lows persisted for longer than 72 h, only 25% of HIGH25 cut-off lows did, despite both systems occurring mostly during the rainy season (May to September). The occurrence of an equatorial moisture plume on the eastern and equatorial edge of cut-off lows is fairly common during high-impact precipitation events, and this mechanism could help explain high-impact precipitation where the occurrence of cut-off lows and moisture plumes is frequent.