Monsoon onset is determined for 42 summer seasons from 1957/58 to 1998/99 for the Southern Hemisphere tropics using a similar definition to that of Drosdowsky (1996), applied to daily NCEP reanalysis zonal winds on a 2.5 degree grid. Mean onset over the 42 seasons shows a steady southward progression from near the equator in September to south of 12^oS by late December, at most longitudes from Africa across the Indian Ocean and into the west Pacific to beyond the dateline. Compared with the Asian Northern Hemisphere monsoon, the Southern Hemisphere system has a greater longitudional extent, but a much more restricted, and slower, relative to the annual cycle, latitudional spread.

The major spatial patterns or modes of variability of onset date anomalies for the 42 seasons are extracted by Empirical Orthogonal Function (EOF) analysis. The first mode accounts for 25% of the total interannual variance in onset dates. Its spatial structure suggests that the largest variability in onset dates occurs over the eastern Indian Ocean southwest of Sumatra, and over the western Pacific Ocean to the northeast of New Guinea, with opposite phase between these two regions. A number of groups of seasons are produced based on the amplitudes of the first four EOFs. Composite anomalies for these groups suggest that the first four EOFs represent real modes of variability, and are not artifacts of the EOF analysis requirements of maximum variance and orthogonality between components.

Correlations between the amplitudes of the first four EOFs and a variety of large scale circulation and SST indices suggest that only the first EOF is related to the El NiZ o/ Southern Oscillation (ENSO). While the simultaneous correlations between the ENSO indices and onset dates are generally strongest, there are some significant correlations at up to three months lag during the winter and early spring prior to onset.