Polar Air Outbreaks in the Americas

Publisher Summary Polar outbreaks have long attracted the attention of meteorologists, and climatologists. This chapter describes their air temperatures, air mass central pressures, and wind characteristics, in relation to the atmospheric circulation, and their trajectories over the continents. Outbreaks of polar air into low latitudes tend to organize tropical convection, and rain in summer, while the cold air tends to produce cooling in lower latitudes, and freezes in the subtropics during winter. Cold waves accompanying polar outbreaks have significant impacts in America, including adverse effects on coffee production in South America, and on citrus production in North America. Synoptic events leading to cold waves are similar in America, particularly the upper air patterns, and forcing, but the intensity of the outbreaks of temperature, and pressure is larger in North America. Typically, they are associated with an amplified ridge lying across the western edge of the continent. Records of freezes in coffee- and citrus-growing areas are used to reconstruct subtropical climate variability, and identify linkages to low-frequency variability mechanisms in the atmospheric circulation. Results suggest that there is little link between the occurrence of E1 Nino, and cold waves, and it freezes either hemisphere. Citrus freeze occurrences in Florida are linked to long-term variability in the Pacific-North American (PNA) teleconnection pattern, and they appear to occur in clusters concentrated near the end of each century. Comparisons are made of possible paleoclimate scenarios for the occurrence of cold waves based on evidence reconstructed from pollen, and lake sediment samples collected in southeastern, and central Brazil.