Synoptic structure of severe Zonda downslope windstorms in Argentina

Abstract

The Zonda wind is a characteristic Argentinian downslope windstorm that occurs on the eastern slopes of the Andes Mountains, creating extremely windy, warm and dry conditions with substantial socioeconomic impacts. This study aims to characterise and understand the synoptic-scale atmospheric dynamics, as well as the vertical structure of the atmosphere on both sides of the Andes, associated with severe and long-duration Zonda events compared with non-severe events. Severe events are most frequent between August and October, comprising 64.8% of occurrences. These events exhibit earlier onset and later cessation, resulting in more late-night and morning Zonda hours compared with non-severe events. The synoptic structure for severe and long Zonda events is associated with a more baroclinic structure, featuring a deep trough at both low and mid-levels and shifted west with altitude. Lower levels present higher anomalies and move faster than mid-level anomalies when the wind crosses the Andes. This is accompanied by a deep trough east of the Andes with a tilted axis and a slower eastward movement. The associated upper-level dynamic shows a stronger convergence and divergence pattern on the windward and lee sides of the Andes respectively, along with a stronger, elongated and persistent jet streak. The precipitation pattern is in agreement with the more intense winds impinging on the Andes, resulting in more intense rising motions and more precipitation on the windward slopes. Analysis of the vertical structure in severe events shows a more humid windward profile, especially at low and mid-levels, with stronger winds. The leeward side presents a more unstable temperature profile, warmer at low levels and colder at upper levels, and extreme dryness between low and mid-levels, indicating the presence of the Zonda at this altitude before the event starts. Given the limited studies on Zonda downslope windstorms in South America, this research represents a major step in our understanding of these severe events and provides valuable insights for weather forecasters.