The team built their classification from the winds at the 850 hectopascal pressure level, roughly 1.5 kilometers above the surface, using the ERA5 reanalysis, the most comprehensive global atmospheric dataset produced by the Copernicus Climate Change Service. The 850 hPa level is a strategic choice for this part of the world because it captures the low-level circulation that steers moisture from the Amazon basin, the South Atlantic, and the South Pacific into the continent’s interior. By grouping thousands of daily wind fields into a small number of representative configurations, the researchers distilled the chaotic day-to-day weather of subtropical South America into a manageable catalogue of seven circulation patterns, or CPs, that together capture the full range of the region’s temporal variability, from individual storms to multi-decadal trends.

What makes the seven patterns compelling is that they are not statistical abstractions; each one corresponds to a physically recognizable feature of South American meteorology. The classification reproduces the behavior of the South American Low Level Jet, the narrow corridor of moist winds that races southward along the eastern flank of the Andes and feeds severe thunderstorms over the plains of Argentina. It captures the South Atlantic Convergence Zone, the vast northwest-to-southeast band of clouds and convection that anchors the summer monsoon. It also tracks the northward march of weather perturbations into the subtropics and the dominant circulation anomalies that set up over the adjacent Atlantic and Pacific Oceans, which act as the region’s great atmospheric switches.

The seasonal behavior of each pattern emerges clearly from the analysis. Certain configurations dominate the austral summer monsoon months, while others characterize the transitional seasons or the drier winter regime. This seasonality matters because it means the same circulation catalogue can be applied year-round without losing physical meaning, a limitation that has hampered previous pattern-based studies focused on single seasons. The authors verified that their seven CPs provide a faithful representation of both the synoptic features and their annual cycle, striking a balance between detail and parsimony that a larger or smaller set of classes failed to achieve.

Perhaps the most immediately useful finding is the tight link between the circulation patterns and rainfall. Each CP is associated with distinct rainfall anomalies and heavy precipitation events across different subregions of subtropical South America. In practical terms, knowing which pattern is in place tells forecasters and water managers which areas face elevated odds of extreme rain and which are likely to stay dry. Because the classification is built on daily data, it can flag the circulation setups that precede flooding episodes in the La Plata Basin, or the persistent blocking configurations that starve central Chile of the frontal rains its Mediterranean-style climate depends on.

The study then connects these daily patterns to the planet’s great climate oscillations. The influence of the El Niño Southern Oscillation, the periodic warming and cooling of the tropical Pacific, is clearly reflected in the interannual variability of the CP frequencies: during El Niño or La Niña years, certain patterns appear more or less often, shifting the regional rainfall odds in characteristic ways. Just as importantly, the researchers documented cross-time-scale interactions, showing that subseasonal drivers such as the Madden Julian Oscillation modulate how ENSO’s influence plays out on the ground. This layered interaction, where a slowly evolving Pacific anomaly conditions the impact of a fast-moving equatorial wave, helps explain why the same ENSO event can produce different rainfall outcomes in different years, a puzzle that has long frustrated seasonal forecasters.

The long-term story is where the study turns from description to attribution. The team found that long-term trends in the Southern Annular Mode, the north-south see-saw of westerly winds around Antarctica, drive frequency changes in one of the circulation patterns typical of transitional seasons, and that this shift promotes a drying trend over central Chile. Meanwhile, the Pacific Decadal Oscillation, a slower oscillation of North Pacific sea surface temperatures with hemisphere-wide reach, alters the frequency of a summer pattern in ways that promote drying over the subtropical eastern Andes. In other words, two of the region’s most alarming climate trends, the central Chile megadrought and the drying of the Andean foothills, can be traced through the circulation patterns to specific remote climate drivers operating on decadal scales. The CP framework thus functions as an attribution tool, converting abstract global indices into concrete statements about which atmospheric configurations are becoming more or less common and what that means for regional water supplies.

This attribution capability arrives at a critical moment. Central Chile has endured one of the most severe multi-year droughts ever recorded in the Southern Hemisphere, with cascading effects on agriculture, hydropower, and the capital city of Santiago’s water security. The La Plata Basin, home to tens of millions of people and much of the continent’s grain production, swung from record floods to a punishing 2019 to 2021 drought within a single decade. Understanding which circulation patterns underpin these swings, and whether their frequencies are shifting under the combined pressure of natural variability and anthropogenic climate change, is essential for anticipating what the coming decades hold. The new classification provides a common vocabulary for those discussions, one grounded in daily weather rather than abstract seasonal averages.

Beyond diagnosis, the authors highlight a second practical application: model evaluation. Because the seven CPs are defined purely from large-scale wind fields, they can be diagnosed equally well in global and regional climate models. Comparing the simulated frequency, persistence, and rainfall associations of each pattern against the ERA5-based benchmark offers a rigorous test of how faithfully models represent the atmospheric engine of South American hydroclimate. Patterns that are missing, overrepresented, or incorrectly linked to precipitation in a model point directly to the physical processes that need improvement. Given that climate projections for the region carry substantial uncertainty, particularly for summertime rainfall, this reference classification gives model developers and downscaling studies a concrete target for validation, complementing earlier work by members of the same team on extreme precipitation and circulation types in southern South America.

The full methodology is transparent and reproducible. The ERA5 reanalysis data are openly available through the Copernicus Climate Data Store, daily precipitation comes from the gauge-based CPC global dataset maintained by NOAA, and all the climate indices used, from ENSO and the Indian Ocean Dipole to the Southern Annular Mode, the Pacific Decadal Oscillation, the Atlantic Multidecadal Oscillation, and the Madden Julian Oscillation indices, are publicly distributed. The R scripts that perform the classification and analysis have been released on GitHub, allowing any researcher to replicate the results, extend them to other regions, or apply the framework to model output. This openness, combined with the elegance of reducing a continent’s weather to seven archetypes, positions the study to become a standard reference for anyone studying, forecasting, or modeling rainfall across subtropical South America, from synoptic meteorologists tracking the next flood season to climate scientists weighing the fingerprints of a warming world on the winds above the Andes.

Subject of Research: Atmospheric circulation patterns and teleconnections controlling rainfall variability in subtropical South America

Article Title: Multi-temporal diagnostic of atmospheric circulation patterns and teleconnections in subtropical South America

Article References: Medina, F. D., Olmo, M. E., & Bettolli, M. L. (2026). Multi-temporal diagnostic of atmospheric circulation patterns and teleconnections in subtropical South America. Climate Dynamics, 64(10), Article 432. https://doi.org/10.1007/s00382-026-08393-9

Image Credits: AI Generated

DOI: 10.1007/s00382-026-08393-9

Keywords: atmospheric circulation patterns, subtropical South America, synoptic climatology, ERA5 reanalysis, ENSO, Southern Annular Mode, Pacific Decadal Oscillation, South American Low Level Jet, South Atlantic Convergence Zone, rainfall variability, drought, climate dynamics