We consider a 396-year record of the Southern Oscillation Index (SOI) the earlier portion of which (1601--1851) was obtained from a dendrochronological reconstruction of Lough and Fritts (1985). Associated with the SOI point processes of upcrossings the positive level and downcrossings the negative level can be interpreted as sequences of El Ni¿o and La Ni¿a occurrences. In the intensities of these processes we emphasize harmonics with periods of 74 and 320 years and prove their statistical significance. ¿ American Geophysical Union 1996