Time-Extrapolated Rainfall Normals for Central Equatorial Pacific Islands

Abstract

Normal annual rainfalls (means and medians) for the period 1910?75 are estimated for islands in the central equatorial Pacific. Ridge regression, with an empirically determined bias constant, is used to establish the relationships among the island rainfalls and those at Fanning, Nauru and Banaba as well as the mean sea level pressure at Darwin. The estimated medians are less than those observed; the estimated means are greater than observed. Since rainfall distributions have large positive skewness, the median is more representative of central tendency, and the difference between the mean and the median is a measure of the variability. These results imply that rainfalls in the central Pacific were smaller and more variable in the earlier part of this century. Variations in the position and intensity of the two major convective bands in the Pacific, the Near Equatorial Convergence Zone and the South Pacific Convergence Zone appear to account for most (88%) of the interannual rainfall variability. Although the interannual rainfall variability is large, the secular variability is not. Most of the adjustments were on the order of 10%, suggesting that even short (20?30 year) data records provide reasonably accurate estimates of normal rainfalls for these islands.