On the Contribution of Major Warming Episodes in the Tropical East Pacific to a Useful Prognostic Relationship Based on the Southern Oscillation

Abstract

It is known that the Southern Oscillation Index (SOI) and the mean sea surface temperature off the Peru Coast are highly coherent and that variations of the latter are dominated by infrequent warming episodes. The present study examines the relative contribution of these warming episodes to the covariance of statistically significant correlations between the fall SOI and winter mean 700 mb heights in the Northern Hemisphere. The degree of dominance of the warming episode years in this context is evaluated by Monte Carlo methods. It was found that, for the 30-year period studied, data pairs following tropical east Pacific warming events contributed disproportionately to major correlation maxima in much of the Northern Hemisphere. Such covariance concentrations, however, were found to be fairly likely outcomes (probability > 9%) if groups of years are chosen at random from the appropriate covariance arrays. Thus, we conclude that the influence of the fall SOI upon the subsequent winter mean 700 mb height distribution is a rather pervasive one, not limited to tropical east Pacific warming situations. In contrast to other areas, correlation maxima in the North American sector received disproportionately small covariance contributions from the warming episode years. In northwest Canada, the contribution of those years was small and opposite in sign to the total covariance.