1. a Instituto de Geociências , University of Campinas , PO Box 6152, 13.083-970, Campinas, Sa˜o Paulo, Brazil E-mail:
2. b CEPAGRI, University of Campinas, 13.083-970 , Campinas, Sa˜o Paulo, Brazil
3. c National Geophysical Data Center/NOAA, 325 Broadway , Boulder, CO 80305, USA