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

Cristian Muñoz,David M Schultz

doi:10.1175/jamc-d-20-0135.1

Abstract

AbstractA study of 500-hPa cutoff lows in central Chile during 1979–2017 was conducted to contrast cutoff lows associated with the lowest quartile of daily precipitation amounts (LOW25) with cutoff lows associated with the highest quartile (HIGH25). To understand the differences between low- and high-precipitation cutoff lows, daily precipitation records, radiosonde observations, and reanalyses were used to analyze the three ingredients necessary for deep moist convection (instability, lift, and 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 cutoff lows was different, with a moisture plume that was more defined and more intense equatorward of HIGH25 cutoff lows as compared with LOW25 cutoff lows where the moisture plume occurred poleward of the cutoff low. The presence of the moisture plume equatorward of HIGH25 cutoff 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 cutoff lows persisted for longer than 72 h, only 25% of HIGH25 cutoff lows did, despite both systems occurring mostly during the rainy season (May–September). The occurrence of an equatorial moisture plume on the eastern and equatorial edge of cutoff lows is fairly common during high-impact precipitation events, and this mechanism could help to explain high-impact precipitation where the occurrence of cutoff lows and moisture plumes is frequent.

Highlights

Cutoff lows are closed upper-level low pressure systems that detach from the main westerlies, mostly by Rossby wavebreaking events (e.g., Ndarana and Waugh 2010) or by stratospheric air intrusions from the polar reservoir to midlatitudes through tropopause folds (e.g., Keyser and Shapiro 1986; Price and Vaughan 1993; Wernli and Sprenger 2007)
As not all cutoff lows are associated with heavy precipitation, we investigate this link for two groups of cutoff lows: those lows associated with low daily precipitation amounts in central Chile and those lows associated with high daily precipitation amounts in central Chile
Let us consider two cutoff lows affecting central Chile that were independent of each other: one identified as a LOW25 cutoff low that left an average precipitation of around 4 mm per station during the heaviest precipitation day associated with the LOW25 cutoff low, and the other identified as a HIGH25 cutoff low that left an average precipitation of around 20 mm per station during the heaviest precipitation day associated with the HIGH25 cutoff low

Summary

Introduction

Cutoff lows are closed upper-level low pressure systems that detach from the main westerlies, mostly by Rossby wavebreaking events (e.g., Ndarana and Waugh 2010) or by stratospheric air intrusions from the polar reservoir to midlatitudes through tropopause folds (e.g., Keyser and Shapiro 1986; Price and Vaughan 1993; Wernli and Sprenger 2007). Studies associating extreme-precipitation events with the presence of moisture plumes around cutoff lows are mostly limited to independent case studies (e.g., Hirota et al 2016; Bozkurt et al 2016), a recent study by Tsuji and Takayabu (2019) suggests the interplay between cutoff lows and atmospheric rivers is key for explaining heavy-precipitation events in Japan. The link between cutoff lows and moisture plumes in initiating heavy-precipitation events, in central Chile, is still unresolved climatologically. This article aims to characterize the link between moisture plumes and cutoff lows affecting central Chile and how this link influences precipitation associated with cutoff lows.

Data and methods

Case studies of LOW25 and HIGH25 cutoff lows

Findings

Conclusions