El Nino-La Nina Dynamics over Southeast Asia

During the past two decades, the Philippines has experienced unusual droughts and floods due to the climatological phenomena called El Nino and La Nina. El Nino occurs approximately once every five years, while La Nina is less frequent. El Nino is a physical events that is generated by anomalously warm sea surface temperatures (SST) over the Central and Eastern Pacific Ocean. The abnormally warm SSTs and overlying air cause changes in the large-scale atmospheric circulations. Normal convergence zones of moisture are shifted during El Nino events causing widespread modifications in the amount of rainfall over certain regions in the Pacific.

For example, in India and Indonesia, below normal rainfall is observed. The same happens in Brazil and in Australia. On the other hand, these episodes produce above normal rainfall in Sri Lanka, Peru and Equador. In Southeast Asia, the frequency and tracks of tropical cyclones are known to be affected. Preliminary analysis indicate that there are less tropical cyclones during El Nino. Furthermore, the tracks of cyclones are displaced generally northward. More detailed descriptions of the effects of El Nino are described bu Hasternath.

In general, the opposite happens during the La Nina period. It occurs during periods of abnormally cold SST over the Central and Eastern Pacific. Usually, where there were droughts during the El Nino, floods occur.

Over the years, these two phenomena have been studied extensively over many regions of the globe. In Southeast Asia, however, there have been extremely few studies done. This paper is an observational and diagnostic study of the dynamics and impact of El Nino and La Nina in Southeast Asia.

[Presented at the Samahang Pisika ng Pilipinas (SPP) Congress, 1999.]