Precipitation Patterns Associated with the High Index Phase of the Southern Oscillation

Abstract

The relationships are examined between precipitation and the high index phase of the Southern Oscillation (SO) for 19 regions of the globe which have documented low SO index-precipitation relationships (Ropelewski and Halpert 1986, 1987). The study reveals that 15 of these regions also show evidence of characteristic precipitation anomalies during the high index phase of the SO. In each of the regions, the high SO index-precipitation relationships show the opposite sign of those documented for the low index. These precipitation relationships were consistent, holding for over 70% of the high SO index years, and statistically significant. In particular, the high index phase of the SO is associated with enhanced precipitation for the monsoons of India and northern Australia as well as for the rainy seasons in northeastern South America and southeastern Africa. High SO index precipitation was found to be less than median in the central Pacific, Minicoy-Sri Lanka, eastern equatorial Africa, the Gulf of Mexico and northern Mexico region, and southeastern South America. The seasons which showed high SO index-precipitation relationship were almost identical to the masons associated with the low index in 13 of the 15 high SO index regions. Thus to a first approximation, this study suggests that the sign of the precipitation anomaly for these regions is linearly related to the phase of the SO. Since there am 25 low index and 19 high index years in the 109 yr analysis period, from 1875 to 1983, these results further suggest that, for over 40% of the years precipitation may be classified and perhaps predicted on the basis of the extreme SO phase.