The Role of the Tropical Convection over the Western Pacific and the Indian Oceans in the Generation of Frost Events in the Southern cone of South America.

MÜLLER GABRIELA V; AMBRIZZI, TÉRCIO

Concepción

Workshop; CLIVAR workshop on the South Pacific; 2005

This work indicates how anomalous convection observed in the western Pacific and South Indian Ocean can play an important role in generating extratropical wave patterns which favor the development of extreme cold events over the southeast of South America (SSA). Numerical simulations using a primitive equation model were carried out and their results were compared with observational data analysis. From the composite analysis it turned out that an anomalous heating observed in the region of the tropical Pacific Ocean to the northeast of Australia during austral winters with maximum occurrence of Generalized Frosts (GF) over SSA can trigger Rossby waves that propagate towards the South American continent favoring the occurrence of these events. Numerical experiments indicate that the extension of the anticyclonic region and the cold advection over SSA generated when the heat forcing is placed over the western Pacific Ocean are similar to that observed during GF winters. However, the amplitudes seem to be insufficient to generate the mass field and motion anomalies observed in the composites of GF winters. When another heat source is placed over the western South Indian Ocean, the expected wave pattern observed during these events is achieved. The results indicate the existence of a double train of Rossby waves which propagate along the subtropical and polar jets, respectively, whose phases coincide just before they reach South America. This pattern generates a strong polar air advection over the southern cone of the continent with the consequent decrease in surface temperature and large areas of generalized frosts in the region. This mechanism suggests that tropical and extratropical wave interaction over the South Pacific is important to generate the appropriate atmospheric dynamical environment for the occurrence of GF in the southern and southeastern regions of South America.