Two Severe Freezes in Brazil: Precursors and Synoptic Evolution

Abstract

Two freezes with repercussions on world coffee markets struck Brazil in recent years, one during FGGE in 1979, the other in 1981. Data from multiple sources including satellites and drifting buoys were analyzed for early warning signs and synoptic evolution of the freeze events. Two important precursors were found: 1) A slowly moving long-wave pattern in the central South Pacific Ocean amplified greatly one to two days before frontogenesis in South America and four to five days before the freezes in Brazil; 2) the long-wave ridge, when at maximum amplitude, was located near the southern Andes, and the next long-wave trough downstream was located in the South Atlantic at the longitude of Brazil. This configuration channeled subantarctic air northward entirely over land into subtropical latitudes. Frontogenesis in low latitudes in the mid-Pacific was the first sign of wave amplification in the 1979 case. Subsequent frontogenesis in northern Argentina was accelerated by the opportune approach of a short-wave trough one day before the downstream long-wave trough reached greatest amplitude in southern Brazil. Vigorous entry of polar air impelled the cold front as far as the equator. As the long-wave trough left Brazil, a hard freeze occurred in four states, cast of the polar anticyclone. In 1981 the long-wave pattern in the Pacific amplified as in 1979, but with a phase relationship displaced some 10° west, relative to the first case. freezes did not occur in eastern Brazil until after the northern portion of the Atlantic trough became cutoff. On the passage of the cold-core vortex through Brazil, freezing temperatures struck a large swath of land north of the Tropic of Capricorn where a freeze is rare. Analysis of Hovmöller ridge-trough diagrams suggests that group-velocity propagation of wave energy from the mid-Pacific contributed to part of the unusual amplification of the cold trough in South America. The opportune superposition of short-and long-wave troughs also contributed to the strong cold air invasion.