Avalia？？o de mudan？as na frequência de sistemas frontais sobre o sul da América do Sul em proje？？es do clima futuro

the frequency of frontal systems reaching the south of south america is compared with reanalysis ncep/ncar and results of gfdl and hadley models for the future climate. three regions were analyzed to identify the frontal systems, in areas of brazil, uruguay, paraguay and argentina. the following criteria were used: increase of sea surface pressure, reduction of the temperature and change of the meridional wind at 850hpa, in two consecutive days. selected cases with temperature drop above five degrees, considered the most intense, were also analyzed. for the present climate it was observed that the models simulated well the synoptic characteristics of frontal systems. however, when comparing the frequency of frontal occurrence between reanalysis data and models simulations, the gfdl and hadley models overestimated their numbers. the hadley model overestimates further more than the ncep and gfdl in present climate. the positive trend observed in area 3 (65ow-60ow, 33os-38os) from ncep reanalysis is not simulated by the models. when the most intense systems are selected, the gfdl model shows higher overestimation in number of frontal systems. both models indicate an increase of frontal systems frequency in future in the three areas, although less in area 3.