Midtropospheric cold vortices in the subtropics of South America

It is not uncommon to observe the development of midtropospheric cold core vortices in the subtropics (20S - 30S) of South America in winter (Jun-Jul-Aug). They have the maximum amplitude in the middle levels, feebly detectable at the 850 hPa level and are almost not detected in the surface pressure analysis. They cause cloudiness and light to moderate rain as they travel east-northeastward over northern Argentina, Paraguay and southern Brazil with an average speed of 8o longitude per day. Their typical horizontal extension (half wavelength) is 1000 km and are fairly symmetrical in the North-South and East-West directions. The temperatures in the core of the vortex are 3o to 4oC cooler than the surroundings at the 500 hPa level. The relative vorticity can attain values of the order - 8X10-5 s-1 and the upward vertical velocity maximum reaches values of - 3X10-3 hPa s-1 (~ 5 cm s-1) and remains to the east and northeast of the vorticity maximum at the 500 hPa level. These systems dissipate soon after they reach the Atlantic ocean near Rio de Janeiro which can be seen through the gradual reduction of the intensity and size of the vorticity and vertical velocity nuclei as they approach the coast. In the months of June and July 1999 two vortices were observed (13-16 June, 6-9 July) and a composite of these two is obtained to represent their common features. The forecasts of these systems with lead times of up to 120 hours by the CPTEC/COLA oprational global model are compared with the NMC analysis.