Relationship between the Subtropical Jet Stream and the generation of Surface Winds and Swells along the Western Coast of South America

Abstract

Extreme swell episodes along the Pacific coast of South America are low-frequency but highimpact phenomena that often originate from distant synoptic disturbances in the South Pacific. These events have the potential to disrupt port operations, fisheries, and coastal infrastructure, particularly when they reach a “very strong” intensity. For instance, the anomalous swell of May 2023 led to flooding and maritime restrictions across several Peruvian localities. Between 2008 and 2023, Peru’s Directorate of Hydrography and Navigation (DIHIDRONAV) issued more than 820 abnormal swell warnings, with only 1% categorized as “very strong” due to their exceptional intensity and impact. This study focuses on the atmospheric conditions associated with these high-impact events, particularly the role of upper-tropospheric dynamics. Using reanalysis data and internal records from DIHIDRONAV, five “very strong” swell events recorded during the austral winter (May–September) were selected for analysis. The methodology integrates synoptic composites, principal component analysis (PCA), and Euclidean distance metrics to assess vertical coherence and anomaly magnitude across multiple pressure levels. Furthermore, a multiparametric jet stream diagnostic is applied for a detailed characterization of the upper-level atmospheric circulation. The results indicate that extreme swell events are preceded by a vertically aligned and dynamically mature baroclinic structure that extends from the surface to the upper troposphere. At 250 hPa, the polar front jet exhibits significantly higher intensity, sharper wind gradients, and enhanced zonal persistence compared to climatology, supporting its role as a modulating factor in swell generation. Moreover, PCA and Euclidean distance analyses suggest the unusual nature of these events—especially aloft—relative to multidecadal climatology. These findings underscore the importance of upper-level dynamics in driving ocean–atmosphere interactions and provide a basis for improving early-warning systems in the region.