The Influence of the Southern Oscillation on the Climate of Western Samoa

Abstract

This thesis is primarily concerned with the influence of the Southern Oscillation (SO) on the climate of Western Samoa. Among the Variables considered are the rainfall at selected stations on the two main islands of Western Samoa (Upolu and Savai'i), surface pressure, surface temperature, surface winds, all of which are recorded at the Apia Observatory, the 850 and 200 mb winds and the thickness between these two levels, recorded at Pagopago in American Samoa and the Southern Oscillation Index (SOI) of sea level pressure (the normalised pressure difference between Tahiti and Darwin). Relationships between suitably normalised climatic variables and the SO were examined for monthly, annual and seasonal data. The results are presented in the form of time series and correlation statistics. We find the relationships between the SO and the surface temperature and also the SO and the 200 - 850 mb thickness to be consistent with conclusions of previous studies by Horel and Wallace, Bjerknes and others. The high-index (low-index) phase of the SO tend to be accompanied by low (high) surface temperature and below (above) normal 200 mb heights. The variations of surface pressure and rainfall were found to be in phase with the fluctuations in the SO. The high correlations between the SOI and the anomalies of these two variables clearly illustrate their positive relationships, and the plots of their corresponding time series also highlight the same relationships. An examination of the correlations among the rainfall anomalies at various sites, reveals distinct regional characteristics of the rainfall distribution, which were not previously known to exist. Easterly component of the mean winds at the law levels were found to be significantly stronger during high SOI and weaker when the SOI is low.