April 1999 freeze event in central South America—A rare case

The April 1999 freeze in central South America is a rare event because an event with such an intensity has never occurred as early as the third week of April in the last 40 years. The minimum temperatures fell by more than 15C in 24 hours to near zero in many places in southern Brazil on the 16th. Many stations have registered the occurrence of freeze during the period 16th, 17th and 18th. On 17th light and moderate snow was reported in and around São Joaquim (28.29oS, 49.92oW). The maximum temperatures were down to 22C in southern Amazon region as far north as 15oS on 18th. There was some damage to agricultaral activity in southern Brazil. However due to accurate forecast guidance from the CPTEC global model with 5 days lead time and close monitoring of the event the losses were kept to a minimum.

A surface high pressure cell in the Pacific near 110oW off the Chile coast remained stationary (or with little movement) for three days from 11th April before it started its eastward movement entering South American continent on 15th. The cell acquired a bean shape as it crossed the Andes, thus splitting itself into two cells. The eastern cell intensified and moved north-northeastward into northern Argentina, Paraguay, central and southern Brazil, Bolivia and southern Amazon region. Two cyclogeneses near the Uruguay-Rio Grande do Sul coast were reponsible for the intensification of the southerlies over the entire stretch of Argentina. These winds were responsible for intense cold advection into central South America. The ageostrophic secondary circulation in the entrance region of the subtropical (outiside) jet north of a deep upper trough over Argentina was responsible for clear skies in central South America which in turn was responsible for the steep nocturnal drop of temperature causing freezes. A trough formation in the cold air sector emanating westwaed from the center of the cyclone in the Atlantic over the uplands of southern Brazil was reponsible for the snow fall on 17th. The baroclinic wave development with the trough over Argentina and the ridge over the Pacific on 15th, prior to the freeze in southern Brazil, was dramatic. During the development phase the vorticity advection dominated over the oceans where as the differencial thermal advection dominated over the continent. The sum total of the two effects was an elongated band of positive geopotential tendency at 850 hPa across northern Argentina, southern Brazil and into the Atlantic, with a northwest to southeast orientation.

The sequence of events, the synoptic development and the precursors of this early event are described and discussed. A tentative conceptual model for the freeze is presented.

Principal author's eMail : saty@cptec.inpe.br