Previous research has discussed the importance of meridional migrations of the North Atlantic Subtropical High (NASH) on U.S. precipitation patterns, but the mechanisms that control these meridionial migrations are virtually unknown. We have observed that, under certain conditions, a southward migration of the NASH is associated with deep tropical incursions of cold surges from the winter hemisphere over South America. When upper tropospheric winds are westerly over Amazonia, cold surges originating from extratropical South America can penetrate deep into the tropics and increase geopotential height over a broad region span from equatorial South America, across the intra-American seas, and into the subtropics of the Northern Atlantic, with anomalies exceeding +1 standard deviation to at least 18°N. The anomalous geopotential and temperature gradients associated with the South American cold surge induces a shallow tropical meridional circulation. The latter in turn increases the lower tropospheric geopotential height over the tropical to subtropical North Atlantic, leading to the equatorward expansion of the NASH. This study uncovers the importance of shallow circulations in the cross-equatorial teleconnection.Geological Science
