Jet stream shift brings winter sunshine to Brazil, cloud to Argentina

The shift in the jet stream in June has brought significant variations in South America’s weather conditions. Rio de Janeiro experienced its sunniest winter in almost two decades, while southern Argentina has seen its cloudiest winter.

Rio de Janeiro recorded the sunniest June in over 18 years, with solar irradiance 15% above the long-term average. This was part of a band of higher than average sunshine extending through Bolivia, Paraguay, and parts of Brazil. The jet stream stayed further south than usual, creating a stable upper atmosphere over these regions that suppressed cloud formation, resulting in sunnier and warmer conditions than typically expected for this time of year.

Conversely, southern Argentina saw an irradiance drop of up to 30% below the long-term average as it experienced its cloudiest June in the same period. The anomalous jet stream position increased atmospheric instability, causing winter-time cold fronts to remain further south. 

In the central-eastern part of Argentina, Buenos Aires saw a more moderate decrease in irradiance, about 10% below the typical level, showing the broader impact of these unusual weather patterns.

June, a winter month in South America, naturally has the lowest total irradiance due to the midday sun reaching its lowest point at the Winter Solstice on June 20. While the sun's position dictates the maximum available sunlight, local cloud cover significantly influences the actual irradiance reaching the surface driving inter-annual variability.

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